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The Sheahan Diamond Literature Reference Compilation - Technical Articles based on Major Region - Africa: 2010 onwards
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcements called the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Region Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In addition most references have been tagged with one or more region words. In an effort to make it easier for users to track down articles related to a specific region, KRO has extracted these region words and developed a list of major region words presented in the Major Region Index to which individual region words used in the article reference have been assigned. Each individual Region Report contains in chronological order all the references with a region word associated with the Major Region word. Depending on the total for each reference type - technical, media and corporate - the references will be either in their own technical, media or corporate Region Report, or combined in a single report. Where there is a significant number of technical references there will be a technical report dedicated to the technical articles while the media and corporate references are combined in a separate region report. References that were added in the most recent monthly update are highlighted in yellow within the Region Report. The Major Region words have been defined by a scale system of "general", "continent", "country", "state or province" and "regional". Major Region words at the smaller scales have been created only when there are enough references to make isolating them worthwhile. References not tagged with a Region are excluded, and articles with a region word not matched with a Major Region show up in the "Unknown" report.
Kimberlite - diamondiferous
Lamproite - diamondiferous
Lamprophyre - diamondiferous
Other - diamondiferous
Kimberlite - non diamondiferous
Lamproite - non diamondiferous
Lamprophyre - non diamondiferous
Other - non diamondiferous
Kimberlite - unknown
Lamproite - unknown
Lamprophyre - unknown
Other - unknown
Future Mine
Current Mine
Former Mine
Click on icon for details about each occurrence. Works best with Google Chrome.
CITATION: Faure, S, 2010, World Kimberlites CONSOREM Database (Version 3), Consortium de Recherche en Exploration Minérale CONSOREM, Université du Québec à Montréal, Numerical Database on consorem.ca. NOTE: This publicly available database results of a compilation of other public databases, scientific and governmental publications and maps, and various data from exploration companies reports or Web sites, If you notice errors, have additional kimberlite localizations that should be included in this database, or have any comments and suggestions, please contact the author specifying the ID of the kimberlite: [email protected]
Aulbach, S., Stachel, T., Craeser, R.A., Heaman, L.M., Shirey, S.B., MUehlenbachs, K., Eichenberg, D., Harris
Sulphide survival and diamond genesis during formation and evolution of Archean subcontinental lithosphere: a comparison between the Slave and Kaapvaal cratons.
Buse, B., Schumacher, J.C., Sparks, R.S.J., Field, M.
Growth of bultfontenite and hydrogarnet in metasomatized basalt xenoliths in the B/K9 kimberlite, Damtshaa: insights into hydrothermal metamorphism pipe
Contributions to Mineralogy and Petrology, Vol. 160, 4, pp. 533-550.
Joint seismic geodynamic mineral physical modeling of African geodynamics: a reconciliation of deep mantle convection with surface geophysical constraints.
Earth and Planetary Science Letters, Vol. 295, 3-4, pp. 329-341.
Crustal structure of the Zimbabwe craton and the Limpopo belt of southern Africa: new constraints from seismic dat a and implications for its evolution.
South African Journal of Geology, Vol. 112, pp. 213-228.
Grantham, G.H., Manhica, A.D.S.T., Armstrong, R.A., Kruger, F.J., Loubser, M.
New SHRIMP, Rb/Sr and Sm/Nd isotope and whole rock chemical dat a from central Mozambique and western Dronning Maud Land: implications for eastern Kalahari
Journal of African Earth Sciences, Vol. 59, 1, pp.74-100.
Guzmics, T., Mitchell, R.H., Szabo, C., Berkesi, M., Milke, R., Abart, R.
Carbonatite melt inclusions in coexisting magnetite, apatite and monticellite in Kerimasi calciocarbonatite, Tanzania: melt evolution and petrogenesis.
Contributions to Mineralogy and Petrology, Vol. 161, 2, pp. 177-196.
Koehm, D., Lindenfeld, M., Rumpker, G., Aanyu, K., Haines, S., Passchier, C.W., Sachu, T.
Active transgression faults in rift transfer zones: evidence for complex stress fields and implications for crustal fragmentation processes in the western branch
International Journal of Earth Sciences, Vol. 99, 7, pp. 1633-1642.
Phanerozoic hot spot traces and paleogeographic reconstructions of the Siberian continent based on interaction with the Africa large low shear velocity province.
Neoproterozoic inheritance during Cainozoic rifting in the western and southwestern branches of the East African Rift system: evidence from carbonatite alkaline
Tanzanian Journal of Earth Science, Vol. 1, Dec. pp. 29-37.
Pivin, M., Valley, J.W., Spicuzza, M.J., Demaiffe, D.
Oxygen isotopic composition of garnet clinopyroxene and zircon megacrysts from kimberlites in Democratic Republic of Congo: insights into their petrogenesis.
International Mineralogical Association meeting August Budapest, abstract p. 560.
Trace element chemistry of mineral inclusions in eclogitic diamonds from the Premier ( Cullinan) and Finsch kimberlites: implications for evolution mantle
Viljoen, K.S., Harris, J.W., Ivanic, T., Richardson, S.H., Whitehead, K.
Trace element geochemistry and Ni thermometry of garnet inclusions in peridotitic diamonds from Premier and Finsch, South Africa: implications - diamond formation
International Mineralogical Association meeting August Budapest, abstract p. 187.
Origin of kimberlites from the Kundelungu region: lithospheric mapping, diamond potential and crustal evolution in southern Democratic Republic of the Congo
Thesis: Macquarie University Phd. ,
Africa, Democratic Republic of Congo
Thesis: note availability based on request to author
Buikin, A.I., Trieloff, M., Korochantseeva, E.V., Hopp, J., Kaliwood, M., Meyer, H-P.,Altherr, R.
Distribution of mantle and atmospheric argon in mantle xenoliths from western Arabian Peninsula: constraints on timing and composition of metasomatizing agents....
Craig, T.J., Jackson, J.A., Priestley, K., McKenzie, D.
Earthquake distribution patterns in Africa: their relationship to variations in lithospheric and geological structure, and their rheological implication
Geophysical Journal International, Vol. 185, 1, pp. 403-404.
Provenance and reconnaissance study of detrital zircons of the Paleozoic Cape Supergroup: revealing the interaction of Kalahari and Rio de la Plat a cratons.
International Journal of Earth Sciences, Vol. 100, 2, pp. 527-541.
Electrical conductivity of continental lithospheric mantle from integrated geophysical and petrological modeling: application to the Kaapvaal craton and Rehoboth
Journal of Geophysical Research, Vol. 116, B10, B10202
Honda, M., Phillips, D., Harris, J.W., Matsumoto, T.
He, Ne and Ar in peridotitic and eclogitic paragenesis diamonds from the Jwaneng kimberlite, Botswana - implications for mantle evolution and diamond formation ages.
Earth and Planetary Science Letters, Vol. 301, 1-2, pp. 43-51.
Petrology of the hypapbyssal kimberlite of the Kroonstad group II kimberlite (orangeite) cluster, South Africa: evolution of the magma within the cluster
The application of airborne, ground and borehole geophysics to the exploration for rare earth elements associated with the Lofdal carbonatite complex, Namibia
Peralk-Carb 2011... workshop June 16-18, Tubingen, Germany, Abstract p.69. (very brief no info)
The application of airborne, ground and borehole geophysics to the exploration for rare earth elements associated with the Lofdal carbonatite complex, Namibia
Peralk-Carb 2011... workshop June 16-18, Tubingen, Germany, Abstract p.69. (very brief no info)
A beautiful mine ( after the horrors of the 90's, when bling was equated with blood, De Beers brings new meaning to diamond clarity. Story based on Orapa
Characterising modal metasomatic processes in young continental lithospheric mantle: a microsampling isotopic and trace element study on xenoliths from the Middle Atlas Mountains, Morocco.
Contributions to Mineralogy and Petrology, Vol. 162, 2, pp. 289-302.
Lithospheric structures and Precambrian terrane boundaries in northeastern Botswana revealed through magnetotelluric profiling as part of southern African
Lithospheric structures and Precambrian terrane boundaries in northeastern Botswana revealed through magnetotelluric profiling as part of Southern Africa...
Journal of Geophysical Research, Vol. 116, B02401 21p.
87 Sr/86Sr- 143Nd/144 Nd systematic and clinopyroxenes host rock disequilibrium in high potassium magmas of the East-African Rift - insight to mantle source heterogeneity.
Peralk-Carb 2011, workshop held Tubingen Germany June 16-18, Poster
The ultrahigh temperature granulites of southern Madagascar in a polymetamorphic context: implications for the amalgamation of the Gondwana supercontinent.
European Journal of Mineralogy, Vol. 23, 2, pp. 127-156.
Mineralogy of heavy minerals concentrates from the unconsolidated deposits of Eledoi and Pello Hill volcanic cones ( Gelai volcano): first preliminary data
Peralk-Carb 2011, workshop held Tubingen Germany June 16-18, Poster
Mineralogy of heavy minerals concentrates from the unconsolidated deposits of Eeldoi and Pello Hill volcanic cones (Gelai volcano, northern Tanzania) prel.
Peralk-Carb 2011... workshop June 16-18, Tubingen, Germany, Abstract p.111-112.
Mineralogy of heavy minerals concentrates from the unconsolidated deposits of Eeldoi and Pello Hill volcanic cones (Gelai volcano, northern Tanzania) prel.
Peralk-Carb 2011... workshop June 16-18, Tubingen, Germany, Abstract p.111-112.
The structural, metamorphic and temporal evolution of the country rocks surrounding Venetia mine, Limpopo belt: evidence for a single paleoproterozoic event
Precambrian Research, Vol. 186, 1-4, pp. 51-69.
Africa, South Africa
Tectonometamorphic - implications for a tectonic model
Electron back scatter diffraction based identification and quantification of diamonds from the RIF gneisses ( Spain and Morocco): economic implications.
Schmadicke, E., Okrusch, M., Rupprecht-Gutowski, P., Will, T.M.
Garnet pyroxenite, eclogite and alkremite xenoliths from the off-craton Gibeon kimberlite field, Namibia: a window into the upper mantle of the Rehoboth Terrane.
Schmadicke, E., Okrusch, M., Rupprecht-Gutpwski, P., Will, T.M.
Garnet pyroxenite, eclogite and alkremite xenoliths from the off-craton Gibeon kimberlite field, Namibia: a window into the upper mantle of Rehoboth Terrane.
The nature of diamonds and their mineral inclusions: a study of diamonds from the PAnd a and Jagersfontein kimberlites and from placer deposits in Brazil.
Thesis, University of Alberta, Earth and Atmospheric Sciences, 214p.
Canada, Northwest Territories, Africa, South Africa, Brazil
Thesis - note availability based on request to author
Hafnium isotope record of the Ancient Gneiss Complex, Swaziland, southern Africa: evidence for Archean crust-mantle formation and crust reworking between 3.66 and 2.73 Ga.
Journal of the Geological Society, Vol. 168, pp. 953-964.
Kahoui, M., Kemainsky, F.V., Griffin, W.L., Belousova, E., Mahdjoub, Y., Chabane, M.
Detrital pyrope garnets from the El Kseibat area, Algeria: a glimpse into the lithospheric mantle beneath the north-eastern edge of the West African Craton.
Journal of African Earth Sciences, In press available, 46p.
Flourine , yttrium and lanthaide rich cerianite (Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosive craters Gregory Rift Tanzania.
Mineralogical Magazine, Vol. 75, 6, pp. 2813-2822.
The origin and evolution of the lithospheric mantle beneath the Makondi fold belt in Botswana: an extensive geochemical study of peridotite xenoliths from the Lethlakane diamond mine.
10th. International Kimberlite Conference Held Bangalore India Feb. 6-11, Poster abstract
Evolution of the South Africa mantle - a case study of garnet peridotites from the Finsch diamond mine ( Kaapvaal craton) part 2: multiple depletion and re-enrichment processes.
Brown, R.J., Buisman, M.I., Fontana, G., Field, M., Mac Niocaill, C., Sparks, R.S.J., Stuart, F.M.
Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on Earth ( the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania).
Brown, R.J., Manya, S., Buisman, I., Fontana, G., Field, M., MacNiocaill, C., Sparks, R.S.J., Stuart, F.M.
Eruption of kimberlite magmas: physical volcanology, geomrphology and age of the youngest kimberlitic volcanoes known on Earth ( the Upper Pleistocene-Holocene Igwisi Hills, volcanoes, Tanzania.
Bulletin of Volcanology, Vol. 74, 7, pp. 1621-1643.
Brown, R.J., Manya, S., Buisman, I., Sparks, R.S.J., Field, M., Stuart, F.M., Fontana, G.
Physical volcanology, geomorphology, and cosmogenic 3HE dating of the youngest kimberlite volcanoes on Earth ( The Holocene Igwisi Hills, Volcanoes, Tanzania.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Eclogitic and peridotitic diamond formation(Kimberley Pool kimberlites, South Africa), as evidenced from C, N stable isotope: a main mantle derived source.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
In situ U Pb dating and Sr Nd isotopic analysis of perovskite: constraints on the age and petrogenesis of the Kuruman kimberlite province, Kaapvaal Craton, South Africa.
Guzmics, T., Mitchell, R.H., Szabo, C., Berkesi, M., Milke, R., Ratter, K.
Liquid immiscibility between silicate, carbonate and sulfide melts in melt inclusions hosted in co-precipitated minerals from Kerimasi volcano (Tanzania): evolution of carbonated nephelinitic magma.
Contributions to Mineralogy and Petrology, Vol. 164, pp. 101-122.
Guzmics, T., Mitchell, R.H., Szabo, C., Berkesi, M., Milke, R., Ratter, K.
Liquid immiscibility between silicate, carbonate and sulfide melts in melt inclusions hosted in co-precipitated minerals from Kerimasi volcano ( Tanzania): evolution of carbonated nephelinitic magma.
Contributions to Mineralogy and Petrology, in press available
Mantle structure beneath Africa and Arabia from adaptively parameterized P-wave tomography: implications for the origin of Cenozoic Afro-Arabian tectonism.
Earth and Planetary Science Letters, Vol. 319-320, pp. 23-34.
Jelsma, H.,Krishnan, S.U., Perritt, S.,Kumar, M., Preston, R., Winter, F., Lemotlo, L., Costa, J., Van der Linde, G., Facatino, M., Posser, A., Wallace, C., Henning, A., Joy, S., Chinn, I., Armstrong, R., Phillips, D.
Kimberlites from central Angola: a case stidy of exploration findings.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Kahoui, M., Kaminsky, F.V., Griffin, W.L., Belousova, E., Mahdjoub, Y., Chabane, M.
Detrital pyrope garnets from the El Kseibat area, Algeria: a glimpse into lithospheric mantle beneath the north eastern edge of the west African Craton.
Journal of African Earth Sciences, Vol. 63, Feb. pp. 1-11.
Evolution of the South Africa mantle - a case study of garnet peridotites from the Finsch diamond mine ( Kaapvaal craton) part 1: inter-mineral trace element and isotopic equilibrium.
Maier, W.D., Peltonen, P., McDonald, I., Barnes, S.J., Barnes, S-J., Hatton, C., Viljoen, F.
The concentration of platinum group elements and gold in southern African and Karelian kimberlite hosted mantle xenoliths: implications for the noble metal content of the Earth's mantle.
Rapid magma ascent and short eruption durations in the Lake Natron-Engaruka monogenetic volcanic field ( Tanzania): a case study of the olivine melilitic Pello Hill scoria cone.
Journal of Volcanology and Geothermal Research, Vol. 247-248, Dec. 1, pp. 16-25.
Carbonate-silicate immiscibility and extremely peralkaline silicate glasses from Nasira cone and recent eruptions at Oldoinyo-Lengai Volcano, Tanzania.
Trace element geochemistry of myerereite and gregoyryite phenocrysts from natrocarbonatite lava, Oldoinyo-Lengai, Tanzania: implications for magma mixing.
Mitchell, R.H., Scott Smith, B.H., Skinner, E.M.W.
Mineralogy of magmaclasts and interclast matrices of Kimberley-type pyroclastic kimberlites from the Kao, Letseng-La-Terae, Lethlakane and Premier kimberlite pipes of southern Africa.
10th. International Kimberlite Conference Held Bangalore India Feb. 6-11, Poster abstract
Monie, P., Bosch, D., Bruguier, O., Vauchez, A., Rolland, Y., Nsungani, P., Buta Nto, A.
The Late Neoproterozoic/Early Palezoic evolution of the West Congo belt of NW Angola: geochronological (U-Pb and Ar-Ar) and petrostructural constraints.
Owona, S., Tichomirowa, M., Ratschbacher, L., Ondoa, J.M., Youmen, D., Pfander, J., Tchoua, F.M., Affaton, P., Ekodeck, G.E.
New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group, Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton.
International Journal of Earth Sciences, Vol. 101, 7, pp. 1689-1703.
Owona, S., Tichomirowa, M., Ratschbacher, L., Ondoa, W.J., Youmen, D., Pfander, J., Tchoua, F.M., Affaton, P., Ekodeck, G.E.
New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group ( Cameron, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton.
International Journal of Earth Sciences, Vol. 101, pp. 1689-1703.
Mineral composition in cognate inclusions in Late Miocene-Early Pliocene potassic lamprophyres with affinities to lamproites from the Denizli region, Western Anatolia, Turkey: implications for uppermost mantle processes in a back arc setting.
Deep carbonate recycling and metasomatic enrichment of the sub-continental lithospheric mantle inferred from mantle xenoliths of the East African Rift system.
Late Ediacaran ( 605-580 Ma) post-collisional alkaline magmatism in the Arabian-Nubian shield: a case study of Serbal ring-shaped intrusion, southern Sinai, Egypt.
Journal of Asian Earth Sciences, Vol. 77, pp. 203-223.
How structure and stress influence kimberlite emplacement.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 51-65.
2.46 Ga kasilite and nepheline syenites from the Awsard plution, Reguibat Rise of the West African Craton, Morocco. Generation of extremely K-rich magmas at the Archean-Proterozoic transition.
El Atrassi, F., Brunet, F., Chazot, G., Bouybaouene, M., Chopin, C.
Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera massif ( northern Morocco): petrography, mineral chemistry and thermobarometry.
El Atrassi, F., Brunet, F., Chazot, G., Chopin, C.
Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera Massif ( northern Morocco): mineralogical, chemical and textural records.
El Bahat, A., Ikenne, M., Soderlund, U., Cousens, B., Youbi, N., Ernst, R., Soulaimani, A., El Janati, M., Hafid, A.
U PB baddeleyite ages and geochemistry of dolerite dykes in the Bas Draa In lier of the Anti-Atlas of Morocco: newly identified Ma event in the West African craton.
Ernst, R.E., Pereira, E., Hamilton, M.A., Pisarevsky, S.A., Rodriques, J., Tasinari, C.C.G., Teixeira, W., Van-Dunem, V.
Mesoproterozoic intraplate magmatic 'barcode' record of the Angola portion of the Congo craton: newly dated magmatic events at 1505 and 1110 Ma and implications for Nuna ( Columbia) supercontinent reconstructions.
Giuliani, A., Kamenetsky, V.S., Kendrick, M.A., Phillips, D., Wyatt, B.A., Maas, R.
Oxide, sulphide and carbonate minerals in a mantle polymict breccia: metasomatism by proto-kimberlite magmas, and relationship to the kimberlite megacrystic suite.
Geology and evaluation of the K2 kimberlite, Koidu mine, Sierra Leone, West Africa.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 191-208.
A structural analysis of the geophysical signature relationship between linear features and plug like bodies of sheets 2229AB and 2229AD in the Limpopo province, South Africa.
Kimberlites from central Angola: a case study of exploration findings.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 173-190.
Kaminsky, F.V., Kahoui, M.,Mahdjoub, Y., Belousova, E., Griffin, W.L.,O'Reilly, S.Y.
Pyrope garnets from the Eglab Shield, Algeria: look inside the Earth's mantle in the West African Craton and suggestions about primary sources of diamond and indicator minerals.
Vladykin, N.V. ed. Deep seated magmatism, its sources and plumes, Russian Academy of Sciences, pp. 73-103.
Konzett, J., Wirth, R., Hauzenberger, C., Whitehouse, M.
Two episodes of fluid migration in the Kaapvaal Craton lithospheric mantle associated with Cretaceous kimberlite activity: evidence from a harzburgite containing a unique assemblage of metasomatic zirconium-phases.
Kouyate, D., Soderlund, U., Youbi, N., Ernst, R., Hafid, A., Ikeene, M., Soulaimani, A., Betrand, H., El Janati, M., Rkha, C.
U Pb baddeleyite and zircon ages of 2040 Ma, 1650 Ma and 885 Ma on dolerites in the West African Craton ( Anti-Atlas inliers) : possible links to break up of Precambrian supercontinents.
Contrasting garnet lherzolite xenoliths suites from the Letseng kimberlite pipes: inferences for the northern Lesotho geotherm.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 29-44.
The lithospheric mantle underneath the Gibeon kimberlite field ( Namibia): a mix of old and young components - evidence from Lu-Hf and Sm-Nd isotope systematics.
New U-Pb SHRIMP ages from the Lubango region, sw Angola: insights into the Paleoproterozoic evolution of the Angolan shield, southern Congo craton, Africa.
Journal of the Geological Society, Vol. 170, pp. 353-363.
Geology of the K1 and K2 kimberlite pipes at Murowa, Zimbabwe.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 35-50.
Moteani, G., Kostitsyn, Y.A., Gilg, H.A., Preinfalk, C., Razakamanana, T.
Geochemistry of phlogopite, diopside, calcite, anhydrite and apatite pegmatites and syenites of southern Madagascar: evidence for crustal silicocarbonatitic (CSC) melt formatio in a Panafrican collisional tectonic setting.
International Journal of Earth Sciences, Vol. 102, 3, pp. 627-645.
Palot, M., Pearson, D.G., Stern, R.A., Stachel, T., Harris, J.W.
Multiple growth events, processes and fluid sources involved in diamond genesis: a micro-analytical study of sulphide bearing diamonds from Finsch mine, RSA.
Geochimica et Cosmochimica Acta, Vol. 106, pp. 51-70.
Nd-Hf isotope systematics of megacrysts from the Mbuji-Mayi kimberlites, D.R. Congo: evidence for a metasomatic origin related to kimberlite interaction with the cratonic lithosphere mantle.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 123-136.
On the iron isotope heterogeneity of lithospheric mantle xenoliths: implications for mantle metasomatism, the origin of basalts and the iron isotope composition of the Earth.
Contributions to Mineralogy and Petrology, Vol. 165, 6, pp. 1243-1258.
Detailed protracted crystallization history of perovskite in Orapa kimberlite.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 211-224.
Geochronological and geochemical constraints on the formation and evolution of the mantle beneath the Kaapvaal craton: Lu Hf and Sm Nd systematics of subcalcic garnets from highly depleted peridotites.
Geochimica et Cosmochimica Acta, Vol. 113, pp. 1-20.
Geological mapping of the Francistown area in northeastern Botswana by surface temperature and spectral emissivity information derived from advanced spaceborn thermal emission and reflection radiometer (ASTER) thermal infrared data.
Youbi, N., Kouyate, D., Soderlund, U., Ernst, R.E., Soulaimani, A., Hafid, A., Ikenne, M., El Bahat, A., Betrand, H., Chaham, K.R., Ben Abbou, M., Mortaji, A., El Ghorfi, M., Zouhair, M., El Janati, M.
The 1750 Ma magmatic event of the West African Craton ( Anti-Atlas) Morocco.
In situ trace element geochemistry and U-Pb dating on perovskite from kimberlites of the Lunda Norte province ( NE Angola): petrogenetic and tectonic implications.
Insights into the mantle structure beneath the Lunda Norte kimberlitic province (NE Angola): petrography and paleothermobarometry of fresh mantle xenoliths.
The provenance of northern Kalahari Basin sediments and growth history of the southern Congo Craton reconstructed by U-Pb ages of zircons from recent river sands.
International Journal of Earth Sciences, Vol. 103, 2, pp. 579-595.
Garzanti, E., Resentini, A., Ando, S., Vezzoli, G., Pereira, A., Vermeesch, P.
Physical controls on sand and composition and relative durability of detrital minerals during ultra-long distance littoral and aeolian transport ( Namibia and southern Angola).
Oxygen isotope and trace element evidence for the origin of sapphire and/or ruby in the Mbuyi-Mayi kimberlite ( FDC) and the Changle alkali basalt China.
Relationships between oxygen fugacity and metasomatism in the Kaapvaal subcratonic mantle, represented by garnet peridotite xenoliths in the Wesselton kimberlite, South Africa.
Hofmann, M., Linnemann, U., Hoffmann, K-H., Gerdes, A., Eckelmann, K., Gartner, A.
The Namuskluft and Dreigratberg sections in southern Namibia ( Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana.
International Journal of Earth Sciences, Vol. 103, pp. 1187-1202.
Gem Diamonds the owner of the highest average dollar-per-carat kimberlite diamond mine in the world continues to defy expectations with its exceptionally large and high-value stones from Botswan and Lesotho.
Determination of sampling configuration for near shore Diamondiferous gravel occurrence using geostatistical methods. Mining area no. 1 - linear beach Namdeb
Journal of South African Institute of Mining and Metallurgy, Vol. 114, Jan. pp. 31-38.
Jelsma, H., Krishnan, U., Perritt, S., Preston, R., Winter, F., Lemotlo, L., van der Linde, G., Armstrong, R., Phillips, D., Joy, S., Costa, J., Facatino, M., Posser, A., Kumar, M., Wallace, C., Chinn, I., Henning, A.
Kimberlites from central Angola: a case study of exploration findings.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, pp. 173-190.
Radiogenic isotope constraints on lithospheric assimilation by sublithospheric melts in the generation of southern African kimberlite megacrysts: a comparsion of on and off craton megacryst suites.
Evolution of silicocrbonatite parental magmas to form the Phalaborwa igneous complex: a complex history of melting of multiple mantle sources, magma mingling, differentiation and magmatic exsolution.
Lithospheric structure of an Archean craton and adjacent mobile belt revealed from 2-D and 3-D inversion of magnetotelluric data: example from southern Congo craton in northern Namibia.
Journal of Geophysical Research, Vol. 118, 8, pp. 4378-4397.
Metasomatism of the Pan-African lithospheric mantle beneath the Damara Belt, Namibia, by the Tristan mantle plume: geochemical evidence from mantle xenoliths.
Contributions to Mineralogy and Petrology, Vol. 168, pp. 1046-
Mangler, M.F., Marks, M.A.W., Zaitsev, A.N., Eby, G.N., Markl, G.
Halogens (F, Cl and Br) at Oldoinyo Lengai volcano ( Tanzania): effects of magmatic differentiation, silicate, natrocarbonatite melt seperation and surface alteration of natrocarbonatite.
Midende, G., Boulais, P., Tack, L., Melcher, F., Gerdes,A., Dewaele, S., Demaiffe, D., Decree, S.
Petrography, geochemistry and U Pb zircon age of the Matongo carbonatite Massif ( Burundi): implication for the Neoproterozoic geodynamic evolution of Central Africa.
Journal of African Earth Sciences, Vol. 100, pp. 656-674.
Isotopic constraints on the nature and circulation of deep mantle C-H-O-N fluids: carbon and nitrogen systematics within super deep diamonds from Kankan Guinea.
Geological Society of America Conference Vancouver Oct. 19-22, 1p. Abstract
Palot, M., Pearson, D.G., Stern, R.A., Stachel, T., Harris, J.W.
Isotopic constraints on the nature and circulation of deep mantle C-H-O-N fluids: Carbon and nitrogen systematics within ultra-deep diamonds from Kankan ( Guinea).
Geochimica et Cosmochimica Acta, Vol. 139, pp. 26-46.
Rooney, T.O., Nelson, W.R., Dosso, L., Furman, T., Hanan, B.
The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates, East African Rift zone.
The geology and emplacement of the Jwaneng DK2 kimberlite, southern Botswana, based on a petrographic study of samples selected from all parts of the kimberlite.
GSSA Kimberley Diamond Symposium and Trade Show provisional programme, Sept. 12, title only
Sommer, H., Wan,Y., Kroner, A., Xie, H., Jacob, D.E.
Shrimp zircon ages and petrology of lower crustal granulite xenoliths from the Letseng-La-Terae kimberlite, Lesotho: further evidence for a Namaquanatal connection.
South Africa Journal of Geology, Vol. 116, 2, pp. 183-198.
Viljoen, K.S., Harris, J.W., Richardson, S.H., Gray, K.
Trace element chemistry of peridotitic garnets in diamonds from the Premier ( Cullinan) and Finsch kimberlites, South Africa: contrasting styles of mantle metasomatism.
Mantle conditions and geochemical environment as controls of diamond survival and grade variation in kimberlitic diamond deposits: Lunda Province NE Angola.
30th. International Conference on Ore Potential of alkaline, kimberlite and carbonatite magmatism. Sept. 29-, http://alkaline2014.com
Review of African Political Economy, Routledge Pub., Vol. 41, no. 142, pp. 500-515.
Africa, Guinea
History
Abstract: The article explores the relationship between mineral resources and conflict management in Guinea. Literature on theories of recent civil wars and/or armed conflicts in West Africa identifies the combination of abundant natural resources and extreme poverty as a significant trigger of violent civil conflicts. In Guinea, however, despite this combination, the state has managed to avoid large-scale civil violence. This gives rise to the question of why this combination has failed to be associated with the onset of large-scale violence in the country. The article identifies mitigating factors that have contributed to political stability in Guinea. It concludes that measures taken by Guinea and its international partners mitigated the security threats posed by these resources, while keeping most Guineans in abject poverty. This is in contrast to findings in recent quantitative studies whereby natural resource abundance alongside extreme poverty is strongly associated with armed conflicts in West African nations.
Abstract: The mantle section of Al'Ays ophiolite consists of heterogeneously depleted harzburgites, dunites and large-sized chromitite pods. Two chromitite-bearing sites (Site1 and Site2), about 10 km apart horizontally from one another, were examined for their upper mantle rocks. Cr-spinels from the two sites have different chemistry; Cr-rich in Site1 and Al-rich in Site2. The average Cr-ratio = (Cr/(Cr + Al) atomic ratio) of Cr-spinels in harzburgites, dunites and chromitites is remarkably high 0.78, 0.77 and 0.87, respectively, in Site1, compared with those of Site2 which have intermediate ratio averages 0.5, 0.56 and 0.6, respectively. The platinum-group elements (PGE) in chromitites also show contrasting patterns from Site1 to Site2; having elevated IPGE (Os, Ir, Ru) and strongly depleted in PPGE (Rh, Pt, Pd) with steep negative slopes in the former, and gentle negative slopes in the latter. The oxygen fugacity (?log fO2) values deduced from harzburgites and dunites of Site1 show a wide variation under reducing conditions, mostly below the FMQ buffer. The Site2 harzburgites and dunites, on the other hand are mostly above the FMQ buffer. Two magmatic stages are suggested for the lithospheric evolution of Al'Ays ophiolite in response to a switch of tectonic setting. The first stage produced a peridotites–chromitites suite with Al-rich Cr-spinels, possibly beneath a mid-ocean ridge setting, or most likely in back-arc rift of a supra-subduction zone setting. The second stage involved higher degrees of partial melting, produced a peridotites–chromitites suite with Cr-rich Cr-spinels, possibly in a fore-arc setting. The coexistence of compositionally different mantle suites with different melting histories in a restricted area of an ophiolite complex may be attributable to a mechanically juxtaposed by mantle convection during recycling. The mantle harzburgites and dunites are apt to be compositionally modified during recycling process; being highly depleted (Site1 case) than their original composition (Site2 case).
Journal of African Earth Sciences, Vol. 100, pp. 20-41.
Africa, Namibia
Geothermometry
Abstract: Intracontinental deformation accommodated along major lithospheric scale shear zone systems and within associated extensional basins has been well documented within West, Central and East Africa during the Late Cretaceous. The nature of this deformation has been established by studies of the tectonic architecture of sedimentary basins preserved in this part of Africa. In southern Africa, where the post break-up history has been dominated by major erosion, little evidence for post-break-up tectonics has been preserved in the onshore geology. Here we present the results of 38 new apatite fission track analyses from the Damara region of northern Namibia and integrate these new data with our previous results that were focused on specific regions or sections only to comprehensively document the thermo-tectonic history of this region since continental break-up in the Early Cretaceous. The apatite fission track ages range from 449 ± 20 Ma to 59 ± 3 Ma, with mean confined track lengths between 14.61 ± 0.1 ?m (SD 0.95 ?m) to 10.83 ± 0.33 ?m (SD 2.84 ?m). The youngest ages (c. 80–60 Ma) yield the longest mean track lengths, and combined with their spatial distribution, indicate major cooling during the latest Cretaceous. A simple numerical thermal model is used to demonstrate that this cooling is consistent with the combined effects of heating caused by magmatic underplating, related to the Paraná-Etendeka continental flood volcanism associated with rifting and the opening of the South Atlantic, and enhanced erosion caused by major reactivation of major lithospheric structures within southern Africa during a key period of plate kinematic change that occurred in the South Atlantic and SW Indian ocean basins between 87 and 56 Ma. This phase of intraplate tectonism in northern Namibia, focused in discrete structurally defined zones, is coeval with similar phases elsewhere in Africa and suggests some form of trans-continental linkage between these lithospheric zones.
Midende, G., Boulvais, P., Tack, L., Melcher, F., Gerdes, A., Dewaele, S., Demaiffe, D., Decree, S.
Petrography, geochemistry and U-Pb zircon age of the Matongo carbonatite Massif ( Burundi): implication for the Neoproterozoic geodynamic evolution of Central Africa.
Journal of African Earth Sciences, Vol. 100, pp. 656-674.
Perceptions of the impact of board members' individual perspectives on the social and environmental performance of companies. ( Based on SA and not junior companies).
Journal of the South African Institute of Mining and Metallurgy, Vol. 114, Nov. pp. 957-969.
Abstract: We reconstruct the history of topographic growth in southern Africa on both sides of the Orange River valley from an integrated analysis of erosion surfaces, crustal deformation and climate change. First, we propose an inventory of erosion surfaces observed in the study area and classify them according to their most likely formative process, i.e. chemical weathering or mechanical erosion. Among the various land units observed we define a new class of landform: the pedivalley, which corresponds to a wide valley with a flat erosional floor. In the Orange River valley, we mapped three low-relief erosion surfaces, each bevelling a variety of lithologies. The oldest and most elevated is (1) a stripped etchplain evolving laterally into (2) a stepped pediplain bearing residual inselbergs; (3) a younger pediplain later formed in response to a more recent event of crustal deformation. These are all Cenozoic landforms: the etchplain is associated with a late Palaeocene to middle Eocene weathering event, and the two pediplains are older than the middle Miocene alluvial terraces of the Orange River. Landscape evolution was first driven by slow uplift (10 m/Ma), followed by a second interval of uplift involving a cumulative magnitude of at least 200 m. This event shaped the transition between the two pediplains and modified the drainage pattern. A final phase of uplift (magnitude: 60 m) occurred after the Middle Miocene and drove the incision of the lower terraces of the Orange River. Climate exerted a major control over the denudation process, and involved very humid conditions responsible for lateritic weathering, followed by more arid conditions, which promoted the formation of pedivalleys. Collectively, these produce pediplains.
Geology and resource potential of the Congo Basin, Springer Regional Geology Reviews, Chapter 17, 9p.
Africa, Democratic Republic of Congo
Overview, history
Abstract: An overview is provided of the exploration history and geological setting of the kimberlites in the Democratic Republic of Congo (DRC). Exploration for diamonds, in what was then known as Congo Belge, started in 1900 and the first diamonds were found in 1903 in Shaba (now Katanga) Province, in 1907 in Kasai Occidental Province near Tshikapa Town and in 1918 in Kasai Oriental Province near Mbuji Mayi Town. While the Kundelungu kimberlites in Katanga Province were discovered in 1908, other kimberlite fields were discovered much later (Mbuji Mayi 1946; Tshibwe 1956; Bas-Congo 1974; Kasendou and Lukashi 2005), during exploration work by Forminière (Société Internationale Forestière et Minière du Congo-Tshikapa), MIBA (Societé Minière de Bakwanga-Mbuji Mayi), the De Beers Group and Bugeco S.A. Published age constraints on the kimberlites show Late Cretaceous ages for the Mbuji Mayi kimberlites (~70 Ma) and Eocene-Oligocene ages for the Kundelungu kimberlites (~32 Ma). Emplacement of the Late Cretaceous kimberlites (Mbuji Mayi, Tshibwe, Kasendou and Lukashi) was concomitant with the deposition of Cretaceous sedimentary sequences. The majority of the pipes show crater-facies preservation and some of the pipes are flared displaying so-called ‘champagne glass-shaped’ morphologies, suggesting emplacement into unconsolidated sediments overlying basement. The age of the Eocene-Oligocene Kundelungu kimberlites corresponds to lithospheric extension associated with the southward propagation of the East African Rift.
Geology and resource potential of the Congo Basin, Springer Regional Geology Reviews, Chapter 16, 20p.
Africa, Democratic Republic of Congo
Overview, history
Abstract: Since the mid-1950s the Kwango River has been a major target for alluvial diamonds which are and continue to be mined from its terraces, younger river flats and present-day river channel. The terraces have maximum ages of Early to Middle Pleistocene. Most of the diamonds have been recovered from large diamond placers in and along the Angolan section of this river—the Cuango River. Smaller deposits have been worked further downstream, where the Kwango River forms the international border between Angola and the Democratic Republic of Congo (DRC), also referred to as the ‘international’ Kwango. The prospecting history of this river goes back to 1906 but a systematic exploration program over the lower Kwango was only initiated in 2005. The application of geophysics to explore the terrace deposits and river flats has been very useful, but a programme of drilling and pitting was required to accurately define gravel and overburden thicknesses, and outline palaeo-channels. A diamond study used to glean information on diamond sizes and characteristics added significant value to the understanding of these alluvial deposits. For most of the international Kwango, terraces and river flats overly aeolian facies of Upper Kwango Group. Basement rocks, providing more favourable sites for diamond concentrations, are only exposed over a relatively short section, just upstream from Tembo. The basal part of the Cretaceous Kwango Group is locally composed of chemically mature gravels with diamonds in economic quantities only proximal to primary sources and no such settings were found in the project area. The size frequency of the diamonds from the international Kwango indicates that these form the distal head of the diamond trail that have been eroded out of the Cretaceous Kwango Group sediments and kimberlites in the Upper Cuango basin in Angola since the Pleistocene. The diamonds below the two major waterfalls along the international Kwango near Tembo, the Guiliame and Francois-Joseph Falls, show a high percentage of breakage and a decline in average diamond size from around 0.25 cts/stn above the falls to between 0.07 and 0.1 cts/stn in the Nzasi Muadi to Kitangu area between 20 km and 130 km below the Falls. However, local variations due to geomorphological influences affect diamond concentrations and sizes. A preliminary assessment of the terrace deposits suggests that these are uneconomic at present using modern mining methods. This is largely due to thick overburden (up to 12 m of sand) combined with thin and hence low-volume, medium-grade basal gravel, and the dominance of small diamonds of lower value.
Abstract: The Blue Moon diamond, discovered in January 2014 at the historic Cullinan mine in South Africa, is of significance from both trade and scientific perspectives. The 29.62 ct rough yielded a 12.03 ct Fancy Vivid blue, Internally Flawless gem. The authors were provided the opportunity to study this rare diamond at the Smithsonian Institution before it went on exhibit at the Natural History Museum of Los Angeles County. Infrared spectroscopy revealed that the amount of uncompensated boron in the diamond was 0.26 ± 0.04 ppm, consistent with measurements of several large type IIb blue diamonds previously studied. After exposure to short-wave ultraviolet light, the Blue Moon displayed orange-red phosphorescence that remained visible for up to 20 seconds. This observation was surprising, as orange-red phosphorescence is typically associated with diamonds of Indian origin, such as the Hope and the Wittelsbach-Graff. Time-resolved phosphorescence spectra exhibited peaks at 660 and 500 nm, typical for natural type II blue diamonds. As with most natural diamonds, the Blue Moon showed strain-induced birefringence.
Contributions to Mineralogy and Petrology, Vol. 169, 18p.
Africa, Tanzania
Carbonatite
Abstract: We have reconstructed the compositional evolution of the silicate and carbonate melt, and various crystalline phases in the subvolcanic reservoir of Kerimasi Volcano in the East African Rift. Trace element concentrations of silicate and carbonate melt inclusions trapped in nepheline, apatite and magnetite from plutonic afrikandite (clinopyroxene-nepheline-perovskite-magnetite-melilite rock) and calciocarbonatite (calcite-apatite-magnetite-perovskite-monticellite-phlogopite rock) show that liquid immiscibility occurred during the generation of carbonatite magmas from a CO2-rich melilite-nephelinite magma formed at relatively high temperatures (1,100 °C). This carbonatite magma is notably more calcic and less alkaline than that occurring at Oldoinyo Lengai. The CaO-rich (32-41 wt%) nature and alkali-"poor" (at least 7-10 wt% Na2O + K2O) nature of these high-temperature (>1,000 °C) carbonate melts result from strong partitioning of Ca (relative to Mg, Fe and Mn) in the immiscible carbonate and the CaO-rich nature (12-17 wt%) of its silicate parent (e.g., melilite-nephelinite). Evolution of the Kerimasi carbonate magma can result in the formation of natrocarbonatite melts with similar composition to those of Oldoinyo Lengai, but with pronounced depletion in REE and HFSE elements. We suggest that this compositional difference results from the different initial parental magmas, e.g., melilite-nephelinite at Kerimasi and a nephelinite at Oldoinyo Lengai. The difference in parental magma composition led to a significant difference in the fractionating mineral phase assemblage and the element partitioning systematics upon silicate-carbonate melt immiscibility. LA-ICP-MS analysis of coeval silicate and carbonate melt inclusions provides an opportunity to infer carbonate melt/silicate melt partition coefficients for a wide range of elements. These data show that Li, Na, Pb, Ca, Sr, Ba, B, all REE (except Sc), U, V, Nb, Ta, P, Mo, W and S are partitioned into the carbonate melt, whereas Mg, Mn, Fe, Co, Cu, Zn, Al, Sc, Ti, Hf and Zr are partitioned into the silicate melt. Potassium and Rb show no preferential partitioning. Kerimasi melt inclusions show that the immiscible calcic carbonate melt is strongly enriched in Sr, Ba, Pb, LREE, P, W, Mo and S relative to other trace elements. Comparison of our data with experimental results indicates that preferential partitioning of oxidized sulfur (as SO4 2?), Ca and P (as PO4 3?) into the carbonate melt may promote the partitioning of Nb, Ta, Pb and all REE, excluding Sc, into this phase. Therefore, it is suggested that P and S enrichment in calcic carbonate magmas promotes the genesis of REE-rich carbonatites by liquid immiscibility. Our study shows that changes in the partition coefficients of elements between minerals and the coexisting melts along the liquid line of descent are rather significant at Kerimasi. This is why, in addition to the REE, Nb, Ta and Zr are also enriched in Kerimasi calciocarbonatites. We consider significant amounts of apatite and perovskite precipitated from melilite-nephelinite-derived carbonate melt as igneous minerals can have high LREE, Nb and Zr contents relative to other carbonatite minerals.
Kleinhanns, I.C., Fullgraf, T., Wilsky, F., Nolte, N., Fliegel, D., Klemd, R., Hansen, B.T.
U-Pb zircon ages and (isotope) geochemical signatures of the Kamanjab In lier ( NW Namibia): constraints on Palaeoproterozoic crustal evolution along the southern Congo craton.
Geological Society of London Special Publication: Continent formation through time., No. 389, pp. 165-195.
Intracontinental anorogenic alkaline magmatism and carbonatites, associated mineral systems and the mantle plume connection. Brandberg, Erongo, Parana-Etendeka, Kruidfontein, Goudini
Gondwana Research, Vol. 27, 3, pp. 1181-1216.
Africa, East Africa, Namibia, South Africa, China, Australia
Journal of Metamorphic Geology, Vol. 33, 5, pp. 463-494.
Africa, Ghana
Geochronology
Abstract: New petrological and geochronological data are presented on high-grade ortho- and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25-2.00 Ga) West African Craton. The study area is located in the interference zone between N-S and NE--SW-trending craton-scale shear zones, formed during the Eburnean orogeny (2.15-2.00 Ga). High-grade metamorphic domains are separated from low-grade greenstone belts by high-strain zones, including early thrusts, extensional detachments and late-stage strike-slip shear zones. Paragneisses sporadically preserve high-pressure, low-temperature (HP-LT) relicts, formed at the transition between the blueschist facies and the epidote-amphibolite sub-facies (10.0-14.0 kbar, 520-600 °C), and represent a low (~15 °C km?1) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite-granulite facies transition. They reveal a clockwise pressure-temperature-time (P-T-t) path characterized by melting at pressures over 10.0 kbar, followed by decompression and heating to peak temperatures of 750 °C at 5.0-8.0 kbar, which fit a 30 °C km?1 apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P-T-t path, characterized by peak metamorphic conditions of 7.0-10.0 kbar at 550-680 °C, which match a 20-25 °C km?1 apparent geotherm. These P-T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in-situ U-Pb dating of monazite crystallization at 2138 ± 7 and 2130 ± 7 Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian ‘basins’ overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono-metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with ‘hot orogeny’ models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.
Giuliani, G.,Pivin, M., Fallick, A.E., Ohnenstetter, D., Song, Y., Demaiffe, D.
Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo and the Changle alkali basalt, China.
Comptes Rendus Geoscience, Vol. 347, 1, pp. 24-34.
Hudgins, T.R., Mukasa, S.B., Simon, A.C., Moore, G., Barifaijo, E.
Melt inclusion evidence for CO2 rich melts beneath the western branch of the East African Rift: implications for long term storage of volatiles in the deep lithospheric mantle.
Contributions to Mineralogy and Petrology, Vol. 169, 5p.
Earth and Planetary Science Letters, Vol. 424, pp. 119-131.
Canada, Northwest Territories, Africa, Tanzania
Deposit - Diavik, Igwisi Hills
Abstract: Olivine xenocrysts are ubiquitous in kimberlite deposits worldwide and derive from the disaggregation of mantle-derived peridotitic xenoliths. Here, we provide descriptions of textural features in xenocrystic olivine from kimberlite deposits at the Diavik Diamond Mine, Canada and at Igwisi Hills volcano, Tanzania. We establish a relative sequence of textural events recorded by olivine during magma ascent through the cratonic mantle lithosphere, including: xenolith disaggregation, decompression fracturing expressed as mineral- and fluid-inclusion-rich sealed and healed cracks, grain size and shape modification by chemical dissolution and abrasion, late-stage crystallization of overgrowths on olivine xenocrysts, and lastly, mechanical milling and rounding of the olivine cargo prior to emplacement. Ascent through the lithosphere operates as a "kimberlite factory" wherein progressive upward dyke propagation of the initial carbonatitic melt fractures the overlying mantle to entrain and disaggregate mantle xenoliths. Preferential assimilation of orthopyroxene (Opx) xenocrysts by the silica-undersaturated carbonatitic melt leads to deep-seated exsolution of CO2-rich fluid generating buoyancy and supporting rapid ascent. Concomitant dissolution of olivine produces irregular-shaped relict grains preserved as cores to most kimberlitic olivine. Multiple generations of decompression cracks in olivine provide evidence for a progression in ambient fluid compositions (e.g., from carbonatitic to silicic) during ascent. Numerical modelling predicts tensile failure of xenoliths (disaggregation) and olivine (cracks) over ascent distances of 2-7 km and 15-25 km, respectively, at velocities of 0.1 to >4 m?s?1. Efficient assimilation of Opx during ascent results in a silica-enriched, olivine-saturated kimberlitic melt (i.e. SiO2 >20 wt.%) that crystallizes overgrowths on partially digested and abraded olivine xenocrysts. Olivine saturation is constrained to occur at pressures <1 GPa; an absence of decompression cracks within olivine overgrowths suggests depths <25 km. Late stage (<25 km) resurfacing and reshaping of olivine by particle-particle milling is indicative of turbulent flow conditions within a fully fluidized, gas-charged, crystal-rich magma.
Abstract: The Ampasindava alkaline province consists of a series of circular and elliptical intrusions, lava flows, dyke swarms and plugs of Cenozoic age emplaced into the Mesozoic-Cenozoic sedimentary rocks of the Antsiranana basin (NW Madagascar) and above the crystalline basement. The magmatism in the Ampasindava region is linked to a NW-SE trending extensional tectonic setting. New 40Ar/39Ar age determinations on feldspar separate of alkali granites and basaltic dykes yielded ages of 18.01?±?0.36 Ma and 26?±?7 Ma, respectively. Alkali basalts and basanites, nepheline syenites and phonolites, and silica saturated-to-oversaturated syenites, trachytes, granites and rhyolites are the main outcropping lithologies. These rocks have sodic affinity. The felsic rocks are dominant, and range from peraluminous to peralkaline. The mantle-normalized incompatible element patterns of the mafic lavas match those of Na-alkaline lavas in within-plate rift settings. The patterns are identical in shape and absolute concentrations to those of the Bobaomby (Cap d’Ambre) and Massif d’Ambre primitive volcanic rocks. These geochemical features are broadly compatible with variable degrees of partial melting of incompatible element-enriched mantle sources. The mineralogical and geochemical variations are consistent with fractional crystallization processes involving removal of olivine, feldspar, clinopyroxene, amphibole, Fe-Ti oxides and apatite. Removal of small amount of titanite explains the concave upward lanthanide pattern in the evolved nepheline syenites and phonolites, which are additionally rich in exotic silicates typical of agpaitic magmas (eudialyte, F-disilicates).
Abstract: U-Pb age data from southwest Madagascar provide a compelling case that the pre-Gondwana Indian plate was stitched with the arc terranes of the Arabian Nubian Shield along a suture that closed between 580 Ma and 520 Ma. The key observations supportive of this interpretation are: (1) metamorphism dated to 630-600 Ma is manifested only on the west side of the suture in rocks that have affinities with the oceanic and island arc terranes of the Arabian Nubian Shield, or which represent continental rocks welded to these terranes prior to the amalgamation of Gondwana, and (2) orogenesis at 580-520 Ma is manifest in rocks on both sides of the suture, an observation taken to mark the timing of collision and to reflect spatial continuity across the suture. In southwest Madagascar the distribution of metamorphic ages places the suture along the Beraketa high-strain zone, the tectonic boundary between the Androyen and Anosyen domains. Similar age relationships allow for the extrapolation of this tectonic boundary into both East Antarctica and Africa.
Abstract: A set of small volcanic edifices with tuff ring and maar morphologies occur in the Catanda area, which is the only locality with extrusive carbonatites reported in Angola. Four outcrops of carbonatite lavas have been identified in this region and considering the mineralogical, textural and compositional features, we classify them as: silicocarbonatites (1), calciocarbonatites (2) and secondary calciocarbonatites produced by the alteration of primary natrocarbonatites (3). Even with their differences, we interpret these lava types as having been a single carbonatite suite related to the same parental magma. We have also estimated the composition of the parental magma from a study of melt inclusions hosted in magnetite microphenocrysts from all of these lavas. Melt inclusions revealed the presence of 13 different alkali-rich phases (e.g., nyerereite, shortite, halite and sylvite) that argues for an alkaline composition of the Catanda parental melts. Mineralogical, textural, compositional and isotopic features of some Catanda lavas are also similar to those described in altered natrocarbonatite localities worldwide such as Tinderet or Kerimasi, leading to our conclusion that the formation of some Catanda calciocarbonatite lavas was related to the occurrence of natrocarbonatite volcanism in this area. On the other hand, silicocarbonatite lavas, which are enriched in periclase, present very different mineralogical, compositional and isotopic features in comparison to the rest of Catanda lavas. We conclude that its formation was probably related to the decarbonation of primary dolomite bearing carbonatites.
Abstract: The circular 625 km2 alkaline Pilanesberg Complex, South Africa, contains coeval eruptive and several distinctive intrusive syenitic and foyaitic components, concentrically arranged at the surface. However, owing to poor outcrop the relationships between the different intrusive rocks, and their shape in the third dimension cannot be convincingly determined in the field. The original interpretation was a laccolith, whereas later models suggested a funnel shape, and appealed to ring-dyke and cone-sheet emplacement mechanisms. However, the radial widths of these coarse-grained bodies are over 1 km and so cannot have been emplaced as ring dykes or cone sheets, which are usually quite thin and fine grained. Creating the space for emplacement and removal of pre-existing country rocks for each postulated subsequent intrusive event presents a major challenge to this latter hypothesis. Extensive previously published and new field relationships are re-evaluated here to suggest that the body is a gently inward-dipping sheet and that subsequent injections of magma merely pumped up an existing and evolving magma chamber rather than intruded into solid rocks. A Bouguer gravity anomaly model is presented that supports the concept of a shallow, flat-bottomed body rather than one that continues to significant depth. There are many analogies with the Kangerlussuaq Intrusion, Greenland.
Abstract: Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr-Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1-2 GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions. The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.
Journal of African Earth Sciences, in press available
Africa, West Africa
Geophysics - magnetics
Abstract: Studies of mafic dyke swarms may simultaneously provide information on the mechanical, geochemical, geochronological and magnetic environments at the time of their formation. The mafic intrusive history of different cratons can also be potentially used to unravel their assembly into their current configuration. The identification and classification of dykes is a first step to all these studies. Fortunately, even in regions with poor outcrop, we can use the strong magnetic response of mafic dykes to identify and map their extent. In West Africa the first maps of mafic dyke distribution were made over 40 years ago, but there are still large areas where there are almost no published data. In this paper we present a significantly updated map of mafic dykes for the West Africa Craton based in large part on new interpretations of the regional airborne magnetic database. This map includes the locations of over three thousand dykes across the craton, which locally shows several orientation clusters that provide a minimum estimate for the total number of dyke swarms in this region. Whilst we will have to wait until systematic dating of the different swarms is completed, we can demonstrate that there is a long and complex history of mafic magmatism across the craton, with up to 26 distinct dyke swarms mapped based according to their orientation. The mapping and dating of these swarms will provide key constraints on the assembly of the fragments that make up the modern continents.
Abstract: The present study contributes new constraints on, and definitions of, the reconstructed plate margins of India and Madagascar based on flexural isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar (ECMM). We have estimated the nature of isostasy and crustal geometry along the two margins, and have examined their possible conjugate structure. Here we utilize elastic thickness (Te) and Moho depth data as the primary basis for the correlation of these passive margins. We employ the flexure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elastic thickness. Gravity inversion and flexure inversion techniques are used to estimate the configuration of the Moho/Crust-Mantle Interface that reveals regional correlations with the elastic thickness variations. These results correlate well with the continental and oceanic segments of the Indian and African plates. The present study has found a linear zone of anomalously low-Te (1-5 km) along the WCMI (~1680 km), which correlates well with the low-Te patterns obtained all along the ECMM. We suggest that the low-Te zones along the WCMI and ECMM represent paleo-rift inception points of lithosphere thermally and mechanically weakened by the combined effects of the Marion hotspot and lithospheric extension due to rifting. We have produced an India-Madagascar paleo-fit representing the initial phase of separation based on the Te estimates of the rifted conjugate margins, which is confirmed by a close-fit correlation of Moho geometry and bathymetry of the shelf margins. The matching of tectonic lineaments, lithologies and geochronological belts between India and Madagascar provide an additional support for the present plate reconstruction.
Abstract: The causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute. Seismic tomography and other studies suggest that hot mantle material rising from the core-mantle boundary (CMB) might play a significant role in the formation of such hotspot trails. An important area to verify this concept is the South Atlantic region, with hotspot trails that spatially coincide with one of the largest low-velocity regions at the CMB, the African large low shear-wave velocity province. The Walvis Ridge started to form during the separation of the South American and African continents at ca. 130 Ma as a consequence of Gondwana breakup. Here, we present the first deep-seismic sounding images of the crustal structure from the landfall area of the Walvis Ridge at the Namibian coast to constrain processes of plume-lithosphere interaction and the formation of continental flood basalts (Paraná and Etendeka continental flood basalts) and associated intrusive rocks. Our study identified a narrow region (<100 km) of high-seismic-velocity anomalies in the middle and lower crust, which we interpret as a massive mafic intrusion into the northern Namibian continental crust. Seismic crustal reflection imaging shows a flat Moho as well as reflectors connecting the high-velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the Etendeka continental flood basalt. We suggest that the observed massive but localized mafic intrusion into the lower crust results from similar-sized variations in the lithosphere (i.e., lithosphere thickness or preexisting structures).
Abstract: The peridotite xenoliths of the Letlhakane kimberlite (Botswana), which intrude the Proterozoic Magondi Belt on the western margin of the Zimbabwe craton, represent highly depleted melting residues. These residues suffered subsequent variable metasomatic overprinting, evidenced by cryptic trace element enrichments in the spinel peridotites to modal addition of phlogopite, clinopyroxene and spinel within the garnet peridotites. In order to assess the robustness of the Re–Os chronometer in such highly metasomatised peridotites, detailed investigations of base metal sulphide (BMS) petrography and single-BMS grain 187Os/188Os analyses have been undertaken in three representative peridotites.
South African Journal of Geology, Vol, 118, no. 3, pp. 285-298.
Africa, East Africa
Geophysics - gravity
Abstract: A three-dimensional interpretation of the central part of the West and Central African Rift System (WCARS) is presented using the Earth Gravity Model 2008 (EGM2008). The 3-D model presented here provides new insights into the regional lithospheric structure of the central sector of the WCARS. The 3-D model reveals a possible crustal thickness and density distribution beneath the rift system, and the depth extent of magmatic activity in the Benue Trough is revealed for the first time. Because of asthenospheric uplift, the crust under the Benue Trough is thinned. The zone of crustal thinning (<30 km) coincides with the trace of an intrusion beneath the trough, and the maximum of which is attained in the Yola Basin. The Adamawa and Kapsiki Plateaus, on the other hand, show by far the largest crustal thickness in the region (34 to 36 km). The inferred zone of intrusion coincides with the maximum gravity anomaly of the rift zone. The great depth (ca. 14 km) to the top of the intrusion along the rift may explain the absence of magmatism in the Benue Trough compared to the Cameroon Volcanic Line (CVL) or the East African Rift System (EARS). The relatively small nature of the intrusion leads to the conclusion that small-scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the Benue Trough.
Abstract: This paper examines existing and newly compiled geophysical representations of the West African Craton (WAC) in terms of its large-scale tectonic architecture. In order to build an interpretation with a significant depth extent we draw upon a range of geophysical data, principally seismic tomographic inversions, receiver functions, gravity and magnetics. We present these results as a series of layers providing a series of depth slices though the cratonic lithosphere. The different geophysical methods suggest partitioning of the WAC into two tectonic elements at the largest scale which is observed in both seismic tomographic images, lithosphere-asthenosphere boundary (LAB) models and long wavelength gravity signals. The different models of the Moho, or crust-mantle boundary, based on these gravity or seismic datasets show little or no correlation, either for short or long-wavelength features, and show little correlation with new receiver function inferred crustal thickness estimates. Manual interpretation of low-wavelength gravity and magnetic data suggest a possible continuation of the WAC across the western margin of the modern boundary, and also highlight distinct domains interpreted to be of Birimian age.
Abstract: This article, based on ethnographic and archival research in the northeastern parklands of the Central African Republic (CAR), explores the area’s history of armed conservation. Critical scholarly accounts of armed conservation practices and projects often starkly contrast the people involved in them: there are agents of the state, or state-like actors, who seek to dominate, territorialize, and discipline, often using violence to do so, and there are local populations who are dispossessed of their lands and resources without compensation and forced into new kinds of poverty, despite rhetoric and practices meant to inculcate "local participation". The case presented here forces us to re-think these accounts. Rather than pursuing authority in the sense of expanding control over other people, people in northeastern CAR (whether putatively in favor of or opposed to conservation) are working to create and maintain access to the status of an income. To do so they engage in practices of threatening and hiding. While the means to use physical force are not equally shared, capacities to threaten and hide are widely held, and organizational and other hierarchies are unstable, making it difficult to describe any of this as a matter of domination and resistance. Expanding on literature that examines processes of green militarization (Lunstrum, 2014), the article focuses on the interactional dynamics of armed conservation to show that threats are as important as acts of physical violence, and that hiding-whether in the bush or plain sight-is critical to understanding armed conservation in an area where the state is largely seen as absent.
Abstract: The West African Craton is highly endowed in minerals, and their spatial and temporal distribution varies from single to multi-phase mineralization events. They are broadly related to three major tectono-metallogenic elements and formed during distinct mineral epochs: (1) In both Archean Shields (Kénéma-Man and Reguibat) and Paleoproterozoic domains (Baoulé-Mossi, Eglab). These are characterized by giant iron ore deposits that formed between ca. 2.5-2.3 Ga, nearly all gold, porphyry copper, lead-zinc and sedimentary manganese ore that developed between 2.2 and 2.1 Ga, and primary diamonds that formed between two intervals at ca. 2.2-2.0 Ga and in the Mesozoic. (2) Across Pan-African and Variscan belts. These are distinguished by major Precambrian IOCG's, copper-gold that formed at ca. 2.1 Ga and approximately 680 Ma, and Neoproterozoic sedimentary iron ore and phosphate deposits. (3) Within intracratonic and coastal basins. These include the development of Cenozoic lateritic bauxites over Mesozoic dolerites, Tertiary/Quaternary mineral sands deposits, oolitic iron ore and sedimentary phosphate deposits. Geological, spatial and temporal correlations using the multi-commodity West African Mineral Deposit Database highlight that gold and non-gold commodities formed in multiple phases. This commenced in the Liberian Orogeny (2.9-2.8 Ga) with the enrichment of iron ore, nickel sulphides, diamonds and gold in the earth's crust. The pre-Eburnean or Tangaean-EoEburnean-Eburnean I Event yielded gold, and the major Eburnean Orogeny yielded gold, iron ore, manganese, diamonds, magmatic nickel sulphides, copper-gold, lead-zinc, and REE minerals. Throughout the Pan-African event sedimentary manganese deposits, lead-zinc, REE minerals, sedimentary phosphates, and again gold were formed. Primary diamonds and magmatic nickel sulphides are related to the break-up of Gondwana, followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin.
Geochemistry, Geophysics, Geosystems: G3, Vol. 16, in press available
Africa, East Africa
Dyke swarms
Abstract: The role of dike intrusion in driving continental breakup is fundamental to our understanding of plate tectonics. Buck (2004) showed that the breakup of thick continental lithosphere requires more than far-field tectonic forces, illustrating the important role of dike opening in driving extension during the earliest stages of rifting. Upper crustal diking in rift environments is often depicted to occur through long (up to 80 km), sub-parallel swarms intruding along the full length of rift basins. These assertions are supported by recent dike-driven rifting events in Iceland and Ethiopia (Wright et al., 2012), and inform numerical modelling studies addressing the mechanical effects of dike intrusion to rift processes (e.g., Buck et al., 2005).
Our current view of dike networks, however, may be biased to evolved (>20 Ma), oceanic (e.g., Krafla) or nascent (e.g., Dabbahu-Manda-Hararo) spreading centers. This is largely because magmatic rifting occurs in these regions with a high enough frequency that it can be persistently captured using modern-day monitoring techniques (e.g., InSAR and seismicity). Dike networks throughout other sectors of the East African Rift may instead exhibit greater complexity, particularly in early-stage rifts (<10 Ma) undergoing infrequent diking episodes (Calais et al., 2008). By unravelling the contributions of dikes in these basins we can further refine our understanding of the role of magmatism during continental rift initiation.
Current geophysical techniques (e.g., seismic reflection) lack the capacity to resolve thin, sub-vertical structures in the sub-surface, and thus reconstructing the geometries of cooled, upper crustal dike swarms poses a significant challenge. Recent structural and volcanological studies in both active and eroded monogenetic volcanic fields have illustrated the utility of volcanic vent alignments and cone morphometrics for inferring the distributions and orientations of upper crustal dikes (Kiyosugi et al., 2012; Le Corvec et al., 2013; Keir et al., 2015). The East African Rift exhibits numerous monogenetic cone fields that may help us understand the distribution and geometry of shallow dike-feeders emplaced in the last few million years (Korme et al., 1997; Mazzarini et al., 2013). Building on these studies and methods, we performed a comparative analysis of upper crustal diking in various rift basins throughout East Africa, based on the distributions, alignments and morphologies of monogenetic cones (Muirhead et al., 2015).
GSA Annual Meeting, Paper 131-2, 1p. Abstract only
Africa
SCLM
Abstract: The subcontinental lithospheric mantle (SCLM) is foundational to understanding the construction, destruction, and division of tectonic plates. Tectonic processes, in turn, both directly and indirectly influence the lithosphere’s thermal, physical and mineralogical properties. Mantle melting and melt/fluid percolation cause fundamental changes to the lithosphere that affect its composition and stability. Specifically, metasomatism by silicate melts and hydrous/carbonated fluids can create lithologies (i.e. pyroxenites) that are denser, more fusible, and less viscous than adjacent peridotite. The resulting density instabilities may lead to lithospheric erosion, topographic uplift and even continental rifting. We explore the link between metasomatized SCLM, mafic volcanism and associated continental rifting in the Ugandan portion of the Western Rift of the East African Rift System using Re-Os isotopes from both alkaline mafic lavas and pyroxenite mantle xenoliths. The lavas record age-corrected 187Os/188Os that range from 0.1421 to 0.2105, which is more radiogenic than primitive mantle (0.1296; Meisel et al., 2001). These data demonstrate that many of the lavas were derived from a metasomatized mantle source though a few have experienced crustal contamination. The mantle xenoliths also record a wide range of 187Os abundances. One peridotite xenolith has a mildly radiogenic signature (187Os/188Os = 0.1342) whereas the pyroxenites span a wide range of 187Os/188Os ratios (0.1401-0.5052). One pyroxenite recorded a mildly unradiogenic 187Os/188Os value (0.127) and has 0.96 ppb of Os. Based on these data, we conclude that the lavas were derived from metasomatized SCLM. Some of the SCLM was sampled by mantle xenoliths but, as a whole, the SCLM is more heterogeneous than the lavas suggest. The widespread, metasomatized SCLM readily contributed to melt generation both in situ as well as during foundering via lithospheric drip (Furman et al., in review). The SCLM-derived volcanism occurred prior to and during Western Rift extension, suggesting that the metasomatized SCLM played a vital role in rift development.
Journal of African Earth Sciences, Vol. 111, pp. 26-40.
Africa, Cameroon
Lherzolite, Harzburgite, Olivine websterite
Abstract: Ultramafic xenoliths (lherzolite, harzburgite and olivine websterite) have been discovered in basanites close to Ngaoundéré in Adamawa plateau. Xenoliths exhibit protogranular texture (lherzolite and olivine websterite) or porphyroclastic texture (harzburgite). They are composed of olivine Fo89-90, orthopyroxene, clinopyroxene and spinel. According to geothermometers, lherzolites have been equilibrated at 880-1060 °C; equilibrium temperatures of harzburgite are rather higher (880-1160 °C), while those of olivine websterite are bracketed between 820 and 1010 °C. The corresponding pressures are 1.8-1.9 GPa, 0.8-1.0 GPa and 1.9-2.5 GPa, respectively, which suggests that xenoliths have been sampled respectively at depths of 59-63 km, 26-33 km and 63-83 km. Texture and chemical compositional variations of xenoliths with temperature, pressure and depth on regional scale may be ascribed to the complex history undergone by the sub-continental mantle beneath the Adamawa plateau during its evolution. This may involve a limited asthenosphere uprise, concomitantly with plastic deformation and partial melting due to adiabatic decompression processes. Chemical compositional heterogeneities are also proposed in the sub-continental lithospheric mantle under the Adamawa plateau, as previously suggested for the whole Cameroon Volcanic Line.
GSA Annual Meeting, Paper 131-14, 1p. Abstract only
Africa, South Africa
Carbonatite
Abstract: Petrologic and geochemical data for carbonatites and associated alkaline igneous rocks are presented for the Zandkopsdrift Carbonatite Complex, Namaqualand. The samples included in this study are relatively fresh, collected by coring at depths of >70 m below the weathered cap zone. The Zandkopsdrift complex is the only locality in the province known to contain significant carbonatite. The carbonatites studied are calico-, ferro- and silico- carbonatites, based on mineralogy, texture, and major element composition. They have low to moderate Mg-numbers (35-65), variable MgO contents (1.2-8.50 wt.%) and high atomic Ca/Ca+Mg (0.73-0.97), indicating that they are not likely simple mantle melts. The carbonatites contain significant apatite, magnetite, pyrochlore and phlogopite. Zandkopsdrift also contains significant amounts of aillikite and olivine melilitite. These rocks have relatively low SiO2 (25-31 wt.%) and Al2O3 (5.3- 6.1 wt.%), high K2O (6-6.3 wt.%) and TiO2 (5.6-9.5 wt.%) and moderate Mg numbers (51-58). ?18O and ?13C isotopes were measured for carbonatites and aillikites. ?13CPDB values are close to those expected for mantle-derived carbonatites (-3.9 to -8.83), while the ?18OSMOW values are significantly higher (+13. 25 to 21.84‰). The high ?18O value observed in carbonatites and aillikites is most likely attributable to secondary alteration by hydrous/hydrothermal fluids. This supports the inference that the Zandkopsdrift carbonatite is magmatic in origin but was later affected by secondary alteration which resulted in the elevated O stable isotopes. The ‘mantle-like’ ?13C is inconsistent with significant assimilation of C-bearing crustal rocks. Chondrite-normalised REE contents in the carbonatites are 2400 to 10,600 for La and 36 to 170 for Lu. The high REE contents of the carbonatites are most likely due to a combination of a source metasomatised by a highly LREE-enriched agent, as well as significant magmatic differentiation. The relatively fractionated composition of the Zandkopsdrift aillikites and melilitites is also consistent with this hypothesis. We propose that the Zandkopsdrift carbonatites were most likely formed by either immiscible liquid separation from or fractional crystallization of a moderately fractionated, carbonate-rich silicate parental magma.
Session No. 131--Booth# 338
Deep-seated magmatism, its sources and plumes, Proceedings of XIII International Workshop held 2014., Vol. 2014, pp. 178-202.
Africa, South Africa, Russia, Yakutia
Deposit - Dike Newlands, Nyurbinskaya
Abstract: Green garnets occur in concentrates of diamondiferous kimberlite bodies in Yakutia (Udachnaya, Mir, etc.), South Africa (Newlands, Bellsbank), Venezuela (Guaniamo sills), and Canada (Mud Lake field). Mantle xenoliths of rocks containing such garnets are very rare. We found peridotite xenoliths with green garnet in situ in kimberlites of the Newlands dike. Xenoliths are irregular in form, 4.5*1.9 cm, 1.5*0.8 cm, and 1.0*0.5 cm in size, and have similar modal compositions: gar(70)+ol(28)+sp(2), gar(9)+ol(90)+sp(1) and gar(50)+ol(30)+sp(20). Rock texture is medium-crystalline, while structure is massive. We also identified a garnet macrocryst of 0.5*0.4 cm in size with a pale green kelyphytic rim. Garnet composition in the studied samples is quite constant and is characterized by the high Cr2O3 content (10.94-11.99%) and CaO content (19.52-24.94%) at the reduced contents of TiO2 (0.24-0.52%). The chrome spinel is high Cr2O3 (55%) content and the low TiO2 (0.5-0.6%) content. Olivine is high-Mg (Fo95), but elevated CaO content (0.09%). Isotopic composition of oxygen in garnet (?18O = 4.05-4.25 pm) and olivine (?18O = 4.91 pm) differs drastically from the mantle values. Rb-Sr and Sm-Nd isotopic composition show the relatively "young" model age of the sample relative to the depleted mantle (1.78 billion years), the age of formation of this rocks is also relatively "young" - probable mezoproterozoic. In kimberlites and placers of the Nyurbinskaya pipe (Nakyn field, Yakutia) there are 4 green garnet grains of 0.5-2.0 mm in size, including one intergrowth gar+sp. Most garnets are characterized by the higher CaO (18.06-22.87%) and TiO2 (1.46, 1.65, 1.75%) contents not noted before for similar garnets. Studied green garnets have the similar "sine wave" type of REE distribution for low-Ti garnets and a "raised" type of REE distribution with enrichment in medium and light REE for high-Ti garnet.
All green garnets are characterized by an increased content of light REE and Sc. High-Ti garnets are characterized by an increased content of light and middle REE, as well as titanium, and a particularly sharply increased content of Zr (!). Paragenesis ol+sp is formed at 805oand 23.4 kbar, and paragenesis ol+gar is formed at 1080oand 23.8 kbar. The rocks are characterized by nonequilibrium paragenesis ol+sp+gar and formation at moderate depths (80-90 km) under conditions of high heat flow (52-55 mW/m2). Judging from modal composition of studied xenoliths (absence of clinopyroxene), variations in chemical compositions and trace element compositions, relatively "young" model age and non-mantle isotopy of oxygen in garnets, these rocks are not "wehrlites" and likely represent metasomatic rocks such as uvarovite-chromite veins or schlierens at the moderate depths of upper mantle - it is similar to uvarovite-chromite veins of the metasomatic or a hydrothermal origin in the crustal serpentinites.
South African Journal of Geology, Vol. 118, 3, pp. 225-248.
Africa, South Africa
Geophysics - seismics
Abstract: Here we present a comprehensive depth and thickness map of the main Karoo and Cape Basins using borehole and reflection seismic data. The depth to the Whitehill Formation, which is the focus of current shale gas interest within the Karoo, is also mapped. Change: The deepest part of the basin is in the south, along the northern boundary of the Cape Fold Belt (~4000 m in the southwest Karoo and ~5000 m in the southeast; ~5500 to 6000 m sediment thickness). The Whitehill Formation along this boundary reaches a depth of ~3000 m in the southwest and ~4000 m in the southeast. Limited borehole data in the southeastern Karoo show a broad deepening of the basin here compared to the southwestern Karoo. In the southeast near East London faulting has resulted in deepening of the basin close to the coast, with the Whitehill Formation deepening to over ~5000 km. Seismic and borehole data show that the Cape Supergroup pinches out below the Karoo Basin around Beaufort West and Graaff-Reinet in the southern Karoo (32.6°S for the Bokkeveld and 32.4°S for the Table Mountain Group). The Cape Supergroup reaches thicknesses of around 4 km in the south. The gravity effect of these sediments does not account for the Cape Isostatic Anomaly (CIA) in the southern part of the Karoo Basin near Willowmore and Steytlerville, i.e., an ~45 mGal Bouguer gravity low. A refraction seismic profile over the anomaly shows this region is associated with a large volume of low velocity/density shallow sediments (4.5 m/s2, 2500 kg/m3), as well as a low velocity/density anomaly associated with a normal fault and the Klein Winterhoek Thrust Fault (5.5 m/s2, 2650 kg/m3). These low density shallow sediments are explained by uplift of Karoo and Cape sediments of ~2 km or greater that is evident on Soekor reflection seismic data. This deformation has brought lower density shales (1800 to 2650 kg/m3) of the Ecca Group closer to the surface. These shallower features along with a deeper lower crust in this region (6.5 m/s2, 2900 kg/m3) are interpreted to account for the CIA.
Journal of African Earth Sciences, Vol. 111, p. 287. 1p.
Africa
CSR - ethics
Abstract: The Journal of African Earth Sciences (Vol. 111, 2015, page 287) has published a "Short Note" entitled "Geoethics: Africa must be respected and heard!" by Lopo Vasconcelos, Jesus Martinez-Frias and Aberra Mogessie, raising an important issue: the need to include ethical issues in the educational systems of Africa. Reference is made to a project aiming at "providing the entire African continent with a roadmap to face the target of geoscience education within a wide scope and a multidisciplinary approach" GEO-ERA (GEOscience Education Roadmap for Africa) promoted and coordinated by the IUGS-Commission on Geoscience Education, Training and Technology Transfer (IUGS-COGE) and the Geological Society of Africa (GSAf). Considering that Education and Ethics must go along together, GSAf and IAGETH (International Association for Geoethics) signed in 2013, after the 24th Colloquium of African Geology, CAG24 (Addis Ababa) a Memorandum of Understanding in order to accomplish this and other goals.
Earth and Planetary Science Letters, Vol. 431, pp. 274-286.
Africa, South Africa
Deposit - Lace
Abstract: Major- and trace-element compositions of eclogite and pyroxenite xenoliths of ?2.5 Ga age (in situ Pb-Pb data on clinopyroxene) from the Lace kimberlite on the Kaapvaal craton were investigated in order to constrain: (1) the nature and evolution of their protoliths; (2) the extent to which they preserve information on the state of the asthenospheric mantle source that gave rise to their low-pressure protoliths; and (3) the effect of their deep recycling on the radiogenic isotope evolution of the convecting mantle. Their elemental relationships are consistent with low-pressure fractionation of olivine ± plagioclase and clinopyroxene during oceanic crust formation, whereby the residual melt was enriched in rare-earth elements (REE), high field-strength elements and Y, producing inverse correlations of ?REE with the size of Eu- and Sr-anomalies. LREE-depletion may indicate loss of on average 20% of a partial melt upon subduction and metamorphism (eclogitisation) of oceanic crust, which did not, however, contribute to juvenile growth of continental crust. The eclogites have median Sm/Nd (0.40) and Lu/Hf (0.27) similar to Depleted Mantle, and lower U/Pb (0.02) and Th/Pb (0.02). If deeply subducted, these rocks cannot explain unradiogenic Nd and Hf, and radiogenic Pb isotope compositions in the sources of some modern ocean island basalts. Low incompatible trace-element contents similar to picrites, and Yb concentrations at a given TiO2 content similar to modern MORB, indicate derivation of the protoliths by average melt fractions of ?0.20-0.25?0.20-0.25 that left a spinel peridotite residue at pressures ?2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of ?1420 to 1470?°C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7±1.24.7±1.2; n=11n=11) indicating lower fO2fO2 (?1.9 relative to the fayalite-magnetite-quartz buffer=?FMQbuffer=?FMQ) than modern MORB. This is in part due to the higher average melt extraction pressure (?1.5 GPa) during formation of their crustal protoliths. Extrapolation to 1 GPa, similar to the average pressure of present-day MORB generation, yields ?FMQ of ?1.7±1.1?1.7±1.1, corresponding to Fe3+# of 0.07±0.040.07±0.04. If these results are correct, they suggest an Archaean ambient mantle more reducing than at present, with implications for the speciation of volatiles, which affects the mantle solidus and the composition of volcanic gases. This has implications for the Archaean atmospheric redox evolution and the recycling of carbon and other volatiles.
Inside Mining, Vol. 109, July, pp. 32-35. Available pdf
Africa
Microdiamonds - responses
Abstract: The recovery and analysis of microdiamonds helps to predict the presence of commercial-sized diamonds in kimberlites. It is a cost-effective method for prioritising targets for bulk sampling.
Abstract: The Matongo carbonatite intrusive body in the Neoproterozoic Upper Ruvubu alkaline plutonic complex (URAPC) in Burundi is overlain by an economic phosphate ore deposit that is present as breccia lenses. The ore exhibits evidence of supergene enrichment but also preserves textures related to the concentration of fluorapatite in the carbonatitic system. Magmatic fluorapatite is abundant in the ore and commonly occurs as millimeter-sized aggregates. It is enriched in light rare earth elements (LREE), which is especially apparent in the final generation of magmatic fluorapatite (up to 1.32 wt% LREE2O3). After an episode of metasomatism (fenitization), which led to the formation of K-feldspar and albite, the fluorapatite-rich rocks were partly brecciated. Oxygen and carbon isotope compositions obtained on the calcite forming the breccia matrix (?18O?=?22.1?- and ?13C?=??1.5?‰) are consistent with the involvement of a fluid resulting from the mixing of magmatic-derived fluids with a metamorphic fluid originating from the country rocks. In a subsequent postmagmatic event, the carbonates hosting fluorapatite were dissolved, leading to intense brecciation of the fluorapatite-rich rocks. Secondary carbonate-fluorapatite (less enriched in LREE with 0.07-0.24 wt% LREE2O3 but locally associated with monazite) and coeval siderite constitute the matrix of these breccias. Siderite has ?18O values between 25.4 and 27.7?- and very low ?13C values (from ?12.4 to ?9.2?, which are consistent with the contribution of organic-derived low ?13C carbon from groundwater. These signatures emphasize supergene alteration. Finally, the remaining voids were filled with a LREE-poor fibrous fluorapatite (0.01 wt% LREE2O3), forming hardened phosphorite, still under supergene conditions. Pyrochlore and vanadiferous magnetite are other minerals accumulated in the eluvial horizons. As a consequence of the supergene processes and fluorapatite accumulation, the phosphate ore, which contains 0.72 to 38.01 wt% P2O5, is also enriched in LREE (LaN/YbN from 47.1 to 83.5; ?REE between 165 and 5486 ppm), Nb (up to 656 ppm), and V (up to 1232 ppm). In the case of phosphate exploitation at Matongo, REE could prove to have a subeconomic potential to be exploited as by-products of phosphates.
MSA Group for Paragon Diamonds, April, 17p. Available pdf
Africa, Lesotho
Microdiamonds - responses
Abstract: The Motete Dyke is a diamondiferous kimberlite situated in the mountains of Lesotho. Total liberation diamond recovery methods were used to analyse representative samples from localities along the 1 500 m outcrop length of the dyke for grade modeling purposes. The microdiamond grade and stone density is remarkably consistent throughout the exposed area of the dyke. A grade of some 60 cpht at +1.18 mm and 90 cpht at +0.85 mm is estimated for the dyke at surface. In order to extend the estimate to deeper levels it will be necessary to drill into the dyke to obtain more sampling material for total liberation analysis. Initial tonnage calculations indicate that the dyke contains an estimated 525 000 carats at +0.85 mm with an additional 50 000 carats in every additional 10 m depth below 2450 m elevation, assuming an average dyke width of 1.44 m and density of 2.57 tons/m3. Bulk sampling of the dyke is recommended to recover a parcel of diamonds for valuation and to confirm the diamond size frequency and content model.
Abstract: This study presents compositional data for a statistically significant number (n=180) of heated and quenched (recreated) carbonate melt inclusions trapped in magnetite and clinopyroxene in jacupirangite from Kerimasi volcano (Tanzania). On the basis of homogenization experiments for clinopyroxene-hosted melt inclusions and forsterite-monticellite-calcite phase relations, a range of 1000 to 900 °C is estimated for their crystallization temperatures. Petrographic observations and geochemical data show that during jacupirangite crystallization, a CaO-rich and alkali-"poor" carbonate melt (relative to Oldoinyo Lengai natrocarbonatite) existed and was entrapped in the precipitating magnetite, forming primary melt inclusions, and was also enclosed in previously crystallized clinopyroxene as secondary melt inclusions. The composition of the trapped carbonate melts in magnetite and clinopyroxene are very similar to the parental melt of Kerimasi calciocarbonatite; i.e., enriched in Na2O, K2O, F, Cl and S, but depleted in SiO2 and P2O5 relative to carbonate melts entrapped at an earlier stage and higher temperature (1050-1100 °C) during the formation of Kerimasi afrikandite. Significant compositional variation is shown by the major minerals of Kerimasi plutonic rocks (afrikandite, jacupirangite and calciocarbonatite). Magnetite and clinopyroxene in the jacupirangite are typically transitional in composition between those of afrikandite and calciocarbonatite. These data suggest that the jacupirangite represents an intermediate stage between the formation of afrikandite and calciocarbonatite. Jacupirangite most probably formed when immiscible silicate and carbonate melts separated from the afrikandite body, although the carbonate melt was not separated completely from the silicate melt fraction. In general, during the evolution of the carbonate melt at Kerimasi, concentrations of P2O5 and SiO2 decreased, whereas volatile content (alkalis, S, F, Cl and H2O) increased. Volatiles were incorporated principally in nyerereite, shortite, burbankite, nahcolite and sulfohalite as identified by Raman spectrometry. These extremely unstable minerals cannot be found in the bulk rock, because of alteration by secondary processes. On the basis of these data, an evolutionary model is developed for Kerimasi plutonic rocks.
Abstract: Everyday international political economy (EIPE) offers an opportunity to rethink the role of individuals and citizenry in shaping governance of natural resources. In South Africa, significant progress has been made by government in re-shaping water governance since the end of apartheid in the early 1990s. The role of government in water governance and water politics has thus been emphasised to a large degree. This study looks at historical material to assess the role that water politics and EIPE has played in shaping the use and management of water resources in the country. Case studies are analysed of two quaternary catchments, A63E and A71L in the Limpopo River Basin, to show how everyday actions by different actors has shaped the current waterscape in the basin. Four events, namely, the politics of the Middle Iron Age State at Mapungubwe; the development of the Mapungubwe National Park and World Heritage Site; the management of water for the De Beers Venetia Diamond Mine; and the establishment of the Coal of Africa Limited colliery are discussed in terms of the agential power at play during each event. The conclusions of the study are that EIPE and reflexive agential power are important factors in water governance that can sometimes be ignored through neoliberal institutionalism. In the current and future governance of water in South Africa they can offer an alternative view of the role and importance of actors and pathways for development.
Abstract: Garnets from kimberlite-hosted mantle and a few xenoliths from the lower crust were investigated for water, major, minor, and trace elements. Xenoliths from the mantle comprise pyroxenite, eclogite, alkremite, and peridotite, and crustal xenoliths are mafic high-pressure granulites. Samples from South Africa, Lesotho, and Namibia comprise two principal settings, Kaapvaal Craton (‘on craton’) and Rehoboth terrane (‘off craton’). The composition of garnet depends on rock type but is unrelated to the setting, except for Ti and Cr. In garnets from ‘off craton’ mantle xenoliths, Ti positively correlates with Cr whereas those from ‘on craton’ samples reveal a negative correlation between both elements. Rare earth element patterns indicative of a metasomatic overprint are observed in garnets from both settings, especially in eclogitic garnet. Water contents in garnet are low and range from <1 to about 40 ppm. No setting-related difference occurs, but a weak correlation between water and rock type exists. Water contents in garnets from eclogite and mafic granulite are lower than those in pyroxenite, alkremite, and peridotite. All garnets are water under-saturated, i.e. they do not contain the maximum amount of water that can be accommodated in the mineral structure. Cratonic and non-cratonic samples also show the same characteristics in the infrared (IR) absorption spectra. An absorption band at 3650 cm-1 is typical for most mantle garnets. Bands at 3520 and 3570 cm-1 are present only in TiO2-rich garnets from the Rehoboth terrane and are ascribed to a Ti-related hydrogen substitution. A number of garnets, especially from the Kaapvaal Craton, contain molecular water in addition to structural water. Molecular water is inhomogeneously distributed at grain scale pointing to local interaction with fluid and to disequilibrium at grain scale. These garnets consistently reveal either submicroscopic hydrous phases or additional IR bands at 3630 and 3610-3600 cm-1 caused by structural water. Both features do not occur in garnets in which molecular water is absent. The observations imply (i) relatively late introduction of fluid, at least in cases where hydrous phases formed, and (ii) a relatively dry environment because only water-deficient garnets are able to incorporate additional structural water. Most importantly, they imply (iii) that the low water contents are primary and not due to water loss during upward transport. This late water influx is not responsible for the metasomatic overprint indicated by garnet REE patterns. The results of this study suggest dry conditions in the lithosphere, including mantle and crustal sections of both the Kaapvaal Craton (‘on craton’) and the Rehoboth terrane (‘off craton’). If the low water contents contributed to the stabilization of the Kaapvaal cratonic root (Peslier et al., 2010) the same should apply to the Rehoboth lithosphere where the same variety of rock types occurs. The extremely low water contents in eclogite relative to pyroxenite may be explained by an oceanic crust origin of the eclogites. Subduction and partial melting would cause depletion of water and incompatible elements. The pyroxenites formed by crystal accumulation in the mantle and did not suffer melt depletion. Such a difference in origin can be reconciled with the low Ti contents in eclogitic garnet and the high Ti contents in pyroxenitic garnet.
Abstract: The West African Craton hosts major resources of gold, iron ore, aluminium ore, diamonds, phosphates and manganese. This portfolio of ore deposits is linked to the formation of Archean -Paleoproterozoic greenstone belts, Jurassic rifting and extended periods of Mesozoic to Cenozoic weathering and erosion. We give a brief overview of the temporal and spatial distribution patterns of West African ore deposits with emphasis on the main commodity types. The oldest ore forming processes generated major resources in iron ore and gold in the Kénéma -Man and Reguibat Shields during the Neo-Archean. The majority of gold, porphyry copper, lead -zinc and sedimentary manganese deposits formed during the Paleoproterozoic, dominantly within the Baoulé-Mossi domain. At the same time diamond-bearing kimberlites developed in Ghana. Another distinct diamond event has been recognized in the Mesozoic of the Kénéma -Man shield. Isolated occurrences of IOCG's as well as copper -gold and gold formed in Pan-African/Variscan belts. During the Neoproterozoic, the majority of mineralization consists of sedimentary iron ore and phosphate deposits located within intracratonic basins. During the Phanerozoic aluminium ore, phosphates and mineral sands concentrated along the margins of the coastal and intracratonic basins.
Abstract: The Mg isotopic compositions of garnet and clinopyroxene mineral separates and whole rocks from 21 xenolithic eclogites (11 low-MgO eclogites and 10 high-MgO eclogites) from the Koidu kimberlite complex, erupted within the Archean Man Shield, Sierra Leone, West Africa, provide new evidence bearing on the origin of cratonic eclogites. Garnet and clinopyroxene in both low-MgO and high-MgO eclogites generally record equilibrium inter-mineral Mg isotope partitioning, with ?26Mg varying from -2.15‰ to -0.46‰ in garnets and from -0.49‰ to +0.35‰ in clinopyroxenes. Bulk ?26Mg values (-1.38‰ to +0.05‰), constructed from garnet and clinopyroxene data, are similar to results from rock powders (-1.60‰ to +0.17‰), suggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositions of the xenoliths. The ?26Mg values of low-MgO eclogites (-0.80‰ to +0.05‰) exceed the range of mantle peridotite xenoliths (-0.25‰ ± 0.04‰), consistent with the eclogite’s derivation from recycled altered oceanic crust. Similarly variable ?26Mg values in high-MgO eclogites (-0.95‰ to -0.13‰), together with their high MgO and low FeO contents, suggest that high-MgO eclogites were produced by Mg-Fe exchange between partially molten low-MgO eclogites and surrounding peridotites. Our study shows that cratonic xenolithic eclogites preserve a record of Mg isotopic compositions produced by low-pressure, surficial isotope fractionations. The recycling of oceanic crust therefore increases the Mg isotope heterogeneity of the mantle.
Geophysical Research Letters, Vol. 42, 20, pp. 8398-8405.
Africa, Botswana
Geophysics - gravity
Abstract: Rifting incorporates the fundamental processes concerning the breakup of continental lithosphere and plays a significant role in the formation and evolution of sedimentary basins. In order to decipher the characteristics of rifting at its earliest stage, we conduct the first teleseismic crustal study of one of the world's youngest continental rifts, the Okavango Rift Zone (ORZ), where the magma has not yet breached the surface. Results from receiver function stacking and gravity modeling indicate that the crust/mantle boundary beneath the ORZ is uplifted by 4-5 km, and the initiation of the ORZ is closely related to lithospheric stretching. Possible decompression melting of the subcrustal lithosphere occurs beneath the ORZ, as evidenced by a relatively low upper mantle density based on the gravity modeling.
Abstract: The Harrat Kishb area of western Saudi Arabia is part of the Cenozoic volcanic fields in the western margin of the Arabian Shield. Numerous fresh ultramafic xenoliths are entrained in the basanite lava of Harrat Kishb, providing an opportunity to study the nature and petrogenetic processes involved in the evolution of the lithospheric mantle beneath the Arabian Shield. Based on the petrological characteristics and mineralogical compositions, the majority of the mantle xenoliths (~ 92%) are peridotites (lherzolites and pyroxene-bearing harzburgites); the remaining xenoliths (~ 8%) are unusual spinel-rich wehrlites containing black Al-spinel micropods. The two types of mantle xenoliths display magmatic protogranular texture. The peridotite xenoliths have high bulk-rock Mg#, high forsterite (Fo90-Fo92) and NiO (0.24-0.46 wt.%) contents of olivine, high clinopyroxene Mg# (0.91-0.93), variable spinel Cr# (0.10-0.49, atomic ratio), and approximately flat chondrite-normalized REE patterns. These features indicate that the peridotite xenoliths represent residues after variable degrees of melt extraction from fertile mantle. The estimated P (9-16 kbar) and T (877-1227 °C) as well as the oxidation state (?logfO2 = ? 3.38 to ? 0.22) under which these peridotite xenoliths originated are consistent with formation conditions similar to most sub-arc abyssal-type peridotites worldwide. The spinel-rich wehrlite xenoliths have an unusual amount (~ 30 vol.%) of Al-spinel as peculiar micropods with very minor Cr2O3 content (< 1 wt.%). Olivines of the spinel-rich wehrlites have low-average Fo (Fo81) and NiO (0.18 wt.%) contents, low-average cpx Mg# (0.79), high average cpx Al2O3 content (8.46 wt.%), and very low-average spinel Cr# (0.01). These features characterize early mantle cumulates from a picritic melt fraction produced by low degrees of partial melting of a garnet-bearing mantle source. The relatively high Na2O and Al2O3 contents of cpx suggest that the spinel-rich wehrlites are formed under high P (11-14 kbar), T (1090-1130 °C), and oxidation state (?logfO2 FMQ = + 0.14 to + 0.37), which occurred slightly below the crust-mantle boundary. The REE patterns of spinel-rich wehrlites are almost similar to those of the associated peridotite xenoliths, which confirm at least a spatial genetic linkage between them. Regarding the formation of Al-spinel micropods in spinel-rich wehrlite cumulates, it is suggested that the melt-rock reaction mechanism is not the only process by which podiform chromitite is formed. Early fractionation of picritic melts produced by partial melting of a mantle source under high P-T conditions could be another mechanism. The cpx composition, not opx, as it was assumed, seems to be the main control of the size and composition of spinel concentrations.
Mineralogy and Petrology, in press available, 17p.
Africa, Burundi
Carbonatite
Abstract: The Matongo carbonatite intrusive body in the Neoproterozoic Upper Ruvubu alkaline plutonic complex (URAPC) in Burundi is overlain by an economic phosphate ore deposit that is present as breccia lenses. The ore exhibits evidence of supergene enrichment but also preserves textures related to the concentration of fluorapatite in the carbonatitic system. Magmatic fluorapatite is abundant in the ore and commonly occurs as millimeter-sized aggregates. It is enriched in light rare earth elements (LREE), which is especially apparent in the final generation of magmatic fluorapatite (up to 1.32 wt% LREE2O3). After an episode of metasomatism (fenitization), which led to the formation of K-feldspar and albite, the fluorapatite-rich rocks were partly brecciated. Oxygen and carbon isotope compositions obtained on the calcite forming the breccia matrix (?18O?=?22.1?‰ and ?13C?=??1.5?‰) are consistent with the involvement of a fluid resulting from the mixing of magmatic-derived fluids with a metamorphic fluid originating from the country rocks. In a subsequent postmagmatic event, the carbonates hosting fluorapatite were dissolved, leading to intense brecciation of the fluorapatite-rich rocks. Secondary carbonate-fluorapatite (less enriched in LREE with 0.07-0.24 wt% LREE2O3 but locally associated with monazite) and coeval siderite constitute the matrix of these breccias. Siderite has ?18O values between 25.4 and 27.7?‰ and very low ?13C values (from ?12.4 to ?9.2?‰), which are consistent with the contribution of organic-derived low ?13C carbon from groundwater. These signatures emphasize supergene alteration. Finally, the remaining voids were filled with a LREE-poor fibrous fluorapatite (0.01 wt% LREE2O3), forming hardened phosphorite, still under supergene conditions. Pyrochlore and vanadiferous magnetite are other minerals accumulated in the eluvial horizons. As a consequence of the supergene processes and fluorapatite accumulation, the phosphate ore, which contains 0.72 to 38.01 wt% P2O5, is also enriched in LREE (LaN/YbN from 47.1 to 83.5; ?REE between 165 and 5486 ppm), Nb (up to 656 ppm), and V (up to 1232 ppm). In the case of phosphate exploitation at Matongo, REE could prove to have a subeconomic potential to be exploited as by-products of phosphates.
Abstract: Kimberlite magmas are of economic and scientific importance because they represent the major host to diamonds and are probably the deepest magmas from continental regions. In addition, kimberlite magmas transport abundant mantle and crustal xenoliths, thus providing fundamental information on the composition of the sub-continental lithosphere. Despite their importance, the composition and ascent mechanism(s) of kimberlite melts remain poorly constrained. Phlogopite is one of the few minerals that preserves a history of fluid migration and magmatism in the mantle and crust and is therefore an invaluable petrogenetic indicator of kimberlite magma evolution. Here we present major and trace element compositional data for phlogopite from the Bultfontein kimberlite (Kimberley, South Africa; i.e. the kimberlite type-locality) and from entrained mantle xenoliths. Phlogopite macrocrysts (~ > 0.3-0.5 mm) and microcrysts (between ~ 0.1 and 0.3 mm) in the Bultfontein kimberlite display concentric compositional zoning patterns. The cores of these phlogopite grains exhibit compositions typical of phlogopite contained in peridotite mantle xenoliths. However, the rims of some grains show compositions analogous to kimberlite groundmass phlogopite (i.e. high Ti, Al and Ba; low Cr), whereas other rims and intermediate zones (between cores and rims) exhibit unusually elevated Cr and lower Al and Ba concentrations. The latter compositions are indistinguishable from matrix phlogopite in polymict breccia xenoliths (considered to represent failed kimberlite intrusions) and from Ti-rich overgrowth rims on phlogopite in other mantle xenoliths. Consequently, it is likely that these phlogopite grains crystallized from kimberlite melts and that the high Ti-Cr zones originated from earlier kimberlite melts at mantle depths. We postulate that successive pulses of ascending kimberlite magma progressively metasomatised the conduit along which later kimberlite pulses ascended, producing progressively decreasing interaction with the surrounding mantle rocks. In our view, these processes represent the fundamental mechanism of kimberlite magma ascent. Our study also indicates that, in addition to xenoliths/xenocrysts and magmatic phases, kimberlite rocks incorporate material crystallized at various mantle depths by previous kimberlite intrusions (mantle-derived ‘antecrysts’).
Abstract: This study presents compositional data for a statistically significant number (n = 180) of heated and quenched (recreated) carbonate melt inclusions trapped in magnetite and clinopyroxene in jacupirangite from Kerimasi volcano (Tanzania). On the basis of homogenization experiments for clinopyroxene-hosted melt inclusions and forsterite-monticellite-calcite phase relations, a range of 1000 to 900 °C is estimated for their crystallization temperatures. Petrographic observations and geochemical data show that during jacupirangite crystallization, a CaO-rich and alkali-"poor" carbonate melt (relative to Oldoinyo Lengai natrocarbonatite) existed and was entrapped in the precipitating magnetite, forming primary melt inclusions, and was also enclosed in previously crystallized clinopyroxene as secondary melt inclusions. The composition of the trapped carbonate melts in magnetite and clinopyroxene is very similar to the parental melt of Kerimasi calciocarbonatite; i.e., enriched in Na2O, K2O, F, Cl and S, but depleted in SiO2 and P2O5 relative to carbonate melts entrapped at an earlier stage and higher temperature (1050-1100 °C) during the formation of Kerimasi afrikandite. Significant compositional variation is shown by the major minerals of Kerimasi plutonic rocks (afrikandite, jacupirangite and calciocarbonatite). Magnetite and clinopyroxene in the jacupirangite are typically transitional in composition between those of afrikandite and calciocarbonatite. These data suggest that the jacupirangite represents an intermediate stage between the formation of afrikandite and calciocarbonatite. Jacupirangite most probably formed when immiscible silicate and carbonate melts separated from the afrikandite body, although the carbonate melt was not separated completely from the silicate melt fraction. In general, during the evolution of the carbonate melt at Kerimasi, concentrations of P2O5 and SiO2 decreased, whereas volatile content (alkalis, S, F, Cl and H2O) increased. Volatiles were incorporated principally in nyerereite, shortite, burbankite, nahcolite and sulfohalite as identified by Raman spectrometry. These extremely unstable minerals cannot be found in the bulk rock, because of alteration by secondary processes. On the basis of these data, an evolutionary model is developed for Kerimasi plutonic rocks.
Geochimica et Cosmochimica Acta, in press available, 14p.
Canada, Africa, Russia
Melting
Abstract: Although terrestrial picritic magmas with FeOTOT ?13 wt.% are rare in the geological record, they were relatively common ca. 2.7 Ga during the Neoarchean episode of enhanced global growth of continental crust. Recent evidence that ferropicritic underplating played an important role in the ca. 2.74-2.70 Ga reworking of the Ungava craton provides the impetus for a comparison of ca. 2.7 Ga ferropicrite occurrences in the global Neoarchean magmatic record. In addition to the Fe-rich plutons of the Ungava craton, volumetrically minor ferropicritic flows, pyroclastic deposits, and intrusive rocks form parts of the Neoarchean greenstone belt stratigraphy of the Abitibi, Wawa, Wabigoon and Vermillion domains of the southern and western Superior Province. Neoarchean ferropicritic rocks also occur on five other Archean cratons: West Churchill, Slave, Yilgarn, Kaapvaal, and Karelia; suggesting that ca. 2.7 Ga Fe-rich magmatism was globally widespread.
In situ oxygen isotope, major-, and trace element constraints on the metasomatic modification and crustal origin of a Diamondiferous eclogite from Roberts Victor, Kaapvaal Craton.
Geochimica et Cosmochimica Acta, in press available, 45p.
Abstract: The Mg isotopic compositions of garnet and clinopyroxene mineral separates and whole rocks from 21 xenolithic eclogites (11 low-MgO eclogites and 10 high-MgO eclogites) from the Koidu kimberlite complex, erupted within the Archean Man Shield, Sierra Leone, West Africa, provide new evidence bearing on the origin of cratonic eclogites. Garnet and clinopyroxene in both low-MgO and high-MgO eclogites generally record equilibrium inter-mineral Mg isotope partitioning, with ?26Mg varying from -2.15‰ to -0.46‰ in garnets and from -0.49‰ to +0.35‰ in clinopyroxenes. Bulk ?26Mg values (-1.38‰-to +0.05-), constructed from garnet and clinopyroxene data, are similar to results from rock powders (-1.60‰ to +0.17‰), suggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositions of the xenoliths. The ?26Mg values of low-MgO eclogites (-0.80‰ to +0.05‰) exceed the range of mantle peridotite xenoliths (-0.25- ± 0.04-), consistent with the eclogite’s derivation from recycled altered oceanic crust. Similarly variable ?26Mg values in high-MgO eclogites (-0.95‰ to -0.13‰), together with their high MgO and low FeO contents, suggest that high-MgO eclogites were produced by Mg-Fe exchange between partially molten low-MgO eclogites and surrounding peridotites. Our study shows that cratonic xenolithic eclogites preserve a record of Mg isotopic compositions produced by low-pressure, surficial isotope fractionations. The recycling of oceanic crust therefore increases the Mg isotope heterogeneity of the mantle.-
Abstract: This article briefly outlines the history of the colonial diamond industry of Sierra Leone from 1930 to 1961, highlighting its contingent aspects and the bonds guiding the decisions and actions taken by local social actors in different contexts and at different times. By drawing on colonial documents and memoirs of colonial officers, it shows how the colonial government of Sierra Leone and the mining company that exercised a monopoly on diamond extraction collaborated on the establishment of a series of legislative and disciplinary devices that encompassed forms of biopolitical expertise.
Abstract: Xenoliths of mantle peridotite have been sampled from four kimberlite intrusions, Melton Wold, Hebron, Uintjiesberg and Markt, emplaced through the Mesoproterozoic Namaqua-Natal Belt, along the southern border of the Kaapvaal Craton. Although many of the xenoliths are heavily altered, constituent clinopyroxene, garnet and phlogopite are fresh and have been analysed by electron microprobe for major elements and by laser ablation ICP-MS for trace elements. Primitive mantle-normalised REE abundances in clinopyroxene are all strongly LREE enriched and show a range of patterns including uniformly MREE-HREE sloped (referred to here as ‘normal’), sinusoidal and humped sinusoidal patterns. HREE abundances are extremely low (Yb = 0.3-0.06 × PM). REEN patterns in coexisting garnets show a similar range of patterns. When normalised to primitive mantle values, trace element patterns in some clinopyroxenes show strong relative depletion in Rb-Ba, Ta-Nb and Ti, with some samples also being relatively depleted in Zr-Hf. These trace element characteristics are indistinguishable from those found in clinopyroxene and garnet from peridotites from the adjacent cratonic mantle. Numerical modelling of reactive porous flow of an enriched metasomatic melt through a geochemically depleted peridotite matrix can account for the full range in observed REEN patterns. The relative depletion in Rb-Ba, Ta-Nb and Ti can be accounted for by an early crystallisation of phlogopite from the percolating melt. The relative depletion in Zr-Hf in some clinopyroxenes requires either zircon to crystallise in the proximal metasomatic assemblage, or metasomatism by a carbonatitic melt. Modelling results, together with the absence of clinopyroxene with depleted or even partially enriched REEN patterns, suggest that all clinopyroxene has been modally introduced through metasomatism into an initially highly depleted harzburgitic protolith. The range in Sr and Pb isotopic composition of the clinopyroxenes indicates regional metasomatism by melts of various compositions. The strong HREEN depletion is interpreted to reflect the effect of initial melt depletion in the early Proterozoic, with melting extending into the spinel stability field requiring an oceanic realm, and again later in the Mesoproterozoic (Namaqua Orogeny). The superimposed incompatible element enrichment indicates subsequent multiple enrichment events by rising alkaline melts similar in composition to kimberlite or ultramafic alkaline lamprophyre, possibly related to Mesozoic plume upwelling beneath the region, that reintroduced clinopyroxene into the depleted Proterozoic harzburgite protolith.
Abstract: This paper provides a review of the Geological exploration of the Angola Coast (from Sumbe to Namibe) from pioneer's first geological descriptions and mining inventory to the most recent publications supported by the oil industry. We focus our attention on the following periods: 1875-1890 (Paul Choffat's work, mainly), 1910-1949 (first maps at country scale), 1949-1974 (detailed mapping of the Kwanza-Namibe coastal series), 1975-2000, with the editing of the last version of the Angola geological map at 1:1 million scale and the progressive completion of previous works. Since 2000, there is a renewal in geological fieldwork publications on the area mainly due to the work of university teams. This review paper thus stands at the frontier between geology, natural resources and the history of geology. It shows how geological knowledge has progressed in time, fueled by economic and scientific reasons.
International Journal of Earth Sciences, in press available, 27p.
Africa, Zimbabwe
Geophysics - magnetics
Abstract: Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological-tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8-2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures, that were exploited by Late Archaean and Proterozoic mafic intrusions. There was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide.
Russian Geology and Geophysics, Vol. 56, pp. 1717-1737.
Africa, Tanzania
Deposit - Oldoinyo Lengai
Abstract: The paper is concerned with study of melt inclusions in minerals of ijolite xenoliths at Oldoinyo Lengai Volcano. Melt inclusions with different phase compositions occur in forsterite macrocrysts and in diopside, nepheline, fluorapatite, Ti-andradite, and Ti-magnetite crystals. Nepheline contains primary melt inclusions (silicate glass + gas-carbonate globule ± submicron globules ± sulfide globule ± daughter/trapped phases, represented by diopside, fluorapatite, Ti-andradite, and alumoakermanite). The gas-carbonate globule consists of a gas bubble surrounded by a fine-grained aggregate of Na-Ca-carbonates (nyerereite and gregoryite). Fluorapatite contains primary carbonate-rich melt inclusions in the core, which consist of nyerereite, gregoryite, thenardite, witherite, fluorite, villiaumite, and other phases. Their mineral composition is similar to natrocarbonatites. Primary melt inclusions (glass + gas bubble ± daughter phases) are rare in diopside and Ti-andradite. Diopside and forsterite have trails of secondary carbonate-rich inclusions. Besides the above minerals, these inclusions contain halite, sylvite, neighborite, Na-Ca-phosphate, alkali sulfates, and other rare phases. In addition, diopside contains sulfide inclusions (pyrrhotite ± chalcopy- rite ± djerfisherite ± galena ± pentlandite). The chemical compositions of silicate glasses in the melt inclusions vary widely. The glasses are characterized by high Na, K, and Fe contents and low Al contents. They have high total alkali contents (16-23 wt.% Na2O + K2O) and peralkalinity index [(Na + K)/Al] ranging from 1.1 to 7.6. The carbonate-rich inclusions in the ijolite minerals are enriched in Na, P, S, and Cl. The data obtained indicate that the parental melt in the intermediate chamber was heterogeneous and contained silicate, natrocarbonate, and sulfide components during the ijolite crystallization. According to heating experiments with melt inclusions, silicate-carbonate liquid immiscibility occurred at temperature over 580 °C.
Abstract: Plate tectonics plays a vital role in the evolution of our planet. Geochemical analysis of Earth’s oldest continental crust suggests that subduction may have begun episodically about 3.8 to 3.2 billion years ago, during the early Archaean or perhaps more than 3.8 billion years ago, during the Hadean. Yet, mantle rocks record evidence for modern-style plate tectonics beginning only in the late Archaean, about 3 billion years ago. Here we analyse the nitrogen abundance, as well as the nitrogen and carbon isotopic signatures of Archaean placer diamonds from the Kaapvaal craton, South Africa, which formed in the upper mantle 3.1 to 3.5 billion years ago. We find that the diamonds have enriched nitrogen contents and isotopic compositions compared with typical mantle values. This nitrogen geochemical fingerprint could have been caused by contamination of the mantle by nitrogen-rich Archaean sediments. Furthermore, the carbon isotopic signature suggests that the diamonds formed by reduction of an oxidized fluid or melt. Assuming that the Archaean mantle was more reduced than the modern mantle, we argue that the oxidized components were introduced to the mantle by crustal recycling at subduction zones. We conclude, on the basis of evidence from mantle-derived diamonds, that modern-style plate tectonics operated as early as 3.5 billion years ago.
Low water contents in diamond mineral inclusions: proto-genetic origin in a dry cratonic lithosphere.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Russia, Africa
Kaapvaal and Siberian SCLMs
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Historical Research, Vol. 89, no 243, pp. 136-157.
Africa, Sierra Leone
History
Abstract: This article briefly outlines the history of the colonial diamond industry of Sierra Leone from 1930 to 1961, highlighting its contingent aspects and the bonds guiding the decisions and actions taken by local social actors in different contexts and at different times. By drawing on colonial documents and memoirs of colonial officers, it shows how the colonial government of Sierra Leone and the mining company that exercised a monopoly on diamond extraction collaborated on the establishment of a series of legislative and disciplinary devices that encompassed forms of biopolitical expertise.
Geological Society of America Bulletin, Vol. 128, 1/2, pp. 169-186.
Africa, Tanzania
Lineaments
Abstract: The North Tanzanian divergence zone along the East African Rift is characterized by active faults and several large volcanoes such as Meru, Ol Doinyo Lengai, and Kilimanjaro. Based on systematic morphostructural analysis of the Shuttle Radar Topographic Mission digital elevation model and targeted field work, 14 debris avalanche deposits were identified and characterized, some of them being - to our knowledge - previously unknown. Our field survey around Mount Meru allowed previous "lahar" deposits to be reinterpreted as debris avalanche deposits and three major collapse events to be distinguished, with the two older ones being associated with eruptions. We used topographic lineaments and faults across the North Tanzanian divergence zone to derive the main tectonic trends and their spatial variations and highlight their control on volcano collapse orientation. Based on previous analogue models, the tectonic regime is inferred from the orientation of the collapse scars and/or debris avalanche deposits. We infer two types of regime: extensional and transtensional/strike-slip. The strike-slip regime dominates along the rift escarpment, but an extensional regime is inferred to have operated for the recent sector collapses. The proposed interpretation of sector collapse scars and debris avalanche deposits therefore provides constraints on the tectonic regime in the region. It is possible that, in some cases, movement on regional faults triggered sector collapse.
Abstract: This study presents compositional data for a statistically significant number (n = 180) of heated and quenched (recreated) carbonate melt inclusions trapped in magnetite and clinopyroxene in jacupirangite from Kerimasi volcano (Tanzania). On the basis of homogenization experiments for clinopyroxene-hosted melt inclusions and forsterite-monticellite-calcite phase relations, a range of 1000 to 900 °C is estimated for their crystallization temperatures. Petrographic observations and geochemical data show that during jacupirangite crystallization, a CaO-rich and alkali-"poor" carbonate melt (relative to Oldoinyo Lengai natrocarbonatite) existed and was entrapped in the precipitating magnetite, forming primary melt inclusions, and was also enclosed in previously crystallized clinopyroxene as secondary melt inclusions. The composition of the trapped carbonate melts in magnetite and clinopyroxene is very similar to the parental melt of Kerimasi calciocarbonatite; i.e., enriched in Na2O, K2O, F, Cl and S, but depleted in SiO2 and P2O5 relative to carbonate melts entrapped at an earlier stage and higher temperature (1050-1100 °C) during the formation of Kerimasi afrikandite. Significant compositional variation is shown by the major minerals of Kerimasi plutonic rocks (afrikandite, jacupirangite and calciocarbonatite). Magnetite and clinopyroxene in the jacupirangite are typically transitional in composition between those of afrikandite and calciocarbonatite. These data suggest that the jacupirangite represents an intermediate stage between the formation of afrikandite and calciocarbonatite. Jacupirangite most probably formed when immiscible silicate and carbonate melts separated from the afrikandite body, although the carbonate melt was not separated completely from the silicate melt fraction. In general, during the evolution of the carbonate melt at Kerimasi, concentrations of P2O5 and SiO2 decreased, whereas volatile content (alkalis, S, F, Cl and H2O) increased. Volatiles were incorporated principally in nyerereite, shortite, burbankite, nahcolite and sulfohalite as identified by Raman spectrometry. These extremely unstable minerals cannot be found in the bulk rock, because of alteration by secondary processes. On the basis of these data, an evolutionary model is developed for Kerimasi plutonic rocks.
Journal of African Earth Sciences, Vol. 113, pp. 35-50.
Africa, Cameroon
Geochemistry
Abstract: Trace element abundances in three indicator minerals (corundum, chrysoberyl, and zircon grains) from the western Mamfe gem placers, as determined by LA-ICP-MS analytical techniques, are shown to be sensitive to their crystallization conditions and source rock types. Corundum is dominantly composed of Al (standardized at 529,300 ppm), Fe (2496-12,899 ppm), and Ti (46-7070 ppm). Among element ratios, Fe/Mg (73-1107), Fe/Ti (0.5-245.0), Ti/Mg (1-175), and Ga/Mg (4-90) are generally higher whereas, Cr/Ga (<0.072) is low. The Fe (?12,899), Ga (?398), Mg (2-62), Cr (1.1-33.0), and V (3.0-93.0) contents (in ppm) mostly typify corundum grains formed in magmatic rocks, although some are metamorphic affiliated. A very higher Ti and significantly low Ga, Ta and Nb contents in some blue grains, suggest interesting concentrations of those high-tech metals in their source rocks. Chrysoberyl is dominantly composed of Al (standardized at 425,000 ppm) and Be (62701-64371 ppm). Iron (7605-9225 ppm), Sn (502-3394 ppm), and Ti (33-2251 ppm) contents are high, whereas Ga (333-608 ppm), Ta (<456.0 ppm), and Nb (<3.0 ppm) are significantly low. The high (Be and Sn) and significantly low Ga-Rb abundances, and Ta > Nb in the western Mamfe chrysoberyls show that they were crystallized in granitic pegmatites, with some of those source rocks being enriched in Ta and Sn. Zirconium oxide (ZrO2: standardized at 66.1 wt.%)) is the only major oxide in analysed coarse-grained zircons. Within the minor elementary suites: Hf (4576-12,565 ppm) and Y (48-2805 ppm) contents are significantly high. The trace element suites include: Th (7-1565 ppm), U (13-687 ppm), and ?REE (50-2161 ppm), whose values are significantly low. The (Yb/Sm)N, Ce/Ce*, and Eu/Eu* anomalies range from 1.0 to 227.0, 0 to 308, and 0.08 to 1.7 respectively. They are Hf-Y-HREE enriched and depleted zircons mainly crystallized in magmatic oxidized environments. They were mainly sorted from granitoids, syenites and kimberlites.
Abstract: This article contributes to evolving debates on Sierra Leone’s post-war “crisis of youth” by providing an extended analysis of the role that young boys and girls assume in negotiating household poverty and enhancing their livelihood opportunities in small-scale mining communities. Child miners – or “half shovels” as they are locally known – are both directly and indirectly involved in small-scale gold extraction in Kono District, Sierra Leone’s main diamond-producing area. But the implications of their involvement are often far more nuanced and complex than international children’s rights advocates understand them to be. Drawing upon recent fieldwork carried out in and around the Kono mining village of Bandafayie, the article argues that children’s participation in the rural economy not only generates much-needed household income, but in many cases is the only way in which they can earn the monies needed to attend school. A blind and uncritical acceptance of international codes and agreements on child labor could have an adverse impact on children and, by extension, poor communities in rural Sierra Leone. Western notions of “progress” and development, as encapsulated in the post-conflict reconstruction programing of international NGOs and donor organizations, often do not match up with the complex realities or competing visions of local people.
Abstract: The Oulad Dlim Massif of the Western Reguibat Shield contains several carbonatite complexes of previously unknown age. The largest and best studied are Gleibat Lafhouda, composed of magnesiocarbonatites, and Twihinate, composed of calciocarbonatites. Gleibat Lafhouda is hosted by Archean gneisses and schists. It has a SHRIMP U-Th-Pb zircon crystallization age of 1.85 ± 0.03 Ga, a Nd model age of TCR = 1.89 ± 0.03 Ga, and a Sm-Nd age of 1.85 ± 0.39 Ga. It forms part of the West Reguibat Alkaline province. Twihinate, on the other hand, is much younger. It is hosted by Late Silurian to Early Devonian deformed granites and has a zircon crystallization age of 104 ± 4 Ma, which is within error of the age of the carbonatites of the famous Richat Structure in the southwest Reguibat Shield. Like these, the Twihinate carbonatites are part of the Mid-Cretaceous Peri-Atlantic Alkaline Pulse. The Twihinate carbonatites contain abundant inherited zircons with ages that peak at ca. 420 Ma, 620 Ma, 2050 Ma, 2466 Ma, and 2830 Ma. This indicates that their substratum has West African rather than, as previously suggested, Avalonian affinities. It has, however, a Paleoproterozoic component that is not found in the neighboring western Reguibat Shield. The 421 Ma to 410 Ma gneissic granites hosting Twihinate are epidote + biotite + Ca-rich garnet deformed I-type to A-type granites derived from magmas of deep origin compatible, therefore, with being generated in a subduction environment. These granites form a body of unknown dimensions and petrogenesis, the study of which will be of key importance for understanding the geology and crustal architecture of this region.
In situ oxygen-isotope, major, and trace element constraints on the metasomatic modification and crust origin of a Diamondiferous eclogite from Roberts Victor, Kaapvaal craton.
Geochimica et Cosmochimica Acta, Vol. 174, pp. 345-359.
Contributions to Mineralogy and Petrology, Vol. 171, 11, 24p.
Africa, South Africa
Deposit - Bellsbank
Abstract: An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and ? 18O values lower than the Earth’s mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4 6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.
Abstract: The cooling history of rift shoulders and the subsidence history of rift basins are cornerstones for reconstructing the morphotectonic evolution of extensional geodynamic provinces, assessing their role in paleoenvironmental changes and evaluating the resource potential of their basin fills. Our apatite fission track and zircon (U-Th)/He data from the Samburu Hills and the Elgeyo Escarpment in the northern and central sectors of the Kenya Rift indicate a broadly consistent thermal evolution of both regions. Results of thermal modeling support a three-phased thermal history since the early Paleocene. The first phase (~65 50?Ma) was characterized by rapid cooling of the rift shoulders and may be coeval with faulting and sedimentation in the Anza Rift basin, now located in the subsurface of the Turkana depression and areas to the east in northern Kenya. In the second phase, very slow cooling or slight reheating occurred between ~45 and 15?Ma as a result of either stable surface conditions, very slow exhumation, or subsidence. The third phase comprised renewed rapid cooling starting at ~15?Ma. This final cooling represents the most recent stage of rifting, which followed widespread flood-phonolite emplacement and has shaped the present-day landscape through rift shoulder uplift, faulting, basin filling, protracted volcanism, and erosion. When compared with thermochronologic and geologic data from other sectors of the East African Rift System, extension appears to be diachronous, spatially disparate, and partly overlapping, likely driven by interactions between mantle-driven processes and crustal heterogeneities, rather than the previously suggested north south migrating influence of a mantle plume.
Geochimica et Cosmochimica Acta, Vol. 179, pp. 156-176.
Africa, South Africa
Deposit - Lace
Abstract: Diamond- and kyanite-bearing eclogites from the Lace kimberlite on the Kaapvaal craton have common picritic to gabbroic oceanic protoliths with bimineralic eclogites, lying on arrays of Eu? and ?REE that are consistent with accumulation and fractionation of plagioclase and olivine. However, they also show significant compositional differences, such as more grossular-rich garnet and aluminous clinopyroxene (cpx), which require the operation of additional processes. Their nature is elucidated using mineral major- and trace-element compositions, as well as Sr isotope ratios determined by in situ techniques.Highly variable major-element compositions across the co-genetic eclogite suites exert a strong effect on the trace-element distribution between garnet and cpx, whereby Sc, Ge, Sr, Y, Cd, REE, Th and U partition more strongly into garnet with increasing grossular-content. Thus, significant differences between the trace-element compositions of garnet can ensue from crystal-chemical effects alone, making their use as petrogenetic indicators potentially ambiguous. After correcting for these compositional effects, garnet in kyanite-/diamond eclogites, and in eclogites devoid of accessory minerals but with similar signatures, shows depletion (or dilution) in Sc, Ge, Y, In, Zr, Hf and the HREE, and enrichment in the LREE and Th compared to garnet in bimineralic eclogites. This is interpreted as the signature of a pelite-derived melt, which was transferred by addition of aluminous cpx that later exsolved kyanite and garnet, as observed in other aluminous eclogite suites. Continental input can explain initial (at 2.9 Ga) 87Sr/86Sr ? 0.714 measured in cpx in eleven samples with low 87Rb/86Sr (<0.01). The association of diamond with kyanite suggests that diamond formation is also linked to this event, possibly due to diamond formation by oxidation of reduced carbon, such as methane, and attendant reduction of Fe3+ in garnet. This model of sediment melt-oceanic crust interaction reconciles evidence for both low- and high-pressure igneous processes in some aluminous eclogites. We suggest that a subduction mélange is a favourable setting for the transfer of a sediment-derived signature into oceanic crust, leading to formation of diamondiferous kyanite-eclogites from bimineralic eclogites. Diapirism, fluxed by the presence of partial melt, may have facilitated dispersal of the eclogites in the lithosphere column, consistent with their widely varying equilibration pressures ranging from ?5 to 8 GPa.
Abstract: The Tundulu and Kangankunde carbonatite complexes in the Chilwa Alkaline Province, Malawi, contain late-stage, apatite-rich lithologies termed quartz-apatite rocks. Apatite in these rocks can reach up to 90 modal% and displays a distinctive texture of turbid cores and euhedral rims. Previous studies of the paragenesis and rare earth element (REE) content of the apatite suggest that heavy REE (HREE)-enrichment occurred during the late-stages of crystallization. This is a highly unusual occurrence in intrusions that are otherwise light REE (LREE) enriched. In this contribution, the paragenesis and formation of the quartz-apatite rocks from each intrusion is investigated and re-evaluated, supported by new electron microprobe (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data to better understand the mechanism of HREE enrichment. In contrast to the previous work at Tundulu, we recognize three separate stages of apatite formation, comprising an “original” euhedral apatite, “turbid” apatite, and “overgrowths” of euhedral late apatite. The crystallization of synchysite-(Ce) is interpreted to have occurred subsequent to all phases of apatite crystallization. The REE concentrations and distributions in the different minerals vary, but generally higher REE contents are found in later-stage apatite generations. These generations are also more LREE-enriched, relative to apatite that formed earlier. A similar pattern of increasing LREE-enrichment and increased REE concentrations toward later stages of the paragenetic sequence is observed at Kangankunde, where two generations of apatite are observed, the second showing higher REE concentrations, and relatively higher LREE contents. The changing REE distribution in the apatite, from early to late in the paragenetic sequence, is interpreted to be caused by a combination of dissolution-reprecipitation of the original apatite and the preferential transport of the LREE complexes by F- and Cl-bearing hydrothermal fluids. Successive pulses of these fluids transport the LREE out of the original apatite, preferentially re-precipitating it on the rim. Some LREE remained in solution, precipitating later in the paragenetic sequence, as synchysite-(Ce). The presence of F is supported by the F content of the apatites, and presence of REE-fluorcarbonates. Cl is not detected in the apatite structure, but the role of Cl is suggested from comparison with apatite dissolution experiments, where CaCl2 or NaCl cause the reprecipitation of apatite without associated monazite. This study implies that, despite the typically LREE enriched nature of carbonatites, significant degrees of hydrothermal alteration can lead to certain phases becoming residually enriched in the HREE. Although at Tundulu the LREE-bearing products are re-precipitated relatively close to the REE source, it is possible that extensive hydrothermal activity in other carbonatite complexes could lead to significant, late-stage fractionation of the REE and the formation of HREE minerals.
Abstract: Perovskite (CaTiO3) has become a very useful mineral for dating kimberlite eruptions, as well as for constraining the compositional evolution of a kimberlitic magma and its source. Despite the undeniable potential of such an approach, no similar study had been done in Angola, the fourth largest diamond producer in Africa. Here we present the first work of in situ U-Pb geochronology and Sr-Nd isotope analyses of perovskite in six Angolan kimberlites, supported by a detailed petrographic and geochemical study of their perovskite populations. Four types of perovskite were identified, differing in texture, major- and trace-element composition, zoning patterns, type of alteration and the presence or absence of inclusions. Primary groundmass perovskite is classified either as anhedral, Na-, Nb- and LREE-poor perovskite (Ia); or euhedral, strongly zoned, Na-, Nb- and LREE-rich perovskite (Ib). Secondary perovskite occurs as reaction rims on ilmenite (IIa) or as high Nb (up to 10.6 wt% Nb2O5) perovskite rims on primary perovskite (IIb). The occurrence of these four types within the Mulepe kimberlites is interpreted as an evidence of a complex, multi-stage process that involved mingling of compositionally different melts. U-Pb dating of these perovskites yielded Lower Cretaceous ages for four of the studied kimberlites: Mulepe 1 (116.2 ± 6.5 Ma), Mulepe 2 (123.0 ± 3.6 Ma), Calonda (119.5 ± 4.3 Ma) and Cat115 (133 ± 10 Ma). Kimberlite magmatism occurred in NE Angola likely due to reactivation of deep-seated translithospheric faults (> 300 km) during the break-up of Gondwana. Sr-Nd isotope analyses of four of these kimberlites indicate that they are Group I kimberlites, which is consistent with the petrological observations.
Abstract: A type of pink pyrope garnet containing vanadium and chromium, believed to have been mined in Tanzania, appeared at the 2015 Tucson shows. The material shows a noticeable color difference from purplish pink under incandescent light (A) to purple under daylight-equivalent light (D65). This study reports a quantitative analysis of the difference in color between the two lighting conditions, based on the use of high-quality visible absorption spectroscopy to calculate CIELAB 1976 colorimetric coordinates. L*, a*, and b* colorimetric parameters were calculated for a wide range of path lengths as extrapolated from visible absorption spectra of thinner samples. Using this method, the path length of light through the stone that produces the optimal color difference can be calculated. This path length can then be used to determine the optimal depth range to maximize color change for a round brilliant of a specific material. The pink pyrope studied here can be designated as "color-change" garnet according to certain classification schemes proposed by other researchers. In many of these schemes, however, the material fails to exceed the minimum requirements for quantitative color difference and hue angle difference to be described as "color-change." Nonetheless, there is no simple solution to the problem of applying color coordinates to classify color-change phenomena. Also presented is a method by which spectra can be corrected for reflection loss and accurately extrapolated to stones with various path lengths.
Abstract: Geological mapping and zircon U-Pb/Hf isotope data from 35 samples from the central Tanzania Craton and surrounding orogenic belts to the south and east allow a revised model of Precambrian crustal evolution of this part of East Africa. The geochronology of two studied segments of the craton shows them to be essentially the same, suggesting that they form a contiguous crustal section dominated by granitoid plutons. The oldest orthogneisses are dated at ca. 2820 Ma (Dodoma Suite) and the youngest alkaline syenite plutons at ca. 2610 Ma (Singida Suite). Plutonism was interrupted by a period of deposition of volcano-sedimentary rocks metamorphosed to greenschist facies, directly dated by a pyroclastic metavolcanic rock which gave an age of ca. 2725 Ma. This is supported by detrital zircons from psammitic metasedimentary rocks, which indicate a maximum depositional age of ca. 2740 Ma, with additional detrital sources 2820 and 2940 Ma. Thus, 200 Ma of episodic magmatism in this part of the Tanzania Craton was punctuated by a period of uplift, exhumation, erosion and clastic sedimentation/volcanism, followed by burial and renewed granitic to syenitic magmatism. In eastern Tanzania (Handeni block), in the heart of the East African Orogen, all the dated orthogneisses and charnockites (apart from those of the overthrust Neoproterozoic granulite nappes), have Neoarchaean protolith ages within a narrow range between 2710 and 2630 Ma, identical to (but more restricted than) the ages of the Singida Suite. They show evidence of Ediacaran "Pan-African" isotopic disturbance, but this is poorly defined. In contrast, granulite samples from the Wami Complex nappe were dated at ca. 605 and ca. 675 Ma, coeval with previous dates of the "Eastern Granulites" of eastern Tanzania and granulite nappes of adjacent NE Mozambique. To the south of the Tanzania Craton, samples of orthogneiss from the northern part of the Lupa area were dated at ca. 2730 Ma and clearly belong to the Tanzania Craton. However, granitoid samples from the southern part of the Lupa "block" have Palaeoproterozoic (Ubendian) intrusive ages of ca. 1920 Ma. Outcrops further south, at the northern tip of Lake Malawi, mark the SE continuation of the Ubendian belt, albeit with slightly younger ages of igneous rocks (ca. 1870-1900 Ma) which provide a link with the Ponte Messuli Complex, along strike to the SE in northern Mozambique. In SW Tanzania, rocks from the Mgazini area gave Ubendian protolith ages of ca. 1980-1800 Ma, but these rocks underwent Late Mesoproterozoic high-grade metamorphism between 1015 and 1040 Ma. One granitoid gave a crystallisation age of ca. 1080 Ma correlating with known Mesoproterozoic crust to the east in SE Tanzania and NE Mozambique. However, while the crust in the Mgazini area was clearly one of original Ubendian age, reworked and intruded by granitoids at ca. 1 Ga, the crust of SE Tanzania is a mixed Mesoproterozoic terrane and a continuation from NE Mozambique. Hence the Mgazini area lies at the edge of the Ubendian belt which was re-worked during the Mesoproterozoic orogen (South Irumide belt), providing a further constraint on the distribution of ca. 1 Ga crust in SE Africa. Hf data from near-concordant analyses of detrital zircons from a sample from the Tanzania Craton lie along a Pb-loss trajectory (Lu/Hf = 0), extending back to ?3.9 Ga. This probably represents the initial depleted mantle extraction event of the cratonic core. Furthermore, the Hf data from all igneous samples, regardless of age, from the entire study area (including the Neoproterozoic granulite nappes) show a shallow evolution trend (Lu/Hf = 0.028) extending back to the same mantle extraction age. This implies the entire Tanzanian crust sampled in this study represents over 3.5 billion years of crustal reworking from a single crustal reservoir and that the innermost core of the Tanzanian Craton that was subsequently reworked was composed of a very depleted, mafic source with a very high Lu/Hf ratio. Our study helps to define the architecture of the Tanzanian Craton and its evolution from a single age-source in the early Eoarchaean.
Abstract: Geosoft is pleased to announce the launch of the Botswana Geoscience Portal, a partnership initiative with the Botswana Geoscience Institute and industry sponsors. Developed and hosted by Geosoft, the portal provides free access to multi-disciplinary datasets from Ngamiland, a district in the country's northwest. It is available online at: http://geoscienceportal.geosoft.com/Botswana/search. The geoscience portal aims to help Botswana attract new investment in resource exploration, improve transparency and stimulate collaboration between government, industry and the public to advance understanding of the economic and social needs of the North-West district. Tiyapo Hudson Ngwisanyi, Chief Executive Officer of the Botswana Geoscience Institute said: “Making geoscientific data more accessible and transparent is critical to furthering understanding of the North-West district, and encouraging new investment in resource exploration within Africa. The portal is a welcome development that will assist us in promoting ongoing, productive collaboration between government and industry.” “Geosoft is excited to be part of an initiative that demonstrates how government and industry can work together to encourage mineral exploration investment and thus downstream economic growth in the country,” said Tim Dobush, Chief Executive Officer of Geosoft. “We are continually engaging with government organizations like the Botswana Geoscience Institute and industry leaders to innovate, and maximize the value of geoscience data for resource discovery and to meet the social/economic needs of the broader public sector.” Data available on the Botswana Geoscience Portal includes airborne geophysics, ground geophysics and geochemistry. Future updates will provide access to borehole data, remote sensing, seismic surveys and information products including interpretations and 3D models.- See more at: http://www.geosoft.com/news/botswana-geoscience-portal-goes-live#sthash.CZGrHC4h.dpuf
Abstract: Perovskite (CaTiO3) has become a very usefulmineral for dating kimberlite eruptions, aswell as for constraining the compositional evolution of a kimberlitic magma and its source. Despite the undeniable potential of such an approach, no similar study had been done in Angola, the fourth largest diamond producer in Africa. Here we present the firstwork of in situ U-Pb geochronology and Sr-Ndisotope analyses of perovskite in six Angolan kimberlites, supported by a detailed petrographic and geochemical study of their perovskite populations. Four types of perovskitewere identified, differing in texture,major- and trace-element composition, zoning patterns, type of alteration and the presence or absence of inclusions. Primary groundmass perovskite is classified either as anhedral, Na-, Nb- and LREE-poor perovskite (Ia); or euhedral, strongly zoned, Na-, Nb- and LREE-rich perovskite (Ib). Secondary perovskite occurs as reaction rims on ilmenite (IIa) or as high Nb (up to 10.6 wt% Nb2O5) perovskite rims on primary perovskite (IIb). The occurrence of these four types within the Mulepe kimberlites is interpreted as an evidence of a complex, multi-stage process that involved mingling of compositionally different melts. U-Pb dating of these perovskites yielded Lower Cretaceous ages for four of the studied kimberlites: Mulepe 1 (116.2±6.5Ma),Mulepe 2 (123.0±3.6Ma), Calonda (119.5±4.3 Ma) and Cat115 (133±10Ma). Kimberlite magmatism occurred in NE Angola likely due to reactivation of deep-seated translithospheric faults (N300 km) during the break-up of Gondwana. Sr-Nd isotope analyses of four of these kimberlites indicate that they are Group I kimberlites, which is consistent with the petrological observations.
Physics of the Earth and Planetary Interiors, Vol. 254, pp. 12-24.
Africa, Europe
Geophysics - seismics, anisotropy, boundary
Abstract: The inner core under Africa is thought to be a region where the nature of inner core texture changes: from the strongly anisotropic ‘western’ part of the inner core to the weakly anisotropic, or isotropic ‘eastern’ part of the inner core. Additionally, observations of a difference in isotropic velocity between the two hemispheres have been made. A very large new dataset of simultaneous PKPdf and PKPbc observations, on which differential travel times have been measured, is used to examine the upper 360 km of the inner core under Europe, Africa and the surrounding oceans. Inversion of the differential travel time data for laterally varying inner core anisotropy reveals that inner core anisotropy is stronger under central Africa and the Atlantic Ocean than under the western Indian Ocean. No hemispherical pattern is present in Voigt isotropic velocities, indicating that the variation in anisotropy is due to differing degrees of crystal alignment in the inner core, not material differences. When anisotropy is permitted to change with depth, the upper east-most part of the study region shows weaker anisotropy than the central and western regions. When depth dependence in the inner core is neglected the hemisphere boundary is better represented as a line at 40°E than one at 10°E, however, it is apparent that the variation of anisotropy as a function of depth means that one line of longitude cannot truly separate the more and less anisotropic regions of the inner core. The anisotropy observed in the part of the inner core under Africa which lies in the ‘western’ hemisphere is much weaker than that under central America, showing that the western hemisphere is not uniformly anisotropic. As the region of low anisotropy spans a significant depth extent, it is likely that heterogeneous heat fluxes in the core, which may cause variations in inner core anisotropy, have persisted for several hundred million years.
Abstract: Why has Zimbabwe, a state that has been notorious for an utter disregard of international agreements, spent resources to implement policies that are in compliance with the Kimberley Process diamond certification scheme? In this research I explain variation in Zimbabwean regulatory policy in response to the Kimberley Process since 2003. This article contends that this variation can be best understood by tracing the political economy of factional rivalries within the ruling Zimbabwe African National Union - Patriotic Front (ZANU-PF) party. This research demonstrates that although the behavior of international regimes and private economic actors matter, in Zimbabwe it is government factions within ZANU-PF that are the main decision makers in relation to Kimberley Process regulations. This explains both why compliance with the Kimberley Process has been lower than other states in Southern Africa and why Zimbabwe has raised its level of compliance with the regulatory regime overtime.
Abstract: Since the end of World War II, there have been 259 armed conflicts in 159 locations (1). Sierra Leone's civil war began 25 years ago, at a time when roughly 25% of all countries worldwide were experiencing civil war (2). How can individuals and groups recover from such violent conflicts? On page 787 of this issue, Cilliers et al. (3) provide rigorous evidence on the efficacy of one postwar reconciliation strategy that was implemented in 100 communities in Sierra Leone (4).
Geochimica et Cosmochimica Acta, in press available
Africa, Ethiopia
Metasomatism - picrites
Abstract: The origin of the Ethiopian-Yemeni Oligocene flood basalt province is widely interpreted as representing mafic volcanism associated with the Afar mantle plume head, with minor contributions from the lithospheric mantle. We reinterpret the geochemical compositions of primitive Oligocene basalts and picrites as requiring a far more significant contribution from the metasomatized subcontinental lithospheric mantle than has been recognized previously. This region displays the fingerprints of mantle plume and lithospheric drip magmatism as predicted from numerical models. Metasomatized mantle lithosphere is not dynamically stable, and heating above the upwelling Afar plume caused metasomatized lithosphere with a significant pyroxenite component to drip into the asthenosphere and melt. This process generated the HT2 lavas observed today in restricted portions of Ethiopia and Yemen now separated by the Red Sea, suggesting a fundamental link between drip magmatism and the onset of rifting. Coeval HT1 and LT lavas, in contrast, were not generated by drip melting but instead originated from shallower, dominantly anhydrous peridotite. Looking more broadly across the East African Rift System in time and space, geochemical data support small volume volcanic events in Turkana (N. Kenya), Chyulu Hills (S. Kenya) and the Virunga province (Western Rift) to be derived ultimately from drip melting. The removal of the gravitationally unstable, metasomatized portion of the subcontinental lithospheric mantle via dripping is correlated in each case with periods of rapid uplift. The combined influence of thermo-mechanically thinned lithosphere and the Afar plume together thus controlled the locus of continental rift initiation between Africa and Arabia and provide dynamic support for the Ethiopian plateau.
Abstract: The lack of consensus on the possible range of initial kimberlite melt compositions and their evolution as they ascend through and interact with mantle and crustal wall rocks, hampers a complete understanding of kimberlite petrogenesis. Attempts to resolve these issues are complicated by the fact that kimberlite rocks are mixtures of magmatic, xenocrystic and antecrystic components and, hence, are not directly representative of their parental melt composition. Furthermore, there is a lack of direct evidence of the assimilation processes that may characterise kimberlitic melts during ascent, which makes understanding their melt evolution difficult. In this contribution we provide novel constraints on the interaction between precursor kimberlite melts and lithospheric mantle wall rocks. We present detailed textural and geochemical data for a carbonate-rich vein assemblage that traverses a garnet wehrlite xenolith [equilibrated at ~ 1060 °C and 43 kbar (~ 140-145 km)] from the Bultfontein kimberlite (Kimberley, South Africa). This vein assemblage is dominated by Ca-Mg carbonates, with subordinate oxide minerals, olivine, sulphides, and apatite. Vein phases have highly variable compositions indicating formation under disequilibrium conditions. Primary inclusions in the vein minerals and secondary inclusion trails in host wehrlite minerals contain abundant alkali-bearing phases (e.g., Na-K bearing carbonates, Mg-freudenbergite, Na-bearing apatite and phlogopite). The Sr-isotope composition of vein carbonates overlaps those of groundmass calcite from the Bultfontein kimberlite, as well as perovskite from the other kimberlites in the Kimberley area. Clinopyroxene and garnet in the host wehrlite are resorbed and have Si-rich reaction mantles where in contact with the carbonate-rich veins. Within some veins, the carbonates occur as droplet-like, globular segregations, separated from a similarly shaped Si-rich phase by a thin meniscus of Mg-magnetite. These textures are interpreted to represent immiscibility between carbonate and silicate melts. The preservation of reaction mantles, immiscibility textures and disequilibrium in the vein assemblage, suggests quenching, probably triggered by entrainment and rapid transport toward the Earth's surface in the host kimberlite magma. Based on the Sr-isotope systematics of vein carbonate minerals, and the close temporal relationship between carbonate-rich metasomatism and kimberlite magmatism, we suggest that the carbonate-rich vein assemblage was produced by the interaction between a melt genetically related to the Bultfontein kimberlite and wehrlitic mantle wall rock. If correct, this unique xenolith sample provides a rare snapshot of the assimilation processes that might characterise parental kimberlite melts during their ascent through the lithospheric mantle.
Contributions to Mineralogy and Petrology, Vol. 171, 7, 20p.
Africa, Tanzania
Tectonics
Abstract: North Tanzanian Divergence is the first stage of continental break-up of East African Rift (<6 Ma) and is one of the most concentrated areas of carbonatite magmatism on Earth, with singular Oldoinyo Lengai and Kerimasi volcanoes. Hanang volcano is the southernmost volcano in the North Tanzanian Divergence and the earliest stage of rift initiation. Hanang volcano erupted silica-undersaturated alkaline lavas with zoned clinopyroxene, nepheline, andradite-schorlomite, titanite, apatite, and pyrrhotite. Lavas are low MgO-nephelinite with low Mg# and high silica content (Mg# = 22.4–35.2, SiO2 = 44.2–46.7 wt%, respectively), high incompatible element concentrations (e.g. REE, Ba, Sr) and display Nb–Ta fractionation (Nb/Ta = 36–61). Major elements of whole rock are consistent with magmatic differentiation by fractional crystallization from a parental melt with melilititic composition. Although fractional crystallization occurred at 9–12 km and can be considered as an important process leading to nephelinite magma, the complex zonation of cpx (e.g. abrupt change of Mg#, Nb/Ta, and H2O) and trace element patterns of nephelinites recorded magmatic differentiation involving open system with carbonate–silicate immiscibility and primary melilititic melt replenishment. The low water content of clinopyroxene (3–25 ppm wt. H2O) indicates that at least 0.3 wt% H2O was present at depth during carbonate-rich nephelinite crystallization at 340–640 MPa and 1050–1100 °C. Mg-poor nephelinites from Hanang represent an early stage of the evolution path towards carbonatitic magmatism as observed in Oldoinyo Lengai. Paragenesis and geochemistry of Hanang nephelinites require the presence of CO2-rich melilititic liquid in the southern part of North Tanzanian Divergence and carbonate-rich melt percolations after deep partial melting of CO2-rich oxidized mantle source.
The Orange and Riet River alluvial diamond deposits in the vicinity of Douglas, Northern Cape Province: geology, evaluation, and exploitation of unique South African large diamond producing deposits.
IGC 35th., Session Mineral Exploration 1p. Abstract
Abstract: Diamonds and their inclusions are unique probes into the deep Earth, tracking the deep carbon cycle to >800?km. Understanding the mechanisms of carbon mobilization and freezing is a prerequisite for quantifying the fluxes of carbon in the deep Earth. Here we show direct evidence for the formation of diamond by redox reactions involving FeNi sulfides. Transmission Kikuchi Diffraction identifies an arrested redox reaction from pyrrhotite to magnetite included in diamond. The magnetite corona shows coherent epitaxy with relict pyrrhotite and diamond, indicating that diamond nucleated on magnetite. Furthermore, structures inherited from h-Fe3O4 define a phase transformation at depths of 320 -330?km, the base of the Kaapvaal lithosphere. The oxidation of pyrrhotite to magnetite is an important trigger of diamond precipitation in the upper mantle, explaining the presence of these phases in diamonds.
Abstract: Reversed-zoned olivines (Fe-richer cores compared to rims), appear to be ubiquitous in kimberlites with a wide distribution. These olivines generally comprise a subordinate population relative to the dominant normally zoned olivines. However, they are notably more abundant in the megacryst-rich mid-Cretaceous Monastery and early Proterozoic Colossus kimberlites, located on the Kaapvaal and Zimbabwe cratons, respectively. The reverse-zoned olivines at these two localities define compositional fields that are closely similar to those for two olivine megacryst populations of the Cr-poor association which have been documented in the Monastery kimberlite. This points to a genetic link between megacrysts and the reversed zoned olivines. The ubiquitous, occurrence of the Fe-rich (relative to the field for rims) olivines in kimberlites with a wide geographic distribution in turn argues for an intimate link between megacrysts and the host kimberlite. Some large olivines have inclusions of rounded Cr-rich clinopyroxenes, garnets and/or spinel, characterized by fine-scale, erratic internal compositional zoning. Olivines with such chemically heterogeneous Cr-rich inclusions are not derived from disaggregated mantle peridotites, but are rather linked to the Cr-rich megacryst suite. Consequently, they cannot be used as evidence that cores of a majority of kimberlitic olivines are derived from disaggregated mantle peridotites.
Earth and Planetary Science Letters, Vol. 447, pp. 60-71.
Africa, South Africa
Deposit - Bultfontein
Abstract: Understanding the mineralogical controls on radiogenic chronometers is a fundamental aspect of all geochronological tools. As with other common dating tools, it has become increasingly clear that the Re -Os system can be impacted by multiple mineral formation events. The accessory and micrometric nature of the Re -Os-bearing minerals has made assessing this influence complex. This is especially evident in cratonic peridotites, where long residence times and multiple metasomatic events have created a complex melting and re-enrichment history. Here we investigate a harzburgitic peridotite from the Bultfontein kimberlite (South Africa) which contains sub-micron Pt -Fe-alloy inclusions within base metal sulphides (BMS). Through the combination of the focused ion beam lift-out technique and low blank mass spectrometry we were able to remove and analyse the Pt -Fe-alloy inclusions for their Re -Os composition and highly siderophile element (HSE) systematics. Six repeats of the whole-rock yield 187Os/188Os compositions of 0.10893 -0.10965, which correspond to Re depletion model ages (TRD) of 2.69 -2.79 Ga. The Os, Ir and Pt concentrations are slightly variable across the different digestions, whilst Pd and Re remain constant. The resulting HSE pattern is typical of cratonic peridotites displaying depleted Pt and Pd. The Pt -Fe-alloys have PUM-like 187Os/188Os compositions of 0.1294±24 (2-s.d.) and 0.1342±38, and exhibit a saw-tooth HSE pattern with enriched Re and Pt. In contrast, their BMS hosts have unradiogenic 187Os/188Os of 0.1084±6 and 0.1066±3, with TRD ages of 2.86 and 3.09 Ga, similar to the whole-rock systematics. The metasomatic origin of the BMS is supported by (i) the highly depleted nature of the mantle peridotite and (ii) their Ni-rich sulphide assemblage. Occurrence of Pt -Fe-alloys as inclusions within BMS grains demonstrates the genetic link between the BMS and Pt -Fe-alloys and argues for formation during a single but continuous event of silicate melt percolation. While the high solubility of HSE within sulphide mattes rules out early formation of the alloys from a S-undersaturated silicate melt and subsequent scavenging in a sulphide matte, the alignment of the Pt -Fe-alloy inclusions attests that they are exsolutions formed during the sub-solidus re-equilibration of the high temperature sulphide phases. The significant difference in 187Os/188Os composition between the included Pt -Fe-alloys and their BMS host can only be accounted for by different Re/Os. This suggests that the formation of Pt -Fe-alloy inclusions within a BMS can result in the fractionation of Re from Os. A survey experiment examining the partitioning of Re and Os confirmed this observation, with the Re/Os of the Pt -Fe-alloy inclusion up to ten times higher than the co-existing BMS. This fractionation implies that, when Re is present in the sulphide melt, the TRD ages of BMS containing alloy inclusions do not date the loss of Re due to partial melting, but rather its fractionation into the Pt -Fe-alloys. As such, BMS ages should be used with caution when dating ancient partial melting events.
Abstract: Blue diamonds are among the rarest and most valuable of naturally occurring gemstones. In this study, 12 rough naturally-sourced type IIb diamonds were subjected to HPHT annealing, three different irradiation energies, and then all were stepwise annealed from 200 °C to 1100 °C and the optical defects were documented by changes in phosphorescence and photoluminescence spectroscopy. Several optical features that are removed from natural type IIb diamonds by HPHT processing, such as 3H, 648.2 nm peak, 776.4 nm peak, and 660 nm band (red) phosphorescence, can be reintroduced into these diamonds with subsequent electron irradiation and annealing at low-to-moderate temperatures. The thermal stability of these centers along with their spatial distribution provided additional insights into their configuration and distinguished them from nitrogen-bearing diamonds.
REM: Revista Escola de Minas, Vol. 68, 3, pp. 279-285.
Africa
Kimberley Process
Abstract: After more than a decade since its creation, the KPCS is undergoing questioning as to efficiency in combating the irregular trade of diamonds, among the countries with significant production for the global market, mainly Angola, the Democratic Republic of the Congo and Zimbabwe. Governments and institutions are considering it to be conducive to serious violations of human rights. In Brazil all activities of the sector have been reduced drastically. It is estimated that there has been a loss in Brazilian production, after implementation of the KPCS rules, in the order of 8.1 million Kts, valued at more than $ 2.0 billion.
Physics of the Earth and Planetary Interiors, Vol. 254, pp. 12-24.
Africa, Europe
Anisotropy
Abstract: The inner core under Africa is thought to be a region where the nature of inner core texture changes: from the strongly anisotropic ‘western’ part of the inner core to the weakly anisotropic, or isotropic ‘eastern’ part of the inner core. Additionally, observations of a difference in isotropic velocity between the two hemispheres have been made. A very large new dataset of simultaneous PKPdf and PKPbc observations, on which differential travel times have been measured, is used to examine the upper 360 km of the inner core under Europe, Africa and the surrounding oceans. Inversion of the differential travel time data for laterally varying inner core anisotropy reveals that inner core anisotropy is stronger under central Africa and the Atlantic Ocean than under the western Indian Ocean. No hemispherical pattern is present in Voigt isotropic velocities, indicating that the variation in anisotropy is due to differing degrees of crystal alignment in the inner core, not material differences. When anisotropy is permitted to change with depth, the upper east-most part of the study region shows weaker anisotropy than the central and western regions. When depth dependence in the inner core is neglected the hemisphere boundary is better represented as a line at 40°E than one at 10°E, however, it is apparent that the variation of anisotropy as a function of depth means that one line of longitude cannot truly separate the more and less anisotropic regions of the inner core. The anisotropy observed in the part of the inner core under Africa which lies in the ‘western’ hemisphere is much weaker than that under central America, showing that the western hemisphere is not uniformly anisotropic. As the region of low anisotropy spans a significant depth extent, it is likely that heterogeneous heat fluxes in the core, which may cause variations in inner core anisotropy, have persisted for several hundred million years.
Canadian Institute of Mining and Metallurgy, Vol. 6, 3, 14p.
Africa, Botswana
Deposit - Orapa
Abstract: The degree of uncertainty associated with a natural diamond resource is important to quantify from the time of discovery through the production lifetime. Data collection occurs during the discovery, exploration, delineation, and production or recovery phases. Quantifying the relationship between data and uncertainty is an important component of project valuation. The value of data is measured as their potential to reduce uncertainty if they are collected. A method is developed using Monte Carlo simulation for predicting resource uncertainty and valuing data during critical phases of development, particularly discovery and exploration. The technique is applied to diamond pipe deposits.
Abstract: In this work we report for the first time the crystallographic orientations of olivine inclusions trapped in diamonds from the Kaapvaal craton (South Africa) determined by single-crystal X-ray diffraction, and analyze them together with all available data in the literature. The overall data set indicates no preferred orientation of the olivine inclusions with respect to their diamond hosts. However, diamonds containing multiple olivine inclusions sometimes show clusters of olivines with the same orientation in the same diamond host. We conclude that such clusters can only be interpreted as the remnants of single olivine crystals pre-dating the growth of the host diamonds.
Abstract: Why has Zimbabwe, a state that has been notorious for an utter disregard of international agreements, spent resources to implement policies that are in compliance with the Kimberley Process diamond certification scheme? In this research I explain variation in Zimbabwean regulatory policy in response to the Kimberley Process since 2003. This article contends that this variation can be best understood by tracing the political economy of factional rivalries within the ruling Zimbabwe African National Union - Patriotic Front (ZANU-PF) party. This research demonstrates that although the behavior of international regimes and private economic actors matter, in Zimbabwe it is government factions within ZANU-PF that are the main decision makers in relation to Kimberley Process regulations. This explains both why compliance with the Kimberley Process has been lower than other states in Southern Africa and why Zimbabwe has raised its level of compliance with the regulatory regime overtime.
Abstract: Diamonds form from fluids or melts circulating at depth in the Earth's mantle. Analysis of these fluids is possible if they remain entrapped in the diamond during its growth, but this is rarely observed in gem-quality stones. We provide the first evidence that typical mineral inclusions in gem-quality diamonds from the Siberian and Kaapvaal cratons are surrounded by a thin film of hydrous silicic fluid of maximum thickness 1.5 ?m. The fluid contains Si2O(OH)6, Si(OH)4, and molecular H2O and was identified using confocal micro-Raman spectroscopy and synchrotron-based X-ray tomographic microscopy. As the solid mineral inclusions have both peridotitic and eclogitic affinities and occur in two cratonic regions, our results demonstrate the strong connection between water-rich fluids and the growth of gem-quality lithospheric diamonds. The presence of the fluid films should be taken into account for a proper evaluation of H2O contents in the mantle based on H2O contents in solid inclusions and for a robust assessment of diamond formation pressures based on the residual pressures of the inclusions.
Abstract: U-Pb and Lu-Hf data are routinely used to trace detrital zircon in clastic sediments to their original source in crystalline bedrock (the protosource), to map out paths of sediment transport, and characterize large-scale processes of crustal evolution. For such data to have a provenance significance, a simple transport route from the protosource in which the zircon formed to its final site of deposition is needed. However, detrital zircon data from Phanerozoic sedimentary cover sequences in South Africa suggest that this “source to sink” relationship has been obscured by repeated events of sedimentary recycling. Phanerozoic sandstones (Cape Supergroup, Karoo Supergroup, Natal Group, Msikaba Formation) and unconsolidated, Cenozoic sands in South Africa share major detrital zircon fractions of late Mesoproterozoic (940-1120 Ma, ?Hf ? 0 to + 15) and Neoproterozoic age (470-720 Ma, ?Hf ? ? 10 to + 8). A Permian age fraction (240-280 Ma, ?Hf ? ? 8 to + 5) is prominent in sandstones from the upper part of the Karoo Supergroup. All of these sequences are dominated by material derived by recycling of older sedimentary rocks, and only the youngest, late Palaeozoic fraction has a clear provenance significance (Gondwanide orogen). The virtual absence of Archaean zircon is a striking feature in nearly all suites of detrital zircon studied in the region. This indicates that significant events in the crustal evolution history of southern African and western Gondwana are not represented in the detrital zircon record. South Africa provides us with a record of recycling of cover sequences throughout the Phanerozoic, and probably back into the Neoproterozoic, in which the “sink” of one sedimentary cycle will act as the “source” in subsequent cycles. In such a setting, detrital zircon may give information on sedimentary processes rather than on provenance.
Abstract: The origin of high topography in southern Africa is enigmatic. By comparing topography in different cratons, we demonstrate that in southern Africa both the Archean and Proterozoic blocks have surface elevation 500-700 m higher than in any other craton worldwide, except for the Tanzanian Craton. An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness. We calculate the model of density structure of the lithospheric mantle in southern Africa and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37-3.41 g/cm3 in the Cape Fold and Namaqua-Natal fold belts, ca. 3.34-3.35 g/cm3 in the Proterozoic Okwa block and the Bushveld Intrusion Complex, ca. 3.34-3.37 g/cm3 in the Limpopo Belt, and ca. 3.32-3.33 g/cm3 in the Kaapvaal and southern Zimbabwe cratons.The results indicate that 0.5-1.0 km of surface topography, with the most likely value of ca. 0.5 km, cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities and requires the dynamic (or static) contribution from the sublithospheric mantle. Given a low amplitude of regional free air gravity anomalies (ca. + 20 mGal on average), we propose that mantle residual (dynamic) topography may be associated with the low-density region below the depth of isostatic compensation. A possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100-200 °C in a ca. 100-150 km thick layer may explain the observed reduction in Vs velocity and may produce ca. 0.5-1.0 km to the regional topographic uplift.
Abstract: We present a new regional model for the depth-averaged density structure of the cratonic lithospheric mantle in southern Africa constrained on a 30? × 30? grid and discuss it in relation to regional seismic models for the crust and upper mantle, geochemical data on kimberlite-hosted mantle xenoliths, and data on kimberlite ages and distribution. Our calculations of mantle density are based on free-board constraints, account for mantle contribution to surface topography of ca. 0.5-1.0 km, and have uncertainty ranging from ca. 0.01 g/cm3 for the Archean terrains to ca. 0.03 g/cm3 for the adjacent fold belts. We demonstrate that in southern Africa, the lithospheric mantle has a general trend in mantle density increase from Archean to younger lithospheric terranes. Density of the Kaapvaal mantle is typically cratonic, with a subtle difference between the eastern, more depleted, (3.31-3.33 g/cm3) and the western (3.32-3.34 g/cm3) blocks. The Witwatersrand basin and the Bushveld Intrusion Complex appear as distinct blocks with an increased mantle density (3.34-3.35 g/cm3) with values typical of Proterozoic rather than Archean mantle. We attribute a significantly increased mantle density in these tectonic units and beneath the Archean Limpopo belt (3.34-3.37 g/cm3) to melt-metasomatism with an addition of a basaltic component. The Proterozoic Kheis, Okwa, and Namaqua-Natal belts and the Western Cape Fold Belt with the late Proterozoic basement have an overall fertile mantle (ca. 3.37 g/cm3) with local (100-300 km across) low-density (down to 3.34 g/cm3) and high-density (up to 3.41 g/cm3) anomalies. High (3.40-3.42 g/cm3) mantle densities beneath the Eastern Cape Fold belt require the presence of a significant amount of eclogite in the mantle, such as associated with subducted oceanic slabs. We find a strong correlation between the calculated density of the lithospheric mantle, the crustal structure, the spatial pattern of kimberlites, and their emplacement ages. (1) Blocks with the lowest values of mantle density (ca. 3.30 g/cm3) are not sampled by kimberlites and may represent the "pristine" Archean mantle. (2) Young (< 90 Ma) Group I kimberlites sample mantle with higher density (3.35 ± 0.03 g/cm3) than the older Group II kimberlites (3.33 ± 0.01 g/cm3), but the results may be biased by incomplete information on kimberlite ages. (3) Diamondiferous kimberlites are characteristic of regions with a low-density cratonic mantle (3.32-3.35 g/cm3), while non-diamondiferous kimberlites sample mantle with a broad range of density values. (4) Kimberlite-rich regions have a strong seismic velocity contrast at the Moho, thin crust (35-40 km) and low-density (3.32-3.33 g/cm3) mantle, while kimberlite-poor regions have a transitional Moho, thick crust (40-50 km), and denser mantle (3.34-3.36 g/cm3). We explain this pattern by a lithosphere-scale (presumably, pre-kimberlite) magmatic event in kimberlite-poor regions, which affected the Moho sharpness and the crustal thickness through magmatic underplating and modified the composition and rheology of the lithospheric mantle to make it unfavorable for consequent kimberlite eruptions. (5) Density anomalies in the lithospheric mantle show inverse correlation with seismic Vp, Vs velocities at 100-150 km depth. However, this correlation is weaker than reported in experimental studies and indicates that density-velocity relationship in the cratonic mantle is strongly non-unique.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 14p.
Africa, Lesotho
Deposit - Letseng
Abstract: Bateman Engineering managed the establishment of a second diamond treatment plant at the Letseng Mine in the Mokhotlong District of northern Lesotho, about 100 km from the town of Buthe Buthe. Letseng Diamonds Proprietary Limited, is owned 70 % by Gem Diamonds and 30 % by the Government of the Kingdom of Lesotho. The second plant, which commenced construction in late 2006, doubled Letseng Mines' hard rock processing capacity from 2.6 million t/yr to 5.2 million t/yr, making it the worlds' seventh largest diamond mine by throughput. Bateman Engineering provided the engineering design, procurement and construction management of the new plant.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 14p.
Africa, South Africa
Deposit - Cullinan
Abstract: There are many underground mining software tools available to plan and schedule the development of underground mines. A shortfall of these tools is the optimization of the underground equipment and strategy in order to maximize the development rate. This paper will illustrate how the SimMine® software was used to maximize underground development at Petra Diamonds' Cullinan Diamond Mine. It will also determine the effect of various development strategies and equipment capacities on the underground development rate and pinpoint potential bottlenecks in the mine development cycle.
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Abstract: Stable (C and O) isotope data from carbonates are one of the most important methods used to infer genetic processes in carbonatites. However despite their ubiquitous use in geological studies, it is suspected that carbonates are susceptible to dissolution-reprecipitation and isotopic resetting, especially in shallow intrusions, and may not be the best records of either igneous or hydrothermal processes. Apatite, however, should be much less susceptible to these resetting problems but has not been used for O isotope analysis. In this contribution, a novel bulk-carbonatite method for the analysis of O isotopes in the apatite PO4 site demonstrates a more robust record of stable isotope values. Analyses of apatite from five carbonatites with magmatic textures establishes a preliminary Primary Igneous Apatite (PIA) field of ?18O = + 2.5 to + 6.0‰ (VSMOW), comparable to Primary Igneous Carbonatite (PIC) compositions from carbonates. Carbonate and apatite stable isotope data are compared in 10 carbonatite samples from Songwe Hill, Malawi. Apatite is heavy rare earth element (HREE) enriched at Songwe and, therefore, oxygen isotope analyses of this mineral are ideal for understanding HREE-related mineralisation in carbonatites. Carbonate C and O isotope ratios show a general trend, from early to late in the evolution, towards higher ?18O values (+ 7.8 to + 26.7‰, VSMOW), with a slight increase in ?13C (? 4.6 to ? 0.1‰, VPDB). Oxygen isotope ratios from apatite show a contrary trend, decreasing from a PIA field towards more negative values (+ 2.5 to ? 0.7‰, VSMOW). The contrasting results are interpreted as the product of the different minerals recording fluid interaction at different temperatures and compositions. Modelling indicates the possibility of both a CO2 rich fluid and mixing between meteoric and deuteric waters. A model is proposed where brecciation leads to depressurisation and rapid apatite precipitation. Subsequently, a convection cell develops from a carbonatite, interacting with surrounding meteoric water. REE are likely to be transported in this convection cell and precipitate owing to decreasing salinity and/or temperature.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 12p.
Africa, South Africa
Deposit - Venetia
Abstract: The nature of diamond deposits are such that most mineral resource reporting codes contain a sub-section unique to diamond estimation and classification. These codes identify a number of criteria which define the uniqueness of diamond deposits, including, the low and variable grade of diamond deposits; the particulate nature of diamonds which affects both size and revenue of individual particles and finally the inherent difficulties and uncertainties in the estimation of diamond resources. These criteria are expanded on to provide an overview of the estimation of kimberlite diamond deposits. Placer deposits have been excluded as they constitute a particularly complex example of particulate distributions. Diamond grade, typically of the order of parts per million (ppm), is dependent on the number of stones per unit volume or mass as well as the diamond size distribution while diamond revenue is dependent on size, model, colour and quality. These parameters of a discrete particle result in a conmlOdily which requires some unique estimation and modelling methodologies. For most commodities "grade" is a measure of concentration and is directly proportional to value. [n the case of diamonds however the same stone grade (e.g. stones per 100 tonnes) may have significantly different carat grades and revenue, depending on the characteristics of the individual stones (size, model, colour and quality). The De _Beers Consolidated Mines Venetia Mine is used as an example of diamond eslimation as well as to highlighl some propriety grade eslimation techniques. The mine has been sampled for grade using a number of different sample supports, from 36" diameter reverse circulation drillholes to micro diamond core drilling. De Beers Group Services (Ply) Lld (DBGS) have developed a technique of mixed (or multiple) support kriging which allows for the combination of samples of different sizes (and therefore grade distributions) in the estimation process. In addition DBGS have researched techniques of both global and local grade estimation using micro diamonds. The estimation of kimberlite diamond deposits has a number of unique components resulting from the discrete nature of the diamond distribution. It is however equally fundamentally important to understand exactly what the sampling data represents; the constraints Wlder which the data were coUected (e.g. bottom cut ofJ) and the adjustments necessary to ensure parity Page 73 The Southern African Institute of Mining and Metallurgy Diamonds - Source to Use 2010 DBush between and within sampling programmes as well as the likely metallurgical process III a production environment.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 8p.
Africa, Botswana
Deposit - AK6
Abstract: The AK6 kimberlite is situated 25 km south of the Debswana Orapa Mine in Botswana and was discovered by De Beers geologists in 1969 during the follow-up of geophysical targets in the Orapa area. The kimberlite was not extensively pursued at the time as the initial bulk sampling indicated it to be of limited size and low grade, factors largely contributed to by the basalt breccia capping. Completion of high resolution integrated geophysical techniques and drill bulk sampling to depth recovered 97 tons of kimberlite during 2003 and 2004, which led to the increased size and grade estimates. Bulk sampling by Large Diameter Drilling (LDD, 23 inch diameter) commenced in 2005; 13 holes were drilled to a cumulative depth of 3,699 m and 689 carats of diamonds were recovered. In July 2006 the De Beers Mineral Resource Classification Committee classified these Phase I LOO results at a High Inferred level with an average grade of 24 carats per hundred tonnes (cpht) at a bottom cut-off of +1 mm, and a modeled average diamond value of 150 dollars per carat. A second phase of LDO drilling was initiated in 2006, and bulk sampling by trenching commenced in 2007 in order to deliver a resource estimate at indicated level. An Indicated Resource of 11.1 million carats at an average grade of 22 cpht was declared for the deposit mining lease application lodged in 2007.
Journal of African Media Studies, Vol. 8, 2, pp. 109-126.
Africa
History
Abstract: This paper argues that the western press often ignores the private sphere of economics in reporting conflict in the developing world. This matters when it comes to resource wars and conflict over natural resources. To explore this concept further I examine American and British press coverage of conflict diamonds in the Angola, Congo and Sierra Leone wars in the pages of four newspapers of record. Overall while conflict diamonds were present in the reporting the press ignored the full extent of private companies and capital in the financing and trading of conflict diamonds. The most common word used was government.
De Wit, M., Bhebhe, Z., Davidson, J., Haggerty, S.E., Hundt, P., Jacob, J., Lynn, M., Marshall, T.R., Skinner, C., Smithson, K., Stiefenhofer, J., Robert, M., Revitt, A., Spaggiari, R., Ward, J.
Abstract: From the discovery of diamonds in South Africa in 1866 until the end of 2013, Africa is estimated to have produced almost 3.2 Bct out of a total global production of 5.03 Bct, or 63.6% of all diamonds that have ever been mined. In 2013 African countries ranked 2nd (Botswana), 3rd (DRC), 6th (Zimbabwe), 7th (Angola), 8th (South Africa), and 9th (Namibia), in terms of carat production and 1st (Botswana), 4th (Namibia), 5th (Angola), 6th (South Africa), 7th (Zimbabwe), and 9th (DRC), in terms of value of the diamonds produced. In 2013 Africa produced 70.6 Mct out of a global total of 130.5 Mct or 54.1%, which was valued at US$ 8.7 billion representing 61.5% of the global value of US$ 14.1 billion.
Abstract: Olivine is the dominant mineral present in kimberlite magmas; however, due to the volatile-rich nature of most kimberlites, they rarely survive late-stage serpentinisation. Here we present major and trace element data for a rare example of ultra-fresh olivine in a macrocrystic calcite kimberlite from the Benfontein kimberlite sill complex. Olivines are characterised by xenocrystic cores surrounded by multiple growth zones representing melt crystallisation and late-stage equilibration. Two distinct core populations are distinguished: Type 1) low Fo (88-89), Ni-rich, Ca- and Na-rich cores, interpreted here to be the result of carbonate-silicate metasomatism potentially as part of the earliest stages of kimberlite magmatism, and Type 2) high Fo (91-93), Ni-rich, low-Ca cores derived from a typical garnet peridotite mantle source. In both cases, the cores have transitional margins (Fo89-90) representing equilibration with a proto-kimberlite melt. Trace element concentrations, in particular Cr, of these transition zones suggest formation of the proto-kimberlite melt through assimilation of orthopyroxene from the surrounding garnet peridotite lithology. Trace element trends in the surrounding melt-zone olivine (Fo87-90) suggest evolution of the kimberlite through progressive olivine crystallisation. The final stages of olivine growth are represented by Fe-rich (Fo85) and P-rich olivine indicating kimberlite evolution to mafic compositions. Fine (< 60 ?m), Mg-rich olivine rims (Fo94-98) represent equilibration with the final stages of kimberlite evolution back to Fe-poor carbonatitic melts. We present a step-by-step model for kimberlite magma genesis and evolution from mantle to crust tracked by the chemistry of olivines in the Benfontein kimberlite. These steps include early stages of metasomatism and mantle assimilation followed by direct crystallisation of the kimberlite melt and late-stage equilibration with the evolved carbonatitic residual liquids. The Ca contents of the Type 1 xenocrystic olivines are the highest yet measured for mantle olivines, and do not overlap with any known mantle xenolith lithologies. These olivines likely represent an important stage of metasomatism directly related to the early stages of kimberlite melt ponding at the base of the lithospheric mantle.
Geological Society of London Special Publication: Magmatic Rifting and Active Volcanism, Vol. 420, pp. 11-30.
Africa
Global finite element model
Abstract: Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 18p.
Africa, South Africa
Deposit - Finsch
Abstract: Finsch Mine is a kimberlite diamond mine located at Lime Acres in the Northern Cape Province of South Africa. The mine was founded in 1961 and started surface mining in 1964. Underground production commenced in 1990 using a modified blast-hole open stoping method for the mining of Blocks 1, 2 and 3. Block 4 is currently being mined as a block cave. The process of identifying and optimizing a method to mine the Block 5 orebody started in 1991, and in 2006 incline caving was identified as being technically feasible. This paper aims to document the process employed in developing this method by the Block 5 pre-feasibility team as well as discuss the technical challenges encountered during this process. The paper commences with a history of Finsch Mine and highlights the complex geology and threat of sidewall failure that prompted the decision to use block caving as the mining method for Block 4. A literature study of mines that implemented mining methods upon which the incline cave was conceptualized is then presented. These practices were then used to form the basis for the designs on which the initial geotechnical modelling was done and built upon through an iterative process of modelling and design changes. The ventilation of the mining area, initial productivity simulation results, and the applicability of automation and comminution processes in the incline cave are also presented. The paper concludes with an investigation into some of the challenges of the mining method, and shows that that incline caving is a technical option available for further investigation in determining the optimal mining method to be employed at Block 5, Finsch Diamond Mine.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 14p.
Africa, South Africa
Deposit - Finsch
Abstract: De Beers' Finsch Mine is situated in the Northern Cape ,province, 170 km northwest of Kimberley. The concentrator facility, designed and constructed by Bateman Engineering, was commissioned in 1967 and upgraded in 1979 using diamond liberation and extraction technology available at the time of design. Since then significant advances in diamond processing and technology have been made and these -have been incorporated into the new main treatment plant and recovery plant flowsheets, making diamond liberation and recovery from the Pre-1979 dumps a viable economic option at Finsch." Significant challenges were experienced as a result of the integration of new technology and its associated infrastructure into an existing plant. Major process flow changes were implemented during the execution phase of the project. The combined effect of these issues resulted in the project being overspent by 25% and the final handover to the Client was some 18 months later than originally planned." The paper highlights some of the difficulties experienced as a result of changes made during the execution phases of the project.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 28p.
Africa, Botswana
Deposit - Orapa
Abstract: Commencing with the Pre-Feasibility Study (PFS) conclusions, the Orapa 3 process design evolved through a phase of value-engineering studies. An overall re-evaluation of the originally proposed process design was necessary both in order to address the interim increase in target throughput from 9.8 Mtpa to 12 Mtpa as well as to increase capital efficiency. In the interests of clarity, the PFS process design for Orapa 3 has been omitted from this paper, as it is no longer relevant. Recognition is however due to those engaged in earlier phases of the Orapa 3 project with respect to fundamental mass~balancing modelling; and ore and diamond characterisation, which fanned the basis for the process design that has evolved from feasibility study activities. The background context for Orapa 3 is of an operation expected to yield operating utilisation and revenue improvements relative to the Orapa 2 operation. Delivering these without undue penalties to capital and operating cost required a shift in thinking, trading excess installed capacity for flexible circuit configuration. The process design adopted is "layered", with the purpose of preserving Run Of Mine (ROM) throughput by reducing in-circuit arisings - particularly to the Dense Medium Separation (DMS) section - rather than simply installing additional DMS capacity on the expectation of low DMS availability. Elsewhere, the ability to monitor and maintain critical sizing activities - particularly desanding - without impact on overall plant throughput is intended to motivate operators to avoid the temptation to trade quality for quantity. At present, since an overall dynamic simulation of the Orapa 3 operation has still to be carried out, the design mass balance has been based on a relatively onerous combination of worst case feed type with 100% front-line process capacity in all plant sections. This means that, whilst the installed DMS capacity is based on routing 100% of sized scrubbing section product directly to the DMS, and with one DMS module always unavailable, the High Pressure Rolls Crushing (HPRC) capacity is based on allowing for 50% of this stream to be instead routed first to the HPRC section, at no more than 75% of maximum roll speed for the two units installed. This is an obvious "belt and braces" approach. Following the dynamic simulation exercise (currently in progress), it is likely that a less conservative approach will be taken. This will not affect the conceptual design of the process plant, being mostly an exercise in refining the number of DMS modules to be installed, and possibly reducing slightly the size of the HPRC roll units. Both of these will have positive capital and operating cost impacts. Page 215 The Southern African Institute of Mining and Metallurgy Diamonds - Source to Use 2010 G Popp/ewell andB Hae/else DMS capacity, despite being split into coarse and fiaes streams, consists of identical modules. Two of the nine modules are set-up to receive either coarse or fine feed, the only difference being that fines modules are rated at lower capacity than the same modules treating coarse feed. Final recovery section capacity is based on entirely wet primary diamond recovery technology. This greatly reduces both the cost of drying. a large amount of recovery section feed and the dust• management issues associated with dry recovery technology. In contrast to the Orapa 2 operations, a scavenginglauditgrease belt section is included as a diamond recovery "goal-keeper" and to provide a process assurance function.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 10p.
Africa, Botswana
Deposit - Jwaneng
Abstract: Debswana's Jwaneng Mine (south-central Botswana) is the richest diamond mine• in the world and has been contributing substantial revenue to Botswana since the mine started operating in 1982. The resource consists of 3 separate volcanic pipes/vents namely North, South and Centre pipes (2 additional small kimberlite bodies have also been intersected within the mining pit) which erupted through Transvaal strata and the overlying Karoo sediments - 245 million years ago. Although earlier drilling and geophysical surveys suggest that the 3 pipes extend to depths greater than 1 km below the surface, the resource is only at an indicated level of confidence to a depth of 400m, and mining activities will start exploiting the inferred resource material below 400m in 2014.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 18p.
Africa, Botswana
Deposit - Jwaneng
Abstract: Slope stability is a fundamental part of a successful mining operation. It impacts directly on the safety of personnel and the economics of the mine. The Jwaneng Mine in Botswana is planning a push back of 644m depth in the south east wall. Mining will commence in 2010 and be completed in 2024. This will put the mine in the deep pit category. The mine has invested in extensive geotechnical data gathering and design programs that started as far back as 2003 for the south east wall. The south east wail of Jwaneng Mine is characterized by foliation that dips adversely into the mining faces. The orienta1ion of the foliation is variable due to intense tectonic movements that have also caused the occurrence of faults that are sub-vertical. A considerable amount of information on the characteristics of structural patterns and the rock mass has been collected. This paper presents an overview of the feasibility level geotechnical design that is about to be concluded.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 12p.
Africa, South Africa
Deposit - Namaqualand mines
Abstract: Alluvial diamond deposits covered with a thick overburden cannot be explored effectively and mined profitably with the available conventional techniques. A novel directional drilling technique has been developed and site trials have been conducted at the e Beers Namaqualand Mines situated at the western coast of South Africa. This paper describes the technique developed as well as the results of the site trials. The main task of the project is to develop a tool and a supporting system to achieve a remote exploration and ore extraction method for inaccessible alluvial diamond deposits. The project involves the acquisition of a system, commissioning on-site and evaluating the information obtained during assessment. The project consists of the design - of certain tools and instrumentation for the system, investigations and desktop studies as well as development of the systems logistics. The operating philosophy of the system is based on the keyhole surgery principle. In this study a pilot hole was drilled first, starting from the surface with an inclination to reach deeply seated diamondiferous gravel layers, followed by drilling into a more or less horizontal gravel layer for a certain length and finally pointing-upwards reaching ground surface at a predetermined exit point. The pilot drill bit was then replaced with a larger diameter reamer/mining tool and an HDPE tail pipe was attached behind the reamer. The drill string was retracted back to enlarge the guide hole and flush the excavated ore to the surface through the tail pipe. The material flow in between the sections of the system was carefully measured to assess and record the gravel recovery rates. Several horizontal holes were drilled at preplanned drill paths to reach and follow the ore layer and the ore extraction was achieved by means of a reamer/mining tool. Principles of the concept have been proven viable in this investigation
Abstract: Kimberlitic magmaclasts are discrete ovoid magmatic fragments that formed prior to emplacement from disrupted kimberlite magma. To provide new constraints on the origin and evolution of the kimberlite melts, we document the mineralogy and petrography of a magmaclast recovered from one of the ca. 520 Ma Venetia kimberlites, South Africa. The sample (BI9883) has a sub-spherical shape and consists of a ~ 10 mm diameter central olivine macrocryst, surrounded by porphyritic kimberlite. The kimberlitic material consists of concentrically aligned, altered olivine phenocrysts, set in a crystalline groundmass of calcite, chromite, perovskite, phlogopite, apatite, ilmenite, titanite, sulphides, rutile and magnetite along with abundant alteration phases (i.e. serpentine, talc and secondary calcite). These features are typical of archetypal hypabyssal kimberlites. We examined primary fluid/melt inclusions in chromite, perovskite and apatite containing a diversity of daughter phases. Chromite and perovskite host polycrystalline inclusions containing abundant alkali-carbonates (i.e. enriched in K, Na, Ba, Sr), phosphates, Na-K chlorides, sulphides and equal to lesser quantities of olivine, phlogopite and pleonaste. In contrast, apatite hosts polycrystalline assemblages with abundant alkali-carbonates and Na-K chlorides and lesser amounts of olivine, monticellite and phlogopite. Numerous solid inclusions of shortite (Na2Ca2(CO3)3), Na-Sr-carbonates and apatite occur in groundmass calcite along with fluid inclusions containing daughter crystals of Na-carbonates and Na-chlorides. The primary inclusions in chromite, perovskite and apatite are considered to represent remnants of fluid(s)/melt(s) trapped during crystallisation of the host minerals, whereas the fluid inclusions in calcite are probably secondary in origin. The component proportions of these primary fluid/melt inclusions were estimated in an effort to constrain the composition of the evolving kimberlite melt. These estimates suggest melt evolution from a silicate-carbonate kimberlite melt that became increasingly enriched in carbonates, phosphates, alkalis and chlorides, in response to the fractional crystallisation of constituent minerals (i.e. olivine to apatite). The concentric alignment of crystals around the olivine kernel and ovoid shape of the magmaclast can be ascribed to the low viscosity of the kimberlite melt and rapid rotation whilst in a liquid or partial crystalline state, or to progressive layer-by-layer growth of the magmaclast. Although the mineralogy of our sample is similar to hypabyssal kimberlites worldwide, it differs from hypabyssal kimberlite units in the main Venetia pipes, which contain monticellite-phlogopite rich assemblages and segregationary matrix textures. Therefore magmaclast BI9883 probably originated from a batch of magma distinct from those that produced known hypabyssal units within the Venetia kimberlite cluster.-
Abstract: The Mesoproterozoic Pilanesberg Complex, South Africa, is built up by several distinct, ring-shaped intrusions of syenite and peralkaline nepheline syenite. A mildly peralkaline ((Na + K) / Al = 1.04–1.09), medium-to coarse grained nepheline syenite makes up the outermost ring in the southwestern part of the complex (“Matooster type white foyaite”). In this rock, mafic silicate minerals (amphibole, biotite, aegirine) and Ti-bearing minerals (ilmenite, astrophyllite, aenigmatite, lorenzenite, bafertisite, jinshajiangite) are interstitial to feldspar and nepheline, and define a series of mineral assemblages reflecting a change from a miaskitic crystallization regime (with Na-Ca amphibole, titanite and ilmenite) to increasingly agpaitic conditions (with arfvedsonite, aegirine, astrophyllite, aenigmatite, lorenzenite). The main driving force behind the evolution was an increase in peralkalinity of the trapped liquid, mainly by adcumulus growth of alkali feldspar and nepheline, which in the later stages of evolution was combined with increases in oxygen fugacity and water activity. Unlike in most other agpaitic rock complexes, Zr remained compatible in aegirine (and to some extent in amphibole) almost to the end of the process, when a hydrous zirconium silicate mineral (hilairite) crystallized as the only mineral in the rock having essential zirconium. The presence of minerals such as hilairite, bafertisite, jinshajiangite and a Na-REE-Sr rich apatite group mineral (fluorcaphite ?) in the latest assemblages suggests that the last remaining interstitial melt or fluid approached a hyperagpaitic composition. The isolated melt pockets in the Pilanesberg white foyaite follow a pattern of evolution that can be seen as a miniature analogue of the fractional crystallization processes controlling magma evolution in large, alkaline igneous rock complexes.-
Abstract: This study gives an analysis of the effect of high temperature annealing on the infrared and photoluminescence (PL) features as well as the inclusions of two hydrogen-rich diamond plates from Zimbabwe that were cut from the same rough. The samples showed strong inclusion-related zoning known as hydrogen clouds which consist of micron-sized particles of as yet undetermined structure. This allowed hydrogen-rich and hydrogen-poor areas to be compared throughout the annealing study. The diamond plates were annealed to temperatures of 300oC, 600oC, 800oC, 1000oC, 1400oC, and 1700oC. The infrared and PL, and Raman maps were collected after every temperature step to study the effects of heat on the defects, and photomicrographs were collected to study the inclusions. Several photoluminescence features were seen to decrease in size including the 637 nm peak, which is the negatively charged nitrogen-vacancy center [NV-] and the 503.2 nm peak, known as the H3 and consists of two nitrogen atoms and vacancy in the neutral charge state and normally has a high thermal stability. The H2 defect at 986.2 nm, which is the negative form of the H3, was shown to increase after annealing. The hydrogen clouds underwent dramatic changes in apparent color and particle size, going from a light translucent gray appearance to an opaque black. The particle size grew from less than one micron to an average of fourteen microns, and the hexagonal outline of the particles became noticeable. Spatial raman spectroscopy was used to show that the color change and size change were due to graphitization of the included particles.
Thesis, University of Pretoria, Ms Metallurgical Engineering 279p. Pdf
Africa, Angola, South Africa
Deposit - AC 56-5-1, Venetia Red
Abstract: Kimberlite is the host rock from which diamonds are mined. The mineralogical features for kimberlites vary greatly with country, origin, depth and type of kimberlite. Kimberlites can contain various clay species with some kimberlites containing predominantly clay minerals. The presence of these clay minerals in the ore can cause difficulty in dewatering due to high flocculant demand, poor supernatant clarity and low settling rates. Identifying critical parameters that can predict the settling behaviour of African kimberlite slurries will assist the process engineer to predict the settling behaviour of different kimberlite slurries. Especially identifying the kimberlites that will most likely not settle with normal flocculant dosage rates is useful. From first principles the settling of a particle is described by Stoke’s law which incorporates the density of the particle and size of the particle as the inherent particle variables. In this case density is assumed constant and therefore the size of particles influence the settling rate of particles to a great extent. This study therefore investigated the influence of particle size on settling rate and whether the particle size distribution showed correlation with settling rate when regression modelling was fitted on the data. Other variables that were tested for correlation with settling were pH when the kimberlite is mixed in water as well as various mineralogical features of the ore. Fitting a simple model to any of these properties or combinations of these properties was attempted which would allow for prediction of settling behaviour. The mineralogical features were classified by evaluating the mineral composition, fractional elemental analysis, cation exchange capacity and the exchangeable sodium percentage of the different kimberlites. These variables were tested as well as their settling behaviour with 18 different African kimberlite samples. The settling rate and slurry bed compaction during natural settling as well coagulant and flocculant assisted settling were measured for the kimberlite slurries. The best performing coagulant and flocculant for each kimberlite were combined to evaluate potential improvements in the settling rates and slurry bed compaction compared to current settling practices that only utilise flocculant additions. Especially the use of coagulant for kimberlites that did not show settling with only flocculants was evaluated. For these 18 kimberlites only 2 kimberlites did not settle with the use of flocculants with settling rates varying between 10.7m/h and 25m/h. Both these kimberlites also did not settle with the combination of coagulant and flocculant, but could only settle with only coagulant additions at settling rates of 1.9 m/h and 2.2 m/h. Regression analysis fitted to the settling rate investigated the influence of particle size, pH and mineralogical features on settling. For representation of the particle size two data points from the Particle Size Distribution (PSD) was utilised which represented the fine material and the coarser material. These two data points were taken at percentage passing 7.5 ?m and 75 ?m. Regression data for kimberlite with flocculant additions showed that particle size and the pH of the slurry were identified as significant parameters in predicting settling. The regression data showed a R2 of 0.78 for the settling rate and an adjusted R2 of 0.79 for the slurry bed depth.
Comptes Rendus Geoscience, in press available 11p.
Africa, Morocco
Peridotite
Abstract: The Beni Bousera peridotite contains a diversity of pyroxenite layers. Several studies have postulated that at least some of them represent elongated strips of oceanic lithosphere recycled in the convective mantle. Some pyroxenites were, however, ascribed to igneous crystal segregation or melt-rock reactions. To further constrain the origin of these rocks, we collected 171 samples throughout the massif and examined their variability in relation with the tectono-metamorphic domains. A major finding is that all facies showing clear evidence for a crustal origin are concentrated in a narrow corridor of mylonitized peridotites, along the contact with granulitic country rocks. These peculiar facies were most likely incorporated at the mantle-crust boundary during the orogenic events that culminated in the peridotite exhumation. The other pyroxenites derive from a distinct protolith that was ubiquitous in the massif before its exhumation. They were deeply modified by partial melting and melt-rock reactions associated with lithospheric thinning.
Abstract: South Africa, and attended by representatives of the diamond industry and leaders of African governments to develop a certification process intended to assure that rough, exported diamonds were free of conflictual concerns. This meeting was supported later in 2000 by the United Nations in a resolution adopted by the General Assembly. By 2002, the Kimberley Process Certification Scheme (KPCS) was ratified and signed by diamond-producing and diamond-importing countries. Over 70 countries were included as members of the KPCS at the end of 2007. To prevent trade in "conflict diamonds" while protecting legitimate trade, the KPCS requires that each country set up an internal system of controls to prevent conflict diamonds from entering any imported or exported shipments of rough diamonds. Every diamond or diamond shipment must be accompanied by a Kimberley Process (KP) certificate and be contained in tamper-proof packaging. The objective of this study was (1) to assess the naturally occurring endowment of diamonds in Mali (potential resources) based on geological evidence, previous studies, and recent field data and (2) to assess the diamond-production capacity and measure the intensity of mining activity. Several possible methods can be used to estimate the potential diamond resource. However, because there is generally a lack of sufficient and consistent data recording all diamond mining in Mali and because time to conduct fieldwork and accessibility to the diamond mining areas are limited, four different methodologies were used: the cylindrical calculation of the primary kimberlitic deposits, the surface area methodology, the volume and grade approach, and the content per kilometer approach. Approximately 700,000 carats are estimated to be in the alluvial deposits of the Kenieba region, with 540,000 carats calculated to lie within the concentration grade deposits. Additionally, 580,000 carats are estimated to have been released from the primary kimberlites in the region. Therefore, the total estimated diamond resources in the Kenieba region are thought to be nearly 1,300,000 carats. The Bougouni zones are estimated to have 1,000,000 carats with more than half, 630,000 carats, contained in concentrated deposits. When combined, the Kenieba and Bougouni regions of Mali are estimated to be host to 2,300,000 carats of diamonds.
Abstract: In May of 2000, a meeting was convened in Kimberley, South Africa, and attended by representatives of the diamond industry and leaders of African governments to develop a certification process intended to assure that rough, exported diamonds were free of conflictual concerns. This meeting was supported later in 2000 by the United Nations in a resolution adopted by the General Assembly. By 2002, the Kimberley Process Certification Scheme (KPCS) was ratified and signed by both diamond-producing and diamond-importing countries. Over 70 countries were included as members at the end of 2007. To prevent trade in "conflict" diamonds while protecting legitimate trade, the KPCS requires that each country set up an internal system of controls to prevent conflict diamonds from entering any imported or exported shipments of rough diamonds. Every diamond or diamond shipment must be accompanied by a Kimberley Process (KP) certificate and be contained in tamper-proof packaging. The objective of this study was to assess the alluvial diamond resource endowment and current production capacity of the alluvial diamond-mining sector in Ghana. A modified volume and grade methodology was used to estimate the remaining diamond reserves within the Birim and Bonsa diamond fields. The production capacity of the sector was estimated using a formulaic expression of the number of workers reported in the sector, their productivity, and the average grade of deposits mined. This study estimates that there are approximately 91,600,000 carats of alluvial diamonds remaining in both the Birim and Bonsa diamond fields: 89,000,000 carats in the Birim and 2,600,000 carats in the Bonsa. Production capacity is calculated to be 765,000 carats per year, based on the formula used and available data on the number of workers and worker productivity. Annual production is highly dependent on the international diamond market and prices, the numbers of seasonal workers actively mining in the sector, and environmental conditions, which influence seasonal farming.
Abstract: In May of 2000, a meeting was convened in Kimberley, South Africa, by representatives of the diamond industry and leaders of African governments to develop a certification process intended to assure that export shipments of rough diamonds were free of conflict concerns. Outcomes of the meeting were formally supported later in December of 2000 by the United Nations in a resolution adopted by the General Assembly. By 2002, the Kimberley Process Certification Scheme (KPCS) was ratified and signed by diamond-producing and diamond-importing countries. The goal of this study was to estimate the alluvial diamond resource endowment and the current production capacity of the alluvial diamond mining sector of Guinea. A modified volume and grade methodology was used to estimate the remaining diamond reserves within Guinea’s diamondiferous regions, while the diamond-production capacity of these zones was estimated by inputting the number of artisanal miners, the number of days artisans work per year, and the average grade of the deposits into a formulaic expression. Guinea’s resource potential was estimated to be approximately 40 million carats, while the production capacity was estimated to lie within a range of 480,000 to 720,000 carats per year. While preliminary results have been produced by integrating historical documents, five fieldwork campaigns, and remote sensing and GIS analysis, significant data gaps remain. The artisanal mining sector is dynamic and is affected by a variety of internal and external factors. Estimates of the number of artisans and deposit variables, such as grade, vary from site to site and from zone to zone. This report has been developed on the basis of the most detailed information available at this time. However, continued fieldwork and evaluation of artisanally mined deposits would increase the accuracy of the results.
Abstract: The Okorusu Alkaline Igneous-Carbonatite Complex is located about 50 km north of Otjiwarongo in North-Central Namibia. The complex was intruded during early Cretaceous into late Precambrian Damaran Series metasedimentary rocks. It is nearly circular in plan view with a diameter of about 8 km. Coarse-grained nepheline syenites and foyaites are exposed in low hills near the northern edge of the complex. Early alkalie-rich fluids pervasively fenitized the metasedimentary rocks along the southern margin of the complex forming an east-west ridge of resistant hills that include Okorusu Mountain. The fenites were subsequently brecciated and intruded by several carbonatites, especially medium-grained iron-rich diopside pyroxene carbonitite and very coarse-grained pegmatitic carbonatite. In addition to predominant calcite, the carbonatites contain titaniferous vanadiferous magnetite crystals and diopside crystals as large as one-third meter and hexagonal pyrrhotite crystals as long as one meter. For the past two decades, Okoruru has been the leading carbonatite-related fluorspar producer in the world. Fluorspar has been mined from five separate ore deposits in open pits A, B, C, D, and E. The deposits formed principally by the replacement of carbonatite as shown by local unreplaced remnants of carbonatite in the fluorspar ores, goethite pseudomorphs in fluorspar ores after carbonatite magnetite, diopside, and pyrrhotite crystals, transitions of the ores into carbonatite, and by elevated phosphorus contents resulting from carbonatite apatite crystals that were incompletely replaced by fluorite. Locally, marbles also are replaced by fluorite to form fluorspar ores that are distinguished from carbonatite-replacement fluorspar ores by their finer grain size and lack of phosphorus contents. Fluid inclusions in the fluorite crystals indicate that the fluorspar ores were deposited from 166 to 128oC from fluids of low salinity with less than 5% NaCl equivalent. The genesis of the fluorspar ores is interpreted to have resulted from deeply circulated ground waters that dissolved fluorine from carbonatite at depth. The fluorine in those ore fluids combined with calcium released during the replacement of calcite in carbonatite and marbles at the sites of the fluorspar ore deposition.
Abstract: Magnesium and oxygen are critical elements in the solid Earth and hydrosphere. A better understanding of the combined behavior of Mg and O isotopes will refine their use as a tracer of geochemical processes and Earth evolution. In this study, the Mg-isotope compositions of garnet and omphacite separated from well-characterized xenolithic eclogites from the Roberts Victor kimberlite pipe (South Africa) have been measured by solution multi-collector ICP-MS. The reconstructed whole-rock ?26Mg values of Type I (metasomatized) eclogites range from ? 0.61‰ to ? 0.20‰ (Type IA) and from ? 0.60‰ to ? 0.30‰ (Type IB) (mean ? 0.43‰ ± 0.12‰), while ?26Mg of Type IIA (fresh, least metasomatized) eclogites ranges from ? 1.09‰ to ? 0.17‰ (mean ? 0.69‰ ± 0.41‰); a Type IIB (fresh, least metasomatized) has ?26Mg of ? 0.37‰. Oxygen-isotope compositions of garnet were analyzed in situ by SIMS (CAMECA 1280) and cross-checked by laser fluorination. Garnets have ?18O of 6.53‰ to 9.08‰ in Type IA, 6.14‰ to 6.65‰ in Type IB, and 2.34‰ to 2.91‰ in Type IIB. The variation of ?26Mg and ?18O in Type IA and IB eclogites is consistent with the previously proposed model for the evolution of these samples, based on major and trace elements and radiogenic isotopes. In this model, the protoliths (Type II eclogites) were metasomatized by carbonatitic to kimberlitic melts/fluids to produce first Type IA eclogites and then Type IB. Metasomatism has changed the O-isotope compositions, but the Mg-isotope compositions of Type IA are mainly controlled by the protoliths; those of Type IB eclogites reflect mixing between the protoliths and the kimberlitic melt/fluid. The combination of a large range of ?26Mg and low ?18O in Type II eclogites cannot be explained easily by seawater alteration of oceanic crust, interaction of carbonate/silicate sediments with oceanic crust, or partial melting of mafic rocks.
Geochimica et Cosmochimica Acta, in press available
Africa, Namibia
Picrite, ferroPicrite
Abstract: We present major and trace element compositions of 154 re-homogenised olivine-hosted melt inclusions found in primitive rocks (picrites and ferropicrites) from the Mesozoic Paraná-Etendeka and Karoo Continental Flood Basalt (CFB) provinces. The major element compositions of the melt inclusions, especially their Fe/Mg ratios, are variable and erratic, and attributed to the re-homogenisation process during sample preparation. In contrast, the trace element compositions of both the picrite and ferropicrite olivine-hosted melt inclusions are remarkably uniform and closely reflect those of the host whole-rocks, except in a small subset affected by hydrothermal alteration. The Paraná-Etendeka picrites and ferropicrites are petrogenetically related to the more evolved and voluminous flood basalts, and so we propose that compositional homogeneity at the melt inclusion scale implies that the CFB parental mantle melts were well mixed prior to extensive crystallisation. The incompatible trace element homogeneity of olivine-hosted melt inclusions in Paraná-Etendeka and Karoo near primitive magmatic rocks has also been identified in other CFB provinces and contrasts with findings from studies of basalts from mid-ocean ridges (e.g. Iceland and FAMOUS on the Mid Atlantic Ridge), where heterogeneity of incompatible trace elements in olivine-hosted melt inclusions is much more pronounced. We suggest that the low variability in incompatible trace element contents of olivine-hosted melt inclusions in near-primitive CFB rocks, and also ocean island basalts associated with moderately thick lithosphere (e.g. Hawaii, Galápagos, Samoa) may reflect mixing along their longer transport pathways during ascent and/or a temperature contrast between the liquidus and the liquid when it arrives in the crust. These thermal paths promote mixing of mantle melts prior to their entrapment by growing olivine crystals in crustal magma chambers. Olivine-hosted melt inclusions of ferropicrites from the Paraná-Etendeka and Karoo CFB have the least variable compositions of all global melt inclusion suites, which may be a function of their unusually deep origin and low viscosity.
Abstract: Magmas forming large igneous provinces (LIP) on continents are generated by extensive melting in the deep crust and underlying mantle and associated with break-up of ancient supercontinents, followed by formation of a new basaltic crust in the mid-oceanic rifts. A lack of the unifying model in understanding the sources of LIP magmatism is justified by lithological and geochemical complexity of erupted magmas on local (e.g. a cross-section) and regional (a single and different LIP) scales. Moreover, the majority of LIP rocks do not fit general criteria for recognizing primary/primitive melts (i.e. < 8 wt% MgO and absence of high-Fo olivine phenocrysts). This study presents the mineralogical (olivine, Cr-spinel, orthopyroxene), geochemical (trace elements and Sr-Nd-Hf-Pb isotopes) and olivine-hosted melt inclusion compositional characteristics of a single primitive (16 wt% MgO), high-Ti (2.5 wt% TiO2) picrite with high-Mg olivine (up to 91 mol% Fo) from the Letaba Formation in the ~ 180 Ma Karoo LIP (south Africa). The olivine compositions (unusually high ?18O (6.17‰), high NiO (0.36-0.56 wt%) and low MnO and CaO (0.12-0.20 and 0.12-0.22 wt%, respectively)) are used to argue for a non-peridotitic mantle source. This is supported by the enrichment of the rock and melts in most incompatible trace elements and depletion in heavy rare earth elements (e.g. high Gd/Yb) that reflects residual garnet in the source of melting. The radiogenic isotopes resemble those of the model enriched mantle (EM-1) and further argue for a long-term enrichment of the source in incompatible trace elements. The enriched high-Ti compositions, strongly fractionated incompatible trace elements, presence of primitive olivine and high-Cr spinel in the Letaba picrites are closely matched by olivine-phyric rocks from the ~ 260 Ma Emeishan (Yongsheng area, SW China) and ~ 250 Ma Siberian (Maimecha-Kotuy region, N Siberia) LIPs. However, many other compositional parameters (e.g. trace element and ?18O compositions of olivine phenocrysts, Fe2 +/Fe3 + in Cr-spinel, Sr-Nd-Hf isotope ratios) only partially overlap or even diverge. We thus imply that parental melts of enriched picritic rocks with forsteritic olivine from three major continental igneous provinces - Karoo, Emeishan and Siberia cannot be assigned to a common mantle source and similar melting conditions. The Karoo picrites also exhibit some mineralogical and geochemical similarities with rocks and glasses in the south Atlantic Ridge and adjacent fracture zones. The geodynamic reconstructions of the continental plate motions since break-up of the Gondwanaland in the Jurassic support the current position of the source of the Karoo magmatism in the southernmost Atlantic. Co-occurrence of modern and recent anomalous rocks with normal mid-ocean ridge basalts in this region can be related to blocks/rafts of the ancient lithosphere, stranded in the ambient upper mantle and occasionally sampled by rifting-related decompressional melting.
Abstract: Alluvial diamonds from the Kasai River, Democratic Republic of the Congo (DRC) are sourced from Cretaceous kimberlites of the Lucapa graben in Angola. Analysis of 40 inclusion-bearing diamonds provides new insights into the characteristics and evolution of ancient lithospheric mantle of the Congo craton. Silicate inclusions permitted us to classify diamonds as peridotitic, containing Fo91-95 and En92-94, (23 diamonds, 70% of the suite), and eclogitic, containing Cr-poor pyrope and omphacite with 11-27% jadeite (6 diamonds, 18% of the suite). Fluid inclusion compositions of fibrous diamonds are moderately to highly silicic, matching compositions of diamond-forming fluids from other DRC diamonds. Regional homogeneity of Congo fibrous diamond fluid inclusion compositions suggests spatially extensive homogenization of Cretaceous diamond forming fluids within the Congo lithospheric mantle. In situ cathodoluminescence, secondary ion mass spectrometry and Fourier transform infrared spectroscopy reveal large heterogeneities in N, N aggregation into B-centers (NB), and ?13C, indicating that diamonds grew episodically from fluids of distinct sources. Peridotitic diamonds contain up to 2962 ppm N, show 0-88% NB, and have ?13C isotopic compositions from ? 12.5‰ to ? 1.9‰ with a mode near mantle-like values. Eclogitic diamonds contain 14-1432 ppm N, NB spanning 29%-68%, and wider and lighter ?13C isotopic compositions of ? 17.8‰ to ? 3.4‰. Fibrous diamonds on average contain more N (up to 2976 ppm) and are restricted in ?13C from ? 4.1‰ to ? 9.4‰. Clinopyroxene-garnet thermobarometry suggests diamond formation at 1350-1375 °C at 5.8 to 6.3 GPa, whereas N aggregation thermometry yields diamond residence temperatures between 1000 and 1280 °C, if the assumed mantle residence time is 0.9-3.3 Ga. Integrated geothermobaromtery indicates heat fluxes of 41-44 mW/m2 during diamond formation and a lithosphere-asthenosphere boundary (LAB) at 190-210 km. The hotter-than-average cratonic mantle may be attributable to contemporaneous rifting of the southern Atlantic, multiple post-Archean reactivations of the craton, and/or proximal Cretaceous plumes.
Earth and Planetary Science Letters, Vol. 454, pp. 103-112.
Mantle, Africa, Morocco
Melting
Abstract: Pyroxenites are often documented among exhumed mantle rocks, and can be found in most tectonic environments, from supra-subduction to sub-continental and sub-oceanic mantle. In particular, websterites, i.e. orthopyroxene-clinopyroxene bearing pyroxenites, are found in parallel layers in most orogenic and ophiolitic peridotites. Their formation is often ascribed to melt infiltration and melt-rock reaction processes accompanied by variable amount of deformation. One outstanding question is whether the ubiquitous occurrence of layered websterites in exhumed rocks is generally linked to the exhumation process or truly represents large-scale melt infiltration processes at depth prior to exhumation. These two hypotheses can be distinguished by comparing the exhumation and formation ages of the websterites. However, determination of the layered websterite formation age is challenging. Here we present a novel approach to constrain the formation age of websterite layers using samples from the Lherz massif (France), where layered websterites and lherzolites have formed through melt-rock reaction. By combining high-resolution REE variations, isotope model ages, and diffusive re-equilibration timescales using REE closure temperatures across the websterite layers, we constrain a minimum age and a maximum age for the formation of layered websterites. We show that layered websterites in Lherz formed 1,500-1,800 Ma ago, and are thus clearly disconnected from the process of exhumation at 104 Ma. Multiple generations of layered websterites commonly found in ultramafic massifs, along with the evidence for ancient melt-rock reaction in Lherz, indicate that melt-rock reactions can happen episodically or continuously in the mantle and that layered websterites found in exhumed mantle rocks record ubiquitous melt infiltration processes in the mantle.
Abstract: T-shaped orogenic triple junctions between mobile belts usually form in two unrelated stages by subsequent and oblique continental collisions separated by a significant time span. Besides these "oblique triple junctions", another type, named "transverse triple junctions", may exist. Such junctions are created by a more complex mechanism of partly contemporaneous convergence of three cratons in a restricted time frame, involving strike slip. The Neoproterozoic-Cambrian Kaoko-Damara junction between the Rio de la Plata, Congo, and Kalahari cratons in Namibia is an example of such a transverse orogenic triple junction, formed by at least four subsequent but partly related deformation events. Initial north-south convergence between the Congo and Kalahari cratons was followed by east-west collision of the Rio de la Plata and Congo cratons. Subsequently, the Kalahari and Congo cratons collided, contemporaneous with sinistral strike-slip motion between the Congo and Rio de la Plata cratons and with the intrusion of large granite-syenite plutons, probably associated with slab detachment aided by the strike-slip movement. Other examples of transverse triple junctions may exist in Gondwana. Transcurrent shear zones, some possibly nucleated on transform faults from the pre-collision setting, are essential for the formation of transverse triple junctions.
Abstract: Type Ib diamonds contain abundant isolated nitrogen and account for less than 0.1% of natural diamonds. This rarity is because over geological time and at temperatures typical of the cratonic lithosphere, isolated nitrogen aggregates to A centres (N pairs) and B centres (4N around a vacancy). Preservation of isolated nitrogen in natural diamonds requires either short mantle residency times or storage at temperatures <700 °C, whereas most cratonic diamonds are billions of years old and resided >1000 °C. Eclogitic sulphides in Zimmi Ib diamonds (Sierra Leone) have 650 Ma Re-Os ages, whose origin is best explained by rapid tectonic exhumation after continental collision to shallower depths in the keel prior to kimberlite eruption (Smit et al., 2016). Here we present SIMS and spectroscopic data for a new suite of Zimmi sulphide-bearing diamonds. These new data allow us to evaluate the speciation of diamond source fluids, and the defects in these diamonds that are responsible for their distinctive canary yellow colours. The combined dataset establishes the link between the spectroscopic features in Zimmi Ib diamonds and their unusual history. Low ?13C values below -15 ‰ in 3 diamonds result from source fluids that originated as organic carbon in the oceanic crust, and were recycled into the lithosphere during subduction. In particular, a trend of decreasing ?13C and N content from core to rim in a single diamond indicates closed-system growth from CH4-rich subducted fluids. Recycled origins for organic carbon supports the temporal link of Zimmi diamond formation with deep underthrusting during Neoproterozoic continental assembly, recorded in the Rokelide orogen. CL imaging of Zimmi diamonds reveals irregular growth patterns with abundant deformation lines, associated with tectonic exhumation of the diamonds. Vacancies formed by deformation were subsequently annealed to form NV centres, where the negative charge state (637 nm) is more abundant due to isolated nitrogen donating an electron. Fancy brownish-yellow to greenish-yellow colours observed in Zimmi Ib diamonds result from a combination of isolated nitrogen, deformation-related amber centres and NV centres. These colour-forming defects can all be attributed to the unique geological history of Zimmi Ib diamonds and their rapid exhumation after formation.
Abstract: The petrogenesis of kimberlites commonly is obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during overprinting by such fluids and on the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa).
Abstract: Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000?m above sea level) but low-relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton and up to 2200?m below surface. Surface samples were also analyzed. AFT dates range between 317?±?33?Ma and 188?±?44?Ma. Alpha (Ft)-corrected AHe dates are between 433?±?24?Ma and 154?±?20?Ma. Modeling of the data reveals two important periods of cooling within the craton: one during the Carboniferous-Triassic (340-220?Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70?Ma). Thermal histories combined with the estimated geothermal gradient of 9°C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20°C indicate removal of up to 9?±?2?km of overburden since the end of Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes.
Geochimica et Cosmochimica Acta, Vol. 196, pp. 36-57.
Africa, Namibia, Angola
Paran-Etendeka, Karoo
Abstract: We present major and trace element compositions of 154 re-homogenised olivine-hosted melt inclusions found in primitive rocks (picrites and ferropicrites) from the Mesozoic Parana ´-Etendeka and Karoo Continental Flood Basalt (CFB) provinces. The major element compositions of the melt inclusions, especially their Fe/Mg ratios, are variable and erratic, and attributed to the re-homogenisation process during sample preparation. In contrast, the trace element compositions of both the picrite and ferropicrite olivine-hosted melt inclusions are remarkably uniform and closely re?ect those of the host whole-rocks, except in a small subset a?ected by hydrothermal alteration. The Parana ´-Etendeka picrites and ferropicrites are petrogenet- ically related to the more evolved and voluminous ?ood basalts, and so we propose that compositional homogeneity at the melt inclusion scale implies that the CFB parental mantle melts were well mixed prior to extensive crystallisation. The incompatible trace element homogeneity of olivine-hosted melt inclusions in Parana ´-Etendeka and Karoo primitive magmatic rocks has also been identi?ed in other CFB provinces and contrasts with ?ndings from studies of basalts from mid- ocean ridges (e.g. Iceland and FAMOUS on the Mid Atlantic Ridge), where heterogeneity of incompatible trace elements in olivine-hosted melt inclusions is more pronounced. We suggest that the low variability in incompatible trace element contents of olivine-hosted melt inclusions in near-primitive CFB rocks, and also ocean island basalts associated with moderately thick lithosphere (e.g. Hawaii, Gala ´pagos, Samoa), may re?ect mixing along their longer transport pathways during ascent and/or a temperature contrast between the liquidus and the liquid when it arrives in the crust. These thermal paths promote mixing of mantle melts prior to their entrapment by growing olivine crystals in crustal magma chambers. Olivine-hosted melt inclusions of ferropicrites from the Parana ´-Etendeka and Karoo CFB have the least variable compositions of all global melt inclusion suites, which may be a function of their unusually deep origin and low viscosity.
Africa, Zimbabwe, Australia, South America, Brazil
Deposit - Murowa, Argyle, Machado River
Abstract: Fourier Transform Infrared (FTIR) spectroscopy is a commonly-used technique for investigating diamonds, that gives the most useful information if spatially-resolved measurements are used. In this paper we discuss the best way to acquire and present FTIR data from diamonds, using examples from Murowa (Zimbabwe), Argyle (Australia) and Machado River (Brazil). Examples of FTIR core-to-rim line scans, maps with high spatial resolution and maps with high spectral resolution that are fitted to extract the spatial variation of different nitrogen and hydrogen defects are presented. Model mantle residence temperatures are calculated from the concentration of A and B nitrogen-containing defects in the diamonds using known times of annealing in the mantle. A new, two-stage thermal annealing model is presented that better constrains the thermal history of the diamond and that of the mantle lithosphere in which the diamond resided. The effect of heterogeneity within the analysed FTIR volume is quantitatively assessed and errors in model temperatures that can be introduced by studying whole diamonds instead of thin plates are discussed. The spatial distribution of VN3H hydrogen defects associated with the 3107 cm? 1 vibration does not follow the same pattern as nitrogen defects, and an enrichment of VN3H hydrogen at the boundary between pre-existing diamond and diamond overgrowths is observed. There are several possible explanations for this observation including a change in chemical composition of diamond forming fluid during growth or kinetically controlled uptake of hydrogen.
Abstract: Oldoinyo Lengai is situated within the Gregory Rift Valley (northern Tanzania) and is the only active volcano erupting natrocarbonatite lava. This study investigates the texture and mineralogy of the June 1993 lava at Oldoinyo Lengai, and presents petrographic evidence of liquid immiscibility between silicate, carbonate, chloride, and fluoride melt phases. The 1993 lava is a porphyritic natrocarbonatite consisting of abundant phenocrysts of alkali carbonates, nyerereite and gregoryite, set in a quenched groundmass, composed of sodium carbonate, khanneshite, Na-sylvite and K-halite, and a calcium fluoride phase. Dispersed in the lava are silicate spheroids (< 2 mm) with a cryptocrystalline silicate mineral assemblage wrapped around a core mineral. We have identified several textural features preserved in the silicate spheroids, melt inclusions, and carbonatite groundmass that exhibit evidence of silicate-carbonate, carbonate-carbonate and carbonate-halide immiscibility. Rapid quenching of the lava facilitated the preservation of the end products of these liquid immiscibility processes within the groundmass. Textural evidence (at both macro- and micro-scales) indicates that the silicate, carbonate, chloride and fluoride phases of the lava unmixed at different stages of evolution in the magmatic system.
International Journal of Earth Sciences, Vol. 105, 8, pp. 2175-2201.
Africa, Zimbabwe
Geophysics - gravity
Abstract: Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological-tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8-2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures, that were exploited by Late Archaean and Proterozoic mafic intrusions. There was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide.
Journal of Structural Geology, Vol. 86, pp. 47-61.
Africa, South Africa
Deposit - Venetia
Abstract: The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.
Abstract: Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.% MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf = 32-35, Ti/Zr = 107-147), producing flat REE and HFSE profiles [(La/Yb)pm = 0.9-1.3], with abundances of 3-10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm = 0.6-0.8] and Th [(Th/La)pm = 0.6-0.9]. Initial isotope compositions (?Nd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping ?50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between ?2830 and ?2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.
Journal of South African Institute of Mining and Metallurgy, Vol. 116, Aug. pp. 731-737.
Africa, Namibia
Deposit - marine placers
Abstract: Over the past 80 years, the Namibian diamondiferous marine placer has been studied extensively to develop solutions for mining and sampling challenges. The types of studies include the statistical modelling of the distributions of the stone counts per sample; investigating the relationship between geology and the grade distribution; assessing the quality potential of the entrapment of the available diamond pulse; using predetermined acceptability of barren samples (zero proportion (Zp) samples) to model distributions; optimal sample sizes; and more. During early-stage project evaluation it is more important to find out if a particular area is likely to be above a specific cut-off grade than to focus on sampling for the purpose of accurate resource estimation. Previous work using mixed Poisson and Sichel distributions to model the abundant onshore diamond data has been very successful in modelling the long-tailed nature of these linear beach deposits. The means of these distributions are, however, sensitive to extreme values. Technical and cost constraints prevent a similar scale of sample collection in an adjacent, geologically equivalent, submerged beach environment. A method not sensitive to extreme values is thus required to make early-stage assessments of the likelihood that the grade of a particular target is above a minimum cut-off grade. The Nachman model describes the functional relationship between the mean population density and proportion of barren patches ( Zp) in a patchy environment. A prerequisite for using the Nachman model is that the underlying data must be modelled using a negative binomial distribution (NBD). The case study data is from an analogous area adjacent to the exploration target and meets the NBD requirement. It is thus appropriate to apply the Nachman model. The Nachman model provides an opportunity to use the observed Zpto predict the mean grade for an area at the very early stage of an exploration project. In future, early-stage exploration data from a homogenous geological zone exhibiting characteristics of the Nachman model assumptions can thus be used to rank and target those areas that show potential to be above the minimum required grade cut-off for follow-up sampling and inclusion in the mine planning cycle.
Journal of South African Institute of Mining and Metallurgy, Vol. 116, July pp. 629-635.
Africa, Namibia
Deposit - marine placers
Abstract: During early-stage diamond exploration projects, hard data underpinning spatial continuity is often very limited. An extreme example of this is a submerged diamondiferous marine placer target area alongside a current onshore mining area in southwestern Namibia. Although an abundance of geological and grade data exists for the adjacent onshore mining area, the target area itself contains no such information. Despite this apparent abundance of data, it is extremely difficult to obtain a variogram (Prins and Jacob, 2014) for use in this study area. The use of traditional simulation techniques is further hindered by the fact that diamond entrapment within the highly gullied footwall is non-stationary. An alternative approach for creating a simulated virtual orebody (VOB) is thus required in order to enable the assessment of sampling strategies. This paper demonstrates how expert opinion is used to generate a composite probability map for diamond concentration using a greyscale hand-sketching technique. The probability map is subsequently calibrated and populated using the diamond distribution for different raised beaches obtained from analog data based on sample results adjacent to the target area. The resultant grade simulation is used to test different sample scenarios and is a first step towards determining an appropriate sampling strategy for the target area. The VOB is used to analyse and rank the efficiency of different sampling strategies for grade determination of submerged diamondiferous linear beach exploration targets.
Journal of South African Institute of Mining and Metallurgy, Vol. 116, Aug. pp. 745-753.
Africa, South Africa
Deposit - Cullinan
Abstract: In 2012, Cullinan Diamond Mine began an expansion programme with the shaft deepening and development of access to the C-Cut 1 block at approximately 839 m below surface. The expansion programme is funded by a combination of bank loans and retained operating profit generated by the mine. Continuous production during deepening of the No. 1 Shaft, which is the rock hoisting shaft, was therefore critical for sustainability and efficiency as well as overall funding of the project. The deepening method, support design and verification, as well as learning outcomes pertaining to the extension of the No. 1 Shaft underneath the existing operating shaft are summarized, with emphasis on the importance of gaining some understanding of the shaft's host rock mass.
Journal of Volcanology and Geothermal Research, in press available 15p.
Africa, Algeria
Lamproite
Abstract: The late Miocene (11-9 Ma) volcanic rocks of Kef Hahouner, ~ 40 km NE of Constantine (NE Algeria), are commonly classified as lamproites in literature. However, these rocks are characterized by an anhydrous paragenesis with plagioclase and Mg-rich olivine phenocrysts, set in a groundmass made up of feldspars, pyroxenes and opaque minerals. Thus, we classify the Kef Hahouner rocks as ultrapotassic shoshonites and latites, having K2O > 3 wt.%, K2O/Na2O > 2.5, MgO > 3-4 wt.%, SiO2 < 55-57 wt.% and SiO2/K2O < 15. All the investigated samples show primitive mantle-normalized multi-element patterns typical of orogenic (arc-type) magmas, i.e. enriched in LILE (e.g. Cs, Rb and Ba) and LREE (e.g. La/Yb = 37-59) with respect to the HFSE, peaks at Pb and troughs at Nb and Ta. Initial isotopic ratios are in the range of 87Sr/86Sr = 0.70874-0.70961, 143Nd/144Nd = 0.51222-0.51223, 206Pb/204Pb = 18.54-18.60, 207Pb/204Pb = 15.62-15.70 and 208Pb/204Pb = 38.88-39.16. The Kef Hahouner volcanic rocks show multi-element patterns similar to the other circum-Mediterranean lamproites and extreme Sr, Nd and Pb isotopic compositions. Nevertheless, the abundant plagioclase, the presence of Al-rich augite coupled with high Al2O3 whole rock compositions (9.6-21.4 wt.%), and the absence of phlogopite are all at inconsistent with the definition of lamproite. We reviewed the rocks classified as lamproites worldwide, and found that many of these rocks, as for the Kef Hahouner samples, should be actually defined as "normal" potassic to ultrapotassic volcanic rocks. Even the grouping of lamproites into "orogenic" and "anorogenic" types appears questionable.
Journal of South African Institute of Mining and Metallurgy, Vol. 116, Apr. pp. 343-348.
Africa, South Africa
Deposit - Cullinan
Abstract: Near-infrared (NIR) spectroscopy sorting technology is incorporated in an automated optical mineral sorter that can discriminate between materials using the differences in characteristics when exposed to near-infrared radiation. During September 2014 to April 2015, a pilot plant that utilized NIR technology to discriminate between kimberlite and waste materials was commissioned to determine the viability of including this technology in the diamond winning process flow sheet at Cullinan Diamond Mine. The plant was used to minimize the waste content in the size fraction -70+35 mm that reports to the crushing section and then to the dense media separation process. This paper describes the initial test work, conducted at Mintek, that led to the decision to conduct a pilot-scale study. The mineralogical characterization of the feed and product streams to establish the sorting criteria and the operational data obtained during the pilot plant campaign are described. The results indicated a good possibility of discriminating between the kimberlite and waste material using NIR technology. However, the consistency of discrimination was not good enough to avoid the risk of potential diamond loss. Furthermore, a lower than expected availability of the machine reduced the throughput capabilities.
International Journal of Earth Sciences, Vol. 105, 8, pp. 2175-2201.
Africa, Zimbabwe
Geophysics - magnetics
Abstract: Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological-tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8-2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures, that were exploited by Late Archaean and Proterozoic mafic intrusions. There was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide.
Smit, K.V., Shirey, S.B., Stern, R.A., Steele, A., Wang, W.
Diamond growth from C-H-N-O recycled fluids in the lithosphere: Evidence from CH4 micro-inclusions and dleta 13 C-Delta 15 N-N content in Marange mixed-habit diamonds.
Geochimica et Cosmochimica Acta, in press available 45p.
Africa, Russia
Geochronology
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ to 0.060‰ with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Abstract: Compared to all published data from carbonatites and granitoids, the fluorapatite compositions in the Songwe Hill carbonatite, determined by EPMA and LA ICP-MS, have the highest heavy (H)REE concentration of any carbonatite apatite described so far. A combination of this fluorapatite and the REE fluorocarbonates, synchysite-(Ce) and parisite-(Ce), which are the other principal REE bearing minerals at Songwe, gives a REE deposit with a high proportion of Nd and a higher proportion of HREE (Eu-Lu including Y) than most other carbonatites. Since Nd and HREE are currently the most sought REE for commercial applications, the conditions that give rise to this REE profile are particularly important to understand. Multiple apatite crystallisation stages have been differentiated texturally and geochemically at Songwe and fluorapatite is divided into five different types (Ap-0-4). While Ap-0 and Ap-1 are typical of apatite found in fenite and calcite-carbonatite, Ap-2, -3 and -4 are texturally atypical of apatite from carbonatite and are progressively HREE-enriched in later paragenetic stages. Ap-3 and Ap-4 exhibit anhedral, stringer-like textures and their REE distributions display an Y anomaly. These features attest to formation in a hydrothermal environment and fluid inclusion homogenisation temperatures indicate crystallisation occurred between 200-350 °C. Ap-3 crystallisation is succeeded by a light (L)REE mineral assemblage of synchysite-(Ce), strontianite and baryte. Finally, late-stage Ap-4 is associated with minor xenotime-(Y) mineralisation and HREE-enriched fluorite. Fluid inclusions in the fluorite constrain the minimum HREE mineralisation temperature to approximately 160 °C. A model is suggested where sub-solidus, carbonatite-derived, (carbo)-hydrothermal fluids remobilise and fractionate the REE. Chloride or fluoride complexes retain LREE in solution while rapid precipitation of apatite, owing to its low solubility, leads to destabilisation of HREE complexes and substitution into the apatite structure. The LREE are retained in solution, subsequently forming synchysite-(Ce). This model will be applicable to help guide exploration in other carbonatite complexes.
Abstract: A regional-scale geophysical inversion of magnetic field data in the Ngamiland region of northwestern Botswana is now available for download from the Botswana Geoscience Portal, a partnership initiative of the Botswana Geoscience Institute (BGI), industry sponsors and Geosoft. The earth modelling results represent Botswana’s latest push to provide geoscience data free of charge to mineral explorers, researchers and other stakeholders from around the world. They were generated by the Geosoft Professional Services Group using VOXI Earth Modelling, the company’s high performance inversion software. "The VOXI inversion results are outstanding, even more so because the area of interest is vast, comprising more than 35 million cells," said Motsamai Tarzan Kwadiba, Principal Geophysicist for the BGI. "The outcome provides a springboard for a variety of research opportunities such as seismotectonic studies for seismic hazard assessment and earthquake risk mitigation, area selection for earth resources exploration and management, and investigations of the anatomy and evolution of the Okavango rift zone." Since its launch in April 2016, over 1000 multi-disciplinary datasets have been downloaded from the Botswana Geoscience Portal. Stakeholders from more than 35 countries have visited the repository to access the 10 gigabytes of pre-competitive geoscience data. The new inversion results add clear 3D images of the shallow crust beneath the Okavango delta region, one of the world’s largest inland deltas. Often referred to as the ‘jewel’ of the Kalahari and Africa's last Eden, the 22000 square-kilometer Okavango delta itself is an alluvial fan contained within a seismically active graben structure at the south-western extremity of the East African Rift system.
Journal of Geochemical Exploration, Vol. 173, pp. 99-109.
Africa, Liberia
Kimberlite
Abstract: This report is brief in context and rich in unexpected discovery. With > 2 km of erosion, kimberlite models predict the near-complete removal of pipes with exposures to the pipe-root-zones of dikes. Exploration in NW Liberia has, indeed, uncovered eight kimberlite dikes (~ 10 m wide) but also an en echelon pipe, comparable in size to the Kimberley pipe and De Beers' pipe in South Africa. Discoveries are in a narrow 200-300 m wide valley of extraordinary thick bush, undergrowth, and organic overburden. Ilmenite and co-existing leucoxene were used as diagnostic tracers for detecting hard rock kimberlite in this tropical terrane. Micro-diamonds show that the redox state of ilmenite is a potentially useful proxy as an index for macro-diamond preservation. The tectonic control of kimberlites is complex, with diverse lithologies. Discoveries include a well-defined regional trend for kimberlite dikes along paleo-fracture zones, Precambrian in age (Liberia Trend), coupled with kimberlite dikes on the craton that are traced to Mesozoic oceanic transform faults (the Sierra Leone Trend). Although long predicted, this is the first report of kimberlite dike-trends in Liberia that are similar in orientation to those in Sierra Leone. An explosive blow on a Liberia-Trend dike demonstrates a similarity to the dynamics attendant in rich (50-500 cpht) diamond-bearing dikes in Sierra Leone, and in South Africa of comparable age. The potentially high grade dikes, along with the pipe (~ 500 × 50 m), now more reasonably accounts for the enormous number of alluvial diamonds (blood and non-conflict), recovered over more than seven decades, downstream from the discovery cluster. A neglected region since the classic work by Bardet (1974), and with few contributions on Liberia since then, an update is considered timely, particularly in the context of discoveries of diamond-bearing kimberlite.
Abstract: In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA) are used to characterize magnetite and ilmenite of the phoscorite-carbonatite association at Phalaborwa. We trace the behavior of the compatible elements for two different generations of magnetite related to (1) a magmatic stage, with variable Ti-V content, which pre-dates the copper mineralization, and (2) a late hydrothermal, low-Ti, low-temperature event, mostly post-dating sulfide formation. Magnetite is shown to be a robust petrogenetic indicator; no influence on its chemical composition is detected from the intergrowth with the accompanying phases, including the interaction with coexisting sulfides. High spatial resolution EPMA characterize the tiny late-stage hydrothermal magnetite veins, as well as the ilmenite granular and lamellar exsolutions mostly developed in the magnetite from the phoscorite. By combining geochemical data with geothermo-oxybarometry calculations for magnetite-ilmenite pairs, we infer that the most primitive magnetite probably formed at oxygen fugacity above the nickel nickel oxide (NNO) buffer, revealing an evolutionary trend of decreasing temperature and oxygen fugacity. Geochemical similarity exists between magnetite from phoscorite and carbonatite, thus supporting a common mantle source for the phoscorite-carbonatite association.
Abstract: The Central African Republic (CAR) is the only source of traditionally defined conflict diamonds in the world today. Since May 2013, exports of its diamonds have been under international embargo by both the United Nations and the Kimberley Process (KP), the initiative that regulates the production and trade of rough diamonds. CAR was suspended from the KP after a March 2013 coup d’état that sparked widespread civil unrest in the country. The coup was the inevitable outcome of years of political instability forged by a coalition of rebel groups, known as Séléka, who attacked the government and incrementally seized territory, including the strategic diamond-mining town of Bria.2 On March 24, 2013 Séléka captured the capital city of Bangui and overthrew the government, initiating a bitter internal conflict that continues to fester to this day. The civil war and regime change forced the United Nations and the international community to impose economic sanctions on CAR. Not only were all diamond exports prohibited, the KP urged diamond-trading countries to exercise enhanced vigilance and ensure that diamonds produced in CAR were seized and not allowed to circulate in legitimate trade.3 While the ban on CAR’s exports was partially lifted in 2016 from regions deemed to be KP compliant, that has not stopped the flow of CAR’s conflict diamonds to international markets—while it was under full embargo or regions still prevented from trading today. This report examines the smuggling of diamonds from the Central African Republic into Cameroon. Further, it focuses on the impact this illicit trade has on Cameroon’s internal controls as well as the broader integrity of the diamond supply chain. The report describes the methods used and the key actors involved in this illicit trade. It concludes that the KP and frontline countries like Cameroon need to do more to interrupt the illicit trade of conflict diamonds from CAR and support each other in taking action.
Journal of African Earth Sciences, Vol. 125, pp. 137-150.
Africa
Tectonics
Abstract: New multi-dimensional discrimination diagrams have been used to identify plate tectonic setting of Precambrian terrains. For this work, nine sets of new discriminant-function based multi-dimensional discrimination diagrams were applied for thirteen case studies of Precambrian basic, intermediate and acid magmas from Africa to highlight the application of these diagrams and probability calculations. The applications of these diagrams indicated the following results: For northern Africa: to Wadi Ghadir ophiolite, Egypt indicated an arc setting for Neoproterozoic (746 ± 19 Ma). For South Africa: Zandspruit greenstone and Bulai pluton showed a collision and a transitional continental arc to collision setting at about Mesoarchaean and Neoarchaean (3114 ± 2.3 Ma and 2610-2577 Ma); Mesoproterozoic (1109 ± 0.6 Ma and 1100 Ma) ages for Espungabera and Umkondo sills were consistent with an island arc setting. For eastern Africa, Iramba-Sekenke greenstone belt and Suguti area, Tanzania showed an arc setting for Neoarchaean (2742 ± 27 Ma and 2755 ± 1 Ma). Chila, Bulbul-Kenticha domain, and Werri area indicated a continental arc setting at about Neoproterozoic (800-789 Ma); For western Africa, Sangmelima region and Ebolowa area, southern Cameroon indicated a collision and continental arc setting, respectively for Neoarchaean (?2800-2900 Ma and 2687-2666 Ma); Finally, Paleoproterozoic (2232-2169 Ma) for Birimian supergroup, southern Ghana a continental arc setting; and Paleoproterozoic (2123-2108 Ma) for Katiola-Marabadiassa, Côte d'Ivoire a transitional continental arc to collision setting. Although there were some inconsistencies in the inferences, most cases showed consistent results of tectonic settings. These inconsistencies may be related to mixed ages, magma mixing, crustal contamination, degree of mantle melting, and mantle versus crustal origin.
Abstract: Mantle flow can cause the Earth’s surface to uplift and subside, but the rates and durations of these motions are, in general, poorly resolved due to the difficulties in making measurements of relatively small vertical movements (hundreds of metres) over sufficiently large distances (about 1,000?km). Here we examine the effect of mantle upwelling through a study of Quaternary uplift along the coast of Angola. Using both optically stimulated luminescence on sediment grains, and radiocarbon dating of fossil shells, we date a 25?m coastal terrace at about 45 thousand years old, when sea level was about 75?m lower than today, indicating a rapid uplift rate of 1.8-2.6?mm?yr?1 that is an order of magnitude higher than previously obtained rates averaged over longer time periods. Automated extraction and correlation of coastal terrace remnants from digital topography uncovers a symmetrical uplift with diameter of more than 1,000?km. The wavelength and relatively short timescale of the uplift suggest that it is associated with a mantle process, possibly convective upwelling, and that the topography may be modulated by rapid short-lived pulses of mantle-derived uplift. Our study shows that stable continental regions far from the effects of glacial rebound may experience rapid vertical displacements of several millimetres per year.
Abstract: Tectonic plate interiors are often regarded as relatively inactive. Yet, reconstructions of marine terrace uplift in Angola suggest that underlying mantle flow can rapidly warp Earth's surface far from obviously active plate boundaries.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Metasomatism
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243 to ?0.204 with an average of ?0.225 ± 0.037 (2?, n = 19), and ?56Fe from ?0.038‰ o 0.060 with an average of ?0.003 ± 0.068 (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (±0.05 for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Journal of African Earth Sciences, Vol. 126, pp. 33-44.
Africa, Tanzania
Deposit - Panda Hill
Abstract: The Mesozoic Panda Hill carbonatite deposit in western Tanzania hosts pyrochlore, an ore and source of niobium. This study was conducted to establish the contents of radioactive elements (uranium and thorium) in pyrochlore along with the concentration of niobium in the ore. The pyrochlore is mainly hosted in sövite and is structurally controlled by NW-SE (SW dipping) or NE-SW (NW dipping) magmatic flow bands with dip angles of between 60° and 90°. Higher concentrations of pyrochlore are associated with magnetite, apatite and/or phlogopite rich flow bands. Electron microprobe analyses on single crystals of pyrochlore yield very low UO2 concentrations that range between 0 and 0.09 wt% (equivalent to 0 atoms per formula unit: a.p.f.u.) and ThO2 between 0.55 and 1.05 wt% (equivalent to 0.1 a.p.f.u.). The analyses reveal high concentrations of Nb2O5 (ranging between 57.13 and 65.50 wt%, equivalent to a.p.f.u. ranging between 1.33 and 1.43) and therefore the Panda Hill Nb-oxide is classified as pyrochlore sensu stricto. These data point to a non radioactive pyrochlore and a deposit rich in Nb at Panda Hill. The Panda Hill pyrochlore has low concentrations of REEs as displayed by La2O3 that range between 0.10 and 0.49 wt% (equivalent to a.p.f.u. ranging between 0 and 0.01) and Ce2O3 ranging between 0.86 and 1.80 wt% (equivalent to a.p.f.u. ranging between 0.02 and 0.03), Pr2O3 concentrations range between 0 and 0.23 wt% (equivalent to 0 a.p.f.u.), and Y2O3 is 0 wt% (equivalent to 0 a.p.f.u.). The abundance of the REEs in pyroclore at the Panda Hill Carbonatite deposit is of no economic significance.
Diamond and Related Materials, Vol. 71, pp. 20-29,
Africa, Zimbabwe
Deposit - Marange
Abstract: This study gives an analysis of the effect of low-pressure, high-temperature annealing on the infrared, Raman, and photoluminescence (PL) features, as well as the inclusion characteristics, of cubo-octahedral diamond plates from the Marange deposits in Zimbabwe. The samples showed strong inclusion-related zoning which consists of micron-sized particles identified as graphite and these grew noticeably larger with annealing at temperatures of 300 °C to 1700 °C. Within the natural diamonds, the graphite inclusions (detected by Raman spectroscopy) had a grain size of approximately 1 ?m, which increased to 3 ?m after 1200 °C and 14 ?m after 1700 °C annealing and their hexagonal morphology was discernible. From the geometry of these grains, we determined that they were oriented within the {111} family of planes. The infrared absorption and PL spatial maps were collected after every temperature step to study the effects of annealing on the defects, and photomicrographs and Raman spectra were collected to study the graphite inclusions. The graphitic inclusions grew much larger as the stressed diamond surrounding them converted to graphite. Many nitrogen-related optical centers, including NV? and H3 are no longer detected after high temperature annealing within the cuboid regions as these may have been transformed to hydrogen-bearing complexes such as NVH and N2VH. The presence of CH4 is detected in the unannealed Marange diamonds, but was no longer observed in Raman spectra after 1200 °C annealing. This CH4 disappearance along with changes in inclusion morphology could provide a method to detect heat treatment if these mixed-habit samples are sourced to create treated black gem diamond.
Abstract: In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA) are used to characterize magnetite and ilmenite of the phoscorite-carbonatite association at Phalaborwa. We trace the behavior of the compatible elements for two different generations of magnetite related to (1) a magmatic stage, with variable Ti-V content, which pre-dates the copper mineralization, and (2) a late hydrothermal, low-Ti, low-temperature event, mostly post-dating sulfide formation. Magnetite is shown to be a robust petrogenetic indicator; no influence on its chemical composition is detected from the intergrowth with the accompanying phases, including the interaction with coexisting sulfides. High spatial resolution EPMA characterize the tiny late-stage hydrothermal magnetite veins, as well as the ilmenite granular and lamellar exsolutions mostly developed in the magnetite from the phoscorite. By combining geochemical data with geothermo-oxybarometry calculations for magnetite-ilmenite pairs, we infer that the most primitive magnetite probably formed at oxygen fugacity above the nickel nickel oxide (NNO) buffer, revealing an evolutionary trend of decreasing temperature and oxygen fugacity. Geochemical similarity exists between magnetite from phoscorite and carbonatite, thus supporting a common mantle source for the phoscorite-carbonatite association.
Abstract: In-situ trace element analyses of fluorapatite, calcite, dolomite, olivine, and phlogopite have been undertaken on representative phoscorite and carbonatite rocks of the Palaeoproterozoic Phalaborwa Complex. Textural and compositional characterization reveals uniformity of fluorapatite and calcite among most of the intrusions, and seems to favor a common genetic origin for the phoscorite-carbonatite association. Representing major repositories for rare earth elements (REE), fluorapatite and calcite exhibit tightly correlated light REE (LREE) abundances, suggesting that partitioning of LREE into these rock forming minerals was principally controlled by simple igneous differentiation. However, light rare earth element distribution in apatite and calcite cannot be adequately explained by equilibrium and fractional crystallization and instead favors a complex crystallization history involving mixing of compositionally distinct magma batches, in agreement with previously reported mineral isotope variability that requires open-system behaviour.
Geochemistry, Geophysics, Geosystems: G3, Vol. 17, 2, pp. 5036-5055.
Africa, Madagascar
Tectonics
Abstract: Accurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models. Here, we provide a new plate tectonic reconstruction of the opening of the Western Somali Basin during the breakup of East and West Gondwana. The model is constrained by a new comprehensive set of spreading lineaments, detected in this heavily sedimented basin using a novel technique based on directional derivatives of free-air gravity anomalies. Vertical gravity gradient and free-air gravity anomaly maps also enable the detection of extinct mid-ocean ridge segments, which can be directly compared to several previous ocean magnetic anomaly interpretations of the Western Somali Basin. The best matching interpretations have basin symmetry around the M0 anomaly; these are then used to temporally constrain our plate tectonic reconstruction. The reconstruction supports a tight fit for Gondwana fragments prior to breakup, and predicts that the continent-ocean transform margin lies along the Rovuma Basin, not along the Davie Fracture Zone (DFZ) as commonly thought. According to our reconstruction, the DFZ represents a major ocean-ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as Madagascar drifted southwards, and offshore Tanzania is an obliquely rifted, rather than transform, margin. New seismic reflection evidence for oceanic crust inboard of the DFZ strongly supports these conclusions. Our results provide important new constraints on the still enigmatic driving mechanism of continental rifting, the nature of the lithosphere in the Western Somali Basin, and its resource potential.
Abstract: Uneconomical two decades ago, the Letseng diamond mine in Lesotho is today a major source of 50-carat-plus Type IIa diamonds. It is also the lowest-yielding diamond mine in the world, averaging about 1.5 carats of diamonds per hundred tons of ore against an industry average of one carat per ton. First opened in 1975, the previous operators of the mine (Rio Tinto and De Beers) struggled to maintain its operation because very large diamonds were mixed in with its very low production. The mine closed in 1982 after seven years of loss-making operation. In the 2000’s, three changes profoundly affected the diamond industry: 1. The price of large diamonds appreciated very strongly compared to more commercial sizes and qualities, 2. Improvements in ore processing technology that reduces potential damage or outright destruction of large crystals, and 3. Improvements in X-Ray technology that enhances the ability to identify low fluorescence, Type IIa and fancy color diamonds. This technology also allows for identification of very large diamonds in kimberlite before primary crushing. Letseng reopened in 2006 and has been operating at a profit despite its very low yield and relatively low volume, of about 100,000 carats yearly. The company continues to upgrade its technology which resulted in the recovery of 90 diamonds larger than 50 carats last year. Letseng also serves as a model for other small “boutique” mines such as Kerowe, the Botswana operation which yielded the 1109 ct Lesedi la Rona diamond in 2015 and the Lulo concession in Angola which have come on line recently.
Nature Communications, Jan. 31, doi 10:1038/ncomms1048
Africa, Mauritius
Hot spots
Abstract: A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U-Pb zircon ages recovered directly from 5.7?Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0?Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U-Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ?700?km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India.
Haissen, F., Cambeses, A., Montero, P., Bea, F., Dilek, Y., Mouttaqi, A.
The Archean kaisilite nepheline syenites of the Awsard intrusive massif ( Reguibat Shield, West African craton, Morocco) and its relationship to alkaline magmatism of Africa.
Journal of African Earth Sciences, Vol. 127, pp. 16-50.
Journal of African Earth Sciences, Vol. 127, pp. 3-15.
Africa, Morocco
Craton - magmatism
Abstract: Located along the northwestern edge of the West African Craton, Morocco exhibits a wide variety of magmatic events from Archean to Quaternary. The oldest magmatic rocks belong to the Archean Reguibat Shield outcrops in the Moroccan Sahara. Paleoproterozoic magmatism, known as the Anti-Atlas granitoids, is related to the Eburnean orogeny and initial cratonization of the WAC. Mesoproterozoic magmatism is represented by a small number of mafic dykes known henceforth as the Taghdout mafic volcanism. Massive Neoproterozoic magmatic activity, related to the Pan-African cycle, consists of rift-related Tonian magmatism associated with the Rodinia breakup, an Early Cryogenian convergent margin event (760-700 Ma), syn-collisional Bou-Azzer magmatism (680-640 Ma), followed by widespread Ediacaran magmatism (620-555 Ma). Each magmatic episode corresponded to a different geodynamic environment and produced different types of magma. Phanerozoic magmatism began with Early Cambrian basaltic (rift?) volcanism, which persisted during the Middle Cambrian, and into the Early Ordovician. This was succeeded by massive Late Devonian and Carboniferous, pre-Variscan tholeiitic and calc-alkaline (Central Morocco) volcanic flows in basins of the Moroccan Meseta. North of the Atlas Paleozoic Transform Zone, the Late Carboniferous Variscan event was accompanied by the emplacement of 330-300 Ma calc-alkaline granitoids in upper crustal shear zones. Post-Variscan alkaline magmatism was associated with the opening of the Permian basins. Mesozoic magmatism began with the huge volumes of magma emplaced around 200 Ma in the Central Atlantic Magmatic Province (CAMP) which was associated with the fragmentation of Pangea and the subsequent rifting of Central Atlantic. CAMP volcanism occurs in all structural domains of Morocco, from the Anti-Atlas to the External Rif domain with a peak activity around 199 Ma. A second Mesozoic magmatic event is represented by mafic lava flows and gabbroic intrusions in the Internal Maghrebian flysch nappes as well as in the external Mesorif. This event consists of Middle-Upper Jurassic MORB tholeiites emplaced during opening of the Alpine Tethys ocean. The Central High Atlas also records Early Cretaceous alpine Tethys magmatism associated with the aborted Atlas rift, or perhaps linked to plume activity on the edge of the WAC. Cenozoic magmatism is associated with Tertiary and Quaternary circum-Mediterranean alkaline provinces, and is characterized by an intermittent activity over 50 Ma from the Anti-Atlas to the Rif Mountain along a SW-NE volcanic lineament which underlines a thinned continental lithosphere.
Abstract: The Man Craton region of West Africa has a rich history of diamonds since they were first discovered in the 1930’s.They are primarily alluvial in source with currently only one kimberlite mine in operation at Koidu in Sierra Leone. The total diamond production from Guinea, Liberia and Sierra Leone over the past 10 years is recorded by the Kimberley Process at around 12.2 million carats with a value of $1.9 billion. The two main producing countries during this period are Guinea, which has yielded 6.7 million carats at an average of $52 per carat, and Sierra Leone where production has reached 5 million carats at a higher value of $277 per carat. Liberia is the smallest producer with 0.4 million carats but these have a high average value of $383 per carat. There are two known age provenances of kimberlites in the Man Craton. The larger, Jurassic age provenance comprises six main clusters of small (generally 10 ha) kimberlite pipes and dykes ranging from the older Bounoudou kimberlites in Guinea, at 153 Ma, through to the younger Tongo kimberlites in Sierra Leone dated at 140 Ma. A single, neo-Proterozoic cluster is known in the Weasua area in Liberia and is dated at 800 Ma. The Jurassic age kimberlites are classified as phlogopite-rich kimberlites with abundant groundmass opaque minerals. The older Weasua kimberlites typically contain less phlogopite and groundmass opaque minerals. Although remnants of diatreme facies are present in some pipes, notably the Banankoro, Koidu and Weasua kimberlites, hypabyssal and transitional facies tend to predominate which would indicate that these kimberlites have been eroded down to the interface between the root and diatreme zones. This suggests potential erosion of up to 2 km over the Man Craton; however geomorphological evidence suggests a lesser amount of erosion has taken place (Skinner et al., 2004). Alluvial diamonds are prevalent throughout the Man Craton and are not restricted to the known kimberlite clusters. This would argue for a wide dispersion of diamonds in the alluvial system as a result of significant landmass uplift and weathering since the time of intrusion. It could also indicate that there are diamondiferous kimberlites yet to be discovered, which is supported by the limited exploration data. It is therefore concluded that there are certain areas of the Man Craton which remain highly prospective for diamondiferous kimberlites.
Earth and Planetary Science Letters, Vol. 463, pp. 178-188.
Africa, Botswana
Deposit - Lethakane
Abstract: Monocrystalline diamonds commonly record complex internal structures reflecting episodic growth linked to changing carbon-bearing fluids in the mantle. Using diamonds to trace the evolution of the deep carbon cycle therefore requires dating of individual diamond growth zones. To this end Rb-Sr and Sm-Nd isotope data are presented from individual eclogitic silicate inclusions from the Orapa and Letlhakane diamond mines, Botswana. ?13C?13C values are reported from the host diamond growth zones. Heterogeneous 87Sr/86Sr ratios (0.7033-0.7097) suggest inclusion formation in multiple and distinct tectono-magmatic environments. Sm-Nd isochron ages were determined based on groups of inclusions with similar trace element chemistry, Sr isotope ratios, and nitrogen aggregation of the host diamond growth zone. Diamond growth events at 0.14±0.090.14±0.09, 0.25±0.040.25±0.04, 1.1±0.091.1±0.09, 1.70±0.341.70±0.34 and 2.33±0.022.33±0.02 Ga can be directly related to regional tectono-magmatic events. Individual diamonds record episodic growth with age differences of up to 2 Ga. Dated diamond zones have variable ?13C?13C values (?5.0 to ?33.6‰ vs PDB) and appear to imply changes in subducted material over time. The studied Botswanan diamonds are interpreted to have formed in different tectono-magmatic environments that involve mixing of carbon from three sources that represent: i) subducted biogenic sediments (lightest ?13C?13C, low 87Sr/86Sr); ii) subducted carbonate-rich sediments (heavy ?13C?13C, high 87Sr/86Sr) and iii) depleted upper mantle (heavy ?13C?13C, low 87Sr/86Sr). We infer that older diamonds from these two localities are more likely to have light ?13C?13C due to greater subduction of biogenic sediments that may be related to hotter and more reduced conditions in the Archaean before the Great Oxidation Event at 2.3 Ga. These findings imply a marked temporal change in the nature of subducted carbon beneath Botswana and warrant further study to establish if this is a global phenomenon.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Craton, Peridotite
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ 0.060 with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Journal of Petrology, Vol. 57, 11-12, pp. 2251-2270.
Africa, Morocco
Deposit - Beni Bousera
Abstract: Correlations between major and minor transition elements in tectonically emplaced orogenic peridotites have been ascribed to variable degrees of melt extraction and melt-rock reaction processes, leading to depletion or refertilization. To elucidate how such processes are recorded in the subcontinental lithospheric mantle, we processed a large geochemical dataset for peridotites from the four tectono-metamorphic domains of the Beni Bousera orogenic massif (Rif Belt, northern Morocco). Our study reveals that variations in bulk-rock major and minor elements, Mg-number and modal mineralogy of lherzolites, as well as their clinopyroxene trace element compositions, are inconsistent with simple partial melting and mainly resulted from different reactions between melts and depleted peridotites. Up to 30% melting at <3 GPa and cryptic metasomatism can account for the geochemical variations of most harzburgites. In Grt-Sp mylonites, melting and melt-rock reactions are masked by tectonic mixing with garnet pyroxenites and subsolidus re-equilibration. In the rest of the massif, lherzolites were mostly produced by refertilization of a refractory protolith (Mg-number = 91, Ol = 70%, Cpx/Opx = 0.4) via two distinct near-solidus, melt- rock reactions: (1) clinopyroxene and orthopyroxene precipitation and olivine consumption at melt/rock ratios <0.75 and variable mass ratio between crystallized minerals and infiltrated melt ®, which are recorded fairly homogeneously throughout the massif; (2) dissolution of orthopyroxene and precipitation of clinopyroxene and olivine at melt/rock ratios <1 and R = 0.2-0.3, which affected mainly the Arie` gite-Seiland and Seiland domains. The distribution of secondary lherzolites in the massif suggests that the first refertilization reaction occurred prior to the differentiation of the Beni Bousera mantle section into petro-structural zones, whereas the second reaction was associated with the development of the tectono-metamorphic domains. Our data support a secondary, refertilization-related origin for most lherzolites in orogenic peridotite massifs.
Abstract: Northern Mozambique (figure 1) has gained attention for its rubies since a major discovery near Montepuez in 2009 (see McClure and Koivula, 2009; Pardieu and Lomthong, 2009; Pardieu and Chauvire, 2012; Pardieu et al., 2009, 2013; Hsu et al., 2014). Until the arrival of Gemfields in 2012, nearly all the production from this deposit came from unlicensed miners, known as garimpeiros. Between 2012 and 2016, Gemfields became a force in the ruby trade, supplying the market through regular auctions in Singapore and Jaipur. In 2016, two new players acquired ruby mining licenses around Montepuez: Mustang Resources and Metals of Africa. During a summer 2016 GIA field expedition, we visited these new sites. We also spent time at the Gemfields operation, in order to follow the development of what is already the world’s largest ruby mine. We also visited an interesting new pink spinel and tourmaline deposit near Ocua.
Journal of African Earth Sciences, Vol. 129, pp. 202-223.
Africa, Morocco
Alkaline rocks
Abstract: The Jbel Boho complex (Anti-Atlas/Morocco) is an alkaline magmatic complex that was formed during the Precambrian-Cambrian transition, contemporaneous with the lower early Cambrian dolomite sequence. The complex consists of a volcanic sequence comprising basanites, trachyandesites, trachytes and rhyolites that is intruded by a syenitic pluton. Both the volcanic suite and the pluton are cut by later microsyenitic and rhyolitic dykes. Although all Jbel Boho magmas were probably ultimately derived from the same, intraplate or plume-like source, new geochemical evidence supports the concept of a minimum three principal magma generations having formed the complex. Whereas all volcanic rocks (first generation) are LREE enriched and appear to be formed by fractional crystallization of a mantle-derived magma, resulting in strong negative Eu anomalies in the more evolved rocks associated with low Zr/Hf and Nb/Ta values, the younger syenitic pluton displays almost no negative Eu anomaly and very high Zr/Hf and Nb/Ta. The syenite is considered to be formed by a second generation of melt and likely formed through partial melting of underplated mafic rocks. The syenitic pluton consists of two types of syenitic rocks; olivine syenite and quartz syenite. The presence of quartz and a strong positive Pb anomaly in the quartz syenite contrasts strongly with the negative Pb anomaly in the olivine syenite and suggests the latter results from crustal contamination of the former. The late dyke swarm (third generation of melt) comprises microsyenitic and subalkaline rhyolitic compositions. The strong decrease of the alkali elements, Zr/Hf and Nb/Ta and the high SiO2 contents in the rhyolitic dykes might be the result of mineral fractionation and addition of mineralizing fluids, allowing inter-element fractionation of even highly incompatible HFSE due to the presence of fluorine. The occurrence of fluorite in some volcanic rocks and the Ca-REE-F carbonate mineral synchysite in the dykes with very high LREE contents (Ce ?720 ppm found in one rhyolitic dyke) suggest the fluorine-rich nature of this system and the role played by addition of mineralizing fluids. The REE mineralization expressed as synchysite-(Ce) is detected in a subalkaline rhyolitic dyke (with ?LREE = 1750 ppm) associated with quartz, chlorite and occasionally with Fe-oxides. The synchysite mineralization is probably the result of REE transport by acidic hydrothermal fluids as chloride complex and their neutralization during fluid-rock interaction. The major tectonic change from compressive to extensional regime in the late Neoproterozoic induced the emplacement of voluminous volcaniclastic series of the Ediacran Ouarzazate Group. The alkaline, within-plate nature of the Jbel Boho igneous complex implies that this extensional setting continued during the early Cambrian.
Journal of African Earth Sciences, Vol. 129, pp. 366-379.
Africa, Mozambique
Craton, Zimbabwe
Abstract: The eastern margin of the Zimbabwe Craton, along the Mozambique-Zimbabwe border, includes the oldest rocks of west-central Mozambique constituting a large terrain of granite-greenstone type dated between 3000 and 2500 Ma. These rocks consist mainly of gneisses and granitoid rocks of tonalitic-trondhjemitic-granodioritic composition belonging to the Mudzi Metamorphic Complex in the northern part and to the Mavonde Complex in the southern part. The latter is associated with a granite-greenstone terrain, which includes the eastern part of Mutare-Odzi-Manica greenstone belt. A volcano-sedimentary sequences cover, belonging to the apparently Mesoproterozoic and Paleoproterozoic Umkondo and Gairezi groups respectively was deposited along the eastern margin of the craton and is exposed in the territory of Mozambique. The Umkondo minimum age is marked by intrusive dolerite in Zimbabwe dated at 1100 Ma while for the Ghairezi it is still not well established. The Gairezi Group was subjected to progressive metamorphism of Pan-African age. At the margin of the Zimbabwe Craton, in its northern part, metasedimentary units occur representing a passive margin of Neoproterozoic age. They make up the Rushinga Group, which includes felsic metavolcanic rocks dated at ca.800 Ma. Granulites and medium- to high-grade paragneisses, and migmatites of the Chimoio, Macossa and Mungari Groups of Neoproterozoic metamorphic age, overly the ortho-metamorphic pre-existing rock of ca. 1100 Ma, which belongs to the Báruè Magmatic Arc. They characterize the N-S trend Mozambique Belt, which appears to the east of the craton tectonically juxtaposed on the Archean rocks. The maximum age of deposition of these rocks, indicated by U-Pb dating of detrital zircons, is ca. 700 Ma and their minimum age is limited by a few monzonitic Cambrian intrusions dated at ca. 500 Ma. The Neoproterozoic bimodal Guro Suite, dated at ca. 850 Ma and composed of felsic and mafic members characterizes the east-dipping outer rim of the craton margin in the north. The felsic member comprises the Serra Banguatere aplitic granite gneiss-migmatite and the mafic member consists of the Magasso metagabbro and mafic gneiss-migmatite. The geochemical signature and bimodality are all characteristics of anorogenic, A-type granites. The tectono-thermal effects of the Pan-African orogenic event, of approximately 500 Ma, are visible along the margin of the Zimbabwe Craton. Deformation and metamorphism are progressive from the craton towards the belt, from greenschist facies to granulite facies. The main suture in the study area shall be placed along the frontal thrusts of the Mungari and Macossa/Chimoio nappes of Neoproterozoic to Cambrian age. To the west of the suture the rejuvenated margin of the craton occurs, indicated by K-Ar dating. To the east, the Mozambique Belt occurs with its paragneisses of the Neoproterozoic overlaying the Mesoproterozoic granitoids of the Báruè magmatic arc.
Journal of African Earth Sciences, Vol. 129, pp. 94-107.
Africa, Cameroon
Craton, Congo
Abstract: We used the World Gravity Map (WGM 2012) data to investigate the Archean Congo craton and the Oubanguides orogenic belt in Cameroon. The Oubanguides orogenic belt constitutes, from northwest to southeast, the Neoproterozoic West Cameroon domain, the Paleoproterozoic-Neoproterozoic Adamawa-Yade domain, and the dominantly Neoproterozoic Yaoundé domain (the crustal expression of the suture zone between the Congo craton and the orogenic terranes). We analyzed the WGM 2012 data to identify different gravity anomalies. We also applied the two-dimensional (2D) radially-averaged power spectral analysis to the WGM 2012 data to estimate the Moho depth. Additionally, we developed a 2D forward gravity model along a Nsbnd S profile to image the lithospheric structure of the Precambrian entities. We found that: (1) the Congo craton, the Yaoundé domain, the southeastern part of the West Cameroon domain, and the northern part of the Adamawa-Yade domain are characterized by low gravity anomaly. (2) the southern part of the Adamawa-Yade domain is marked by a pronounced E-W trending high gravity anomaly. (3) the crust is thicker beneath the Congo craton, the Yaoundé domain and the southern part of the Adamawa-Yade domain. (4) the presence of a denser lower crust material beneath the southern part of the Adamawa-Yade domain. We propose that this denser crustal material is an under-thrusted portion of the Congo craton that has been densified through metacratonization processes that accompanied collision between the craton and the orogenic terranes. This is in good agreement with geological and geochemical observations indicating that the northern edge of the Congo craton and the Adamawa-Yade domain had undergone metacratonization during the Neoproterozoic. Our suggestion is also in good agreement with observations which show that the margins of many cratons worldwide have been decratonized due to subduction processes. Our work highlights the importance of potential field geophysical data in mapping the metacratonized margins of cratons.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 5540 Abstract
Africa, Botswana
Deposit - Letlhakane
Abstract: The diamondiferous Letlhakane kimberlites are part of the Orapa kimberlite cluster (˜ 93.1 Ma) in north-eastern Botswana, located on the edge of the Zimbabwe Craton, close to the Proterozoic Magondi Mobile Belt. Here we report the first Re-Os ages of six individual eclogitic sulphide inclusions (3.0 to 35.7?g) from Letlhakane diamonds along with their rhenium, osmium, iridium and platinum concentrations, and carbon isotope, nitrogen content and N-aggregation data from the corresponding growth zones of the host diamonds. For the first time, Re-Os data will be compared to Sm-Nd ages of individual eclogitic silicate inclusions recovered from the same diamonds using a Triton Plus equipped with four 1013? amplifiers. The analysed inclusion set currently encompasses pairs of individual sulphides from two diamonds (LK040 sf4 & 5, LK113 sf1 & 2) and two sulphide inclusions from separate diamonds (LK048, LK362). Ongoing work will determine the Sm-Nd ages and element composition of multiple individual eclogitic garnets (LK113/LK362, n=4) and an eclogitic clinopyroxene (LK040) inclusion. TMA ages of the six sulphides range from 1.06 to 2.38 Ga (± 0.1 to 0.54 Ga) with Re and Os contents between 7 and 68 ppb and 0.03 and 0.3 ppb, respectively. The host diamond growth zones have low nitrogen abundances (21 to 43 ppm N) and high N-aggregation (53 to 90% IaB). Carbon isotope data suggests the involvement of crustal carbon (?13C between -19.3 to -22.7 ± 0.2 per mill) during diamond precipitation. Cathodoluminescence imaging of central plates from LK040 and LK113 displays homogenous internal structure with no distinct zonation. The two sulphide inclusions from LK040 define an 'isochron' of 0.92 ± 0.23 Ga (2SD) with initial 187Os/188Os = 1.31 ± 0.24. Sulphides from LK113 have clear imposed diamond morphology and indicate diamond formation at 0.93 ± 0.36 Ga (2SD) with initial 187Os/188Os = 0.69 ± 0.44. The variation in the initial 187Os/188Os does not justify including these inclusions (or any from other diamonds) on the same isochron and implies an extremely heterogeneous diamond crystallisation environment that incorporated recycled Os. C1-normalized osmium, iridium and platinum (PGE) compositions from the analysed sulphide inclusions display enrichment in Ir (3.4 to 33) and Pt (2.3 to 28.1) in comparison to eclogitic xenolith data from Orapa that are depleted relative to chondrite. The Re-Os isochrons determined in this study are within error of previously reported ages from the adjacent (˜40km) Orapa diamond mine (1.0 to 2.9 Ga) based on sulphide inclusions and a multi-point 990 ± 50 Ma (2SD) isochron for composite (n=730) silicate inclusions. Together with additional new Sm-Nd isochron age determinations from individual silicate inclusions from Letlhakane (2.3 ± 0.02 (n = 3); 1.0 ± 0.14 (n = 4) and 0.25 ± 0.04 Ga (n = 3), all 2SE) these data suggest a phase of Mesoproterozoic diamond formation as well as Neoarchean/Paleoproterozoic and Mesozoic diamond growth, in punctuated events spanning >2.0 Ga.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 16438 Abstract
Africa, Zimbabwe, Australia, South America, Brazil
Deposit - Murowa, Argyle, Machado River
Abstract: Fourier Transform Infrared (FTIR) spectroscopy is a commonly-used technique for investigating diamonds. It gives the most useful information if spatially-resolved measurements are used [1]. In this contribution we discuss the best way to acquire and present FTIR data from diamonds, using examples from Murowa (Zimbabwe), Argyle (Australia) and Machado River (Brazil). Examples of FTIR core-to-rim line scans, maps with high spatial resolution and maps with high spectral resolution that are fitted to extract the spatial variation of different nitrogen and hydrogen defects are presented. Model mantle residence temperatures are calculated from the concentration of A and B nitrogen-containing defects in the diamonds using known times of annealing in the mantle. A new, two-stage thermal annealing model is presented that better constrains the thermal history of the diamond and that of the mantle lithosphere in which the diamond resided. The effect of heterogeneity within the analysed FTIR volume is quantitatively assessed and errors in model temperatures that can be introduced by studying whole diamonds instead of thin plates are discussed. The kinetics of platelet growth and degradation will be discussed and the potential for two separate, kinetically-controlled defect reactions to be used to constrain a full thermal history of the diamond will be assessed. [1] Kohn, S.C., Speich, L., Smith, C.B. and Bulanova, G.P., 2016. FTIR thermochronometry of natural diamonds: A closer look.
International Journal of Earth Sciences, in press available 17p.
Africa, Namibia
Alkaline rocks
Abstract: Rb-Sr whole-rock and mineral isotope data from nepheline syenite, tinguaite, and carbonatite samples of the Kalkfeld Complex within the Damaraland Alkaline Province, NW Namibia, indicate a date of 242?±?6.5 Ma. This is interpreted as the age of final magmatic crystallization in the complex. The geological position of the complex and the spatially close relationship to the Lower Cretaceous Etaneno Alkaline Complex document a repeated channeling of small-scale alkaline to carbonatite melt fractions along crustal fractures that served as pathways for the mantle-derived melts. This is in line with Triassic extensional tectonic activity described for the nearby Omaruru Lineament-Waterberg Fault system. The emplacement of the Kalkfeld Complex more than 100 Ma prior to the Paraná-Etendeka event and the emplacement of the Early Cretaceous Damaraland intrusive complexes excludes a genetic relationship to the Tristan Plume. The initial ?Sr-?Nd pairs of the Kalkfeld rocks are typical of younger African carbonatites and suggest a melt source, in which EM I and HIMU represent dominant components.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 9187 Abstract
Africa, Zimbabwe, Sierra Leone
Deposit - Marange, Zimmi
Abstract: Traditional models for diamond formation within the lithospheric mantle invoke either carbonate reduction or methane oxidation. Both these mechanisms require some oxygen exchange with the surrounding wall-rock at the site of diamond precipitation. However, peridotite does not have sufficient buffering capacity to allow for diamond formation via these traditional models and instead peridotitic diamonds may form through isochemical cooling of H 2 O-rich CHO fluids [1]. Marange mixed-habit diamonds from eastern Zimbabwe provide the first natural confirmation of this new diamond growth model [2]. Although Marange diamonds do not contain any silicate or sulphide inclusions, they contain Ni-N-vacancy complexes detected through photoluminescence (PL) spectroscopy that suggest the source fluids equilibrated in the Ni-rich depleted peridotitic lithosphere. Cuboid sectors also contain abundant micro-inclusions of CH 4 , the first direct observation of reduced CH 4-rich fluids that are thought to percolate through the lithospheric mantle [2]. In fluid inclusion-free diamonds, core-to-rim trends in ? 13 C and N content are used to infer the speciation of the diamond-forming fluid. Core to rim trends of increasing ? 13 C with decreasing N content are interpreted as diamond growth from oxidized CO 2-or carbonate-bearing fluids. Diamond growth from reduced species should show the opposite trends-decreasing ? 13 C from core to rim with decreasing N content. Within the CH 4-bearing growth sectors of Marange diamonds, however, such a 'reduced' trend is not observed. Rather, ? 13 C increases from core to rim within a homogeneously grown zone [2]. These contradictory observations can be explained through either mixing between CH 4-and CO 2-rich end-members of hydrous fluids [2] or through closed system precipitation from an already mixed CH 4-CO 2 H 2 O-maximum fluid with XCO 2 (CO 2 /[CO 2 +CH 4 ]) between 0.3 and 0.7 [3]. These results demonstrate that Marange diamonds precipitated from cooling CH 4-CO 2-bearing hydrous fluids rather than through redox buffering. As this growth mechanism applies to both the fluid-rich cuboid and gem-like octahedral sectors of Marange diamonds, a non-redox model for diamond formation from mixed CH 4-CO 2 fluids is indicated for a wider range of gem-quality peridotitic diamonds. Indeed, at the redox conditions of global diamond-bearing lithospheric mantle (FMQ-2 to-4; [4]), CHO fluids are strongly water-dominated and contain both CH 4 and CO 2 as dominant carbon species [5]. By contrast diamond formation in eclogitic assemblages, through either redox buffering or cooling of carbon-bearing fluids, is not as well constrained. Zimmi diamonds from the West African craton have eclogitic sulphide inclusions (with low Ni and high Re/Os) and formed at 650 Ma, overlapping with the timing of subduction [6]. In one Zimmi diamond, a core to rim trend of decreasing ? 13 C (-23.4 to-24.5 %¸) and N content is indicative of formation from reduced C 2 H 6 /CH 4-rich fluids, likely derived from oceanic crust recycled during Neoproterozoic subduction. Unlike mixed CH 4-CO 2 fluids near the water maximum, isochemical cooling or ascent of such reduced CHO fluids is not effficient at diamond precipitation. Furthermore, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the ?1 internal variation seen. Consequently, this Zimmi eclogitic diamond likely formed through redox buffering of reduced subduction-related fluids, infiltrating into sulphide-bearing eclogite.
Geochimica et Cosmochimica Acta, in press available 55p.
Africa, South Africa
Deposit - Roberts Victor
Abstract: Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2 °C/km to reach peak conditions of 5.8 GPa and 890 °C for the kyanite eclogite. The kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak pressure (5.3 GPa) and was associated with a rise in temperature to 1380 °C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. The ?18O values of garnets are consistently higher than normal mantle values. Each sample has its characteristic trend of ?18O variance between garnets in the kyanite-bearing sections and those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic lithosphere. As kyanite contains around 100 ppm of H2O it is suggested that the kyanite-out reaction, once initiated by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite. The steep (high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of eclogites into the lithospheric mantle. Such a trajectory is best explained in a model where strong lateral compression forces eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and compression.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 18924 Abstract
Africa
Geochronology
Abstract: Kimberlites provide rich information about the composition and evolution of cratonic lithosphere. Accurate geochronology of these eruptions is key for discerning spatiotemporal trends in lithospheric evolution, but kimberlites can sometimes be difficult to date with available methods. We explored whether (U-Th)/He dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages. We obtained zircon and/or perovskite (U-Th)/He (ZHe, PHe) dates from 16 southern African kimberlites. Most samples with abundant zircon yielded reproducible ZHe dates (?15% dispersion) that are in good agreement with published eruption ages. The majority of dated zircons were xenocrystic. Zircons with reproducible dates were fully reset during eruption or resided at temperatures above the ZHe closure temperature prior to entrainment in the kimberlite magma. Not dating hazy and radiation damaged grains can help avoid anomalous results for more shallowly sourced zircons that underwent incomplete damage annealing and/or partial He loss during the eruptive process. All seven kimberlites dated with PHe yielded reproducible (?15% dispersion) and reasonable results. We conducted two preliminary perovskite 4He diffusion experiments, which suggest a PHe closure temperature of >300°C. Perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that PHe dates will typically record emplacement rather than postemplacement processes. ZHe and PHe geochronology can effectively date kimberlite emplacement and provide useful complements to existing techniques.
Romisch-Germanisches Zentralmuseum Leibniz-Forschungsinstitut fur Archaologie Mainz International Conference Oct. 20-22, 2015, pp. 155-217.
Africa, Europe
Gemology - gemstones
Abstract: Archaeology and discoveries of new gemstones and new gem sources in recent decades attest to the need for critical review and updating of literature in translation concerning gems of the ancient world. The origins and identities of gemstones used in ancient glyptic have been inferred almost exclusively from literary descriptions available in secondary or even tertiary sources after now-lost ancient original texts. To date, no epigraphical or philological study has verified the ancient gem cutters’repertoire of materials against empirical gemological examination of extant material in public or private collections. However, such objective data should improve interpretation of literary source material that is often fragmentary or contains descriptions fraught with lexical ambiguities and contradictions. A carefully qualified perspective is needed. Whether in original form or in translation, manuscripts, from antiquity to the present day, reflect some degree of current knowledge about geography and gems in the contemporary world of the author/epigrapher/translator. Contemporary knowledge attributed to earlier cultures is an unwitting bias that frequently eludes both translators and scholars. Together with critical examination of the imprint of authorial bias, a gemological review of extant material is discussed in relation to the important treatises on gemstone nomenclature, identity, and geographic origin.
van den Heuvel, Q., Matveev, S., Drury, M., Gress, M., Chinn, I., Davies, G.
Genesis of diamond inclusions: an integrated cathodluminescence ( CL) and electron backscatter diffraction (EBSD) study on eclogitic and peridotitic inclusions and their diamond host.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 6564 Abstract
Abstract: The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano's recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490-595 °C, which is 250-450 °C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8-9 wt%, i.e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000-1050 °C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of ?48% calcite, ?12% apatite, and ?2 wt% clinopyroxene. SiO2 solubility only increases from 0.2 to 2.9 wt% at 750-1200 °C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism.
Mineralogy and Petrology, Vol. 111, 3, pp. 373-381.
Africa, Tanzania
mineralogy
Abstract: Jörgkellerite, ideally Na3Mn3+ 3(PO4)2(CO3)O2•5H2O, is a new layered phosphate-carbonate from the Oldoinyo Lengai volcano in the Gregory Rift (northern Tanzania). The mineral occurs as spherulites, up to 200 ?m in diameter, consisting of plates up to 10 ?m in thickness in shortite-calcite and calcite carbonatites. Jörgkellerite is brown with a vitreous lustre and has a perfect micaceous cleavage on {001}, Mohs hardness is 3. The calculated density is 2.56 g/cm3. Jörgkellerite is uniaxial (-), ? = 1.700(2), ? = 1.625(2) (Na light, 589 nm) with distinct pleochroism: O = dark brown, E = light brown. The empirical formula of the mineral (average of 10 electron microprobe analyses) is (Na2.46K0.28Ca0.08Sr0.04Ba0.02)?2.88(Mn3+ 2.39Fe3+ 0.56)?2.95((PO4)1.95(SiO4)0.05))?2.00(CO3)(O1.84(OH)0.16)?2.00•5H2O. The oxidation state of Mn has been determined by XANES. Jörgkellerite is trigonal, space group P-3, a = 11.201(2) Å, c = 10.969(2) Å, V = 1191.9(7) Å3 and Z = 3. The five strongest powder-diffraction lines [d in Å, (I/I o), (hkl)] are: 10.970 (100) (001), 5.597 (15) (002), 4.993 (8) (111), 2.796 (14) (220) and 2.724 (20) (004). The crystal structure is built up of the layers composed of disordered edge-sharing [MnO6] octahedra. Each fourth Mn site in octahedral layer is vacant that results in appearance of ordered system of hexagonal "holes" occupied by (CO3) groups. The overall composition of the layer can be expressed as [Mn3O8(CO3)]. These manganese-carbonate layers are linked in the third dimension by (PO4) tetrahedra and Na-polyhedra. The origin of jörgkellerite is related to low-temperature oxidative alteration of gregoryite-nyerereite carbonatites.
Abstract: Kimberlitic magmaclasts are discrete ovoid magmatic fragments that formed prior to emplacement from disrupted kimberlite magma. To provide new constraints on the origin and evolution of the kimberlite melts, we document the mineralogy and petrography of a magmaclast recovered from one of the ca. 520 Ma Venetia kimberlites, South Africa. The sample (BI9883) has a sub-spherical shape and consists of a ~ 10 mm diameter central olivine macrocryst, surrounded by porphyritic kimberlite. The kimberlitic material consists of concentrically aligned, altered olivine phenocrysts, set in a crystalline groundmass of calcite, chromite, perovskite, phlogopite, apatite, ilmenite, titanite, sulphides, rutile and magnetite along with abundant alteration phases (i.e. serpentine, talc and secondary calcite). These features are typical of archetypal hypabyssal kimberlites. We examined primary fluid/melt inclusions in chromite, perovskite and apatite containing a diversity of daughter phases. Chromite and perovskite host polycrystalline inclusions containing abundant alkali-carbonates (i.e. enriched in K, Na, Ba, Sr), phosphates, Na-K chlorides, sulphides and equal to lesser quantities of olivine, phlogopite and pleonaste. In contrast, apatite hosts polycrystalline assemblages with abundant alkali-carbonates and Na-K chlorides and lesser amounts of olivine, monticellite and phlogopite. Numerous solid inclusions of shortite (Na2Ca2(CO3)3), Na-Sr-carbonates and apatite occur in groundmass calcite along with fluid inclusions containing daughter crystals of Na-carbonates and Na-chlorides. The primary inclusions in chromite, perovskite and apatite are considered to represent remnants of fluid(s)/melt(s) trapped during crystallisation of the host minerals, whereas the fluid inclusions in calcite are probably secondary in origin. The component proportions of these primary fluid/melt inclusions were estimated in an effort to constrain the composition of the evolving kimberlite melt. These estimates suggest melt evolution from a silicate-carbonate kimberlite melt that became increasingly enriched in carbonates, phosphates, alkalis and chlorides, in response to the fractional crystallisation of constituent minerals (i.e. olivine to apatite). The concentric alignment of crystals around the olivine kernel and ovoid shape of the magmaclast can be ascribed to the low viscosity of the kimberlite melt and rapid rotation whilst in a liquid or partial crystalline state, or to progressive layer-by-layer growth of the magmaclast. Although the mineralogy of our sample is similar to hypabyssal kimberlites worldwide, it differs from hypabyssal kimberlite units in the main Venetia pipes, which contain monticellite-phlogopite rich assemblages and segregationary matrix textures. Therefore magmaclast BI9883 probably originated from a batch of magma distinct from those that produced known hypabyssal units within the Venetia kimberlite cluster.
Abstract: The Mesoproterozoic Pilanesberg Complex, South Africa, is built up by several distinct, ring-shaped intrusions of syenite and peralkaline nepheline syenite. A mildly peralkaline ((Na + K) / Al = 1.04–1.09), medium-to coarse grained nepheline syenite makes up the outermost ring in the southwestern part of the complex (“Matooster type white foyaite”). In this rock, mafic silicate minerals (amphibole, biotite, aegirine) and Ti-bearing minerals (ilmenite, astrophyllite, aenigmatite, lorenzenite, bafertisite, jinshajiangite) are interstitial to feldspar and nepheline, and define a series of mineral assemblages reflecting a change from a miaskitic crystallization regime (with Na-Ca amphibole, titanite and ilmenite) to increasingly agpaitic conditions (with arfvedsonite, aegirine, astrophyllite, aenigmatite, lorenzenite). The main driving force behind the evolution was an increase in peralkalinity of the trapped liquid, mainly by adcumulus growth of alkali feldspar and nepheline, which in the later stages of evolution was combined with increases in oxygen fugacity and water activity. Unlike in most other agpaitic rock complexes, Zr remained compatible in aegirine (and to some extent in amphibole) almost to the end of the process, when a hydrous zirconium silicate mineral (hilairite) crystallized as the only mineral in the rock having essential zirconium. The presence of minerals such as hilairite, bafertisite, jinshajiangite and a Na-REE-Sr rich apatite group mineral (fluorcaphite ?) in the latest assemblages suggests that the last remaining interstitial melt or fluid approached a hyperagpaitic composition. The isolated melt pockets in the Pilanesberg white foyaite follow a pattern of evolution that can be seen as a miniature analogue of the fractional crystallization processes controlling magma evolution in large, alkaline igneous rock complexes.
Journal of African Earth Sciences, Vol. 134, pp. 10-23.
Africa, Malawi
carbonatite - Songwe Hill
Abstract: Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 x 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1–C3). Early C1 carbonatite is scarce and is composed of medium–coarse-grained calcite carbonatite containing zircons with a U–Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.
Abstract: The Beni Bousera peridotite contains a diversity of pyroxenite layers. Several studies have postulated that at least some of them represent elongated strips of oceanic lithosphere recycled in the convective mantle. Some pyroxenites were, however, ascribed to igneous crystal segregation or melt–rock reactions. To further constrain the origin of these rocks, we collected 171 samples throughout the massif and examined their variability in relation with the tectono-metamorphic domains. A major finding is that all facies showing clear evidence for a crustal origin are concentrated in a narrow corridor of mylonitized peridotites, along the contact with granulitic country rocks. These peculiar facies were most likely incorporated at the mantle–crust boundary during the orogenic events that culminated in the peridotite exhumation. The other pyroxenites derive from a distinct protolith that was ubiquitous in the massif before its exhumation. They were deeply modified by partial melting and melt–rock reactions associated with lithospheric thinning.
South African Journal of Geology, Vol. 119, 4, pp. 585-606.
Africa, South Africa
deposit - Lichtenburg Ventersdorp
Abstract: Diamond-bearing gravels of the Lichtenburg-Ventersdorp area of the North West Province are associated with north-south orientated sinuous ‘runs’ that occur almost entirely on a flat erosional surface of the Malmani dolomites (Transvaal Supergroup) at some 1,500 m elevation. East to west, this dolomite plain measures 150 km, and north-south it is on average 40 km wide. This unconformity, which first developed before the Pretoria Group sedimentation over a period of at least 80 Myr, is marked by siliceous breccias (palaeo-karst infill) and conglomerates (reworked breccias). It was exhumed in pre-Karoo and post-Gondwana times. Glacial pavements and remnants of thin Lower Karoo sediments are also found on this polyphase surface. The gravels that make up these ‘runs’ and sinkholes directly or indirectly linked to these runs, are coarse-grained, very poorly-sorted, and are best described as diamictites. The ‘runs’ are narrow, elongated, generally positive ridges that meander across the dolomite surface and are up to 30 km long and between 80 to 300 m wide. They have always been regarded as post-Cretaceous drainage features linked to southward-flowing river systems. Diamonds were discovered in these ‘runs’ and they have produced some 12 million carats. However, no Cainozoic fossils or artefacts have ever been found in almost 90 years of mining. From new field evidence, geomorphological studies, age dating from inclusions in diamond and zircon and clay analyses, it is proposed that these coarse-grained runs represent proximal palaeoeskers of the last deglaciation of the Dwyka continental ice sheet, that are preserved on this ancient ‘palimpsest’ surface. The age of the deposit is constrained by two populations of agate within the diamictites that are linked to two separate volcanic units of the Pretoria Group. In addition, the youngest crustal zircon ages from the gravels are 1 Ba, but mantle zircons from Lichtenburg suggest that these have been derived from Cambrian age kimberlites. Analysis of inclusions in diamond support a Neoproterozoic to Cambrian source for the diamonds, so the absence of diamonds from Mesozoic kimberlites and Cainozoic fossils within the gravels support the conclusion that the runs are of Karoo age.
Abstract: The Pilanesberg Complex (South Africa) is one of the world's largest but least studied alkaline complexes. It consists of trachytes, phonolites, syenites and nepheline syenites (foyaites) and the preservation of the volcanic carapace makes it unique among the larger alkaline complexes. The intrusive history of the Pilanesberg Complex shows similarities to the Greenland Kangerlussuaq Intrusion, and our new whole-rock major and trace element analyses, combined with existing data, show that the complex belongs to the Sr-rich type of evolved alkaline rocks, more similar to the complexes of the Kola Peninsula than Ilímaussaq. Despite the absence of mafic lithologies, comparison with experimental studies shows that the parental magma was most likely an alkali basalt. Significant iron enrichment is caused by an early stage of fractionation involving clinopyroxene and amphibole rather than olivine and plagioclase, reflecting water-rich compositions and intermediate levels of oxygen fugacity. This fractionation trend has led to strong enrichment in Sr and Ba, but only moderate levels of Y and middle to heavy rare earth elements, and minimal Eu-anomaly. Late-stage water-rich fluids caused significant autometasomatism in most units. New U-Pb dating of titanite constrains the age of the Pilanesberg Complex as 1395 + 10/? 11 Ma. Initial 87Sr/86Sr isotope ratios around 0.7028 (?Sr1395 = ? 1) are typical for a moderately depleted mantle source, unlike the local lithospheric mantle. The combination of an enriched trace-element signature and depleted isotopic characteristics is evidence for small degrees of partial melting. Epsilon Sr values are similar to those reported for other alkaline complexes worldwide, emplaced in crust with contrasting geological histories; this likely reflects lithospheric metasomatism shortly before magmatism and minimal crustal contamination.
Abstract: The 2060 Ma old Palabora Carbonatite Complex (PCC), South Africa, comprises diverse REE mineral assemblages formed during different stages and reflects an outstanding instance to understand the evolution of a carbonatite-related REE mineralization from orthomagmatic to late-magmatic stages and their secondary post-magmatic overprint. The 10 rare earth element minerals monazite, REE-F-carbonates (bastnäsite, parisite, synchysite), ancylite, britholite, cordylite, fergusonite, REE-Ti-betafite, and anzaite are texturally described and related to the evolutionary stages of the PCC. The identification of the latter five REE minerals during this study represents their first described occurrences in the PCC as well as in a carbonatite complex in South Africa. The variable REE mineral assemblages reflect a multi-stage origin: (1) fergusonite and REE-Ti-betafite occur as inclusions in primary magnetite. Bastnäsite is enclosed in primary calcite and dolomite. These three REE minerals are interpreted as orthomagmatic crystallization products. (2) The most common REE minerals are monazite replacing primary apatite, and britholite texturally related to the serpentinization of forsterite or the replacement of forsterite by chondrodite. Textural relationships suggest that these two REE-minerals precipitated from internally derived late-magmatic to hydrothermal fluids. Their presence seems to be locally controlled by favorable chemical conditions (e.g., presence of precursor minerals that contributed the necessary anions and/or cations for their formation). (3) Late-stage (post-magmatic) REE minerals include ancylite and cordylite replacing primary magmatic REE-Sr-carbonates, anzaite associated with the dissolution of ilmenite, and secondary REE-F-carbonates. The formation of these post-magmatic REE minerals depends on the local availability of a fluid, whose composition is at least partly controlled by the dissolution of primary minerals (e.g., REE-fluorocarbonates). This multi-stage REE mineralization reflects the interplay of magmatic differentiation, destabilization of early magmatic minerals during subsequent evolutionary stages of the carbonatitic system, and late-stage fluid-induced remobilization and re-/precipitation of precursor REE minerals. Based on our findings, the Palabora Carbonatite Complex experienced at least two successive stages of intense fluid–rock interaction.
Abstract: The petrogenesis of kimberlites is commonly obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during fluid interaction and the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa). Sample BK consists of abundant macrocrysts (> 1 mm) and (micro-) phenocrysts of olivine and lesser phlogopite, smaller grains of apatite, serpentinised monticellite, spinel, perovskite, phlogopite and ilmenite in a matrix of calcite, serpentine and dolomite. As in kimberlites worldwide, BK olivine grains consist of cores with variable Mg/Fe ratios, overgrown by rims that host inclusions of groundmass phases (spinel, perovskite, phlogopite) and have constant Mg/Fe, but variable Ni, Mn and Ca concentrations. Primary multiphase inclusions in the outer rims of olivine and in Fe-Ti-rich (‘MUM’) spinel are dominated by dolomite, calcite and alkali carbonates with lesser silicate and oxide minerals. Secondary inclusions in olivine host an assemblage of Na-K carbonates and chlorides. The primary inclusions are interpreted as crystallised alkali-Si-bearing Ca-Mg-rich carbonate melts, whereas secondary inclusions host Na-K-rich C-O-H-Cl fluids. In situ Sr-isotope analyses of groundmass calcite and perovskite reveal similar 87Sr/86Sr ratios to perovskite in the Bultfontein and the other Kimberley kimberlites, i.e. magmatic values. The ?18O composition of the BK bulk carbonate fraction is above the mantle range, whereas the ?13C values are similar to those of mantle-derived magmas. The occurrence of different generations of serpentine and occasional groundmass calcite with high 87Sr/86Sr, and elevated bulk carbonate ?18O values indicate that the kimberlite was overprinted by hydrothermal fluids, which probably included a significant groundwater component. Before this alteration the groundmass included calcite, monticellite, apatite and minor dolomite, phlogopite, spinel, perovskite and ilmenite. Inclusions of groundmass minerals in olivine rims and phlogopite phenocrysts show that olivine and phlogopite also belong to the magmatic assemblage. We therefore suggest that the crystallised kimberlite was produced by an alkali-bearing, phosphorus-rich, silica-dolomitic melt. The alkali-Si-bearing Ca-Mg-rich carbonate compositions of primary melt inclusions in the outer rims of olivine and in spinel grains with evolved compositions (MUM spinel) support formation of these melts after fractionation of abundant olivine, and probably other phases (e.g., ilmenite and chromite). Finally, the similarity between secondary inclusions in kimberlite olivine of this and other worldwide kimberlites and secondary inclusions in minerals of carbonatitic, mafic and felsic magmatic rocks, suggests trapping of residual Na-K-rich C-O-H-Cl fluids after groundmass crystallisation. These residual fluids may have persisted in pore spaces within the largely crystalline BK groundmass and subsequently mixed with larger volumes of external fluids, which triggered serpentine formation and localised carbonate recrystallisation.
Abstract: Magmas forming large igneous provinces (LIP) on continents are generated by extensive melting in the deep crust and underlying mantle and associated with break-up of ancient supercontinents, followed by formation of a new basaltic crust in the mid-oceanic rifts. A lack of the unifying model in understanding the sources of LIP magmatism is justified by lithological and geochemical complexity of erupted magmas on local (e.g. a cross-section) and regional (a single and different LIP) scales. Moreover, the majority of LIP rocks do not fit general criteria for recognizing primary/primitive melts (i.e. < 8 wt% MgO and absence of high-Fo olivine phenocrysts). This study presents the mineralogical (olivine, Cr-spinel, orthopyroxene), geochemical (trace elements and Sr-Nd-Hf-Pb isotopes) and olivine-hosted melt inclusion compositional characteristics of a single primitive (16 wt% MgO), high-Ti (2.5 wt% TiO2) picrite with high-Mg olivine (up to 91 mol% Fo) from the Letaba Formation in the ~ 180 Ma Karoo LIP (south Africa). The olivine compositions (unusually high ?18O (6.17‰), high NiO (0.36–0.56 wt%) and low MnO and CaO (0.12–0.20 and 0.12–0.22 wt%, respectively)) are used to argue for a non-peridotitic mantle source. This is supported by the enrichment of the rock and melts in most incompatible trace elements and depletion in heavy rare earth elements (e.g. high Gd/Yb) that reflects residual garnet in the source of melting. The radiogenic isotopes resemble those of the model enriched mantle (EM-1) and further argue for a long-term enrichment of the source in incompatible trace elements. The enriched high-Ti compositions, strongly fractionated incompatible trace elements, presence of primitive olivine and high-Cr spinel in the Letaba picrites are closely matched by olivine-phyric rocks from the ~ 260 Ma Emeishan (Yongsheng area, SW China) and ~ 250 Ma Siberian (Maimecha-Kotuy region, N Siberia) LIPs. However, many other compositional parameters (e.g. trace element and ?18O compositions of olivine phenocrysts, Fe2 +/Fe3 + in Cr-spinel, Sr-Nd-Hf isotope ratios) only partially overlap or even diverge. We thus imply that parental melts of enriched picritic rocks with forsteritic olivine from three major continental igneous provinces – Karoo, Emeishan and Siberia cannot be assigned to a common mantle source and similar melting conditions. The Karoo picrites also exhibit some mineralogical and geochemical similarities with rocks and glasses in the south Atlantic Ridge and adjacent fracture zones. The geodynamic reconstructions of the continental plate motions since break-up of the Gondwanaland in the Jurassic support the current position of the source of the Karoo magmatism in the southernmost Atlantic. Co-occurrence of modern and recent anomalous rocks with normal mid-ocean ridge basalts in this region can be related to blocks/rafts of the ancient lithosphere, stranded in the ambient upper mantle and occasionally sampled by rifting-related decompressional melting.
Geochimica et Cosmochimica Acta, available in press 23p.
Africa, Botswana
deposit - Orapa
Abstract: Base metal sulphide (BMS) inclusions in diamonds provide a unique insight into the chalcophile and highly siderophile element composition of the mantle. Entombed within their diamond hosts, these provide a more robust (closed system) sample, from which to determine the trace element, Re-Os and S-isotopic compositions of the mantle than mantle xenoliths or orogenic peridotites, as they are shielded from alteration during ascent to the Earth’s crust and subsequent surface weathering. However, at temperatures below 1100 °C some BMS inclusions undergo subsolidus re-equilibration from an original monosulphide solid solution (Mss) and this causes fractionation of the major and trace elements within the inclusions. Thus to study the subjects noted above, current techniques require the entire BMS inclusion to be extracted for analyses. Unfortunately, ‘flaking’ of inclusions during break-out is a frequent occurrence and hence the risk of accidentally under-sampling a portion of the BMS inclusion is inherent in current practices. This loss may have significant implications for Re-Os isotope analyses where incomplete sampling of a Re-rich phase, such as chalcopyrite that typically occurs at the outer margins of BMS inclusions, may induce significant bias in the Re-Os and 187Os/188Os measurements and resulting model and isochron ages. We have developed a method for the homogenisation of BMS inclusions in diamond prior to their break-out from the host stone. Diamonds are heated to 1100 °C and then quenched to chemically homogenise any sulphide inclusions for both major and trace elements. Using X-ray Computed Microtomography (µCT) we determine the shape and spatial setting of multiple inclusions within a host stone and crucially show that the volume of a BMS inclusion is the same both before and after homogenisation. We show that the homogenisation process significantly reduces the inherent variability of in situ analysis when compared with unhomogenised BMS, thereby widening the scope for multiple methods for quantitative analysis, even on ‘flakes’ of single BMS inclusions. Finally we show that the trace elements present in peridotite (P-type) and eclogitic (E-type) BMS are distinct, with P-type diamonds having systematically higher total platinum-group element (particularly Os, Ir, Ru) and Te and As concentrations. These distinctions suggest that the PGE and semi-metal budgets of mantle-derived partial melts will be significantly dependent upon the type(s) and proportions of sulphides present in the mantle source.
Abstract: Oldoinyo Lengai is situated within the Gregory Rift Valley (northern Tanzania) and is the only active volcano erupting natrocarbonatite lava. This study investigates the texture and mineralogy of the June 1993 lava at Oldoinyo Lengai, and presents petrographic evidence of liquid immiscibility between silicate, carbonate, chloride, and fluoride melt phases. The 1993 lava is a porphyritic natrocarbonatite consisting of abundant phenocrysts of alkali carbonates, nyerereite and gregoryite, set in a quenched groundmass, composed of sodium carbonate, khanneshite, Na-sylvite and K-halite, and a calcium fluoride phase. Dispersed in the lava are silicate spheroids (< 2 mm) with a cryptocrystalline silicate mineral assemblage wrapped around a core mineral. We have identified several textural features preserved in the silicate spheroids, melt inclusions, and carbonatite groundmass that exhibit evidence of silicate-carbonate, carbonate-carbonate and carbonate-halide immiscibility. Rapid quenching of the lava facilitated the preservation of the end products of these liquid immiscibility processes within the groundmass. Textural evidence (at both macro- and micro-scales) indicates that the silicate, carbonate, chloride and fluoride phases of the lava unmixed at different stages of evolution in the magmatic system.
Earth and Planetary Science Letters, Vol. 473, pp. 44-51.
Africa, Zimbabwe
deposit - Marange
Abstract: Because of the inability of depleted cratonic peridotites to effectively buffer oxygen fugacities when infiltrated by CHO or carbonatitic fluids, it has been proposed recently (Luth and Stachel, 2014) that diamond formation in peridotites typically does not occur by rock-buffered redox reactions as previously thought but by an oxygen-conserving reaction in which minor coexisting CH4 and CO2 components in a water-rich fluid react to form diamond (CO2 + CH4 = 2C + 2H2O). In such fluid-buffered systems, carbon isotope fractionation during diamond precipitation occurs in the presence of two dominant fluid carbon species. Carbon isotope modelling of diamond precipitation from mixed CH4CH4- and CO2-bearing fluids reveals unexpected fundamental differences relative to diamond crystallization from a single carbon fluid species: (1) irrespective of which carbon fluid species (CH4 or CO2) is dominant in the initial fluid, diamond formation is invariably associated with progressive minor (<1‰) enrichment of diamond in 13C as crystallization proceeds. This is in contrast to diamond precipitation by rock-buffered redox processes from a fluid containing only a single carbon species, which can result in either progressive 13C enrichment (CO2 or carbonate fluids) or View the MathML sourceC13 depletion (CH4 fluids) in the diamond. (2) Fluid speciation is the key factor controlling diamond ?13C?13C values; as XCO2 (XCO2 = CO2/[CO2 + CH4]) in the initial fluid increases from 0.1 to 0.9 (corresponding to an increase in fO2fO2 of 0.8 log units), the carbon isotope composition of the first-precipitated diamond decreases by 3.7‰. The tight mode in ?13C of ?5 ±1‰?5 ±1‰ for diamonds worldwide places strict constraints on the dominant range of XCO2 in water-rich fluids responsible for diamond formation. Specifically, precipitation of diamonds with ?13C values in the range ?4 to ?6‰ from mantle-derived fluids with an average ?13C value of ?5‰ (derived from evidence not related to diamonds) requires that diamond-forming fluids were relatively reduced and had methane as the dominant carbon species (XCO2 = 0.1–0.5). Application of our model to a recently published set of in-situ carbon isotope analyses for peridotitic diamonds from Marange, Zimbabwe (Smit et al., 2016), which contain CH4 fluid inclusions, allows us to perfectly match the observed co-variations in ?13C?13C, ?15N?15N and N content and at the same time explain the previously counter-intuitive observation of progressive View the MathML sourceC13 enrichment in diamonds that appear to have grown from a fluid with methane as the dominant carbon species. Similarly, the almost complete absence in the published record of progressive View the MathML sourceC13 depletion trends within diamonds likely reflects ubiquitous precipitation from CH4- and CO2-bearing water-rich fluids, rather than diamond formation exclusively by carbonate-bearing and CH4-free oxidized fluids or melts.
Abstract: The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano’s recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490–595 °C, which is 250–450 °C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8–9 wt%, i.e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000–1050 °C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of ?48% calcite, ?12% apatite, and ?2 wt% clinopyroxene. SiO2 solubility only increases from 0.2 to 2.9 wt% at 750–1200 °C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism.
Abstract: The Karowe mine, owned and operated by Lucara Diamond Corporation, located in the Republic of Botswana, achieved commercial diamond production in July 2012. The AK06 kimberlite discovered in 1969 is the ore source at Karowe. The AK06 kimberlite within the Orapa Kimberlite Field is a roughly north-south elongate kimberlite body with a near surface expression of approximately 3.3 ha and a maximum area of approximately 7 ha at approximately 120 m below surface. The body comprises three geologically distinct, coalescing pipes that taper with depth. These pipes are referred to as the North Lobe, Centre Lobe, and South Lobe. The AK6 kimberlite is an opaque-mineral-rich monticellite kimberlite, texturally classified primarily as fragmental volcaniclastic kimberlite with lesser macrocrystic hypabyssal facies kimberlite of the Group 1 variety. The nature of the kimberlite differs between each lobe with distinctions apparent in the textural characteristics. The South Lobe is considered to be distinctly different from the North and Centre Lobes that are similar to each other in terms of their geological characteristics. The North and Centre Lobes exhibit internal textural complexity whereas the bulk of the South Lobe is more massive and internally homogeneous. The South Lobe forms the majority of the resource and displays the coarsest diamond size distribution of the three lobes. In three years of production, Karowe has established a continuing production of high value diamonds including coloured diamonds. In March 2013 a 239 ct gem quality diamond was recovered which was the first in a continuing population of large high value Type IIa diamonds recovered from the Centre Lobe, and more importantly the South Lobe, of the Karowe mine. Large diamonds, 50 ct in size, are spatially distributed horizontally and vertically within the South Lobe. Since commissioning to mid-December 2015, approximately 1.5 Mct have been produced and specials (diamonds greater than 10.8 ct in weight) represent circa 4.6% by weight of all diamond production. Life of Mine average stone size for the specials is 29.6 ct/stone. In 2015, a plant optimization project was completed to modify the process plant to treat harder kimberlite at depth and improve the recovery of exceptional diamonds via bulk sorters for primary run of mine large diamond recovery. In November 2015, the Karowe mine recovered an 813 ct stone and the world’s second largest gem quality diamond in over 100 years weighing 1,111 ct
Geochemical and petrographical study of megacrysts and mantle xenoliths from Gemsbok Hollow and Gruendorn kimberlites in the Warmbad kimberlite Province, South Africa
Slope stability challenges and solutions for mining kimberlite resources hosted in structurally complex country rock: dip slope mining at Jwaneng mine, Botswana.
Abstract: The widespread occurrence of pyrope garnet in Archean lithospheric mantle remains one of the ‘holy grails’ of mantle petrology. Most garnets found in peridotitic mantle equilibrated with incompatible-trace-element-enriched melts or fluids and are the products of metasomatism. Less common are macroscopic intergrowths of pyrope garnet formed by exsolution from orthopyroxene. Spectacular examples of these are preserved in both mantle xenoliths and large, isolated crystals (megacrysts) from the Kaapvaal craton of southern Africa, and provide direct evidence that some garnet in the sub-continental lithospheric mantle initially formed by isochemical rather than metasomatic processes. The orthopyroxene hosts are enstatites and fully equilibrated with their exsolved phases (low-Cr pyrope garnet ±± Cr-diopside). Significantly, P-TP-T estimates of the post-exsolution orthopyroxenes plot along an unperturbed conductive Kaapvaal craton geotherm and reveal that they were entrained from a large continuous depth interval (85 to 175 km). They therefore represent snapshots of processes operating throughout almost the entire thickness of the sub-cratonic lithospheric mantle. New rare-earth element (REE) analyses show that the exsolved garnets occupy the full spectrum recorded by garnets in mantle peridotites and also diamond inclusions. A key finding is that a few low-temperature exsolved garnets, derived from depths of ~90 km, are more depleted in light REEs than previously observed in any other mantle sample. Importantly, the REE patterns of these strongly LREE-depleted garnets resemble the hypothetical composition proposed for pre-metasomatic garnets that are thought to pre-date major enrichment events in the sub-continental lithospheric mantle, including those associated with diamond formation. The recalculated compositions of pre-exsolution orthopyroxenes have higher Al22O33 and CaO contents than their post-exsolution counterparts and most likely formed as shallow residues of large amounts of adiabatic decompression melting in the spinel-stability field. It is inferred that exsolution of garnet from Kaapvaal orthopyroxenes may have been widespread, and perhaps accompanied cratonization at ~ 2.9 to 2.75 Ga. Such a process would considerably increase the density and stability of the continental lithosphere.
Tracing mantle metasomatism using combined stable (S,O) and radiogenic (Sr, Nd, Hf, Pb) isotope geochemistry: case studies from mantle xenoliths of the Kimberley kimberlites.
Abstract: The diamondiferous Letlhakane kimberlites are part of the Orapa kimberlite cluster (˜ 93.1 Ma) in north-eastern Botswana, located on the edge of the Zimbabwe Craton, close to the Proterozoic Magondi Mobile Belt. Here we report the first Re-Os ages of six individual eclogitic sulphide inclusions (3.0 to 35.7?g) from Letlhakane diamonds along with their rhenium, osmium, iridium and platinum concentrations, and carbon isotope, nitrogen content and N-aggregation data from the corresponding growth zones of the host diamonds. For the first time, Re-Os data will be compared to Sm-Nd ages of individual eclogitic silicate inclusions recovered from the same diamonds using a Triton Plus equipped with four 1013? amplifiers. The analysed inclusion set currently encompasses pairs of individual sulphides from two diamonds (LK040 sf4 & 5, LK113 sf1 & 2) and two sulphide inclusions from separate diamonds (LK048, LK362). Ongoing work will determine the Sm-Nd ages and element composition of multiple individual eclogitic garnets (LK113/LK362, n=4) and an eclogitic clinopyroxene (LK040) inclusion. TMA ages of the six sulphides range from 1.06 to 2.38 Ga (± 0.1 to 0.54 Ga) with Re and Os contents between 7 and 68 ppb and 0.03 and 0.3 ppb, respectively. The host diamond growth zones have low nitrogen abundances (21 to 43 ppm N) and high N-aggregation (53 to 90% IaB). Carbon isotope data suggests the involvement of crustal carbon (?13C between -19.3 to -22.7 ± 0.2 per mill) during diamond precipitation. Cathodoluminescence imaging of central plates from LK040 and LK113 displays homogenous internal structure with no distinct zonation. The two sulphide inclusions from LK040 define an 'isochron' of 0.92 ± 0.23 Ga (2SD) with initial 187Os/188Os = 1.31 ± 0.24. Sulphides from LK113 have clear imposed diamond morphology and indicate diamond formation at 0.93 ± 0.36 Ga (2SD) with initial 187Os/188Os = 0.69 ± 0.44. The variation in the initial 187Os/188Os does not justify including these inclusions (or any from other diamonds) on the same isochron and implies an extremely heterogeneous diamond crystallisation environment that incorporated recycled Os. C1-normalized osmium, iridium and platinum (PGE) compositions from the analysed sulphide inclusions display enrichment in Ir (3.4 to 33) and Pt (2.3 to 28.1) in comparison to eclogitic xenolith data from Orapa that are depleted relative to chondrite. The Re-Os isochrons determined in this study are within error of previously reported ages from the adjacent (˜40km) Orapa diamond mine (1.0 to 2.9 Ga) based on sulphide inclusions and a multi-point 990 ± 50 Ma (2SD) isochron for composite (n=730) silicate inclusions. Together with additional new Sm-Nd isochron age determinations from individual silicate inclusions from Letlhakane (2.3 ± 0.02 (n = 3); 1.0 ± 0.14 (n = 4) and 0.25 ± 0.04 Ga (n = 3), all 2SE) these data suggest a phase of Mesoproterozoic diamond formation as well as Neoarchean/Paleoproterozoic and Mesozoic diamond growth, in punctuated events spanning >2.0 Ga.
Abstract: In this study, a private collection of diamondiferous eclogite xenoliths has been made available for non-destructive investigation. All samples have at least one diamond visible. The samples are predominantly sourced from the Excelsior and Newlands mines (South Africa), with additional samples from Roberts Victor mine (South Africa) and Orapa (Botswana). 3D volume models of the samples were created using X-ray tomography. The 3D images reveal abundant secondary veining that is clearly younger than the eclogite. Diamonds are located in fluid pathways and occur in both altered garnet and altered clinopyroxene. Most of the veining is unrelated to the spatial positioning of diamond in the samples. In some instances, early veining has annealed or partially annealed, suggesting a range in timing of at least some of the several metasomatic events that have affected the rock. Importantly, in the most graphic examples, a clear distinction can be seen between diamond-bearing and non-diamond-bearing veins, even where sulphide is present in abundance in the non-diamond-bearing veins. The amount of diamond detected in the xenoliths varies from a single crystal to well over 50 diamonds forming more than 9% of the rock. This extreme value contrasts with the diamond recovery from currently viable diamond mines of less than 2ppm or 0.0002%. The morphology of the diamonds includes step-faced flat-faced octahedra, single crystals and aggregates. This is particularly a feature of diamonds in the Excelsior specimens. In the samples from Newlands and Orapa, in contrast, diamond surfaces reflect resorption processes such as rounding and corrosion of the diamonds. The following conclusions can be drawn from this study: Diamonds in this collection, sourced from within the Kalahari craton, appear to have formed by a metasomatic process during which fluids infiltrated pre-existing mantle-derived eclogite; Several metasomatic events have occurred during the residence of the eclogite in the mantle; Some of these metasomatic events have been diamond-friendly, whilst others have been diamond-neutral or diamond-destructive; Diamond can be present at very high concentrations along particular metasomatic fluid pathways in eclogitic mantle rocks; The absolute timing of diamond formation is still to be determined.
Abstract: The Letšeng Diamond Mine comprises two adjacent kimberlites, the Main and Satellite pipes. Very low grade and low frequency of high value stones preclude use of standard evaluation methods, increasing the need for high confidence geology models. New results of drillcore investigations, including core logging, country rock dilution measurements, indicator mineral abundances and petrography are integrated with open pit mapping and macrodiamond production data to present updated 3D geological models of the Main and Satellite pipes. Letšeng was emplaced ~90 Ma and forms part of a Cretaceous kimberlite province extending across southern Africa. The Letšeng bodies are steep-sided volcanic pipes that are infilled with multiple asymmetric phases of kimberlite separated by near-vertical, sharp, cross-cutting internal contacts. There are associated marginal carbonate-cemented country-rock breccias and sub-vertical kimberlite dykes that can occur within a zone of well-developed carbonate veining adjacent to the pipes. The pipe infills are dominated by kimberlite closely resembling Kimberley-type pyroclastic kimberlite (KPK; formerly tuffisitic kimberlite; Scott Smith et al. 2013) that contains common large megaxenoliths of massive and brecciated country rock basalt. Other textural varieties include hypabyssal kimberlite (HK), transitional HK-KPK and resedimented volcaniclastic kimberlite (RVK). Each phase of kimberlite represents a separate magmatic system. In each KPK there is a continuum of crystallization from the magmaclast selvages to the intermagmaclast matrix. As documented elsewhere, the HK-KPK transition involves an increasing (i) degree of deuteric replacement of olivine, (ii) amount of incorporated country rock xenoliths and (iii) textural modification of the magma prior to solidification subsurface within the diatreme. These conclusions negate some other proposed modes of emplacement.
Abstract: An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and ?18O values lower than the Earth’s mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4–6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.
Geochemistry of the Namaqualand, Busmanland and Warmbad melillitite and kimberlite provinces of South Africa and Namibia: the southern extension of the African kimberlitic megalineament.
Abstract: The Koffiefontein kimberlite is one of the classic locales on the Kaapvaal Craton and best known for being the first locality where very deep diamonds were recovered. Koffiefontein diamonds were last studied in 1980s and, based on major advances in micro-analytical techniques, here we revisit the mine to provide improved inclusion-based geothermobarometry. Currently, ~70 peridotitic diamonds have been studied with ~150 olivine, Cr-rich pyrope garnet, enstatite, and diopside inclusions liberated. So far, the inclusions were analysed (EPMA) for their major element compositions. The peridotitic inclusion suite is dominated by a highly-depleted harzburgitic association: olivine Mg-number ranges between 93 and 95, harzburgitic garnets contain < 3.00 wt % CaO, and 14 % of garnet inclusions are lherzolitic in paragenesis. Conventional mineral exchange geothermobarometry on non-touching inclusion pairs reveals that diamonds with harzburgitic inclusions derive from close to the base of the lithosphere (1143-1283 °C and 55-65 kbar; 12 diamonds) whilst much rarer lherzolitic inclusions last equilibrated in the upper portion of diamond stable lithosphere (983-1158 °C and 47-52 kbar; 5 diamonds). Both associations fall along a 39 mW/m2 reference geotherm (Hasterok and Chapman, 2011), implying a total lithosphere thickness of ~200 km at 90 Ma. To expand our geothermobarometry data beyond rare garnet-opx associations we conducted high-precision trace element analysis of Al and Ca in olivine inclusions via EPMA. Using long count times (300 sec on peak and each background) and a 200 nA beam current, detection limits of 8 ppm for Al and 6 ppm for Ca were achieved. This technique allowed for trace element analysis of olivine inclusions as small as 20 ?m in diameter. Based on these high-precision Al analyses, the updated Al-inolivine thermometer of Bussweiler et al (2017) extrapolated to a 39 mW/m2 geotherm yielded temperatures of 1124-1278 °C (30 diamonds).
Mineral inclusions in diamonds from Karowe mine, Botswana: examining the mantle sources of a diamond population containing exceptionally large crystals.
Geochemistry of peridotitic clinopyroxene from the Diamondiferous Mbuji-Mayi and Tshibwe kimberlites ( DRC): insight into the compositional and thermal state of the SCLM beneath the eastern Kasai craton.
Abstract: Mantle xenoliths hosted in Cenozoic alkaline rocks of northern Madagascar (Massif d'Ambre and Bobaomby volcanic fields) are spinel lherzolites, harzburgites and rare websterites. Petrography, electron microprobe, LA-ICP-MS and thermal ionization mass spectrometry techniques allowed to recognize domains characterized by variable degree of partial melting and extent of re-enrichment processes: 1) refractory spinel-to-spinel + plagioclase-lherzolites, with clinopyroxenes having marked LREE (Light Rare Earth Elements) depletion ((La/Yb)N ~ 0.2) and very high 143Nd/144Nd (0.513594), which represent a limited and shallow portion of old mantle that suffered low degree partial melting (2–3%) and was later accreted to the lithosphere. These lherzolites acted as a low-porosity region, being, in places, percolated by small volumes of melts shortly before eruption; 2) lherzolites and harzburgites that suffered variable degrees of partial melt extraction (up to 15%), assisted and/or followed by pervasive, porous flow infiltration of alkaline melts in a relatively large porosity region, leading to the creation of a wide area rich in secondary mineral phases (i.e. olivine, clinopyroxene and pargasitic amphibole), enriched in incompatible elements (e.g., LaN/YbN in clinopyroxene up to 15) and having radiogenic Sr and unradiogenic Nd; 3) websterites and wehrlite-bearing samples that record differentiation processes of alkaline melts highly enriched in Th, U and LREE, not yet documented in the erupted volcanics of northern Madagascar. The mantle xenoliths of northern Madagascar show a regional decrease of the equilibration temperature from to SW (up to 1180 °C, Nosy Be Archipelago) to the NE (up to 900 °C, Bobaomby district). A significant lithologic and geochemical variation of the shallow lithospheric mantle beneath northern Madagascar is noted, in contrast with the relatively uniform geochemical and isotopic composition of the host alkali basalt and basanite lavas.
Contrasting thermal structure, melt depletion and metasomatism of mantle lithosphere beneath two Proterozoic terranes west of the Kaapvaal craton, southern Africa.
Abstract: The petrogenesis of kimberlites is commonly obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during fluid interaction and the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa). Sample BK consists of abundant macrocrysts (N1 mm) and (micro-) phenocrysts of olivine and lesser phlogopite, smaller grains of apatite, serpentinised monticellite, spinel, perovskite, phlogopite and ilmenite in a matrix of calcite, serpentine and dolomite. As in kimberlites worldwide, BK olivine grains consist of cores with variable Mg/Fe ratios, overgrown by rims that host inclusions of groundmass phases (spinel, perovskite, phlogopite) and have constant Mg/Fe, but variable Ni, Mn and Ca concentrations. Primary multiphase inclusions in the outer rims of olivine and in Fe-Ti-rich (‘MUM’) spinel are dominated by dolomite, calcite and alkali carbonates with lesser silicate and oxide minerals. Secondary inclusions in olivine host an assemblage of Na-K carbonates and chlorides. The primary inclusions are interpreted as crystallised alkali-Si-bearing Ca-Mg-rich carbonate melts, whereas secondary inclusions host Na-K-rich C-O-H-Cl fluids. In situ Sr-isotope analyses of groundmass calcite and perovskite reveal similar 87Sr/86Sr ratios to perovskite in the Bultfontein and the other Kimberley kimberlites, i.e. magmatic values. The ?18O composition of the BK bulk carbonate fraction is above the mantle range, whereas the ?13C values are similar to those of mantle-derived magmas. The occurrence of different generations of serpentine and occasional groundmass calcite with high 87Sr/86Sr, and elevated bulk carbonate ?18O values indicate that the kimberlite was overprinted by hydrothermal fluids, which probably included a significant groundwater component. Before this alteration the groundmass included calcite, monticellite, apatite and minor dolomite, phlogopite, spinel, perovskite and ilmenite. Inclusions of groundmass minerals in olivine rims and phlogopite phenocrysts show that olivine and phlogopite also belong to the magmatic assemblage. We therefore suggest that the crystallised kimberlite was produced by an alkali-bearing, phosphorus-rich, silica-dolomitic melt. The alkali-Si-bearing Ca-Mg-rich carbonate compositions of primary melt inclusions in the outer rims of olivine and in spinel grains with evolved compositions (MUM spinel) support formation of these melts after fractionation of abundant olivine, and probably other phases (e.g., ilmenite and chromite). Finally, the similarity between secondary inclusions in kimberlite olivine of this and other worldwide kimberlites and secondary inclusions in minerals of carbonatitic, mafic and felsic magmatic rocks, suggests trapping of residual Na-K-rich C-O-H-Cl fluids after groundmass crystallisation. These residual fluids may have persisted in pore spaces within the largely crystalline BK groundmass and subsequently mixed with larger volumes of external fluids, which triggered serpentine formation and localised carbonate recrystallisation.
Abstract: The concept of vertical pit mining has developed as an alternative mining method for the exploitation from surface of small, vertical or near vertical, massive orebodies to depths of approximately 100 m where ore extraction can become uneconomical due to high stripping ratios. Support of the sidewalls is ensured with systematic anchoring depending on the condition of the insitu rock mass together with the installation of mesh and shotcrete in order to prevent small-scale ravelling and spalling which, with the pit at depth, can prove hazardous to operations at the base of the pit. Hoisting of the ore is carried out either by a vertical A Frame hoist at the crest of the pit, which requires tramming of ore at the base of the pit to the hoist. Alternatively, a Blondin Cableway system, which allows for loading of kibbles at any location within the pit, negating the need for a tram to the hoist located on the side of the pit, can be utilised. For various reasons the technique has been successfully applied to the mining of a chrome deposit in Zimbabwe to a depth of 95m and a kimberlite pit in West Africa where the pit was terminated at a depth of 85m due to a sidewall failure. Feasibility studies for two further vertical pits were conducted for orebodies in South Africa and Australia, kimberlite and nickel respectively, but due to economic considerations, have not been mined.
The Archean sedimentary sulfur recycling under the Kaapvaal craton revisited from 4S- isotopic compositions in sulfide inclusions in diamonds from Kimberley Pool.
Abstract: Fibrous diamond growth zones can contain abundant high density fluid inclusions (HDFs) and these provide the most direct information on the source and composition of diamond-forming fluids. Four different fluid end-members have been defined; silicic, low-Mg carbonatitic, high-Mg carbonatitic, and saline. Continuous arrays exist between the silicic and low-Mg carbonatitic end-member and between the saline and high-Mg carbonatitic end-member. Different processes have been proposed to explain the two major element compositional arrays, but the origin of and relation between the various fluid end-members is still uncertain. In this study we provide new constraints on the evolution and origin of these diamond-forming fluids by combining noble gas systematics with ?13C, N concentrations, and fluid inclusion compositions.
Abstract: The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano’s recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490–595 °C, which is 250–450 °C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8–9 wt%, i.e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000–1050 °C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of ?48% calcite, ?12% apatite, and ?2 wt% clinopyroxene. SiO2 solubility only increases from 0.2 to 2.9 wt% at 750–1200 °C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism.
Geochemical Perspectives Letters, Vol. 4, pp. 1-6.
Africa, South Africa, Zimbabwe
metasomatism, geochronology
Abstract: Metasomatism, the chemical alteration of rocks by a variety of melts and fluids, has formed a key concept in studies of the Earth’s mantle for decades. Metasomatic effects are often inferred to be far-reaching and yet the evidence for their occurrence is usually based upon individual hand specimens or suites of rocks that display considerable heterogeneity. In rare cases, however, we are offered insights into larger-scale chemical modifications that occur in the mantle. Here we utilise the Lu–Hf systematics of zircon megacrysts erupted in kimberlite magmas to discern two temporally and compositionally discrete metasomatic events in the mantle beneath southern Africa, each having an influence extending over an area exceeding one million km2. These data provide unambiguous evidence for metasomatic processes operating at continental scales and seemingly unperturbed by the age and composition of the local lithospheric mantle. The most recent of these events may be associated with the major Jurassic-Karoo magmatism in southern Africa.
Journal of Asian Earth Sciences, in press available, 47p.
India, Africa, Madagascar
tectonis
Abstract: It has long been recognised that Madagascar was contiguous with India until the Late Cretaceous. However, the timing and nature of the amalgamation of these two regions remain highly contentious as is the location of Madagascar against India in Gondwana. Here we address these issues with new U-Pb and Lu-Hf zircon data from five metasedimentary samples from the Karwar Block of India and new Lu-Hf data from eight previously dated igneous rocks from central Madagascar and the Antongil-Masora domains of eastern Madagascar. New U-Pb data from Karwar-region detrital zircon grains yield two dominant age peaks at c. 3100 Ma and c. 2500 Ma. The c. 3100 Ma population has relatively juvenile ?Hf(t) values that trend toward an evolved signature at c. 2500 Ma. The c. 2500 Ma population shows a wide range of ?Hf(t) values reflecting mixing of an evolved source with a juvenile source at that time. These data, and the new Lu-Hf data from Madagascar, are compared with our new compilation of over 7000 U-Pb and 1000 Lu-Hf analyses from Madagascar and India. We have used multidimensional scaling to assess similarities in these data in a statistically robust way. We propose that the Karwar Block of western peninsular India is an extension of the western Dharwar Craton and not part of the Antananarivo Domain of Madagascar as has been suggested in some models. Based on ?Hf(t) signatures we also suggest that India (and the Antongil-Masora domains of Madagascar) were palaeogeographically isolated from central Madagascar (the Antananarivo Domain) during the Palaeoproterozoic. This supports a model where central Madagascar and India amalgamated during the Neoproterozoic along the Betsimisaraka Suture.
Geochimica et Cosmochimica Acta, Vol. 213, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and ?18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy ?18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu?, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like ?18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ?3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Earth and Planetary Science Letters, Vol. 474, pp. 283-295.
Africa, South Africa
deposit - Lace
Abstract: Reconstructing the redox state of the mantle is critical in discussing the evolution of atmospheric composition through time. Kimberlite-borne mantle eclogite xenoliths, commonly interpreted as representing former oceanic crust, may record the chemical and physical state of Archaean and Proterozoic convecting mantle sources that generated their magmatic protoliths. However, their message is generally obscured by a range of primary (igneous differentiation) and secondary processes (seawater alteration, metamorphism, metasomatism). Here, we report the Fe3+/?Fe ratio and ?18 O in garnet from in a suite of well-characterised mantle eclogite and pyroxenite xenoliths hosted in the Lace kimberlite (Kaapvaal craton), which originated as ca. 3 Ga-old ocean floor. Fe3+/?Fe in garnet (0.01 to 0.063, median 0.02; n = 16) shows a negative correlation with jadeite content in clinopyroxene, suggesting increased partitioning of Fe3+ into clinopyroxene in the presence of monovalent cations with which it can form coupled substitutions. Jadeite-corrected Fe3+/?Fe in garnet shows a broad negative trend with Eu*, consistent with incompatible behaviour of Fe3+ during olivine-plagioclase accumulation in the protoliths. This trend is partially obscured by increasing Fe3+ partitioning into garnet along a conductive cratonic geotherm. In contrast, NMORB-normalised Nd/Yb - a proxy of partial melt loss from subducting oceanic crust (<1) and metasomatism by LREE-enriched liquids (>1) - shows no obvious correlation with Fe3+/?Fe, nor does garnet ?18OVSMOW (5.14 to 6.21‰) point to significant seawater alteration. Median bulk-rock Fe3+/?Fe is roughly estimated at 0.025. This observation agrees with V/Sc systematics, which collectively point to a reduced Archaean convecting mantle source to the igneous protoliths of these eclogites compared to the modern MORB source. Oxygen fugacites (fO2) relative to the fayalite-magnetite-quartz buffer (FMQ) range from ?log ? fO2 = FMQ-1.3 to FMQ-4.6. At those reducing conditions, the solubility of carbon in the fluids released by dehydration is higher than in fluids closer to FMQ. The implication is that Archean processes of C transport and deposition would have differed from those known in modern-style subduction zones, and diamond would have formed from methane-rich fluids. In addition, such reducing material could drive redox melting or freezing upon deep recycling and migration of CH4-bearing fluids into the ambient mantle.
Journal of Structural Geology, Vol. 86, pp. 47-61.
Africa, South Africa
deposit - Venetia
Abstract: The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo™ provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.
Abstract: The nitrogen isotopic composition of mantle samples lie between -15 to -5 ‰ and, is different from subducted sediments that varies between +6 to +15 ‰. As a result, N can be an excellent tracer of a subducted component [e.g. 1]. We have studied N from Indian carbonatites of adjacent locations in Hogenakal (2700 Ma) and Sevattur (770 Ma) with established mantle origin. We have also studied two diamonds representative of the Witwatersrand basin (2900–2700 Ma) and Jagersfontein (1100-1700 Ma) belonging to the Kapvaal supergroup. Both India and the Kapvaal craton formed part of a supercontinent that persisted as coherent units until the breakup of Pangea. Comparing the N in these diamonds with the carbonatites will put constraints on differences in their sources and the role of subduction in their generation. The nitrogen isotopic composition of the carbonatites are comparable ranging between -3 to +13 ‰, although value as low as -22 ‰ is observed from an apatite from Hogenakal. The N contents vary from 130 to 6000 ppb. Of the diamonds, Jagersfontein have ?15N of +2.3 and +9.3 ‰, while those from the Witwatersrand basin are –2.3 and 0 ‰. Their nitrogen lie between 500-900 ppm. These results have important implications for their formation and sources.
Earth and Planetary Science Letters, Vol. 474, pp. 215-225.
Africa, Algeria
diamond inclusions
Abstract: Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U-Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 ?m in size) analyzed in situ provide a U-Pb age of 32.4 ± 3.3 Ma interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (?30 ?m) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740-810 °C, most likely during HP retrograde metamorphism. U-Pb analyses provide a significantly younger age of 20.7 ± 2.3 Ma attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica-Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.
Abstract: Carbonatites are the primary source of LREE worldwide. Here we describe evidence from the Okorusu mine in NorthCentral Namibia, based on results from a suite of techniques including SEM-EDS and SEM-CL imaging, EPMA, LA-ICPMS on minerals and fluid inclusions, bulk rock chemistry and microthermometry. This provides indications of hydrothermal reworking in a carbonatite-related REE deposit. The Okorusu deposit is part of a ring complex consisting of syenites, nepheline syenites, and carbonatite with hydrothermal fluorite ore mineralisation formed principally by replacing carbonatite bodies. The primary carbonatites show a typical LREE enriched pattern. Primary REE mineralisation is contained in the magmatic phases apatite, pyrochlore and calcite. These phases have been partially broken down by hydrothermal activity. Most of the REE in the carbonatite samples now occur in secondary hydrothermal phases, mainly synchysite-(Ce). The REE occur also as synchysite-(Ce) in the hydrothermal fluorite but additionally they are incorporated into the fluorite structure resulting in cathodoluminescence zoning. Fluid inclusions are observed in both magmatic phases (apatite, calcite and clinopyroxene) and in hydrothermal phases (fluorite, calcite and quartz). The fluid inclusions associated with secondary REE mineralisation in fluorite consist of liquid-vapour inclusion with a constant liquid/bubble ratio and often a small daughter mineral. This suggests that the REE were transported by a relatively concentrated aqueous fluid. Fluid and melt inclusions hosted in the magmatic phases show a wider range in composition. The Okorusu carbonatite deposit shows primary and secondary features common to carbonatite deposits worldwide, and so the results reported here may be of wider significance.
Abstract: Ihouhaouene area in In Ouzzal terrane (Hoggar, South Algeria) is exceptional by numerous carbonatite complexes systematically associated to syenites. They constitute one of the oldest carbonatite emplaced at 2 Ga. Various types of carbonatites are distinguished by their successive placement and pegmatitic to brecciated appearance. The first-generation of carbonatites are always brecciated with elements of syenite and carbonate cement with calcite, apatite, alkali feldspar, wollastonite, clinopyroxene +/- sphene, allanite, quartz and garnet. Late carbonatite intrusions appear in small pegmatitic veins rich in apatite (3-50 mm). All carbonatites are calciocarbonatites (38-50 wt% CaO) with silica content ranging from 5 to 21 wt% SiO2. The high silica content is interpreted as assimilation of syenite material during emplacement. Carbonatites have high Rare Earth Element (REE) concentrations with high Ligh REE/Heavy REE fractionation (e.g. 1088 ppm La, La/Yb= 144-198) and variable concentrations in Th (26.5-197 ppm). The REE concentrations are mainly controlled by apatite phenocrysts (30-40 vol.%) with 4-9 wt% REE. In late pegmatitic carbonatite, REE-rich apatites are green-yellow phenocrysts with britholite exsolution (up to 40 vol.%, Ca4(REE)6 (SiO4,PO4)6 (OH,F,Cl)2). Britholites are hexagonal and occur as fine lamellar exsolutions (<10 um) in the same crystallographic axis (001) than apatites or as irregularshaped grains (10-200 um). All britholites contain 8-16 wt% La, 21-43 wt% Ce and 7-12 wt% Nd. The apatite-britholite exsolutions correspond to a substitution of the trivalent rareearth elements (REE3+) and Si4+ for Ca2+ and P5+. The REE substitution is accompanied by a change in volatile composition with F-rich apatite and Cl-rich britholite indicating that Si and Cl-rich hydrothermal fluids are present at the late stage of carbonatite evolution leading to REEenrichment and the crystallization of REE minerals.
Abstract: The Pilanesberg Alkaline Complex (South Africa) consists of a partially eroded phonolitic-trachytic package of lavas and tuffs, intruded by consanguinous syenites and nepheline syenites (foyaites). The latter have been divided in several units, based on their colour and mineralogy. Most of the foyaitic units are sodic in composition, but whole rock analyses show that some samples are more potassic, with Na2O/K2O<0.8. This observation, together with old reports of leucite-bearing lavas [1], could suggest the existence of a second, potassic magmatic lineage. To investigate whether the observed potassium-enrichment is a primary feature, or the result of deuteric alteration, the mineralogical distinction between sodic and potassic samples was investigated. The mineralogy of the sodic samples is dominated by nepheline, alkali-feldspar and aegirine, ± titanite, amphibole, biotite, and late agpaitic phases [2]. Within the potassic samples, the main primary ferromagnesian mineral is biotite, which shows conspicuous zoning in thin section; nepheline has been extensively replaced by sodalite and cancrinite, but alkali-feldspar appears relatively unaltered. No agpaitic minerals were observed. U-Pb isotope systematics of titanite are similar for sodic and potassic samples in terms of the age (ca. 1.4 Ga) and composion of common Pb; Ar-Ar dating of biotite also gives ca. 1.4 Ga, showing that biotite is a primary magmatic phase. Compositions of the biotite in sodic and potassic samples are similar, with the sodic samples having slightly higher Fe# (independent of whole rock Fe#), higher Na, but lower (Na+K) and Ba. Zoning in biotite from potassic samples is related to a decrease in Mg, Ti and F in the rim of the crystals. Despite the primary character of the biotite, the question whether the potassic samples reflect a combination of alteration and perhaps minor crustal contamination, or a separate mag
Feneyrol, J., Giuliani, G., Demaiffe, D., Ohenstetter, D., Fallick, A.E., Dubessy, J., Martelet, J-E., Rakotondrazafy, A.F.M., Omito, E., Ichangi, D., Nyamai, C., Wamunyu, W.
Abstract: The genetic model previously proposed for tsavorite- (and tanzanite-) bearing mineralization hosted in the Neoproterozoic Metamorphic Mozambique Belt (stretching from Kenya through Tanzania to Madagascar) is refined on the basis of new Sm-Nd age determinations and detailed Sr-O-S isotope and fluid-inclusion studies. The deposits are hosted within meta-sedimentary series composed of quartzites, graphitic gneisses, calc-silicate rocks intercalated with meta-evaporites, and marbles. Tsavorite occurs either in nodules (also called “boudins”) oriented parallel to the metamorphic foliation in all of the deposits in the metamorphic belt or in quartz veins and lenses located at the hinges of anticlinal folds (Lelatema fold belt and Ruangwa deposits, Tanzania). Gem tanzanite occurs in pockets and lenses in the Lelatema fold belt of northern Tanzania. The Sm-Nd isotopic data for tsavorites and tanzanites hosted in quartz veins and lenses from Merelani demonstrate that they formed at 600 Ma, during the retrograde metamorphic episode associated with the East African Orogeny. The tsavorites hosted in nodules do not provide reliable ages: their sedimentary protoliths had heterogeneous compositions and their Sm-Nd system was not completely rehomogenized, even at the local scale, by the fluid-absent metamorphic recrystallization. The initial 87Sr/86Sr isotopic ratios of calcite from marble and tanzanites from Merelani fit with the strontium isotopic composition of Neoproterozoic marine carbonates. Seawater sediment deposition in the Mozambique Ocean took place around 720 Ma. The quartz-zoisite O-isotopic thermometer indicates a temperature of formation for zoisite between 385 and 448 °C. The sulfur isotopic composition of pyrite (between –7.8 and –1.3‰ V-CDT) associated with tsavorite in the Lelatema fold belt deposits suggests the contribution of reduced marine sulfate. The sulfur in pyrite in the marbles was likely derived from bacterial sulfate reduction which produced H2S. Fluid inclusion data from tsavorite and tanzanite samples from the Merelani mine indicate the presence of a dominant H2S-S8±(CH4)±(N2)±(H2O)-bearing fluid. In the deposits in Kenya and Madagascar, the replacement of sulfate by tsavorite in the nodules and the boron isotopic composition of tourmaline associated with tsavorite are strong arguments in favor of the participation of evaporites in garnet formation.
Abstract: Carbonatites are rare magmas containing almost no silica; the corresponding crystallized rocks represent the main rare earth elements (REE) deposits in production. Oldoinyo Lengai (Tanzania) is the only active carbonatite volcano on Earth, and may be used as a natural laboratory to identify the parameters responsible for the genesis of the >500 reported fossil occurrences of carbonatite magmas. Nevertheless the carbonatites emitted at Oldoinyo Lengai are unique as alkalirich (natrocarbonatites), and their origin may not be representative of the fossil carbonatites (calcio-, ferro-, magnesio-carbonatites). Here we use three-phases melt inclusions trapped in clinopyroxenes and nephelines from cognate cumulates – that sample the active magma chamber of Oldoinyo Lengai – emitted during the 2007-08 sub-plinian explosive eruption to track the carbonatite presence within the plumbing system, and to eventually quantify its composition at depth. We show that although natrocarbonatites are emitted at Oldoinyo Lengai summit, more classical calciocarbonatites are present at magma chamber depth, consistent with rare natrocarbonatites being derived from more classical calciocarbonatites by further magma differentiation. Those unique cognate samples allows us to provide the first direct measurements of partition coefficients for major and trace elements of natural coexisting in equilibrium carbonatite and silicate melts. Partition coefficients suggests that natrocarbonatites derive from calciocarbonatites by fractionating Ca-rich, and Na-poor phases. The Oldoinyo Lengai can therefore be used as a perfect analogue of fossil igneous systems that are now exhumed, commonly lacking any relation with the initial geodynamic setting, and form REE mineral deposits.
Abstract: The widespread occurrence of pyrope garnet in Archean lithospheric mantle remains one of the ‘holy grails’ of mantle petrology. Most garnets found in peridotitic mantle equilibrated with incompatible-trace-element-enriched melts or fluids and are the products of metasomatism. Less common are macroscopic intergrowths of pyrope garnet formed by exsolution from orthopyroxene. Spectacular examples of these are preserved in both mantle xenoliths and large, isolated crystals (megacrysts) from the Kaapvaal craton of southern Africa, and provide direct evidence that some garnet in the sub-continental lithospheric mantle formed initially by isochemical rather than metasomatic processes. The orthopyroxene hosts are enstatites and fully equilibrated with their exsolved phases (low-Cr pyrope garnet?±?Cr-diopside). Significantly, P-T estimates of the post-exsolution orthopyroxenes plot along an unperturbed conductive Kaapvaal craton geotherm and reveal that they were entrained from a large continuous depth interval (85 to 175?km). They therefore represent snapshots of processes operating throughout almost the entire thickness of the sub-cratonic lithospheric mantle. New rare-earth element (REE) analyses show that the exsolved garnets occupy the full spectrum recorded by garnets in mantle peridotites and also diamond inclusions. A key finding is that a few low-temperature exsolved garnets, derived from depths of ?90?km, are more depleted in light rare-earth elements (LREEs) than previously observed in any other mantle sample. Importantly, the REE patterns of these strongly LREE-depleted garnets resemble the hypothetical composition proposed for pre-metasomatic garnets that are thought to pre-date major enrichment events in the sub-continental lithospheric mantle, including those associated with diamond formation. The recalculated compositions of pre-exsolution orthopyroxenes have higher Al2O3 and CaO contents than their post-exsolution counterparts and most probably formed as shallow residues of large amounts of adiabatic decompression melting in the spinel-stability field. It is inferred that exsolution of garnet from Kaapvaal orthopyroxenes may have been widespread, and perhaps accompanied cratonization at ?2.9 to 2.75 Ga. Such a process would considerably increase the density and stability of the continental lithosphere.
Abstract: The Palabora Carbonatite Complex (PCC) in South Africa intruded at 2060 Ma into Archean basement. The tripartite pipe-like intrusion is represented by a northern and southern pyroxenite and the central Loolekop pipe. Carbonatites and phoscorites of the Loolekop pipe experienced at least 4 stages of mineralization, recrystallization and redistibution reflected by an (1) orthomagmatic, (2) late-magmatic, (3) sulphide and (4) post-magmatic phase (Giebel et al., 2017). These four stages exhibit considerable variability of REE mineralization and especially stages 2 and 4 show intense fluid-rock interaction textures. We present microtextural and compositional data on apatite and phlogopite along a 2 km depth profile through the Loolekop pipe and investigate how these data reflect fluidmineral interaction with depth during stage (2). A special focus lies on understanding the behaviour, sources and sinks of REE elements. While fluid-apatite interaction causes a dissolution of apatite coupled with a precipitation of monazite at apatite rims, the fluid-phlogopite interaction induces a chloritization of phlogopite and an occasional formation of britholite along strongly dissolved phlogopite rims. We suspect that REE are transported into the system by this late-magmatic fluid rather than being released by the dissolution of orthomagmatic REE-bearing minerals. Combining these observations with fluid inclusion textures and microthermometry, we will investigate the nature and composition of the involved fluids and will try to model REE mineralisation processes during late-magmatic fluidmineral ineraction
Abstract: The origin of intraplate carbonatitic to alkaline volcanism in Africa is controversial. A tectonic control, i.e., decompression melting associated with far-field stress, is suggested by correlation with lithospheric sutures, repeated magmatic cycles in the same areas over several million years, synchronicity across the plate, and lack of clear age progression patterns. Conversely, a dominant role for mantle convection is supported by the coincidence of Cenozoic volcanism with regions of lithospheric uplift, positive free-air gravity anomalies, and slow seismic velocities. To improve constraints on the genesis of African volcanism, here we report the first radiometric and isotopic results for the Catanda complex, which hosts the only extrusive carbonatites in Angola. Apatite (U-Th-Sm)/He and phlogopite 40Ar/39Ar ages of Catanda aillikite lavas indicate eruption at ca. 500–800 ka, more than 100 m.y. after emplacement of abundant kimberlites and carbonatites in this region. The lavas share similar high-? (HIMU)–like Sr-Nd-Pb-Hf isotope compositions with other young mantle-derived volcanics from Africa (e.g., Northern Kenya Rift; Cameroon Line). The position of the Catanda complex in the Lucapa corridor, a long-lived extensional structure, suggests a possible tectonic control for the volcanism. The complex is also located on the Bié Dome, a broad region of fast Pleistocene uplift attributed to mantle upwelling. Seismic tomography models indicate convection of deep hot material beneath regions of active volcanism in Africa, including a large area encompassing Angola and northern Namibia. This is strong evidence that intraplate late Cenozoic volcanism, including the Catanda complex, resulted from the interplay between mantle convection and preexisting lithospheric heterogeneities.
Abstract: Metasomatism by fluid or melt is commonly attributed as the cause of chemical and modal heterogeneity observed in peridotite xenoliths from the sub-continental lithospheric mantle. Documented manifestations are (1) perturbation of the oxygen fugacity (fO2), which may affect the stability of carbon-bearing phases, and (2) trace-element enrichment, typified by the shape of REEN patterns. Garnet, which contains Fe2+ and Fe3+ in measurable quantities, and exhibits prominent variation in REEN patterns between samples, may record the metasomatic history of the mantle. Here we report variations of fO2 and trace element concentrations for a suite of 22 garnet-bearing peridotite xenoliths from the Louwrensia kimberlite, south-central Namibia. The xenoliths span an estimated pressure range between 2.7 and 4.5 GPa. Fe3+/?Fe of garnet was determined by Fe K-edge XANES spectroscopy. Concomitant fO2 was calculated using the oxybarometer calibration of Miller et al. [1]. The trace element concentrations of all phases were determined by LA-ICP-MS. A global dataset comprising 454 garnet REEN patterns from 19 kimberlites has been compiled. The REEN pattern of each sample was fit to orthogonal polynomial functions that parameterise the abundance, slope, quadratic curvature, and cubic curvature [2]. Quadratic and cubic curvature correlate with abundance, albeit with considerable scatter. There is, however, an absence of correlation between REEN patterns and fO2, depth, or modal abundance. This is in contrast to correlations and trends observed for basaltic melts that clearly identify petrogenetic trends. The partitioning of REEs between garnet and co-existing phases in these samples highlights pronounced trace-element disequilibrium and hence question the validity of considering garnet REEN in isolation as a means of discerning metasomatic history
South America, Brazil, Africa, Central African Republic
carbonado
Abstract: Carbonado diamond is found only in Brazil and the Central African Republic. These unusual diamond aggregates are strongly bonded and porous, with melt-like glassy patinas unlike any conventional diamond from kimberlites-lamproites, crustal collisional settings, or meteorite impact. Nearly two centuries after carbonado's discovery, a primary host rock compatible with the origin of conventional diamond at high temperatures and pressures has yet to be identified. Models for its genesis are far-reaching and range from terrestrial subduction to cosmic sources.
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Abstract: Metasomatised regions of Earth’s sub-continental lithospheric mantle potentially represent a large volatile reservoir. Nevertheless, the mechanisms involved in the storage and upward transport of volatiles such as C and S, from the convecting mantle and/or subducting slabs, are poorly understood. We have carried out a systematic microanalytical study of a suite of sulfide-rich mantle peridotites from the Bultfontein diamond mine, Kimberley. EDS mapping of large (>2mm), interstitial base metal sulfides in the Bultfontein xenoliths has identified distinct Ni-rich regions (pentlandite). The Ni-rich sulfides are adjacent to olivine with Ni poor rims (<0.2 wt% NiO). Diffusion profiles between the protolith olivines and adjacent sulfides are used to estimate the timing of the S-rich metasomatic event. The presence of large unequilibrated olivine indicates that Nisulfides were introduced immediately prior to kimberlite emplacement. The calculated composition of melt in equilibrium with metasomatic clinopyroxenes in the Bultfontein sulphide-bearing peridotites shows a strong correlation to high-density carbonatitic fluids trapped in diamonds. This association is extended by the wealth of metasomatic sulfides in the Bultfontein xenoliths. Moreover, Ni-rich sulfides (~25 wt%) are the most common mineral inclusion in peridotitic diamonds, implying that diamonds crystallise from a S-saturated fluid. Many studies attribute the metasomatism at Bultfontein to silicate melts associated with the kimberlite, but we explore the possibility of metasomatism by reactive percolation of a volatile-rich agent with carbonatitic affinity. The S-rich nature of the metasomatism and the correlation with diamond high-density fluids has great implications for the transport of volatiles from the deep mantle to shallow regions of the craton.
Abstract: Polycrystalline diamond aggregates (diamondites) are produced by rapid crystal nucleation caused by extreme carbon supersaturation in mantle fluids. They may form episodically and under variable chemical conditions, providing snapshots of diamond formation in the Earth’s mantle. Diamondites, thus, represent an extreme end member of diamond formation mechanisms, while forming via the same processes and ingredients as the gem-sized diamonds. We present results on a large suite of diamondites from the Venetia mine (South Africa), comprising a complete characterisation of the diamonds and their silicate inclusions and intergrowths. The highlighted characteristic of this sample suite is its heterogeneity in all aspects, from affiliated silicate to diamond composition and texture of the diamond aggregates. The diamond grains in the samples are intergrown with silicates (garnets, clinopyroxenes, phlogopites) comprising a websteritic-eclogitic and a peridotiticpyroxenitic suite of minerals. Diamonds, regardless of their affiliation based on their silicate phases, overlap in carbon and nitrogen composition and have ?13C values between -28 and -8 ‰, ?15N values of 0.8 to 16.3 ‰ and nitrogen contents of 4 to 2329 ppm. The entire range of carbon and nitrogen variability of the suite is also reflected in some individual samples. Cathodoluminescence imaging visualizes different zones in the samples that can be interpreted as different growth events with differing nitrogen contents and ?15N decoupled from ?13C values, in line with the variability off nitrogen aggregation states. Electron backscatter diffraction analyses identify an original texture of randomly intergrown diamond grains that is partly changed by deformation and newly grown smaller diamond grains. The large overall variability suggesting episodic formation of diamondite with nitrogen from crustal sources.
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 731-737.
Africa, Namibia
deposit - Orange River
Abstract: Over the past 80 years, the Namibian diamondiferous marine placer has been studied extensively to develop solutions for mining and sampling challenges. The types of studies include the statistical modelling of the distributions of the stone counts per sample; investigating the relationship between geology and the grade distribution; assessing the quality potential of the entrapment of the available diamond pulse; using predetermined acceptability of barren samples (zero proportion (Zp) samples) to model distributions; optimal sample sizes; and more. During early-stage project evaluation it is more important to find out if a particular area is likely to be above a specific cut-off grade than to focus on sampling for the purpose of accurate resource estimation. Previous work using mixed Poisson and Sichel distributions to model the abundant onshore diamond data has been very successful in modelling the long-tailed nature of these linear beach deposits. The means of these distributions are, however, sensitive to extreme values. Technical and cost constraints prevent a similar scale of sample collection in an adjacent, geologically equivalent, submerged beach environment. A method not sensitive to extreme values is thus required to make early-stage assessments of the likelihood that the grade of a particular target is above a minimum cut-off grade. The Nachman model describes the functional relationship between the mean population density and proportion of barren patches ( Zp) in a patchy environment. A prerequisite for using the Nachman model is that the underlying data must be modelled using a negative binomial distribution (NBD). The case study data is from an analogous area adjacent to the exploration target and meets the NBD requirement. It is thus appropriate to apply the Nachman model. The Nachman model provides an opportunity to use the observed Zpto predict the mean grade for an area at the very early stage of an exploration project. In future, early-stage exploration data from a homogenous geological zone exhibiting characteristics of the Nachman model assumptions can thus be used to rank and target those areas that show potential to be above the minimum required grade cut-off for follow-up sampling and inclusion in the mine planning cycle.
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 745-752.
Africa, South Africa
deposit - Cullinan
Abstract: In 2012, Cullinan Diamond Mine began an expansion programme with the shaft deepening and development of access to the C-Cut 1 block at approximately 839 m below surface. The expansion programme is funded by a combination of bank loans and retained operating profit generated by the mine. Continuous production during deepening of the No. 1 Shaft, which is the rock hoisting shaft, was therefore critical for sustainability and efficiency as well as overall funding of the project. The deepening method, support design and verification, as well as learning outcomes pertaining to the extension of the No. 1 Shaft underneath the existing operating shaft are summarized, with emphasis on the importance of gaining some understanding of the shaft's host rock mass.
Abstract: The use of confocal HR-Raman mapping opens new perspectives in studying melt inclusions. Our major goal is to show advantages of this powerful technique through case studies carried out on alkaline and carbonatite rocks of Kerimasi volcano (East African Rift). Raman spectrometry is one of the few methods that enable qualitative nondestructive analysis of both solid and fluid phases, therefore it is widely used for the identification of minerals and volatiles within melt and fluid inclusions. For better understanding of petrogenetic processes in carbonatite systems it is essential to find all mineral phases in the melt inclusions trapped in intrusive or volcanic rocks. Previous Raman spectroscopic point measurements in melt inclusions revealed the presence of daughter phases (e.g. alkali carbonates, hydrocarbonates) [1] but utilizing Raman mapping on them even provides information on their size, shape and distribution. Raman 3D mapping were applied on unheated multiphase melt inclusions of intrusive and volcanic rocks with high spatial resolution (XY plane < 1 micron) with a depth scan (Z step) as low as 0.5 micron at every XY point, parallel to the surface of the host minerals. Analysis below the surface of the host mineral is especially useful because we can avoid the loss of sensitive (e.g. water soluble) phases and contamination of the melt inclusions, moreover unexposed melt inclusions are suitable for further analytical measurements (e.g. EPMA, microthermometry). By scanning multiple layers 2D or 3D Raman images can be gained, thus we can get an insight into post entrapment crystallization processes that contribute to a more precise description of the evolution of alkaline and carbonatite rocks.
Abstract: Diamond inclusions are the only samples from the mantle transition zone (410-660 km) and the lower mantle. Majoritic garnet is a rare inclusion, limited to pressures of ~8-20 Gpa with Si content being indicative of depth of re-equilibration. These garnet inclusions can therefore provide information on properties of the transition zone such as oxidation state. In this study, we used Synchrotron Mössbauer Source (SMS) to determine the ferric-ferrous ratios of 13 small (30 to 100 micrometers diameter) majoritic inclusions in diamonds from Jagersfontein. The studied inclusions have pyroxenitic affinities [1], with compositions intermediate between typical peridotite and eclogite. They contain 4.62-11.2 wt% CaO, 0.03-0.34 wt% Cr2O3 and Mg# of 0.65-0.81. Minimum pressures for their equilibration using Beyer and Frost [2] barometer are between 8 and 18 GPa with at least 4 of these inclusions being formed in the transition zone. The Fe3+/Fetotal ratios in the garnets increase from 0.08±0.01 to 0.30±0.03 with increasing pressure. These values define a clear extension of the trend apparent in the data from peridotite xenoliths crystallised at lower pressures. Thermodynamic calculations suggest that these high ferric contents correspond to oxygen fugacities above the FeFeO (IW) buffer, which means that the high Fe3+ contents were not generated by disproportionation of Fe2+ to Fe3+ and Fe0 . It is more likely that carbonate was the oxidising agent responsible for generating the high Fe3+ of these garnets.
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 723-731.
Africa, Lesotho
deposit - Letseng
Abstract: The steepening of slope angles on an open pit mining operation has a material impact on improving the economics of mining. Steepening of slope angles can also increase the risk of slope failure. Slope failures are inherently costly events, because they can be catastrophic, resulting in multiple fatalities, equipment damage, and temporary or permanent closure of a mine. The steepening of the basalt slope angles at Letseng Diamond Mine followed operational improvements that were introduced through improved blasting practices and geotechnical controls. The steeper slope design resulted in a 6 Mt/a reduction in the peak waste mining compared with the previous mine plan coupled with an increase in the net present value and life of mine. This paper is an outline of the steps that were taken at Letseng to increase slope angles in waste and the resulting improvements to the mine plan.
South African Institute of Mining and Metallurgy, Vol. 116, 4, pp. 343-350.
Africa, South Africa
deposit - Cullinan
Abstract: Near-infrared (NIR) spectroscopy sorting technology is incorporated in an automated optical mineral sorter that can discriminate between materials using the differences in characteristics when exposed to near-infrared radiation. During September 2014 to April 2015, a pilot plant that utilized NIR technology to discriminate between kimberlite and waste materials was commissioned to determine the viability of including this technology in the diamond winning process flow sheet at Cullinan Diamond Mine. The plant was used to minimize the waste content in the size fraction -70+35 mm that reports to the crushing section and then to the dense media separation process. This paper describes the initial test work, conducted at Mintek, that led to the decision to conduct a pilot-scale study. The mineralogical characterization of the feed and product streams to establish the sorting criteria and the operational data obtained during the pilot plant campaign are described. The results indicated a good possibility of discriminating between the kimberlite and waste material using NIR technology. However, the consistency of discrimination was not good enough to avoid the risk of potential diamond loss. Furthermore, a lower than expected availability of the machine reduced the throughput capabilities.
Mollex, G., France, L., Furi, E., Bonnet, R., Botcharnikov, R.E., Zimmermann, L., Wilke, S., Deloule, E., Chazot, G., Kazimoto. E.O., Marty, B., Burnard, P.
Abstract: Cognate xenoliths that have been emitted during the last sub-plinian eruption in 2007-08 at Oldoinyo Lengai (OL) represent a unique opportunity to document the igneous processes occuring within the active magma chamber. Detailed petrographic descriptions coupled to a thermobarometric approach, and to the determination of volatile solubility models, allow us to identify the melt evolution at magma chamber conditions, and the storage parameters (P, T). Results indicate that a fresh phonolite melt (~1060°C) was injected into a crustal magma chamber at 11.5 ±3.5 km depth, in agreement with geophysical surveys performed during the eruption. The phonolite contains high volatile contents: 3.2 wt.% H2O and 1.4 wt.% CO2. The liquid line of descent highlights an evolution to nephelinite compositions by cooling down to 880°C. Our results support previous results related to this eruption, and are similar to the historical products emitted during the whole volcano history, allowing us to suggest that no major modification in the plumbing system has occured during the OL evolution. New noble gas results show that: i. fumaroles display constant He isotopic signature since 1988; ii. Cognate xenoliths documenting the active magma chamber and fumaroles display similar He isotopic values (6.58±0.46RA, and 7.31±0.40RA, respectively); iii. OL He isotopic composition is similar to that of other silicate volcanoes of the Arusha region, and comparable to the typical subcontinental lithospheric mantle (SCLM) range (5.2 to 7.0 RA); iv. Ne isotopic ratio of OL is following the MORB signature. Those results are interpreted as showing that 1/ no major modification in the hydrothermal system architecture has occured since 1988 despite major modification of the summit crater morphology, 2/ no contamination by either the atmospheric gases, or crustal material assimilation has occured between the magma chamber and the surface, and 3/ the source of OL and of the other silicate volcanoes in the Arusha region is a SCLM metasomatized by asthenospheric fluids.
Abstract: The 525 Ma Marinkas Quellen (MQ) Complex of southern Namibia, part of the Kuboos-Bremen Line (KBL) of alkaline igneous centers [1] consists of granites, nepheline syenites and carbonatites and is the only carbonatite locality in the KBL [1]. MQ carbonatite variteties include calciocarbonatites, magnesiocarbonatites and ferrocarbonatites. The enrichments in Ba, Nb and the REE vary widely in the carbonatites, with La ranging from 45 to 11154 ppm. All the carbonatites are characterised by large Zr, Hf, Ti depletions. Zr/Hf ratios ranges from 40 to 500, all greater than the chondritic value of 36. Such large Zr/Hf fractionations are often associated with carbonatite metasomatism. The values of carbon and oxygen isotope ratios of bulk carbonate in Marinkas Quellen carbonatites vary significantly (e.g., ?13C = -3.95 to -6.02‰; ?18 O = 8.84 to 22.22‰). The carbon isotope compositions are in the mantle range, while the oxygen isotope values extend to higher than typical mantle values, presumably due to interaction with hydrous fluids. All but two of the carbonatite samples have initial 87Sr/86Sr ratios falling in the range of 0.70236 to 0.70408. Of the remaining samples, one, a ferrocarbonatite, has a higher value of 0.70503 that is likely due to contamination by the surrounding rock or assimilation in the lower crust or Sr exchange with groundwater. The other, a magnesiocarbonatite, appears to have experienced an increase in its Rb/Sr ratio due to alteration, resulting in an over-corrected initial 87Sr/86Sr value. The relatively low Sr isotope ratios of most samples, plus their HNd(t) values (+3.9 to +4.8) values suggest that the carbonatite magma was generated from a long-lived low Rb/Sr, high Sm/Nd, relatively depleted mantle source. The radiogenic Pb isotope composition of the carbonatites (206Pb/204Pbi ratios from 18.06 to 22.38), suggests a high U/Pb source, akin to the HIMU mantle end member. This points to a sub-lithospheric (asthenospheric) source with only a relatively minor contribution from enriched lithospheric mantle
Abstract: North Tanzanian Divergence is the first stage of continental break-up of East African Rift and one of the most concentrated areas of carbonatite magmatism with Natron basin in the North (2 Ma-present - Lengai) and Manyara basin in the southern part (0.4-0.9 Ma). The Manyara basin has volcanic activities with mafic magmas as melilitites (Labait), Mg-nephelinites and carbonatite (Kwaraha), and more differentiated magmas as Mg-poor nephelinites (Hanang) recording deep magmatic environment and differentiation in the crust of CO2-rich alkaline magmas. Melilitite and Mg-nephelinite with olivine-cpx-phlogopite record mantle environment at 1.5 GPa-1300°C with water content in melt of 0.1- 0.4 wt% H2O (1-4 ppm in olivine, FTIR). Although fractional crystallization can be considered as an important process during ascent, leading to Mg-poor nephelinite with cpx-melanite-nepheline, complex zonation of cpx (e.g. abrupt change of Mg#, Nb/Ta, and H2O) recorded open system with multiple carbonate-rich silicate immiscibility and melilititic melt replenishment. The low water content of cpx (25 ppm H2O; FTIR) indicates that 0.3 wt% H2O was present during carbonate-rich nephelinite crystallization at crustal level (600 MPa - 1050°C). The interstitial melt entrapped as melt inclusions (MI) in nepheline evolved to CO2-rich and H2O-poor phonolitic composition with 6 wt% CO2 and 1 wt% S at logfO2=FMQ+1 to 1.5 (Fe3+/?Fe=0.3 - S6+/?S=0.55, XANES). At 200 MPa, phonolitic melt in MI reaches carbonate saturation and immiscibility process leads to trachytic melt with high CO2, S and halogen content (0.43 wt% CO2, SIMS; 2 wt% S, 0.84 wt% Cl; 2.54 wt% F) and very low H2O content (<0.1wt%, Raman) and an anhydrous Ca-Na±S,K carbonate liquid. The Ca-Na carbonatitic liquid in Mg-poor nephelinite represents an early stage of the evolution path towards carbonatitic magmatism as observed in Kwaraha and Lengai. Manyara volcanism has similarities with the Natron volcanism with multistage evolution and silicate-carbonatite magmatism but differ by their volatile components (up to 10 H2O wt% in Lengai nephelinite). This can be interpreted in term of depth of partial melting with H2O-CO2 lithospheric mantle source (Natron) and deep anhydrous CO2-rich asthenospheric mantle source in the southern part of rift initiation (Manyara) and percolation of deep CO2-rich silicate liquid leading to lithospheric mantle metasomatism.
Abstract: Despite advances in understanding the evolution of the West African Craton (WAC), much debate still hinges over its geodynamic evolution. In the case of the Paleoproterozoic Baoulé-Mossi domain, part of the problem is that most studies are localized and fail to present large-scale correlations. To address this, we present the integrated results of zircon U-Pb geochronology, O and Hf isotopes, and whole-rock geochemistry datasets obtained from felsic intrusions across Ghana, Burkina Faso, Mali and Guinea. Combining the new and existing U-Pb age data reveals that magmatism was continuous for approximately 150 Myrs, and involved migration of a magmatic front from east to west. Mafic and felsic magmatism was coeval, and a small amount of inherited zircons as old as 3.5 Ga were identified. The Hf- isotope data reveal the existence of two different crustal domains, with subtlely different Hf isotope signatures. These data also hint at a crustal source component potentially as old as 2.8 Ga in some rocks. The two isotopic domains are separated by a less radiogenic sliver of crust across the Banfora Belt. O isotope compositions (zircon d18O from 6.5 to 11 ‰), corroborate evidence for a crustal source. Geochemical data from felsic intrusions indicates that the studied rocks are not TTG equivalents as previously described but are more akin to modern granitic intrusions. The intrusions are either two mica (muscovite/biotite) or one mica (biotite), and some are amphibole rich. These intrusions are mainly calc-alkaline and magnesian and range from metaluminous (SiO2<65%) to peraluminous (SiO2>65%). They predominantly show arc-type trace element signatures. The combination of the different spatially extensive data sets favors a plate tectonic regime for the period between 2.3 and 2.0 Ga. Under this scenario, two, predominately juvenile, crustal blocks developed in an arc-type setting. This evolved into a continental arc-type setting and later, upon the indentation of the Archean Kenema-Man domain at 2.1-2.0 Ga, into a continent-continent collisional setting.
Abstract: Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2 °C/km to reach peak conditions of 5.8 GPa and 890 °C for the kyanite eclogite. The kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak pressure (5.3 GPa) and was associated with a rise in temperature to 1380 °C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. The ?18O values of garnets are consistently higher than normal mantle values. Each sample has its characteristic trend of ?18O variance between garnets in the kyanite-bearing sections and those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic lithosphere. As kyanite contains around 100 ppm of H2O it is suggested that the kyanite-out reaction, once initiated by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite. The steep (high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of eclogites into the lithospheric mantle. Such a trajectory is best explained in a model where strong lateral compression forces eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and compression.
Geochimica et Cosmochimica Acta, in press available, 22p.
Africa, South Africa
deposit - Venetia
Abstract: Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533 Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n = 13) display an approximately isochronous relationship and fall on a 3.28 ± 0.17 Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35–3.28 Ga. The group I samples derive from ?50 to ?170 km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on the formation and subsequent evolution of Venetia lithosphere. First, melt depletion and remobilisation of Re and Os must have occurred within error of the 3.28 Ga mean TMA age. Second, the refractory peridotites contain significant Re despite recording >40 % melt extraction. Third, assuming that Si-enrichment and Re-Os mobility in the Venetia lithospheric mantle were linked, this process must have occurred within ?100 Myr of initial melt depletion in order to preserve the isochronous relationship. Based on the regional geological evolution, we propose a rapid recycling model with initial melt depletion at ?3.35 Ga to form a tholeiitic mafic crust that is recycled at ?3.28 Ga, resulting in the intrusion of a TTG suite and Si-enrichment of the lithospheric mantle. The non-zero primary Re contents of the Venetia xenoliths imply that TRD model ages significantly underestimate the true depletion age even for highly depleted peridotites. The overlap of the ?2.6 Ga TRD ages with the time of the Kaapvaal-Limpopo collision is purely fortuitous and has no geological significance. Hence, this study underlines the importance of scrutiny if age information is to be derived from whole rock Re-Os analyses.
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 709-714.
Africa, Botswana
deposit - Karowe
Abstract: Historically, the recovery of large diamonds in conventional treatment plant flow sheets has been associated with dense media separation (DMS). This is attributed mainly to DMS's highly efficient and proven track record in the concentration and separation of ores with variable solids densities. In most instances, DMS has been utilized as a pre-concentration step ahead of any recovery plant, due to its ability and versatility in reducing feed within a specific size range to manageable volumes for downstream X-ray processing and subsequent diamond recovery. The benefit of using carbon-signature-based detection equipment for retrieving large stones upfront in the flow sheet not only equates to earlier recovery of diamonds from the system, but also lessens the exposure of diamond-bearing ore to additional materials handling, pumping, and/or crushing, which has been known to damage or even break diamonds and decrease revenue.
Abstract: The geochemistry and mineralogy of REE deposits is diverse, from carbonatite-related deposits, alkaline rocks, mineral sands and ion adsorption clays to potential by-products of phosphate and bauxite, and reuse of waste materials. Despite the large number of prospects that have been explored recently, very little additional REE production has started. A major challenge is to design effective, cost-efficient and environmentally-friendly processing and extraction. Processing flow sheets have to be constructed carefully for each deposit. Translating geochemistry and mineralogy studies, including quantitative mineralogy results, into processing characteristics can be illustrated using results from the Songwe Hill carbonatite, Malawi. Combining results with other published data then allows us to make some general conclusions about the common REE ore minerals and their geological environment, including the REE fluorcarbonate series, monazite and xenotime. The use of chemicals for REE extraction is often the largest environmental burden to mitigate. A new issue is that certain REE, such as Ce, are in oversupply, and are not being recovered in some proposed processing flowsheets. It will be important to understand the environmental and commercial implications of this development.
Geochimica et Cosmochinica Acta, Vol. 213, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and ?18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy ?18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu?, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like ?18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ?3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Geochimica et Cosmochinica Acta, Vol. 215, pp. 432-446.
Africa, Zimbabwe
craton
Abstract: Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9–2.8 Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon ?18O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In ?Hf-time space, 3.8–3.6 Ga grains define an array consistent with reworking of a mafic reservoir (176Lu/177Hf ?0.015) that separated from chondritic mantle at ?3.9 Ga. Crustal domains formed after 3.6 Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35 Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral accretion of arc-related crustal blocks until 3.35 Ga.
Journal of Volcanology and Geothermal Research, Vol. 322, pp. 14-25.
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: Volcanic ash leachate studies have been conducted on various volcanoes on Earth, but few have been done on African volcanoes until now. Tephra emissions may affect the environment and the health of people living in this area, and therefore we conducted a first tephra (ash and lapilli sized) leachate study on the Oldoinyo Lengai volcano, situated in northern Tanzania. The recent explosive eruption in 2007-2008 provided us with fresh samples from the first three weeks of the eruption which were used for this study. In addition, we also used a natrocarbonatitic sample from the activity prior to the explosive eruption, as the major activity at Oldoinyo Lengai is natrocarbonatitic. To compare the leaching process affecting the natrocarbonatitic lavas and the tephras from Oldoinyo Lengai, the 2006 natrocarbonatitic lava flow was resampled 5 years after the emplacement and compared to the initial, unaltered composition. Special interest was given to the element fluorine (F), since it is potentially toxic to both humans and animals. A daily intake of fluoride (F?) in drinking water of > 1.5 mg/l can lead to dental fluorosis, and higher concentrations lead to skeletal fluorosis. For this reason, a guideline value for fluoride in drinking water was set by the WHO (2011) to 1.5 mg/l. However, surface waters and groundwaters in the Gregory Rift have elevated fluoride levels of up to 9.12 mg/l, and as a consequence, an interim guideline value for Tanzania has been set at 8 mg/l. The total concentration of fluorine in the samples from the natrocarbonatitic lava flow is high (3.2 wt%), whereas we observed a significant decrease of the fluorine concentration (between 1.7 and 0.5 wt%) in the samples collected three days and three weeks after the onset of the explosive 2007-08 eruption. However, the total amount of water-extractable fluoride is lower in the natrocarbonatitic lavas (319 mg/l) than in the nephelinitic tephra (573-895 mg/l). This is due to the solubility of the different F-bearing minerals. In the natrocarbonatites, fluorine exists predominantly in fluorite (CaF2), and in the early tephra as Na-Mg bearing salts such as neighborite (NaMgF3) and sellaite (MgF2). All these three minerals have very low solubility in water (16-130 mg/l). The later nephelinitic tephras contain surface coating of villiaumite (NaF), which is highly soluble (42,200 mg/l) in water and can thus release the fluoride more readily upon contact with water. Although there is still the need for further data and a more precise study on this topic in Tanzania, we can already draw a first conclusion that the intake of water during or directly following the deposition of the tephra is not advisable and should be avoided, whereas the release of fluoride from the lava flow has less influence on the river waters.-
Abstract: Kimberlite magmas, the primary source of diamonds, have many features indicative of explosive eruptions and high volatile contents. The main approaches used to establish exsolution of fluid during magma ascent include theoretical modeling and experimental estimates of volatile solubility in kimberlite-like melts. Both approaches are hampered by the poorly constrained composition of kimberlite melts. Resorption features on diamonds are very sensitive to the presence and composition of the kimberlite fluid as well as to temperature and pressure. Here, we use direct evidence from diamond resorption features as a new method for investigating the parameters of fluid exsolution. The method is based on experimental reproduction of diamond resorption in kimberlite melts with and without an exsolved fluid phase. We studied 802 diamonds from two kimberlites (BK1 and AK15) from the Orapa cluster, Botswana. Samples from the BK1 pipe include three lithologies: two coherent kimberlites (CK-A and CK-B) and a pyroclastic kimberlite (massive volcaniclastic kimberlite, MVK). The known depth of diamond samples in each kimberlite lithology allows us to demonstrate an increase in the intensity of kimberlite-induced resorption with depth of diamond recovery in the drill holes. Each kimberlite lithology has a different proportion of diamonds with kimberlite-induced resorption, which is unique in style in each lithology: glossy surfaces in MVK due to reaction with C-O-H fluid, rough corroded surfaces in CK-B due to reaction with volatile-undersaturated melt, and a combination of glossy surfaces with corroded features in CK-A due to an overprint of melt resorption after fluid resorption. Both diamond resorption and kimberlite textures in the BK1 kimberlite show evidence of fluid exsolution only in CK-A and MVK lithologies, but no fluid presence in CK-B. The observed diamond resorption features may be controlled by (1) a temporary separation of the rising magma column into a bubble-rich head and bubble-poor volatile-depleted tail and (2) fluid exsolution at depths greater than decompressional degassing. We discuss how the depth of fluid exsolution from kimberlite melt may affect the diamond grade and the resorption of diamond populations in a kimberlite.
Earth and Planetary Science Letters, Vol. 475, pp. 143-151.
Africa
peridotites
Abstract: Continental Flood Basalts (CFB) result from voluminous outpourings of magma that often precede continental break-up. Notwithstanding the petrogenetic importance of CFBs, the nature of the mantle source for such magmas is contentious, particularly with regard to picrites with Ni-rich olivine phenocrysts. Previous studies have suggested that Ni-rich olivines associated with plume volcanism in regions of thickened (>90 km) lithosphere are related to either source mineralogy differences (peridotite versus pyroxenite) or change in olivine-melt partitioning due to pressure increase. In order to evaluate these two hypotheses, we present trace element data for olivines from the Karoo CFB Tuli and Mwenezi picrites and the Etendeka CFB Horingbaai/LTZ-L type picrites, all of which erupted in regions of thickened (>90 km) lithosphere in southern Africa. Karoo picrite olivines are Ni-rich, Ca- and Mn-poor, and have low (<1.4) 100*Mn/Fe. These compositions are consistent with a pyroxenitic source. Etendeka Horingbaai/LTZ-L picrite olivines do not show Ni-enrichment, but are characterized by high Al and Cr, and high (>1.4) 100*Mn/Fe, which is more consistent with high temperature melting of a dominantly peridotitic source. We also show that the Karoo and Etendeka olivines are characterized by distinct Mn/Zn ratios of <13 and >15, respectively.In addition, bulk rock geochemical data compilations and previously reported olivine for Karoo and Etendeka CFBs are discussed in order to further constrain source components based on previously described pyroxenite melt geochemical indices such as MgO-CaO systematics, FeO/MnO, Zn/Fe, and FC3MS (FeO/CaO-3*MgO/SiO2). These geochemical indices suggest a pyroxenite-dominated source for Karoo CFBs as well as for Etendeka ferropicrites whereas a peridotite-dominated source is indicated for Etendeka Horingbaai/LTZ-L type picrites analyzed in this study. Based on our data, Ni-enrichment of olivine in plume-related magmas in regions of thickened lithosphere in southern Africa is not ubiquitous. We therefore suggest that mineralogical variation of the source is a more likely major control of olivine chemistry and parent melt variations for Karoo and Etendeka CFBs. We also show that olivine Mn-Zn correlations are a useful discriminator for source variation and recommend the use of olivine for a pyroxenite-dominated source relative to olivine for a peridotite-dominated source.
Abstract: Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95?Ga) and one Proterozoic (1.15?Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1?Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.
Barium isotopic composition of the mantle constrained by carbonatites.
Goldschmidt Conference, 1p. Abstract
Africa, Tanzania, east Africa, Canada, Europe, Germany, Greenland
carbonatite
Abstract: Deep mantle origin and ultra-reducing conditions in podiform chromitite: diamonds, moissanite, and other unusual minerals in podiform chromitites from the Pozanti-Karsanti ophiolite, southern Turkey
deposits - Tchiuzo, Anomaly 116, Catoca, Alt Cuilo-4, Cuilo-63, Cucumbi-79.
Abstract: In this study, we compare the major- and trace-element compositions of olivine, garnet, and clinopyroxene that occur as single crystals (142 grains), with those derived from xenoliths (51 samples) from six kimberlites in the Lucapa area, northeastern Angola: Tchiuzo, Anomaly 116, Catoca, Alto Cuilo-4, Alto Cuilo-63 and Cucumbi-79. The samples were analyzed using electron probe microanalysis (EPMA) and laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). The results suggest different paragenetic associations for these kimberlites in the Lucapa area. Compositional overlap in some of the macrocryst and mantle xenolith samples indicates a xenocrystic origin for some of those macrocrysts. The presence of mantle xenocrysts suggests the possibility of finding diamond. Geothermobarometric calculations were carried out using EPMA data from xenoliths by applying the program PTEXL.XLT. Additional well calibrated single-clinopyroxene thermobarometric calculations were also applied. Results indicate the underlying mantle experienced different equilibration conditions. Subsequent metasomatic enrichment events also support a hypothesis of different sources for the kimberlites. These findings contribute to a better understanding of the petrogenetic evolution of the kimberlites in northeastern Angola and have important implications for diamond exploration.
Van der Meer, Q., Klaver, M., Reisberg, L., Riches, A. J.V., Davies, G.R.
Preservation of an Archean whole rock Re-Os isochron for the Venetia lithospheric mantle: evidence for rapid crustal recycling and lithosphere stabilization at 3.3 Ga.
Geochimica et Cosmochimica Acta, Vol. 216, pp. 242-263.
Abstract: Magnesian ilmenite is a common kimberlite indicator mineral, although its use in diamond exploration is still controversial. Complex crystallisation and replacement processes have been invoked to explain the wide compositional and textural ranges of ilmenite found in kimberlites. This work aims to shed light on these processes, as well as their implications for diamond exploration. Petrographic studies were combined for the first time with both major- and trace-element analyses to characterise the ilmenite populations found in xenoliths and xenocrysts in two Angolan kimberlites (Congo-Kasai craton). A multi-stage model describes the evolution of ilmenite in these pipes involving: i) crystallisation of ferric and Mg-rich ilmenite either as metasomatic phases or as megacrysts, both in crustal and in metasomatised mantle domains; ii) kimberlite entrainment and xenolith disaggregation producing at least two populations of ilmenite nodules differing in composition; iii) interaction of both types with the kimberlitic magma during eruption, leading to widespread replacement by Mg-rich ilmenite along grain boundaries and fractures. This process produced similar major-element compositions in ilmenites regardless of their primary (i.e., pre-kimberlitic) origin, although the original enrichment in HFSE (Zr, Hf, Ta, Nb) observed in Fe3 +-rich xenocrysts is preserved. Finally (iv) formation of secondary Mn-ilmenite by interaction with a fluid of carbonatitic affinity or by infiltration of a late hydrothermal fluid, followed in some cases by subsolidus alteration in an oxidising environment. The complexities of ilmenite genesis may lead to misinterpretation of the diamond potential of a kimberlite during the exploration stage if textural and trace-element information is disregarded. Secondary Mg-enrichment of ilmenite xenocrysts is common and is unrelated to reducing conditions that could favour diamond formation/preservation in the mantle. Similarly, Mn-rich ilmenite should be disregarded as a diamond indicator mineral, unless textural studies can prove its primary origin.
Journal of Volcanology and Geothermal Research, in press available, 19p.
Africa, Angola, Democratic Republic of Congo
carbonatites
Abstract: The South Atlantic Igneous Province comprises the Paraná Basalts, Rio Grande Rise, Tristan archipelago and surrounding guyot province,Walvis Ridge, Etendeka basalts and, in somemodels, the alkaline igneous lineament in the Lucapa corridor, Angola. Although these volcanics are often considered to have a single generic origin, complexities that suggest otherwise are observed. The Paraná Basalts erupted ~5 Ma before sea-floor spreading started in the neighborhood, and far more voluminous volcanic margins were emplaced later. A continental microcontinent likely forms much of the Rio Grande Rise, and variable styles of volcanism built the Walvis Ridge and the Tristan da Cunha archipelago and guyot province. Such complexities, coupled with the northward-propagating mid-ocean ridge crossing amajor transverse transtensional intracontinental structure, suggest that fragmentation of Pangaea was complex at this latitude and that the volcanism may have occurred in response to distributed extension. The alternative model, a deep mantle plume, is less able to account for many observations and no model variant can account for all the primary features that include eruption of the Paraná Basalts in a subsiding basin, continental breakup by rift propagation that originated far to the south, the absence of a time-progressive volcanic chain between the Paraná Basalts and the Rio Grande Rise, derivation of the lavas from different sources, and the lack of evidence for a plume conduit in seismic-tomography- and magnetotelluric images. The region shares many common features with the North Atlantic Igneous Province which also features persistent, widespread volcanismwhere a propagating mid-ocean ridge crossed a transverse structural discontinuity in the disintegrating supercontinent.
South African Journal of Geology, Vol. 120, 1, pp. 1-20.
Africa, South Africa
deposit - Jagersfontein
Abstract: Our earlier studies continued in a diligent search for rare ultradeep xenoliths in the kimberlite diatreme at Jagersfontein. The search has met with moderate success insofar as 20 majorite-bearing (decomposed to *gt + lamellar px) xenoliths are identified. Discrete gts (1 to 2 cm), gt-rich layers (2 to 3 cm) in lherzolites, and rare megacrystic gts (>3 cm) from xenoliths characterize the ultradeep suite. Pyroxene lamellae are crystallographically controlled along {111} gt planes, but px may also be prismatic, lensoidal, coarsely graphic, or annular to gt; jigsaw, rather than 120° dihedral textures, are typical. Gt ranges from Pyr68-74 mole% and CaO - Cr2O3 relations, with two exceptions, are distinctly lherzolitic. Cpx37-46 = Wo mole%, Jd 3-19 mole%, with 0.4 to 2.4 wt% Cr2O3; opx = 92 to 95 mole% en, and ol averages 92.5 mole% with maximum wt% 0.1 CaO, 0.4 NiO, and 0.1 Cr2 O3. A new class of 10 ultradeep xenoliths has lamellar spinel (Cr/Cr + Al = 0.74; Mg/Mg + Fe = 0.58) in addition to cpx with gt >3wt% Cr2O3 (c.f. 0.5 to 1.5 for sp-free types). Five samples are texturally linked but are compositional outliers to the central body of data: two are sp hosts (Cr# 0.69, Mg# 0.76) and (Cr# 0.74, Mg# 0.57) to gt (Pyr72) + cpx (Jd14); one is a gt megacryst (Pyr80) with sp (Cr# 57, Mg# 69); and the remaining two are unusually rich in chromium with gt = 7.3 to 8.2 wt% Cr2O3, rimmed by cpx (2.3 to 3.3 wt% Cr2O3). In addition, there are 17 xenoliths with compositional affinities to the ultradeep suite but lacking the texturally diagnostic lamellar intergrowths of cpx in gt are possibly completely equilibrated to gt + irregular cpx. Results from the new collection substantiate our earlier conclusions that the mantle was sampled by the Jagersfontein kimberlite from the lower lithosphere (250 to 350 km) and the transition zone (435 km) with diagnostic high P-T majorite in lherzolite that decomposed to gt + px at one or more interruptive stations (e.g. lithosphere-asthenosphere boundary), and at one or another time, en route to the surface. Homogeneous majorite occurs as diamond inclusions at Jagersfontein but these are eclogitic, leading to the proposition that the source region in the asthenosphere was an unassimilated mixture of lherzolite and eclogite in the Mid-Cretaceous at the time of kimberlite sampling. Important questions arise: Is majorite primordial; did majorite form exclusively from the transition of pyroxene; or did subsequent dissolution into coexisting garnet take place? Why has majorite not been identified in eclogite, nor diamonds of lherzolitic affinity? Does the formation of majorite and the crystallization of encapsulating diamond imply distinct high P-T events?
International Journal of Earth Sciences, Vol. 106, pp. 2797-2813.
Africa, Namibia
carbonatites
Abstract: Rb-Sr whole-rock and mineral isotope data from nepheline syenite, tinguaite, and carbonatite samples of the Kalkfeld Complex within the Damaraland Alkaline Province, NW Namibia, indicate a date of 242?±?6.5 Ma. This is interpreted as the age of final magmatic crystallization in the complex. The geological position of the complex and the spatially close relationship to the Lower Cretaceous Etaneno Alkaline Complex document a repeated channeling of small-scale alkaline to carbonatite melt fractions along crustal fractures that served as pathways for the mantle-derived melts. This is in line with Triassic extensional tectonic activity described for the nearby Omaruru Lineament-Waterberg Fault system. The emplacement of the Kalkfeld Complex more than 100 Ma prior to the Paraná-Etendeka event and the emplacement of the Early Cretaceous Damaraland intrusive complexes excludes a genetic relationship to the Tristan Plume. The initial ?Sr-?Nd pairs of the Kalkfeld rocks are typical of younger African carbonatites and suggest a melt source, in which EM I and HIMU represent dominant components.
Abstract: The Precambrian geology of eastern Zambia and Malawi is highly complex due to multiple episodes of rifting and collision, particularly during the formation of Greater Gondwana as a product of the Neoproterozoic Pan-African Orogeny. The lithospheric structure and extent of known Precambrian tectonic entities of the region are poorly known as there have been to date few detailed geophysical studies to probe them. Herein, we present results from electromagnetic lithospheric imaging across Zambia into southern Malawi using the magnetotelluric method complemented by high-resolution aeromagnetic data of the upper crust in order to explore the extent and geometry of Precambrian structures in the region. We focus particularly on determining the extent of subcontinental lithospheric mantle (SCLM) beneath the Archean-Paleoproterozoic cratonic Bangweulu Block and the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide Orogenic Belts. We also focus on imaging the boundaries between these tectonic entities, particularly the boundary between the Irumide and Southern Irumide Belts. The thickest and most resistive lithosphere is found beneath the Bangweulu Block, as anticipated for stable cratonic lithosphere. Whereas the lithospheric thickness estimates beneath the Irumide Belt match those determined for other orogenic belts, the Southern Irumide Belt lithosphere is substantially thicker similar to that of the Bangweulu Block to the north. We interpret the thicker lithosphere beneath the Southern Irumide Belt as due to preservation of a cratonic nucleus (the pre-Mesoproterozoic Niassa Craton). A conductive mantle discontinuity is observed between the Irumide and Southern Irumide Belts directly beneath the Mwembeshi Shear Zone. We interpret this discontinuity as modified SCLM relating to a major suture zone. The lithospheric geometries determined from our study reveal tectonic features inferred from surficial studies and provide important details for the tectonothermal history of the region.
Contributions to Mineralogy and Petrology, Vol. 172, 96
Africa, Malawi
carbonatite
Abstract: Carbonatites host some of the largest and highest grade rare earth element (REE) deposits but the composition and source of their REE-mineralising fluids remains enigmatic. Using C, O and 87Sr/86Sr isotope data together with major and trace element compositions for the REE-rich Kangankunde carbonatite (Malawi), we show that the commonly observed, dark brown, Fe-rich carbonatite that hosts REE minerals in many carbonatites is decoupled from the REE mineral assemblage. REE-rich ferroan dolomite carbonatites, containing 8-15 wt% REE2O3, comprise assemblages of monazite-(Ce), strontianite and baryte forming hexagonal pseudomorphs after probable burbankite. The 87Sr/86Sr values (0.70302-0.70307) affirm a carbonatitic origin for these pseudomorph-forming fluids. Carbon and oxygen isotope ratios of strontianite, representing the REE mineral assemblage, indicate equilibrium between these assemblages and a carbonatite-derived, deuteric fluid between 250 and 400 °C (?18O + 3 to + 5‰VSMOW and ?13C ? 3.5 to ? 3.2‰VPDB). In contrast, dolomite in the same samples has similar ?13C values but much higher ?18O, corresponding to increasing degrees of exchange with low-temperature fluids (< 125 °C), causing exsolution of Fe oxides resulting in the dark colour of these rocks. REE-rich quartz rocks, which occur outside of the intrusion, have similar ?18O and 87Sr/86Sr to those of the main complex, indicating both are carbonatite-derived and, locally, REE mineralisation can extend up to 1.5 km away from the intrusion. Early, REE-poor apatite-bearing dolomite carbonatite (beforsite: ?18O + 7.7 to + 10.3‰ and ?13C ?5.2 to ?6.0‰; 87Sr/86Sr 0.70296-0.70298) is not directly linked with the REE mineralisation.
Journal of African Earth Sciences, Vol. 137, pp. 9-21.
Africa, Botswana
deposit - Jwaneng
Abstract: Country rock at Jwaneng Diamond Mine provides a rare insight into the deformational history of the Transvaal Supergroup in southern Botswana. The ca. 235 Ma kimberlite diatremes intruded into late Archaean to Early Proterozoic, mixed, siliciclastic-carbonate sediments, that were subjected to at least three deformational events. The first deformational event (D1), caused by NW-SE directed compression, is responsible for NE-trending, open folds (F1) with associated diverging, fanning, axial planar cleavage. The second deformational event (D2) is probably progressive, involving a clockwise rotation of the principal stress to NE-SW trends. Early D2, which was N-S directed, involved left-lateral, oblique shearing along cleavage planes that developed around F1 folds, along with the development of antithetic structures. Progressive clockwise rotation of far-field forces saw the development of NW-trending folds (F2) and its associated, weak, axial planar cleavage. D3 is an extensional event in which normal faulting, along pre-existing cleavage planes, created a series of rhomboid-shaped, fault-bounded blocks. Normal faults, which bound these blocks, are the dominant structures at Jwaneng Mine. Combined with block rotation and NW-dipping bedding, a horst-like structure on the northwestern limb of a broad, gentle, NE-trending anticline is indicated. The early compressional and subsequent extensional events are consistent throughout the Jwaneng-Ramotswa-Lobatse-Thabazimbi area, suggesting that a large area records the same fault geometry and, consequently, deformational history. It is proposed that Jwaneng Mine is at or near the northernmost limit of the initial, northwards-directed compressional event.
Abstract: A series of terraces flanking the Lower Orange River in the study area were deposited after ca. 90% of the incision had occurred, thus only the late stage incision/depositional history of this margin is able to be addressed here. Two principal suites of river terraces are distinguished by their palaeo-courses, bedrock strath levels, overall geometry and clast assemblages: an older, higher lying Proto suite and a younger Meso suite. The Proto suite represents a long, post-Eocene, through the Oligocene into the Early Miocene, phase of incision, followed by a prolonged period of aggradation where up to 90 m of fluvial, diamondiferous deposits accumulated during the Early-Middle Miocene. The Meso suite of deposits represents shorter phrases of incision and aggradation in the Pilo-Pleistocene. The Proto and Meso deposits were built in response to both base level rise and increased supply of material from tributaries draining the Great Escarpment locally, with clast assemblage and downstream fining data indicating the latter to be the more important variable. River incision into bedrock is a topic of great interest to fluvial geomorphologists, although most data are derived from active tectonic settings. The incision of a large river into a plateau surface is relatively rare, the best known example being the Colorado River in the young (6 Ma) Grand Canyon. The Orange River in the study area represents a long-lived example of this setting, with the present day dissected topography having evolved from more confined canyon-like walls following the early incision of the Orange River in the Early Tertiary. Although a long-lived incision, the modern channel is not graded in the study area, and is actively incising. The world-wide dataset of incision rates in modern rivers indicates that the Orange River could have completed its entire incision within less than a million years. The continued downcutting of this river so long after the initial incision event is indicative of the roles of intermittent, ongoing epeirogenesis and/or eustatic influences (both of which cannot be proven at this stage), tributary input from the plateau rim (Great Escarpment) or merely the long tag time involved in landscape adjustment following incision into a plateau surface.
Earth and Planetary Science Letters, Vol. 481, pp. 143-153.
Africa, South Africa
deposit - Wesselton
Abstract: Rare garnet crystals from a peridotite xenolith from the Wesselton kimberlite, South Africa, have distinct zones related to two separate episodes of mantle metasomatism. The garnet cores were firstly depleted through melt extraction, then equilibrated during metasomatism by a potentially diamond-forming carbonate-bearing or proto-kimberlitic fluid at 1100-1300?°C and 4.5-5.5 GPa. The garnet rim chemistry, in contrast, is consistent with later overgrowth in equilibrium with a kimberlite at around and . This suggests that the rock was physically moved upwards by up to tens of kilometres between the two metasomatic episodes. Preserved high Ca, Al and Cr contents in orthopyroxenes suggest this uplift was tectonic, rather than magmatic. Diffusion profiles were measured over the transitions between garnet cores and rims using electron microprobe (Mg, Ca, Fe for modelling, plus Cr, Mn, Ti, Na, Al) and nano Secondary Ion Mass Spectrometry (NanoSIMS; 89Y, along with 23Na, Ca, Cr, Fe, Mn and Ti) analyses. The short profile lengths (generally <10 ?m) and low Y concentrations (0.2-60 ppm) make the NanoSIMS approach preferable. Diffusion profiles at the interface between the zones yield constraints on the timescale between the second metasomatic event and eruption of the kimberlite magma that brought the xenolith to the surface. The time taken to form the diffusion profiles is on the order of 25 days to 400 yr, primarily based on modelling of Y diffusion along with Ca, Fe and Mg (multicomponent diffusion) profiles. These timescales are too long to be produced by the interaction of the mantle xenolith with the host kimberlite magma during a single-stage ascent to the crust (hours to days). The samples offer a rare opportunity to study metasomatic processes associated with failed eruption attempts in the cratonic lithosphere.
Geochemistry, Geophysics, Geosystems, Vol. 18, 10, pp. 3556-3575.
Africa, South Africa
geodynamics
Abstract: The deep mantle African Superswell is considered to contribute to the topographic uplift of the Southern African Plateau, but dynamic support estimates vary wildly depending on the approach and data used. One reason for these large disparities is that the role of lithospheric structure, key in modulating deep dynamic contributions to elevation, is commonly ignored or oversimplified in convection studies. We use multiple high-quality geophysical data coupled with xenolith-based geochemical constraints to compute the isostatic lithospheric contribution to the elevation of the Plateau, facilitating isolation of the current dynamic component from the total observed elevation. We employ a multiobservable stochastic algorithm to invert geoid anomaly, surface-wave dispersion data, magnetotelluric data, and surface heat flow to predict elevation in a fully thermodynamically and internally-consistent manner. We find that a compositionally layered 230?±?7 km thick lithosphere is required to simultaneously fit all four data types, in agreement with abundant independent xenolith evidence. Our stochastic modeling indicates a lithospheric contribution to elevation of the order of 670 m, which implies dynamic support arising from the convecting sublithospheric mantle of ?650 m. Our results have important implications for the understanding of lithospheric-deep mantle feedback mechanisms and for calibrating dynamic topography estimates from global convection studies.
Abstract: The fragmentary Phanerozoic geological record of the anomalously elevated Zimbabwe Craton makes reconstructing its history difficult using conventional field methods. Here we constrain the cryptic Phanerozoic evolution of the Zimbabwe Craton using a spatially extensive apatite (U-Th-Sm)/He (AHe), apatite fission track (AFT), and zircon (U-Th)/He (ZHe) data set. Joint thermal history modeling reveals that the region experienced two cooling episodes inferred to be the denudational response to surface uplift. The first and most significant protracted denudation period was triggered by stress transmission from the adjacent ~750-500 Ma Pan-African orogenesis during the amalgamation of Gondwana. The spatial extent of this rejuvenation signature, encompassing the current broad topographic high, could indicate the possible longevity of an ancient topographic feature. The ZHe data reveal a second, minor denudation phase which began in the Paleogene and removed a kilometer-scale Karoo cover from the craton. Within our data set, the majority of ZHe ages are younger than their corresponding AHe and AFT ages, even at relatively low eU. This unexpectedly recurrent age “inversion” suggests that in certain environments, moderately, as well as extremely, damaged zircons have the potential to act as ultra-low-temperature thermochronometers. Thermal history modeling results reveal that the zircon radiation damage accumulation and annealing model (ZRDAAM) frequently overpredicts the ZHe age. However, the opposite is true for extremely damaged zircons where the ZHe and AHe data are also seemingly incompatible. This suggests that modification of the ZRDAAM may be required for moderate to extreme damage levels.
Abstract: The Damara Orogen is composed of the Damara, Kaoko and Gariep belts developed during the Neoproterozoic Pan-African Orogeny. The Damara Belt contains Neoproterozoic siliciclastic and carbonate successions of the Damara Supergroup that record rift to proto-ocean depositional phases during the Rodinia supercontinent break up. There are two conflicting interpretations of the geotectonic framework of the Damara Supergroup basin: i) as one major basin, composed of the Outjo and Khomas basins, related to rifting in the Angola-Congo-Kalahari paleocontinent or, ii) as two independent passive margin basins, one related to the Angola-Congo and the other to the Kalahari proto-cratons. Detrital zircon provenance studies linked to field geology were used to solve this controversy. U-Pb zircon age data were analyzed in order to characterize depositional ages and provenance of the sediments and evolution of the succession in the northern part of the Outjo Basin. The basal Nabis Formation (Nosib Group) and the base of the Chuos Formation were deposited between ca. 870 Ma and 760 Ma. The upper Chuos, Berg Aukas, Gauss, Auros and lower Brak River formations formed between ca. 760 Ma and 635 Ma. It also includes the time span recorded by the unconformity between the Auros and lower Brak River formations. The Ghaub, upper Brak River, Karibib and Kuiseb formations were deposited between 663 Ma and 590 Ma. The geochronological data indicate that the main source areas are related to: i) the Angola-Congo Craton, ii) rift-related intrabasinal igneous rocks of the Naauwpoort Formation, iii) an intrabasinal basement structural high (Abbabis High), and iv) the Coastal Terrane of the Kaoko Belt. The Kalahari Craton units apparently did not constitute a main source area for the studied succession. This is possibly due to the position of the succession in the northern part of the Outjo Basin, at the southern margin of the Congo Craton. Comparison of the obtained geochronological data with those from the literature shows that the Abbabis High forms part of the Kalahari proto-craton and that Angola-Congo and Kalahari cratons were part of the same paleocontinent in Rodinia times.
Textural evidences of late stage carbonate dissolution precipitation and stable isotope exchange re-equilibration in the Kangankunde carbonatite complex, Malawi.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 39-40.
Africa, Malawi
deposit - Kangankunde
Abstract: The Kangankunde carbonatite complex, southern Malawi is an eroded remnant of a carbonatite depocentre belonging to the Lower Cretaceous Chilwa Alkaline Province (Garson and Campbell-Smith, 1965; Karmalkar et al., 2010). The carbonatite complex consists of fenites, carbonated agglomeratic breccias and ankeritic-ferron dolomite carbonatite with sporadic patches of REE minerals. Coarse pegmatitic siderite and ankeritic carbonatite hosts exotic cavities and vugs that contain RE minerals like monazite, synchysite, bastnasite, and florencite-goyazite along with barite and strontianite. Bright green monazite occurs with thick quartz veins and as disseminations within host carbonatites (Garson and Campbell-Smith, 1965; Holt, 1965). Texturally, the host carbonatite exhibits euhedral to subhedral rhombohedra of ankerite-calcite and dolomite set in a fine groundmass. However samples close to mineralized zones show streaky textures, streaming effects and patterns that resemble remobilization and fluidization. Textural evidences such as presence of pseudomorphs containing the REE mineral assemblage, veinlets and drusy cavities indicate that REE mineralization replaced earlier formed carbonates (Wall et al. 1994). SEM micromorphology of such carbonatites show several dissolution-precipitation features indicating that there was a pervasive fluid interaction with the host subsequent to cooling and crystallization. Within cavities and vugs, the precipitation is closely associated with monazite veins and formation of minerals like collinsite, synchysite and rare aragonite (Duraiswami and Shaikh, 2010). Carbon and oxygen isotope ratios were determined on 8 bulk carbonatite powders from Kangankunde using Thermo Fisher Scientific GasBench II, equipped with autosampler (CTC Analytics AG, Zwingen, Switzerland), and coupled to a Delta Plus XP Mass Spectrometer at NGRI, Hyderabad. International Standards NBS-19-1 and NBS-18-1 plus internal standards were used. The internal precision (1s) measured for raw d18O and d13C was 0.04–0.08‰ and 0.03 to 0.06 respectively. The ?13C VS V-PDB show restricted range (-0.31 to -2.76) but ?18O VS V-SMOW values vary widely (8.22 to 24.5). The samples analysed in the present study plot outside the Primary Igneous Carbonatite field (Demény et al., 2004) and are related either to carbo-hydrothermal fluids or alteration of dolomite by a later, cooling, deuteric fluid (Fendley et al., 2017). This study provides conclusive textural evidences (pseudomorphic replacements, atolls, iron-oxide exsolution fringes from dolomite/ankerite, coloform banding, secondary veinlets, etc.) to supports geochemical and stable isotopic inferences about the role of late stage hydrothermal fluids and subsequent alterations in the Kangankunde carbonatite complex (Wall and Mariano, 1996, Wall et al., 1994, Doroshkevich et al., 2009, Fendley et al., 2017).
Abstract: The origin of intraplate carbonatitic to alkaline volcanism in Africa is controversial. A tectonic control, i.e., decompression melting associated with far-field stress, is suggested by correlation with lithospheric sutures, repeated magmatic cycles in the same areas over several million years, synchronicity across the plate, and lack of clear age progression patterns. Conversely, a dominant role for mantle convection is supported by the coincidence of Cenozoic volcanism with regions of lithospheric uplift, positive free-air gravity anomalies, and slow seismic velocities. To improve constraints on the genesis of African volcanism, here we report the first radiometric and isotopic results for the Catanda complex, which hosts the only extrusive carbonatites in Angola. Apatite (U-Th-Sm)/He and phlogopite 40Ar/39Ar ages of Catanda aillikite lavas indicate eruption at ca. 500-800 ka, more than 100 m.y. after emplacement of abundant kimberlites and carbonatites in this region. The lavas share similar high-? (HIMU)-like Sr-Nd-Pb-Hf isotope compositions with other young mantle-derived volcanics from Africa (e.g., Northern Kenya Rift; Cameroon Line). The position of the Catanda complex in the Lucapa corridor, a long-lived extensional structure, suggests a possible tectonic control for the volcanism. The complex is also located on the Bié Dome, a broad region of fast Pleistocene uplift attributed to mantle upwelling. Seismic tomography models indicate convection of deep hot material beneath regions of active volcanism in Africa, including a large area encompassing Angola and northern Namibia. This is strong evidence that intraplate late Cenozoic volcanism, including the Catanda complex, resulted from the interplay between mantle convection and preexisting lithospheric heterogeneities.
Abstract: There are two types of emerged relief on the Earth: high elevation areas (mountain belts and rift shoulders) in active tectonic settings and low elevation domains (anorogenic plateaus and plains) characteristic of the interior of the continents i.e. 70% of the Earth emerged relief. Both plateaus and plains are characterized by large erosional surfaces, called planation surfaces that display undulations with middle (several tens of kilometres) to very long (several thousands of kilometres) wavelengths, i.e. characteristic of lithospheric and mantle deformations respectively. Our objective is here (1) to present a new method of characterization of the very long and long wavelength deformations using planation surfaces with an application to Central Africa and (2) to reconstruct the growth of the very long wavelength relief since 40 Ma, as a record of past mantle dynamics below Central Africa. (i) The African relief results from two major types of planation surfaces, etchplains (weathering surfaces by laterites) and pediplains/pediments. These planation surfaces are stepped along plateaus with different elevations. This stepping of landforms records a local base level fall due to a local tectonic uplift. (ii) Central Africa is an extensive etchplain-type weathering surface - called the African Surface - from the uppermost Cretaceous (70 Ma) to the Middle Eocene (45 Ma) with a paroxysm around the Early Eocene Climatic Optimum. Restoration of this surface in Central Africa suggests very low-elevation planation surfaces adjusted to the Atlantic Ocean and Indian Ocean with a divide located around the present-day eastern branch of the East African Rift. (iii) The present-day topography of Central Africa is younger than 40 -30 Ma and records very long wavelength deformations (1000 -2000 km) with (1) the growth of the Cameroon Dome and East African Dome since 34 Ma, (2) the Angola Mountains since 15 -12 Ma increasing up to Pleistocene times and (3) the uplift of the low-elevation (300 m) Congo Basin since 10 -3 Ma. Some long wavelength deformations (several 100 km) also occurred with (1) the low-elevation Central African Rise since 34 Ma and (2) the Atlantic Bulge since 20 -16 Ma. These very long wavelength deformations record mantle dynamics, with a sharp increase of mantle upwelling around 34 Ma and an increase of the wavelength of the deformation and then of mantle convection around 10 -3 Ma.
The Bulhoek carbonatite complex, South Africa: evidence for magmatic crystallization of dolomite at low pressures and the petogenetic implications.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 5-7.
Africa, South Africa
deposit - Bulhoek
Abstract: In many plutonic carbonatite complexes the dominant carbonatite type is calcitic and there is a tendency to assume that dolomitic carbonatites are insignificant relative to calcitic types. While calcitic carbonatites dominate in regions of rifting, e.g. the East African Rift System, dolomitic carbonatites are more abundant in the Archaean Kaapvaal and Zimbabwean Cratons of southern Africa and the Archaean parts of the Canadian Shield (Harmer and Gittins, 1997). The ~1.4Ga Newania carbonatite, the only dolomite carbonatite complex described from India, is located within the Archaean Aravalli Craton (Ray et al., 2013). Increased exploration activity on carbonatites in response to the short-lived 2009-2012 REE boom has highlighted the importance of dolomitic carbonatites as targets for REE (and P) deposits: for example, of the 10 advanced African REE projects (i.e. deposits with a defined resource) associated with carbonatites, 7 are hosted within dolomite carbonatites (Harmer and Nex, 2016). This study reports a detailed petrological study of the Bulhoek Carbonatite Complex, one of a number of carbonatite and alkaline igneous complexes that were emplaced into the central Kaapvaal Craton at 1.4 Ma (Hanson et al., 2006). The Bulhoek Complex comprises three centres of intrusion of magnesian carbonatite into granitic and granophyric components of the 2.05Ga Bushveld Complex over a linear 20 km long zone. At all three centres, areas of fenitised granite enclose sheeted intrusions of dolomitic carbonatite of similar composition and intrusive style. No alkaline silicate magmatic rocks have been in the vicinity of the intrusive centres. Carbonatites were emplaced in three discrete intrusive episodes: initial intrusion was accompanied by significant shattering of the country rocks and produced a sequence of medium-grained, flow foliated dolomite carbonatites (C1) containing clasts of fenitised country rock granites along with xenoliths of fenitised ferrogabbro and magnetite transported from Bushveld Upper Zone cumulates underlying the granites. A second intrusive pulse produced cross-cutting sheets of medium- to coarse-grained dolomitic carbonatite free of xenoliths (C2) that make up the bulk of the carbonatite at each intrusive centre. Evidence that both C1 and C2 carbonatites intruded as crystal-rich mushes include strongly developed sub-vertical foliations, deflection of foliations around large fenite xenoliths and the common occurrence of folded foliations through drag effects at contacts with country rock and in C2 intrusions along contacts with C1 carbonatites.
Abstract: Lenses of retrogressed eclogites occur in a 100 km wide zone of the Nyong Complex, a remnant of the Eburnian-Transamazonian orogen, marking a Palaeoproterozoic suture between the Congo and São Francisco Cratons. The eclogites show trace element pattern (depleted in LREE) similar to those of mid-ocean ridge basalts, indicating that the precursor melts formed in a depleted mantle source and the eclogites formed from oceanic crust. Despite numerous plagioclase ‘exsolutions’ up to 25 mol% jadeite component is preserved in omphacite and points to minimum pressures of 16 kbar at c. 800 °C. Pressures may have been 18-20 kbar as indicated by estimated compositions of peak omphacite. The age of eclogite metamorphism has been constrained by U-Pb SHRIMP dating of zircon at 2093 ± 45 Ma. The eclogites are associated with 2.05 Ga old charnockites and mafic granulites containing textures characteristic for near-isobaric cooling. These rocks may represent the plate above a subduction zone in which the eclogites were tectonically emplaced. With an age of 2.09 Ga the eclogites of the Nyong Complex are older than other subduction related Palaeoproterozoic eclogites of the Ubendian (1.88 Ga) and Usagaran belts (2.0 Ga) at the southern border of the Tanzania Craton. They are also older than eclogites in the Belomorian province (1.9 Ga; Russia) and thus represent the oldest known eclogites outcropping in an orogenic belt. The African eclogites (all with MORB chemistry) indicate that during the formation of the Nuna supercontinent the Palaeoproterozoic oceanic lithosphere around the Congo-Tanzania Craton was thick, cold and rigid enough to become subducted similar to cold oceanic lithosphere in the modern plate tectonic regime. However, apparent geothermal gradients of 12-14 °C/km for the Palaeoproterozoic eclogites are higher than those of Neoproterozoic and Phanerozoic eclogites and are interpreted as the result of warm subduction in a hotter Palaeoproterozoic Earth.
Platinum group element and rhenium osmium geochemistry of selected carbonatites from India, USA and East africa.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 22-23.
India, United States, Africa, East Africa
carbonatites
Abstract: Carbonatites and associated alkaline silicate rocks might have potential economic impact for a large variety of metals such as Cu, Ni, Fe and platinum-group elements (PGE - Os, Ir, Ru, Pd, Pt) as it is demonstrated in South Africa (Phalaborwa; Taylor et al. 2009) or Brazil (Ipanema; Fontana 2006). In addition, determined PGE contents along with Re-Os isotopic compositions may also provide important information about PGE fractionation during the genesis of upper mantle-derived carbonatitic melts and nature of their sources. Nevertheless, the existing PGE data for carbonatites are extremely rare, limited mostly to Chinese localities and they are not paralleled by Re-Os isotopic data (Xu et al. 2008). Therefore, in this study, we present the first complete PGE datasets together with Re-Os determinations for a suite of selected carbonatite bodies worldwide. We have chosen eight carbonatite sites with different alkaline rock association, age and geotectonic position. Among these, the youngest samples are from East African rift system and include Oldoinyo Dili, Tanzania with an age spanning from ~0 to 45 Ma; same as Tororo and Sukulu in Uganda (Woolley and Kjarsgaard 2008). These carbonatites are in association with pyroxenites and nepheline syenites. Another young carbonatitic complex is Amba Dongar in west India with Cretaceous age of ~65 Ma associated with alkaline volcanic rocks such as trachybasalts within Deccan Traps (Sukheswala and Udas 1963). Proterozoic bodies are represented by Iron Hill, USA carbonatites associated with pyroxenite, melitolite and ijolite with age ranging from ~520 to 580 Ma (Nash 1972). These carbonatites are famous for their intensive and varied fenitization. Last and the oldest carbonatites in this study comes from Samalpatti and Sevattur, South India having the age of ~800 Ma (Schleicher et al. 1997) and outcropping as small bodies within alkaline rocks such as pyroxenite, syenite and gabbro. The PGE concentrations and Re-Os isotopic ratios were determined by standard methods consisting of decarbonatization using HCl, decomposition of samples in Carius Tubes in the presence of reverse aqua regia and spikes (isotopic dilution), separation of Os by CHCl3 followed by N-TIMS measurements and Ir, Ru, Pd, Pt, Re isolation by anion exchange chromatography followed by ICP-MS measurements. All analysed carbonatites exhibit extremely low PGE contents (? PGE up to 1 ppb), even in the samples with high S contents (up to 1.5 wt. %). Such values are much lower than other determined so far for upper mantle-derived melts such as basalts, komatiites, etc. (Day et al. 2016). Such signatures indicate very low partitioning of PGE into carbonatitic melts and/or early separation of PGE-bearing fraction. Elements from iridium-group I-PGE; Os, Ir and Ru; mostly < 0.1 ppb) are distinctly lower compared to palladiumgroup elements and Re (PPGE; Pt, Pd, Re; mostly > 0.1 ppb) with some rocks being largely enriched in Re (up to ~6 ppb). Most of the analysed carbonatites exhibit progressive enrichment from Os to Re and consequently, PdN/ReN < 0.1 except south India carbonatites and associated alkaline rocks (> 0.30). Rocks analysed so far for Os have OsN/IrN up to 6.2 that might suggest that the carbonatites might concentrate Os over Ir. The highest HSEtot contents have been found in Mg-Cr-rich silicocarbonatites from South India (up to 40 ppb) and taking into account their only slightly radiogenic 187Os/188Os ratios (0.14-0.57), these rocks represents mixture of CO2-rich alkaline mantle melts and country rocks. Very high concentrations of HSE have been also found in magnetite separated from Fe-carbonatite from Amba Dongar, India (0.2-0.5 ppb of I-PGE and 0.9-9 ppb of P-PGE). The 187Os/188Os ratios determined so far for carbonatites from South India vary from 0.24 to 6.5 and calculated ?Os values range from +100 up to +5000. Such wide range of values suggest extremely heterogenous source of the melts and/or possible contamination by 187Os-rich crustal materials.
Earth and Planetary Science Letters, Vol. 458, 1, pp.405-417.
Africa, Madagascar
geophysics - seismics
Abstract: The crust and upper mantle of the Madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments. An island-wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations, derived from surface waves, across the entire island. Late Cenozoic alkaline intraplate volcanism has occurred in three separate regions of Madagascar (north, central and southwest), with the north and central volcanism active until <1 Ma, but the sources of which remains uncertain. Combined analysis of three complementary surface wave methods (ambient noise, Rayleigh wave cross-correlations, and two-plane-wave) illuminate the upper mantle down to depths of 150 km. The phase-velocity measurements from the three methods for periods of 8-182 s are combined at each node and interpolated to generate the first 3-D shear-velocity model for sub-Madagascar velocity structure. Shallow (upper 10 km) low-shear-velocity regions correlate well with sedimentary basins along the west coast. Upper mantle low-shear-velocity zones that extend to at least 150 km deep underlie the north and central regions of recent alkali magmatism. These anomalies appear distinct at depths <100 km, suggesting that any connection between the zones lies at depths greater than the resolution of surface-wave tomography. An additional low-shear velocity anomaly is also identified at depths 50-150 km beneath the southwest region of intraplate volcanism. We interpret these three low-velocity regions as upwelling asthenosphere beneath the island, producing high-elevation topography and relatively low-volume magmatism.
Simon, S.J., Wei, C.T., Viladkar, S.G., Ellmies, R., Soh, Tamech, L.S., Yang, H., Vatuva, A.
Metamitic U rich pyrochlore from Epembe sovitic carbonatite dyke, NW Namibia.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 12.
Africa, Namibia
deposit - Epembe
Abstract: The Epembe carbonatite dyke is located about 80 km north of Opuwo, NW Namibia. The 10 km long dyke is dominated by massive and banded sövitic carbonatite intrusions. Two distinct type of sövite have been recognized: (1) coarse-grained light grey Sövite I which is predominant in brecciated areas and (2) medium- to fine-grained Sövite II which hosts notable concentrations of pyrochlore and apatite. The contact between the carbonatite and basement gneisses is marked by K-feldspar fenite. The pyrochlore chemistry at Epembe shows a compositional trend from primary magmatic Ca-rich pyrochlore toward late hydrothermal fluid enriched carbonatite phase, giving rise to a remarkable shift in chemical composition and invasion of elements such as Si, U, Sr, Ba, Th and Fe. Enrichment in elements like U, Sr and Th lead to metamictization, alteration and A-site vacancy. It is therefore suggested that the carbonatite successive intrusive phases assimilated primary pyrochlore leading to extreme compositional variation especially around the rims of the pyrochlore. The genesis of the Epembe niobium deposit is linked to the carbonatite magmatism but the mechanism that manifested such niobium rich rock remains unclear and might be formed as a result of cumulate process and/or liquid immiscibility of a carbonate-silicate pair.
Abstract: Geodynamics of crustal growth and evolution consist in one of the thorniest questions of the early Earth. In order to solve it, Archean cratons are intensively studied through geophysical, geochemical and geochronological investigations. However, timing and mechanisms leading to accretion and stabilization of crustal blocks are still under question. In this study, new information on the evolution of Archean cratons is provided through complementary approaches applied to the northern margin of the Archean Kaapvaal craton (KC). The study area comprises the Pietersburg Block (PB) and the terrane immediately adjacent to the North: the Southern Marginal Zone of the Limpopo Complex (SMZ). We present a comprehensive petro-metamorphic study coupled with LA-ICP-MS U-Pb isotope examination of both Na- and K-rich granitoids from the two areas. This dataset points toward a new interpretation of the northern KC (PB?+?SMZ). Two significant magmatic events are newly recognized: (i) a ca. 3.2?Ga event, and (ii) a protracted magmatic event between ca. 2.95–2.75?Ga. These events affected in both investigated areas and are unrelated to the ca. 2.7?Ga-old event usually attributed to the SMZ. More importantly, phase equilibrium modelling of several lithologies from the SMZ basement points to middle-amphibolite facies conditions of equilibration instead of granulite-facies conditions historically assumed. This study has both important regional and global implications. Firstly, the presence of a continuous basement from the Thabazimbi-Murchison Lineament to the Palala Shear Zone, different than Central Zone of the Limpopo Complex basement, implies a complete reviewing of the whole Limpopo Complex concept. Secondly, the geometry observed in the northern Kaapvaal craton is assumed to testify for a complete accretionary orogenic sequence with formation of both mafic and TTG lithologies through arc-back arc geodynamic. This was followed by a long-lived lateral compression triggering partial melting of the lower continental crust and emplacement of Bt-granitoids bodies that stabilizes the continental crust. Lastly, partial melting of the underlying enriched mantle stabilized the entire lithosphere allowing long-term preservation of the crustal block.
Abstract: Cratons are strong and their preservation demonstrates that they resist deformation and fragmentation. Yet several cratons are rifting now, or have rifted in the past. We suggest that cratons need to be weakened before they can rift. Specifically, metasomatism of the depleted dehydrated craton mantle lithosphere is a potential weakening mechanism. We use 2D numerical models to test the efficiency of simulated melt metasomatism and coeval rehydration to weaken craton mantle lithosphere roots. These processes effectively increase root density through a parameterized melt-peridotite reaction, and reduce root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by adjacent standard Phanerozoic lithosphere. We find that cratons only become vulnerable to rifting following large-volume melt metasomatism (~ 30% by volume) and thinning of the gravitationally unstable cratonic lithosphere from > 250 km to ~ 100 km; at which point its residual crustal strength is important. Furthermore, our results indicate that rifting of cratons depends on the timing of extension with respect to metasomatism. An important effect in the large-volume melt models is the melt-induced increase in temperature which must have time to reach peak values in the uppermost mantle lithosphere before rifting. Release of water stored in the transition zone at the base of a big mantle wedge may provide a suitable natural setting for both rehydration and refertilization of an overlying craton and is consistent with evidence from the eastern North China Craton. An additional effect is that cratons subside isostatically to balance the increasing density of craton mantle lithosphere where it is moderately metasomatized. We suggest that this forms intracratonic basins and that their subsidence and subsequent uplift, and cratonic rifting constitute evidence of progressive metasomatism of cratonic mantle lithosphere.
Abstract: We investigate the along-axis variations in architecture, segmentation and evolution of the Main Ethiopian Rift (MER), East Africa, and relate these characteristics to the regional geology, lithospheric structure and surface processes. We first illustrate significant along-axis variations in basin architecture through analysis of simplified geological cross-sections in different rift sectors. We then integrate this information with a new analysis of Ethiopian topography and hydrography to illustrate how rift architecture (basin symmetry/asymmetry) is reflected in the margin topography and has been likely amplified by a positive feedback between tectonics (flexural uplift) and surface processes (fluvial erosion, unloading). This analysis shows that ~70% of the 500 km-long MER is asymmetric, with most of the asymmetric rift sectors being characterized by a master fault system on the eastern margin. We finally relate rift architecture and segmentation to the regional geology and geophysical constraints on the lithosphere. We provide strong evidence that rift architecture is controlled by the contrasting nature of the lithosphere beneath the homogeneous, strong Somalian Plateau and the weaker, more heterogeneous Ethiopian Plateau, differences originating from the presence of pre-rift zones of weakness on the Ethiopian Plateau and likely amplified by surface processes. The data provided by this integrated analysis suggest that asymmetric rifts may directly progress to focused axial tectonic-magmatic activity, without transitioning into a symmetric rifting stage. These observations have important implications for the asymmetry of continental rifts and conjugate passive margins worldwide.
Mineralogical Magazine, Vol. 81, 6, pp. 1367-1395.
Africa, Malawi
carbonatite - Chilwa
Abstract: Carbonatites are enriched in critical raw materials such as the rare earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and RE minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study (Natural History Museum, London collection BM1968 P37). In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, RE minerals and pyrochlore in vein networks. Petrography using SEM-EDX showed that the RE minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence of REE mobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by LA-ICP-MS) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallisation of plagioclase and co-crystallisation with K-feldspar in the fenite. The fenite minerals have consistently higher mid REE/light REE ratios (La/Sm = ~1.3 monazite, ~1.9 bastnäsite, ~1.2 parisite) than their counterparts in the carbonatites (La/Sm = ~2.5 monazite, ~4.2 bastnäsite, ~3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few µm in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and barite were found in the inclusions. Decrepitation of inclusions occurred at around 200?C before homogenisation but melting temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of around 24 wt.% NaCl equivalent was determined. Among the trace elements in whole rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, Y, and REE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct RE mineral micro-assemblages in fenite at some distance from carbonatite could be developed as an exploration indicator of REE enrichment.
Abstract: The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow-moving African continent, which last experienced orogeny during the Pan-African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas-rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises ~20% new magmatic material ponded in the lower crust and intruded as sills and dikes at shallower depths. In the Main Ethiopian Rift, intrusions comprise 30% of the crust below axial zones of dike-dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges. Our comparisons suggest that transitional crust, including seaward dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15-20 km thick crust. In the 30 Ma Recent CVL, which lacks a hot spot age progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hot spot toward the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology, and contributing significantly to global carbon and water fluxes.
Abstract: This study focuses on the causes, modalities and obstacles of sediment transfer in the longest cell of littoral sand drift documented on Earth so far. Sand derived from the Orange River is dragged by swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal Namibia to Angola. All four dunefields are terminated by river valleys, where aeolian sand is flushed back to the ocean; and yet sediment transport continues at sea, tracing an 1800 km long submarine sand highway. Sand drift would extend northward to beyond the Congo if the shelf did not become progressively narrower in southern Angola, where drifting sand is funnelled towards oceanic depths via canyon heads connected to river mouths. Garnet-magnetite placers are widespread along this coastal stretch, indicating systematic loss of the low-density feldspatho-quartzose fraction to the deep ocean. More than half of Moçamedes Desert sand is derived from the Orange River, and the rest in similar proportions from the Cunene River and from the Swakop and other rivers draining the Damara Orogen in Namibia. The Orange fingerprint, characterized by basaltic rock fragments, clinopyroxene grains and bimodal zircon-age spectra with peaks at ca 0•5 Ga and ca 1•0 Ga, is lost abruptly at Namibe, and beach sands further north have abundant feldspar, amphibole-epidote suites and unimodal zircon-age spectra with a peak at ca 2•0 Ga, documenting local provenance from Palaeoproterozoic basement. Along with this oblique-rifted continental margin, beach placers are dominated by Fe-Ti-Cr oxides with more monazite than garnet and thus have a geochemical signature sharply different from beach placers found all the way along the Orange littoral cell. High-resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of kilometres, providing essential information for the correct reconstruction of ‘source to sink’ relationships in hydrocarbon exploration and to predict the long-term impact of man-made infrastructures on coastal sediment budgets.
Russian Geology and Geophysics, Vol. 58, pp. 1305-1316.
Africa, Angola
eclogites
Abstract: We studied the Sm-Nd, Rb-Sr, and Re-Os isotope compositions of mantle xenoliths (eclogites and peridotites) from diamondiferous kimberlites of the Catoca cluster of the Kasai Craton. In the eclogites, the primary strontium isotope composition 87Sr/86Sr varies from 0.7056 to 0.7071, and the neodymium isotope composition eNd, from 1.8 to 2.6. The 187Re/188Os and 187Os/188Os ratios range from 135 to 80 and from 1.3110 to 1.9709, respectively, which indicates a significant portion of radiogenic Os: yOs = 129-147. These isotope values exceed the values assumed for model reservoirs (primitive upper mantle (PUM) and bulk silicate Earth (BSE)) and those of chondrites. The isotope composition of the studied systems indicates the formation of eclogites from a rhenium-enriched source, namely, the subducted oceanic crust transformed as a result of metasomatism and/or melting under upper-mantle conditions.
Abstract: The Precambrian geology of eastern Zambia and Malawi is highly complex due to multiple episodes of rifting and collision, particularly during the formation of Greater Gondwana as a product of the Neoproterozoic Pan-African Orogeny. The lithospheric structure and extent of known Precambrian tectonic entities of the region are poorly known as there have been to date few detailed geophysical studies to probe them. Herein, we present results from electromagnetic lithospheric imaging across Zambia into southern Malawi using the magnetotelluric method complemented by high-resolution aeromagnetic data of the upper crust in order to explore the extent and geometry of Precambrian structures in the region. We focus particularly on determining the extent of subcontinental lithospheric mantle (SCLM) beneath the Archean-Paleoproterozoic cratonic Bangweulu Block and the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide Orogenic Belts. We also focus on imaging the boundaries between these tectonic entities, particularly the boundary between the Irumide and Southern Irumide Belts. The thickest and most resistive lithosphere is found beneath the Bangweulu Block, as anticipated for stable cratonic lithosphere. Whereas the lithospheric thickness estimates beneath the Irumide Belt match those determined for other orogenic belts, the Southern Irumide Belt lithosphere is substantially thicker similar to that of the Bangweulu Block to the north. We interpret the thicker lithosphere beneath the Southern Irumide Belt as due to preservation of a cratonic nucleus (the pre-Mesoproterozoic Niassa Craton). A conductive mantle discontinuity is observed between the Irumide and Southern Irumide Belts directly beneath the Mwembeshi Shear Zone. We interpret this discontinuity as modified SCLM relating to a major suture zone. The lithospheric geometries determined from our study reveal tectonic features inferred from surficial studies and provide important details for the tectonothermal history of the region.
The diamond bearing territories of Africa and their importance for expansion of the raw material base of the Russian diamond mining industry. ***IN RUS
Mineral Resources of Russia: economics and Management *** IN RUS, No. 6, pp. 66-72. pdf
Abstract: MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) xenoliths are transported from the deep-cratonic lithosphere to the Earth's surface by Cretaceous kimberlites emplaced in the Kimberley area of the Kaapvaal Craton. MARID xenoliths have high modal abundances (70-80?vol%) of mica and amphibole, indicating their origin from a hydrous source. The ?18O values (4.7????18O???6.9‰) of phlogopite micas from 14 MARID samples indicate that these minerals are both 18O-enriched and 18O-depleted with respect to the average upper mantle ?18O value of 5.8?±?0.6‰. The range of ?2H values of phlogopites (?83????2H????53‰, n?=?14) of MARID xenoliths are slightly larger than the average mantle ?2H value (?70?±?10‰). The oxygen (?18Ophlogopites-amphibole?=??0.4 and 0.4‰) and hydrogen (?2Hphlogopite-amphibole?=?14 and 36‰) isotopic disequilibrium recorded from two MARID xenoliths suggests the duration of the last isotopic exchange, possibly just before the kimberlite emplacement, between these minerals and metasomatic fluids was too short to reach isotopic equilibrium. Our model calculation indicates that the phlogopites of MARID xenoliths underwent isotopic exchange with fluids of ?18O?=?5.5 to 10‰, ?2H?=??62 to ?90‰. The range of ?18O value of the calculated metasomatic fluids resembles the oxygen isotopic composition of the primary mantle carbonate (~ 6-9‰) suggesting interaction between carbonatic melt and MARID xenoliths in the continental lithospheric mantle. Furthermore, ?18O values of phlogopites together with previously published nitrogen isotope data (?11 ? ?15N ? 9‰; Banerjee et al., 2015) indicates incorporation of inhomogeneously distributed recycled crustal material from subducted crust within their source magma. Therefore, O-H-N isotope data for MARID xenoliths indicates their crystallization from geochemically heterogeneous magma in the upper continental mantle and subsequent metasomatism with mantle fluids.
Anais da Academia Brasileira de Ciencas, Vol. 78, 2, pp. 309-315.
Africa, Angola
deposit - Camafuca
Abstract: This work presents a geochemical study of a set of garnets, selected by their colors, from the Camafuca-Camazambo kimberlite, located on northeast Angola. Mantle-derived garnets were classified according to the scheme proposed by Grütter et al. (2004) and belong to the G1, G4, G9 and G10 groups. Both sub-calcic (G10) and Ca-saturated (G9) garnets, typical, respectively, of harzburgites and lherzolites, were identified. The solubility limit of knorringite molecule in G10D garnets suggests they have crystallized at a minimum pressure of about 40 to 45 kbar (4-4.5 GPa). The occurrence of diamond stability field garnets (G10D) is a clear indicator of the potential of this kimberlite for diamond. The chemistry of the garnets suggests that the source for the kimberlite was a lherzolite that has suffered a partial melting that formed basaltic magma, leaving a harzburgite as a residue.
Mineralogy and Petrology, in press available, 16p.
Africa, Botswana
diamond exploration
Abstract: Bechuanaland/Botswana has a long and colourful history in exploration and mining. Here these activities are subdivided into three phases: pre-historic, historic and modern. Quarrying stone in Botswana was ongoing 500,000 years ago during the Early Stone Age (ESA). Actual mining of stones probably only started during the Middle Stone Age (MSA) i.e. post 250,000 BP, and the first prehistoric hard rock mining of specularite and limonite, likely started during the Late Stone Age (LSA) 20,000 to 2,000 BP. In east Botswana iron and copper were mined from AD 800 onwards; the mining of gold started in the thirteenth century. Historic mining started with the re-discovery of gold close to Francistown in 1865 and lasted until the 1950s. Rumours of diamonds in Bechuanaland had already surfaced in the 1880s, and it was Ngamiland, in the northwest, that was first explored systematically for diamonds and gold between 1896 and 1899. A joint initiative between Anglo American and De Beers started serious prospecting parts of eastern Bechuanaland between 1932 and 1938; and in 1938 the first diamond finds in Bechuanaland were reported. Modern mining and exploration started with the signing of an agreement in 1959, allowing Consolidated African Selection Trust Ltd. (CAST) into the Bamangwato Tribal Reserve. CAST found a few diamonds in the Motloutse River, but concluded that these were reworked and dropped the exploration rights. De Beers believed that these diamonds had come from west of the Motloutse headwaters, across the watershed in the Kalahari. This ultimately led to the discovery of the Orapa kimberlite field in 1967, a year after Botswana became independent. This discovery triggered a major exploration boom across Botswana adding important diamond-bearing kimberlites such as at Letlhakane (1968), Jwaneng (1973), Gope (1981) and Lerala (1991).
The Extractive Industries and Society, Vol. 4, pp. 489-496.
Africa, Guinea
artisanal mining
Abstract: The period of protracted conflict in Sierra Leone and Liberia brought the politics of alluvial diamond mining in West Africa to the forefront of academic and policy-oriented discussions. Using social contract theory, this paper moves away from discussions on how minerals have perpetuated conflict in the region, and interrogates how the governance of diamond mining in Guinea impacts regime stability and social insecurity. More importantly, it attempts to illustrate how artisanal diamond mining contributes to stability. The paper situates this discussion within the broad spectrum of the social contract between state and citizens and an analysis of how these are at play in diamond mining areas. It illustrates how artisanal diamond mining enables specific social contracts to emerge and how this in turn contributes to stability in the regions where they are extracted.
Contrasting termite transported indicator mineral concentrations in the Kgalafadi of central district Botswana: Macrotermes micaelseni vs Hodotermes mossambicus.
Vancouver Kimberlite Cluster, March 9, 1p. Abstract
Mineralogical Magazine, Vol. 81, 6, pp. 1367-1395.
Africa, Malawi
carbonatite
Abstract: Carbonatites are enriched in critical raw materials such as the rare earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and RE minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study (Natural History Museum, London collection BM1968 P37). In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, RE minerals and pyrochlore in vein networks. Petrography using SEM-EDX showed that the RE minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence of REE mobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by LA-ICP-MS) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallisation of plagioclase and co-crystallisation with K-feldspar in the fenite. The fenite minerals have consistently higher mid REE/light REE ratios (La/Sm = ~1.3 monazite, ~1.9 bastnäsite, ~1.2 parisite) than their counterparts in the carbonatites (La/Sm = ~2.5 monazite, ~4.2 bastnäsite, ~3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few µm in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and barite were found in the inclusions. Decrepitation of inclusions occurred at around 200?C before homogenisation but melting temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of around 24 wt.% NaCl equivalent was determined. Among the trace elements in whole rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, Y, and REE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct RE mineral micro-assemblages in fenite at some distance from carbonatite could be developed as an exploration indicator of REE enrichment.
Earth and Planetary Science Letters, Vol. 401, pp. 132-147.
Africa, South Africa
metasomatism
Abstract: The Karoo igneous rocks (174-185 Ma) of southern Africa represent one of the largest continental flood basalt provinces on Earth. Available evidence indicates that Karoo magmas either originated in the asthenosphere and were extensively modified by interaction with the lithospheric mantle prior to emplacement in the upper crust; or were produced by partial melting of enriched mantle lithosphere. However, no direct evidence of interaction by Karoo melts (or their precursors) with lithospheric mantle rocks has yet been identified in the suites of mantle xenoliths sampled by post-Karoo kimberlites in southern Africa. Here we report U-Pb ages for lindsleyite-mathiasite (LIMA) titanate minerals (crichtonite series) from three metasomatised, phlogopite and clinopyroxene-rich peridotite xenoliths from the ?84 Ma Bultfontein kimberlite (Kimberley, South Africa), located in the southern part of the Karoo magmatic province. The LIMA minerals appear to have formed during metasomatism of the lithospheric mantle by fluids enriched in HFSE (Ti, Zr, Hf, Nb), LILE (K, Ba, Ca, Sr) and LREE. LIMA U-Pb elemental and isotopic compositions were measured in situ by LA-ICP-MS methods, and potential matrix effects were evaluated by solution-mode analysis of mineral separates. LIMA minerals from the three samples yielded apparent U-Pb ages of , and (). A single zircon grain extracted from the ?190 Ma LIMA-bearing sample produced a similar U-Pb age of , within uncertainty of the LIMA ages. These data provide the first robust evidence of fluid enrichment in the lithospheric mantle beneath the Kimberley region at ?180-190 Ma, and suggest causation of mantle metasomatism by Karoo melts or their precursor(s). The results further indicate that U-Pb dating of LIMA minerals provides a new, accurate tool for dating metasomatic events in the lithospheric mantle.
Earth and Planetary Science Letters, Vol. 482, pp. 253-264.
Africa, South Africa
metasomatism
Abstract: Radiogenic isotope variations unrelated to radiogenic ingrowth are common between minerals found in metasomatised mantle xenoliths entrained in kimberlite, basalts and related magmas. As the metasomatic minerals are assumed to have been in isotopic equilibrium originally, such variations are typically attributed to contamination by the magma host and/or interaction with mantle fluids during or before xenolith transport to surface. However, the increasing evidence of metasomatism by multiple, compositionally distinct fluids permeating the lithospheric mantle, coeval with specific magmatic events, suggests that isotopic disequilibrium might be a consequence of discrete, though complex, metasomatic events. Here we provide clear evidence of elemental and Sr isotope heterogeneity between coeval Ti-rich LIMA (lindsleyite–mathiasite) minerals at the time of their formation in the mantle. LIMA minerals occur in close textural association with clinopyroxene and phlogopite in low-temperature (?800–900?°C), strongly metasomatised mantle xenoliths from the ?84 Ma Bultfontein kimberlite (South Africa). Previous U/Pb dating of the LIMA phases was used to argue that each xenolith recorded a single event of LIMA crystallisation at ?180–190 Ma, coeval with the emplacement of Karoo magmas. SEM imaging reveals that up to four types of LIMA phases coexist in each xenolith, and occasionally in a single LIMA grain. Major element and in situ Sr isotope analyses of the different LIMA types show that each phase has a distinct elemental composition and initial 87Sr/86Sr ratio (e.g., 0.7068–0.7086 and 0.7115–0.7129 for two LIMA types in a single xenolith; 0.7053-0.7131 across the entire sample suite). These combined age and isotopic constraints require that multiple fluids metasomatised these rocks at broadly the same time (i.e. within a few thousands to millions of years), and produced similar mineralogical features. Elemental and isotopic variations between different LIMA types could be due to interaction between one (or more) Karoo-related Ti-rich silicate melts and previously metasomatised, phlogopite-rich lithospheric mantle. This study demonstrates that mantle metasomatic assemblages seemingly generated in a single event may instead result from the infiltration of broadly coeval fluids with variable compositions. This in turn implies that the isotopic variations recorded in mantle rocks may be an inherent feature of metasomatism, and that hot fluids infiltrating a rock do not necessarily cause equilibration at the cm scale, as has been assumed previously. Simple modelling of solid-state diffusion in mantle minerals shows that isotopic disequilibrium may be preserved for up to hundreds of Myr at mantle lithosphere temperatures (?1100–1200?°C), unless subsequently affected by transient heating and/or fluid infiltration events. Radiogenic isotope disequilibrium associated with mantle metasomatism may therefore be a common feature of mantle xenoliths.
Abstract: The Neoproterozoic peridotite-chromitite complexes in the Central Eastern Desert of Egypt, being a part of the Arabian-Nubian Shield, are outcropped along the E-W trend from Wadi Sayfayn, Wadi Bardah, and Jabal Al-Faliq to Wadi Al-Barramiyah, from east to west. Their peridotites are completely serpentinized, and the abundance of bastite after orthopyroxene suggests harzburgite protoliths with subordinate dunites, confirmed by low contents of Al2O3, CaO and clinopyroxene (< 3 vol%) in bulk peridotites. The primary olivine is Fo89.3-Fo92.6, and the residual clinopyroxene (Cpx) in serpentinites contains, on average, 1.1 wt% Al2O3, 0.7 wt% Cr2O3, and 0.2 wt% Na2O, similar in chemistry to that in Izu-Bonin-Marian forearc peridotites. The wide range of spinel Cr-number [Cr/(Cr + Al)], 0.41-0.80, with low TiO2 (0.03 wt%), MnO (0. 3 wt%) and YFe [(Fe3 +/(Cr + Al + Fe3 +) = 0.03 on average)] for the investigated harzburgites-dunites is similar to spinel compositions for arc-related peridotites. The partial melting degrees of Bardah and Sayfayn harzburgites range mainly from 20 to 25% and 25 to 30% melting, respectively; this is confirmed by whole-rock chemistry and Cpx HREE modelling (~ 20% melting). The Barramiyah peridotite protoliths are refractory residues after a wide range of partial melting, 25-40%, where more hydrous fluids are available from the subducting slab. The Neoproterozoic mantle heterogeneity is possibly ascribed mainly to the wide variations of partial melting degrees in small-scale areas, slab-derived inputs and primordial mantle compositions. The Sayfayn chromitites were possibly crystallized from island-arc basaltic melts, followed by crystallization of Barramiyah chromitites from boninitic melt in the late stage of subduction. The residual Cpx with a spoon-shape REE pattern is rich in both LREE and fluid-mobile elements (e.g., Pb, B, Li, Ba, Sr), but poor in HFSE (e.g., Ta, Nb, Zr, Th), similar to Cpx in supra-subduction zone (SSZ) settings, where slab-fluid metasomatism is a prevalent agent. The studied chromitites and their host peridotites represent a fragment of sub-arc mantle, and originated in an arc-related setting. The systematic increase in the volume of chromitite pods with the increasing of their host-peridotite thickness from Northern to Southern Eastern Desert suggests that the thickness of wall rocks is one factor controlling the chromitite size. The factors controlling the size of Neoproterozoic chromitite pods are the thickness, beside the composition, of the host refractory peridotites, compositions and volumes of the supplied magmas, the amount of slab-derived fluids, and possibly the partial melting degree of the host peridotites.
International Journal of Earth Sciences, Vol. 106, 8, pp. 1-36.
Africa, Namibia
craton
Abstract: Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko-Dom Feliciano-Gariep orogenic system along the coasts of the southern Atlantic Ocean. In the Coastal Terrane (Kaoko Belt, Namibia), the bimodal character of the ca. 820-785 Ma magmatic suite and associated sedimentation sourced in the neighbouring pre-Neoproterozoic crust are taken as evidence that the Coastal Terrane formed as the shallow part of a developing back arc/rift. The arc-like chemistry of the bimodal magmas is interpreted as inherited from crustal and/or lithospheric mantle sources that have retained geochemical signature acquired during an older (Mesoproterozoic) subduction-related episode. In contrast, the mantle contribution was small in ca. 800-770 Ma plutonic suites in the Punta del Este Terrane (Dom Feliciano Belt, Uruguay) and in southern Brazil; still, the arc-like geochemistry of the prevalent felsic rocks seems inherited from their crustal sources. The within-plate geochemistry of a subsequent, ca. 740-710 Ma syn-sedimentary volcanism reflects the ongoing crustal stretching and sedimentation on top of the Congo and Kalahari cratons. The Punta del Este-Coastal Terrane is interpreted as an axial part of a Neoproterozoic “Adamastor Rift”. Its opening started in a back-arc position of a long-lasting subduction system at the edge of a continent that fragmented into the Nico Pérez-Luís Alves Terrane and the Congo and Kalahari cratons. The continent had to be facing an open ocean and consequently could not be located in the interior of the Rodinia. Nevertheless, the early opening of the Adamastor Rift coincided with the lifetime of the circum-Rodinia subduction system.
International Journal of Earth Sciences, Vol. 106, 8, pp. 2797-2813.
Africa, Namibia
carbonatite
Abstract: Rb-Sr whole-rock and mineral isotope data from nepheline syenite, tinguaite, and carbonatite samples of the Kalkfeld Complex within the Damaraland Alkaline Province, NW Namibia, indicate a date of 242 ± 6.5 Ma. This is interpreted as the age of final magmatic crystallization in the complex. The geological position of the complex and the spatially close relationship to the Lower Cretaceous Etaneno Alkaline Complex document a repeated channeling of small-scale alkaline to carbonatite melt fractions along crustal fractures that served as pathways for the mantle-derived melts. This is in line with Triassic extensional tectonic activity described for the nearby Omaruru Lineament-Waterberg Fault system. The emplacement of the Kalkfeld Complex more than 100 Ma prior to the Paraná-Etendeka event and the emplacement of the Early Cretaceous Damaraland intrusive complexes excludes a genetic relationship to the Tristan Plume. The initial ?Sr-?Nd pairs of the Kalkfeld rocks are typical of younger African carbonatites and suggest a melt source, in which EM I and HIMU represent dominant components.
The Extractive Industries and Society, Vol. 4, pp. 172-179.
Africa, Zimbabwe
artisanal mining
Abstract: The deployment of soldiers’ in diamond mining areas in Zimbabwe is beginning to receive much scholarly attention, partly because scholars researching on such issues are mainly driven by the meta-narratives of ‘artisanal miners’, popularly known as "magweja" in the Zimbabwean context. In many cases soldiers are presented as perpetrators of violence within deployed areas, with magweja and villagers, as victims of military deployment. While this is not untrue, this article explores the ways in which soldiers developed mutual relationships with magweja, friendships and interactions with villagers, including sexual relationships in the communities surrounding the diamond area. However, we assert that these relationships were always implicitly characterised by unequal power dynamics in which the authority of deployed soldiers was ominously present and fear sometimes informed the actions of villagers. Soldiers’ power was always materially represented by the authority vested in them by a powerful demagogic state. It was visibly represented through their combat uniforms, and the gun as part of their daily standing orders, including other soldiering routines such as patrols, and guard duties. We argue that even beyond these circumscribed conditions of relations building, the people behind the combat uniforms had their own agencies that never always resonated with that of the state. Their actions were informed by the chain of command and the consequences wrought by failing to abide by standing orders. In this article, we illuminate those hidden narratives and expose the manner in which the dictatorial state instrumentalises its armed forces despite the disparate personal agencies of those soldiers which are suppressed by the very nature of military service. The paper is based on interviews with soldiers, "magweja" and villagers.
Abstract: Establishing relationships between the long-term landscape evolution of drainage basins and the fill of sedimentary basins benefits from analysis of bedrock river terrace deposits. These fragmented detrital archives help to constrain changes in river system character and provenance during sediment transfer from continents (source) to oceans (sink). Thick diamondiferous gravel terrace deposits along the lower Orange River, southern Namibia, provide a rare opportunity to investigate controls on the incision history of a continental-scale bedrock river. Clast assemblage and heavy mineral data from seven localities permit detailed characterisation of the lower Orange River gravel terrace deposits. Two distinct fining-upward gravel terrace deposits are recognised, primarily based on mapped stratigraphic relationships (cross-cutting relationships) and strath and terrace top elevations, and secondarily on the proportion of exotic clasts, referred to as Proto Orange River deposits and Meso Orange River deposits. The older early to middle Miocene Proto Orange River gravels are thick (up to 50 m) and characterised by a dominance of Karoo Supergroup shale and sandstone clasts, whereas the younger Plio-Pleistocene Meso Orange River gravels (6-23 m thick) are characterised by more banded iron formation clasts. Mapping of the downstepping terraces indicates that the Proto gravels were deposited by a higher sinuosity river, and are strongly discordant to the modern Orange River course, whereas the Meso deposits were deposited by a lower sinuosity river. The heavy minerals present in both units comprise magnetite, garnet, amphibole, epidote and ilmenite, with rare titanite and zircon grains. The concentration of amphibole-epidote in the heavy minerals fraction increases from the Proto to the Meso deposits. The decrease in incision depths, recorded by deposit thicknesses above strath terraces, and the differences in clast character (size and roundness) and type between the two units, are ascribed to a more powerful river system during Proto-Orange River time, rather than reworking of older deposits, changes in provenance or climatic variations. In addition, from Proto- to Meso-Orange River times there was an increase in the proportion of sediments supplied from local bedrock sources, including amphibole-epidote in the heavy mineral assemblages derived from the Namaqua Metamorphic Complex. This integrated study demonstrates that clast assemblages are not a proxy for the character of the matrix, and vice versa, because they are influenced by the interplay of different controls. Therefore, an integrated approach is needed to improve prediction of placer mineral deposits in river gravels, and their distribution in coeval deposits downstream.
Abstract: The compositions of kimberlite melts at depth and upon emplacement in the upper crust remain elusive. This can be attributed to the unquantified effects of multiple processes, such as alteration, assimilation, xenocryst contamination, and fractional crystallisation. The inability to accurately constrain the composition and physical properties of kimberlite melts prevents a comprehensive understanding of their petrogenesis. To improve constraints on the compositions of kimberlite melts, we have combined modal analysis including the discrimination of xenocrystic from magmatic phases, with mineral chemistry determinations to reconstruct a whole-rock composition. We apply this approach to a sample of “fresh” macrocrystic hypabyssal kimberlite (sample BK-1) from the Bultfontein mine (Kimberley, South Africa). The accuracy of this whole-rock reconstruction method is validated by the similarity between reconstructed and measured whole-rock compositions. A series of corrections are then applied to account for the effects of post-emplacement serpentinisation, pre-emplacement olivine crystallisation, and the inclusion and assimilation of mantle material. This approach permits discernment of melt compositions at different stages of kimberlite evolution. The primitive melt parental to the Bultfontein kimberlite is estimated to contain 17.4-19.0?wt% SiO2, 20.2-22.8?wt% MgO, 20.9-21.9?wt% CaO, 2.1-2.3?wt% P2O5, 1.2-1.4?wt% TiO2, 0.9-1.1?wt% Al2O3, and 0.6-0.7?wt% K2O, and has a Mg# of 83.4-84.4. Primary volatile contents (i.e., after an attempt to account for volatile loss) are tentatively estimated at ~2.1-2.2?wt% H2O and ~22.9-25.4?wt% CO2. This composition is deficient in SiO2, MgO and H2O, but enriched in CaO and CO2 compared with most previous estimates of primitive kimberlite melts. We suggest that the primitive melt parental to the Bultfontein kimberlite was a transitional silicate-carbonate melt, which was progressively enriched in SiO2, MgO, Al2O3, Cr2O3, and Na2O through the assimilation of lithospheric mantle material. Comparisons with experimentally produced low-degree melts of carbonated lherzolite indicate that the Bultfontein kimberlite could have formed by ~0.5% melting of asthenospheric mantle at ~6.0-8.6?GPa (i.e., ~190-285?km) and ~1400-1500?°C. The low calculated Na2O contents (<0.2?wt%), which are inconsistent with derivation from low-degree melting of lherzolite, suggest that an alkali-bearing, volatile-rich fluid was exsolved during ascent or released after emplacement, and subsequently removed.
gssaconferences.co.za /compliance -jse-reporting, June 28,29 th
Africa, South Africa
diamond resource and reserve reporting
Abstract: This two-day Workshop (28th and 29th June 2018) comprises an introduction to the SAMREC and SAMVAL Codes, and JSE Reporting. This course is aimed at geologists, mining engineers and other technical specialists, who include sign-off as Competent Persons ("CPs") or Competent Valuators ("CVs") in their job description. Day one focuses on the basics of the SAMREC and SAMVAL Codes, and concentrates on the requirements for CPs who compile documents, specifically Competent Persons Reports and Integrated Annual Reports, for companies listed on the Johannesburg Stock Exchange (applicable primarily to Solid Minerals). Day two focuses on the Section 12 Listing rules applicable to both the Main Board and the Alt-X as well as the requirements of SAMREC and SAMVAL, highlighting some of the issues experienced by CPs over the years as well as some of the updated requirements as a result of the implementation of the 2016 SAMREC/SAMVAL Codes. An introduction to the JSE Readers Panel and a discussion of some of the on-going compliance issues identified by the panel.
Abstract: Using low?temperature thermochronology on apatite and zircon crystals, we show that the western Reguibat Shield, located in the northern part of the West African Craton, experienced significant cooling and heating events between Jurassic and present times. The obtained apatite fission track ages range between 49 and 102 Ma with mean track lengths varying between 11.6 and 13.3 ?m and Dpar values between 1.69 and 3.08 ?m. Zircon fission track analysis yielded two ages of 159 and 118 Ma. Apatite (U-Th)/He uncorrected single?grain ages range between 76 and 95 Ma. Thermal inverse modelling indicates that the Reguibat Shield was exhumed during the Early Cretaceous, Late Cretaceous, Palaeocene-Eocene and Quaternary. These exhumation events were coeval with regional tectonic and geodynamic events, and were probably driven by a combined effect of plate tectonics and mantle dynamics.
South African Journal of Geology, Vol. 120, 1, pp. 1-20.
Africa, South Africa
deposit - Jagersfontein
Abstract: Our earlier studies continued in a diligent search for rare ultradeep xenoliths in the kimberlite diatreme at Jagersfontein. The search has met with moderate success insofar as 20 majorite-bearing (decomposed to *gt + lamellar px) xenoliths are identified. Discrete gts (1 to 2 cm), gt-rich layers (2 to 3 cm) in lherzolites, and rare megacrystic gts (>3 cm) from xenoliths characterize the ultradeep suite. Pyroxene lamellae are crystallographically controlled along {111} gt planes, but px may also be prismatic, lensoidal, coarsely graphic, or annular to gt; jigsaw, rather than 120° dihedral textures, are typical. Gt ranges from Pyr68-74 mole% and CaO - Cr2O3 relations, with two exceptions, are distinctly lherzolitic. Cpx = Wo37-46 mole%, Jd3-19 mole%, with 0.4 to 2.4 wt% Cr2O3; opx = 92 to 95 mole% en, and ol averages 92.5 mole% with maximum wt% 0.1 CaO, 0.4 NiO, and 0.1 Cr2O3. A new class of 10 ultradeep xenoliths has lamellar spinel (Cr/Cr + Al = 0.74; Mg/Mg + Fe = 0.58) in addition to cpx with gt >3wt% Cr2O3 (c.f. 0.5 to 1.5 for sp-free types). Five samples are texturally linked but are compositional outliers to the central body of data: two are sp hosts (Cr# 0.69, Mg# 0.76) and (Cr# 0.74, Mg# 0.57) to gt (Pyr72) + cpx (Jd14); one is a gt megacryst (Pyr80) with sp (Cr# 57, Mg# 69); and the remaining two are unusually rich in chromium with gt = 7.3 to 8.2 wt% Cr2O3, rimmed by cpx (2.3 to 3-3 wt% Cr2O3). In addition, there are 17 xenoliths with compositional affinities to the ultradeep suite but lacking the texturally diagnostic lamellar intergrowths of cpx in gt are possibly completely equilibrated to gt + irregular cpx. Results from the new collection substantiate our earlier conclusions that the mantle was sampled by thejagersfontein kimberlite from the lower lithosphere (250 to 350 km) and the transition zone (435 km) with diagnostic high P-T majorite in lherzolite that decomposed to gt + px at one or more interruptive stations (e.g. lithosphere-asthenosphere boundary), and at one or another time, en route to the surface. Homogeneous majorite occurs as diamond inclusions at Jagersfontein but these are eclogitic, leading to the proposition that the source region in the asthenosphere was an unassimilated mixture of lherzolite and eclogite in the Mid-Cretaceous at the time of kimberlite sampling. Important questions arise: Is majorite primordial; did majorite form exclusively from the transition of pyroxene; or did subsequent dissolution into coexisting garnet take place? Why has majorite not been identified in eclogite, nor diamonds of lherzolitic affinity? Does the formation of majorite and the crystallization of encapsulating diamond imply distinct high P-T events?
South African Journal of Geology, Vol. 120, 3, pp. 371-384.
Africa, Zimbabwe, South Africa
diamond morphology
Abstract: The morphological, chemical impurities and carbon isotope properties of diamonds may reveal subtle details of their mantle source and growth characteristics, supporting efforts towards identifying their original place of harvesting. Here we investigate the mantle carbon and nitrogen sources and growth patterns from selected diamonds mined from four kimberlites: macro-sized diamonds from River Ranch kimberlite in Zimbabwe and the Swartruggens and Klipspringer kimberlitic deposits from South Africa, and micro-sized diamonds from the Klipspringer and Premier kimberlite intrusions in South Africa. Type IaAB diamonds are found in all the samples; Type IaB diamonds only occur in samples from the Swartruggens, River Ranch and Premier kimberlites. A single Type II diamond (nitrogen below the detection limit) was also observed in the River Ranch and Premier kimberlites. Both the micro- and macro-sized diamonds from Klipspringer have similar nitrogen contents. Based on the % B-defect, the diamonds from Klipspringer are grouped into low- and high-nitrogen aggregates (i.e. % of B-defect <40% and >56%, respectively) that likely represent two different diamond forming episodes. Time averaged mantle storage temperatures for Type IaAB diamonds are calculated to have been: 1060°C for Swartruggens; 1190°C for River Ranch; 1100°C (low aggregated); and 1170°C (highly aggregated) for Klipspringer, and 1210°C for Premier diamonds. The CL-images of the River Ranch, Klipspringer and Premier diamonds reveal multi-oscillatory growth zones. The carbon isotopic analyses on the diamonds reveal an average ?13CVPDB value of: -4.5‰ for Swartruggens; -4.7‰ for River Ranch; -4.5‰ for Klipspringer; and -3‰ for Premier. With the exception of the diamond from Premier, the average ?13C value of the diamonds are similar to the average ?13C value of the mantle (-5‰), which is similar to the occurrence of diamonds in the other kimberlites. The internal carbon isotopic variation of individual diamonds from Swartruggens, Klipspringer and Premier are less than 4‰, which is similar to the variability of most other diamond occurrences reported from elsewhere in the world. Up to 6.7‰ internal carbon isotopic variation was observed in a single diamond from River Ranch. The internal carbon isotopic studies of the diamonds reveal that the primary carbon in the Swartruggens and Klipspringer was derived from an oxidation of CH4-bearing fluid, whereas in the River Ranch the primary carbon was derived from the reduction of carbonate-or CO2-bearing fluids. The Swartruggens diamonds also reveal a secondary carbon sourced from a reduction of CO2- or carbonate-rich fluid or melt. Diamonds from Klipspringer exhibit a cyclic change in ?13C values that reflects fluctuation in a complex mantle perturbation system or periodic change in fugacity of the mantle. Based on this study, we conclude that, in principle, a selected range of diamond signatures might be used to fingerprint their origins; especially when linked to their other physical properties such as a low temperature magnetic signature.
Abstract: Using numerical thermo?mechanical experiments we analyse the role of an active mantle plume and pre?existing lithospheric thickness differences in the structural development of the central and southern East African Rift system. The plume?lithosphere interaction model setup captures the essential features of the studied area: two cratonic bodies embedded into surrounding lithosphere of normal thickness. The results of the numerical experiments suggest that localization of rift branches in the crust is mainly defined by the initial position of the mantle plume relative to the cratons. We demonstrate that development of the Eastern branch, the Western branch and the Malawi rift can be the result of non?uniform splitting of the Kenyan plume, which has been rising underneath the southern part of the Tanzanian craton. Major features associated with Cenozoic rifting can thus be reproduced in a relatively simple model of the interaction between a single mantle plume and pre?stressed continental lithosphere with double cratonic roots.
Abstract: The mantle beneath the Cullinan kimberlite (formerly known as "Premier") is a unique occurrence of diamondiferous cratonic mantle where diamonds were generated contemporaneously and shortly following a mantle upwelling that led to the formation of a Large Igneous Province that produced the world's largest igneous intrusion - the 2056?Ma Bushveld Igneous Complex (BIC). We studied 332 diamond inclusions from 202 Cullinan diamonds to investigate mantle thermal effects imposed by the formation of the BIC. The overwhelming majority of diamonds come from three parageneses: (1) lithospheric eclogitic (69%), (2) lithospheric peridotitic (21%), and (3) sublithospheric mafic (9%). The lithospheric eclogitic paragenesis is represented by clinopyroxene, garnet, coesite and kyanite. Main minerals of the lithospheric peridotitic paragenesis are forsterite, enstatite, Cr-pyrope, Cr-augite and spinel; the sublithospheric mafic association includes majorite, CaSiO3 phases and omphacite. Diamond formation conditions were calculated using an Al-in-olivine thermometer, a garnet-clinopyroxene thermometer, as well as majorite and Raman barometers. The Cullinan diamonds may be unique on the global stage in recording a cold geotherm of 40?mW/m2 in cratonic lithosphere that was in contact with underlying convecting mantle at temperatures of 1450-1550?°C. The studied Cullinan diamonds contain a high proportion of inclusions equilibrated at temperatures exceeding the ambient 1327?°C adiabat, i.e. 54% of eclogitic diamonds and 41% of peridotitic diamonds. By contrast, ? 1% of peridotitic diamond inclusions globally yield equally high temperatures. We propose that the Cullinan diamond inclusions recorded transient, slow-dissipating thermal perturbations associated with the plume-related formation of the ~2?Ga Bushveld igneous province. The presence of inclusions in diamond from the mantle transition zone at 300-650?km supports this view. Cullinan xenoliths indicative of the thermal state of the cratonic lithosphere at ~1.2?Ga are equilibrated at the relatively low temperatures, not exceeding adiabatic. The ability of diamonds to record super-adiabatic temperatures may relate to their entrainment from the deeper, hotter parts of the upper mantle un-sampled by the kimberlite in the form of xenoliths or their equilibration in a younger lithosphere after a decay of the thermal disturbance.
Abstract: Lenses of retrogressed eclogites occur in a 100 km wide zone of the Nyong Complex, a remnant of the Eburnian-Transamazonian orogen, marking a Palaeoproterozoic suture between the Congo and São Francisco Cratons. The eclogites show trace element pattern (depleted in LREE) similar to those of mid-ocean ridge basalts, indicating that the precursor melts formed in a depleted mantle source and the eclogites formed from oceanic crust. Despite numerous plagioclase ‘exsolutions’ up to 25 mol% jadeite component is preserved in omphacite and points to minimum pressures of 16 kbar at c. 800 °C. Pressures may have been 18-20 kbar as indicated by estimated compositions of peak omphacite. The age of eclogite metamorphism has been constrained by U-Pb SHRIMP dating of zircon at 2093 ± 45 Ma. The eclogites are associated with 2.05 Ga old charnockites and mafic granulites containing textures characteristic for near-isobaric cooling. These rocks may represent the plate above a subduction zone in which the eclogites were tectonically emplaced. With an age of 2.09 Ga the eclogites of the Nyong Complex are older than other subduction related Palaeoproterozoic eclogites of the Ubendian (1.88 Ga) and Usagaran belts (2.0 Ga) at the southern border of the Tanzania Craton. They are also older than eclogites in the Belomorian province (1.9 Ga; Russia) and thus represent the oldest known eclogites outcropping in an orogenic belt. The African eclogites (all with MORB chemistry) indicate that during the formation of the Nuna supercontinent the Palaeoproterozoic oceanic lithosphere around the Congo-Tanzania Craton was thick, cold and rigid enough to become subducted similar to cold oceanic lithosphere in the modern plate tectonic regime. However, apparent geothermal gradients of 12-14 °C/km for the Palaeoproterozoic eclogites are higher than those of Neoproterozoic and Phanerozoic eclogites and are interpreted as the result of warm subduction in a hotter Palaeoproterozoic Earth.
Abstract: The detrital zircon population in quartzitic conglomerates from the northern Tanzania Craton yield ages between 2640 Ma and 2790 Ma which includes most of the igneous history from this part of the craton. The igneous evolution is characterised by mafic volcanism with an oceanic plateau?like geochemical signature at ~2800 Ma followed by diorite and tonalite-trondhjemite-granodiorite dominated magmatism between 2790 and 2700 Ma, which transitioned into more evolved high?K magmatism between 2700 and 2620 Ma. The ?Hf values of the detrital zircons range from +2.4 to ?1.4 and change with time from radiogenic Hf pre?2700 Ma (98% positive ?Hf) to unradiogenic Hf post?2700 Ma (41% positive ?Hf). The petrological progression from mafic to felsic crust is reflected in the detrital age distribution and Hf isotopes and is consistent with juvenile mafic crust slowly maturing into more evolved felsic crust through a series of successive partial melting events in an oceanic?plateau?like environment.
Earth Planetary Science Letters, Vol. 491, pp. 148-159.
Africa, Australia, Canada, Europe
geothermometry
Abstract: The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100-150 km. The metasomatic component can consist of 0.5-1 wt% water bound in amphibole, antigorite and chlorite, ?0.2 wt% water plus potassium to form phlogopite, or ?5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100-200?°C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.
Ikenne, M., Lahna, A.A., Soderlund, U., Tassinar, C.C.G., Ernst, R.E., Pin, Ch., Youbi, N., El Aouli, EH., Hafid, A., Admou, H., Mata, J., Bouougri, EH., Boumehdi, M.A.
New Mesoproterozoic age constraints for the Taghdout Group, Anti-Atlas ( Morocco): toward a new lithostratigra[hic framework for the Precambrian in the NW margin of the West African Craton.
The First West African Craton and Margins International Workshop WACMA, Held Apr. 24-29. 1p. Abstract
University of Helsinki, Scientific Report, Vol. 8, 1, DOI:10.1038/s41598-018-23661-3
Africa, Europe, Finland
magmatism
Abstract: In the Karoo large igneous province, the geochemical assessment of mantle source variability and structure is hampered by probable crustal contamination overprinting of compositionally diverse flood basalts. Mantle source characteristics have been defined only for exceptional, primitive rock types. Here I use a compiled dataset for over 800 samples to demonstrate that the abundance of Nb relative to Zr, Ti, and Y provides a useful geochemical tracer of mantle sources for variably contaminated rock types of the Karoo province. Variations in the relative abundance of Nb reveal emplacement of distinctive, Nb-undepleted and Nb-depleted magmas in the North Karoo and South Karoo sub-provinces, respectively, and clarify correlation between flood basalts and previously proposed mantle source components. Judging from plate tectonic reconstructions and the compositions of plausible mantle source components, the geochemical bilateral asymmetry in Karoo may reflect tapping of contrasting plume and upper mantle reservoirs in the two sub-provinces.
Reviews of African Political Economy, Vol. 44, 152, pp. 272-291.
Africa, South Africa
history
Abstract: This article examines the history of South African mining over the last 30 years. It notes the declining contribution of mining to the economy, and a drop in employment levels and labour migration. It considers political, legislative and macro-economic changes, as well as mine ownership and control. It addresses the question why a democratically elected government, progressive labour legislation, trade-unionisation and Black Economic Empowerment have made remarkably little difference to working conditions. After examining the trajectories of individual commodities, such as gold, platinum, coal and diamonds, it concludes there has been no fundamental change in the relationship between state and capital.
Journal of African Earth Sciences, Vol. 142, pp. 193-206.
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: Oldoinyo Lengai, a stratovolcano in northern Tanzania, is most famous for being the only currently active carbonatite volcano on Earth. The bulk of the volcanic edifice is dominated by eruptive products produced by silica-undersaturated, peralkaline, silicate magmas (effusive, explosive and/or as cumulates at depth). The recent (2007-2008) explosive eruption produced the first ever recorded pyroclastic flows at this volcano and the accidental lithics incorporated into the pyroclastic flows represent a broad variety of different rock types, comprising both extrusive and intrusive varieties, in addition to various types of cumulates. This mix of different accidental lithics provides a unique insight into the inner workings of the world's only active carbonatite volcano. Here, we focus on the magnetic mineralogy and the rock magnetic properties of a wide selection of samples spanning the spectrum of Oldoinyo Lengai rock types compositionally, as well from a textural point of view. Here we show that the magnetic properties of most extrusive silicate rocks are dominated by magnetite-ulvöspinel solid solutions, and that pyrrhotite plays a larger role in the magnetic properties of the intrusive silicate rocks. The natrocarbonatitic lavas, for which the volcano is best known for, show distinctly different magnetic properties in comparison with the silicate rocks. This discrepancy may be explained by abundant alabandite crystals/blebs in the groundmass of the natrocarbonatitic lavas. A detailed combination of petrological/mineralogical studies with geophysical investigations is an absolute necessity in order to understand, and to better constrain, the overall architecture and inner workings of the subvolcanic plumbing system. The results presented here may also have implications for the quest in order to explain the genesis of the uniquely natrocarbonatitic magmas characteristic of Oldoinyo Lengai.
The Journal of the Southern African Institute of Mining and Metallurgy, Vol. 118, Jan. pp. 1-6.
Africa, South Africa
diamond size frequency
Abstract: Diamond size frequency distribution (SFD) curves, combined with the associated dollar per carat per size class, play an important role in the diamond industry. Value per size class is unique for each deposit and typically varies from less than a dollar per carat to several thousands of dollars per carat for special stones. Recovery of large stone therefore contributes significantly to the bottom line of a large diamond producer. While the design of the process plant should prevent damage and possible breakage of large stones, it should also ensure adequate liberation of the finer diamonds. Innovative solutions are required to protect and recover type I and II diamonds if prominent within the resource. X-ray transmission (XRT) sorting presents the opportunity to develop flow sheet designs that incorporate a balance between exploitation of the resource and process efficiency, as well as practical capital and operating costs. This paper serves to illustrate the role and application of SFD curves in determining optimum cut-off and re-crush sizes within the flow sheet of a large diamond producer. A thorough understanding of the unique technical and economic aspects of a deposit provides the basis from where new and innovative technologies can be proposed, allowing mining companies to maintain and improve profit margins. It highlights the results of various plant trials and newly commissioned XRT sorters for larger size fractions. It also provides recommendations for future applications of XRT machines in the diamond process flow sheet.
Abstract: It is well established that the cratonic subcontinental lithospheric mantle (C-SCLM) represents a residue of extensively melted peridotite. The widespread occurrence of garnet in C-SCLM remains a paradox because experiments show that it should be exhausted beyond ?20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene; however, the few documented examples of garnet exsolution in cratonic samples are exotic and do not afford a direct link to garnet in granular harzburgite. We report crystallographic, petrographic, and chemical data for an exceptionally well preserved orthopyroxene megacryst containing garnet lamellae, juxtaposed against granular harzburgite. Garnet lamellae are homogeneously distributed within the host orthopyroxene and occur at an orientation that is unrelated to orthopyroxene cleavage, strongly indicating that they formed by exsolution. Garnet lamellae are subcalcic Cr-pyrope, and the orthopyroxene host is high-Mg enstatite; these phases equilibrated at 4.4 GPa and 975 °C. The reconstructed precursor is a high-Al enstatite that formed at higher pressure and temperature conditions of ?6 GPa and 1750 °C. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. We hypothesize that high-Al enstatite was a common phase in the C-SCLM and that exsolution during cooling and stabilization of the C-SCLM could be the origin of most subcalcic garnets in depleted peridotites.
Earth Planetary Science Letters, Vol. 487, pp. 210-220.
Africa
tectonics
Abstract: Rifts are often associated with ancient traces of hotspots, which are supposed to participate to the weakening of the lithosphere. We investigated the expected past trajectories followed by three hotspots (Afar, East-Africa and Lake-Victoria) located around the Red Sea. We used a hotspot reference frame to compute their location with respect to time, which is then compared to mantle tomography interpretations and geological features. Their tracks are frequently situated under continental crust, which is known to strongly filter plume activity. We looked for surface markers of their putative ancient existence, such as volcanism typology, doming, and heat-flow data from petroleum wells. Surface activity of the East-Africa hotspot is supported at 110 Ma, 90 Ma and 30 Ma by uplift, volcanic activity and rare gas isotopic signatures, reminiscent of a deep plume origin. The analysis of heat-flow data from petroleum wells under the Arabian plate shows a thermal anomaly that may correspond to the past impact of the Afar hotspot. According to derived hotspot trajectories, the Afar hotspot, situated (at 32 Ma) 1000 km north-east of the Ethiopian-Yemen traps, was probably too far away to be accountable for them. The trigger of the flood basalts would likely be linked to the East-Africa hotspot. The Lake-Victoria hotspot activity appears to have been more recent, attested only by Cenozoic volcanism in an uplifted area. Structural and thermal weakening of the lithosphere may have played a major role in the location of the rift systems. The Gulf of Aden is located on inherited Mesozoic extensional basins between two weak zones, the extremity of the Carlsberg Ridge and the present Afar triangle, previously impacted by the East-Africa hotspot. The Red Sea may have opened in the context of extension linked to Neo-Tethys slab-pull, along the track followed by the East Africa hotspot, suggesting an inherited thermal weakening.
Earth Planetary Science Letters, Vol. 491, pp. 134-147.
Africa, South Africa
deposit - De Beers-Pool
Abstract: Fluid/melt inclusions in diamonds, which were encapsulated during a metasomatic event and over a short period of time, are isolated from their surrounding mantle, offering the opportunity to constrain changes in the sub-continental lithospheric mantle (SCLM) that occurred during individual thermo-chemical events, as well as the composition of the fluids involved and their sources. We have analyzed a suite of 8 microinclusion-bearing diamonds from the Group I De Beers Pool kimberlites, South Africa, using FTIR, EPMA and LA-ICP-MS. Seven of the diamonds trapped incompatible-element-enriched saline high density fluids (HDFs), carry peridotitic mineral microinclusions, and substitutional nitrogen almost exclusively in A-centers. This low-aggregation state of nitrogen indicates a short mantle residence times and/or low mantle ambient temperature for these diamonds. A short residence time is favored because, elevated thermal conditions prevailed in the South African lithosphere during and following the Karoo flood basalt volcanism at ?180 Ma, thus the saline metasomatism must have occurred close to the time of kimberlite eruptions at ?85 Ma. Another diamond encapsulated incompatible-element-enriched silicic HDFs and has 25% of its nitrogen content residing in B-centers, implying formation during an earlier and different metasomatic event that likely relates to the Karoo magmatism at ca. 180 Ma. Thermometry of mineral microinclusions in the diamonds carrying saline HDFs, based on Mg-Fe exchange between garnet-orthopyroxene (Opx)/clinopyroxene (Cpx)/olivine and the Opx-Cpx thermometer, yield temperatures between 875-1080?°C at 5 GPa. These temperatures overlap with conditions recorded by touching inclusion pairs in diamonds from the De Beers Pool kimberlites, which represent the mantle ambient conditions just before eruption, and are altogether lower by 150-250?°C compared to P-T gradients recorded by peridotite xenoliths from the same locality. Oxygen fugacity differs as well. The calculated for the saline HDF compositions (to ?1.34) are higher by about a log unit compared with that recorded by xenoliths at 4-7 GPa. We conclude that enriched saline HDFs mediated the metasomatism that preceded Group I kimberlite eruptions in the southwestern Kaapvaal craton, and that their ‘cold and oxidized’ nature reflects their derivation from a deep subducting slab. This event had little impact on the temperature and redox state of the Kaapvaal lithosphere as a reservoir, however, it likely affected its properties along limited metasomatized veins and their wall rock. To reconcile the temperature and oxygen fugacity discrepancy between inclusions in diamonds and xenoliths, we argue that xenoliths did not equilibrate during the last saline metasomatic event or kimberlite eruption. Thus the P-T-gradients they record express pre-existing lithospheric conditions that were likely established during the last major thermal event in the Kaapvaal craton (i.e. the Karoo magmatism at ca. 180 Ma).
Russian Geology and Geophysics, Vol. 59, pp. 512-524.
Africa, Australia, South America, India
craton
Abstract: This is a synopsis of available data the on crustal structure and properties of thirteen Archean cratons of Gondwanaland (the cratons of Africa, Australia, Antarctica, South America, and the Indian subcontinent). The data include estimates of surface area, rock age and lithology, Moho depth, thickness of lithosphere and sediments, as well as elevations, all summarized in a table. The cratons differ in size from 0.05 x 106 km2 (Napier craton) to 4 x 106 km2 (Congo craton) and span almost the entire Archean period from 3.8 to 2.5 Ga. Sediments are mostly thin, though reach 7 km in the Congo and West African cratons. Elevations above sea level are from 0 to 2 km; some relatively highland cratons (Kaapvaal, Zimbabwe, and Tanzanian) rise to more than 1 km. On the basis of regional seismic data, the Moho map for cratons has been improved. The Moho diagrams for each craton are constructed. The analysis of the available new data shows that the average Moho depth varies from 33 to 44 km: Pilbara (33 km), Grunehogna (35 km), Sao Francisco (36 km), Yilgarn (37 km), Dharwar (38 km), Tanzanian (39 km), Zimbabwe (39 km), Kaapvaal (40 km), Gawler (40 km), Napier (40 km), West Africa (40 km), Congo (42 km), and Amazon (44 km) cratons. The Moho depth within the cratons is less uniform than it was assumed before: from 28 to 52 km. The new results differ significantly from the earlier inference of a relatively flat Moho geometry beneath Archean cratons. According to the new data, early and middle Archean undeformed crust is characterized by a shallow Moho depth (28-38 km), while late Archean or deformed crust may be as thick as 52 km.
Mineralogy and Petrology, in press available, 11p.
Africa, Botswana
deposit - Orapa
Abstract: Spatially resolved analyses reveal considerable isotopic heterogeneity within and among diamonds ranging in size from 0.15 to 4.75 mm from the Orapa Mine, Botswana. The isotopic data are interpreted in conjunction with nitrogen aggregation state data and growth zone relationships from cathodoluminescence images. The integrated information confirms that a distinct diamond growth event (with low IaAB nitrogen aggregation states, moderately high nitrogen contents and ?13C and ?15N values compatible with average mantle values) is younger than the dominant population(s) of Type IaAB diamonds and cores of composite diamonds with more negative and positive ?13C and ?15N values, respectively. A significant proportion of the older diamond generation has high nitrogen contents, well outside the limit sector relationship, and these diamonds are likely to reflect derivation from subducted organic matter. Diamonds with low ?13C values combined with high nitrogen contents and positive ?15N values have not been previously widely recognised, even in studies of diamonds from Orapa. This may have been caused by prior analytical bias towards inclusion-bearing diamonds that are not necessarily representative of the entire range of diamond populations, and because of average measurements from heterogeneous diamonds measured by bulk combustion methods. Two distinct low nitrogen/Type II microdiamond populations were recognised that do not appear to continue into the macrodiamond sizes in the samples studied. Other populations, e.g. those containing residual singly-substituted nitrogen defects, range in size from small microdiamonds to large macrodiamonds. The total diamond content of the Orapa kimberlite thus reflects a complex assortment of multiple diamond populations.
Mineralogy and Petrology, in press available, 14p.
Africa, South Africa
deposit - Bultfontein
Abstract: MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) xenoliths are thought to be formed by intense Bprimary^ mantle metasomatism. These rocks also display secondary features, such as cross-cutting veins and geochemical zonation of matrix minerals, which probably reflect latermetasomatic events. To investigate the nature and origin(s) of these secondary features, 28 MARID and PIC xenoliths from southern African kimberlites and orangeites have been studied. MARID-hosted veins contain both carbonate and Ti-rich phases (e.g., titanite, phlogopite), suggesting that they formed by the infiltration of a carbonated silicate melt. Elevated TiO2 contents in MARID matrix mineral rims are spatially associated with carbonate-dominated veins, suggesting a genetic relationship between vein formation and geochemical zonation. Spongy rims around primaryMARID and PIC clinopyroxene are depleted in Na2O andAl2O3 relative to their cores, possibly reflecting mineral dissolution in the xenoliths during ascent and emplacement of the entraining kimberlite. The preservation of compositional differences between primary and secondary phases in MARID and PIC xenoliths indicates that metasomatism occurred shortly before, or broadly coeval with, kimberlite/orangeite magmatism; otherwise, at typical mantle temperatures, such features would have quickly re-equilibrated. Increased Na2O in some mineral rims (e.g., K-richterite) may therefore reflect equilibration with a more Na-enriched primitive kimberlite melt composition than is commonly suggested. Vein-hosted clinopyroxene 87Sr/86Sri (0.70539 ± 0.00079) in one MARID sample is intermediate between primary clinopyroxene in the sample (0.70814 ± 0.00002) and the host Bultfontein kimberlite (0.70432 ± 0.00005), suggesting that vein minerals are derived from interactions between primary MARID phases and kimberlite-related melts/fluids. Sulfur isotope compositions of barite (?34SVCDT = +4.69 ‰) and sulfides (?34SVCDT = ?0.69 ‰) in carbonate veins reflect equilibration at temperatures of 850-900 °C, consistent with sulfurrich melt/fluid infiltration in the lithospheric mantle. In contrast, vein carbonate C-O isotope systematics (?13CVPDB = ?9.18 ‰ ?18OVSMOW = +17.22‰) are not typical of kimberlites or other mantle carbonates (?13CVPDB = ?3 to ?8‰ ?18OVSMOW = 6 to 9 ), and may represent post-emplacement hydrothermal interactions of the cooling kimberlite with crustal fluids. These constraints suggest protracted metasomatism of MARID rocks shortly before and during entrainment by the host kimberlite.
Geophysical Research Abstracts www.researchgate.net, Vol. 20, EGU2018-128291p. Abstract
Africa, South Africa
diamond inclusions
Abstract: Changing recycling budgets of surface materials and volatiles by subduction of tectonic plates influence the compositions of Earth’s major reservoirs and affect climate throughout geological time. Fluids play a key role in processes governing subduction recycling, but quantifying the exact fate of volatiles introduced into the mantle at ancient and recent destructive plate boundaries remains difficult. Here, we report on the role of fluids and the fate of volatiles and other elements at two very different tectonic settings: 1) at subduction settings, and 2) within the subcontinental lithospheric mantle (SCLM). We will show how olivine-hosted melt inclusions from subduction zones and mineral inclusions in diamond from the SCLM are used to reveal how changing tectonic settings influence volatile cycles with time. Melt inclusions from the complex Italian post-collisional tectonic setting are used to identify changing subduction recycling through time. The use of CO2 in deeply trapped melt inclusions instead of in lavas or volcanic gases provides a direct estimate of deep recycling, minimizing possible effects of contamination during transfer through the crust. The aim is to distinguish if increased recycling of sediments from the down-going plate at continental subduction settings results in increased deep CO2 recycling or if the increased CO2 flux results from crustal degassing of the overriding plate. Both processes likely affected climate through Earth history but could thus far not be discriminated. The study of mineral inclusions and their host diamonds from the SCLM can link changes in the cycling of carbon-rich fluids and the time and process through which the carbon redistribution took place. We use Sm-Nd isotope techniques to date the mineral inclusions and use the carbon isotope data of the host diamonds to investigate the growth conditions. I will present case-studies of peridotitic and eclogitic diamonds from three mines in Southern Africa.
Geochimica et Cosmochimica Acta, Vol. 232, pp. 303-328.
Africa, Morocco, Europe, Spain
UHP
Abstract: We present an integrated geochemical study of ultra-high pressure (UHP) garnet pyroxenites from the Ronda and Beni Bousera peridotite massifs (Betic-Rif Belt, westernmost Mediterranean). Based on their Sr-Nd-Pb-Hf isotopic systematics, we classify UHP garnet pyroxenites into three groups: Group A pyroxenites (Al 2 O 3 : 15-17.5 wt. %) have low initial 87 Sr/ 86 Sr, relatively high ? Nd , ? Hf and 206 Pb/ 204 Pb ratios, and variable 207 Pb/ 204 Pb and 208 Pb/ 204 Pb. Group B pyroxenites (Al 2 O 3 < 14 wt. %) are characterized by high initial 87 Sr/ 86 Sr and relatively low ? Nd , ? Hf and 206 Pb/ 204 Pb ratios. Group C pyroxenites (Al 2 O 3 ~ 15 wt. %) have depleted radiogenic signatures with relatively low initial 87 Sr/ 86 Sr and 206 Pb/ 204 Pb, high ? Nd and ? Hf , and their 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios are similar to those of Group B pyroxenites. The major and trace element and isotopic compositions of UHP garnet pyroxenites support their derivation from ancient (1.5-3.5 Ga) oceanic crust recycled into the mantle and intimately stirred with peridotites by convection. However, the genesis of these pyroxenites requires also the involvement of recycled continental lower crust with an isotopic composition akin to the lower crustal section of the lithosphere where these UHP garnet pyroxenites now reside in. These oceanic and continental crustal components were stirred in different proportions in the convective mantle, originating pyroxenites with a more marked geochemical imprint of either oceanic (Group A) or continental lower crust (Group B), or hybrid compositions (Group C). The pyroxenite protoliths likely underwent several melting events, one of them related to the formation of the subcontinental lithospheric mantle and continental crust, generating restitic UHP garnet pyroxenites now preserved in the Ronda and Beni Bousera orogenic peridotites. The extent of melting was mostly 3 controlled by the bulk Mg-number (Mg#) of the pyroxenite protoliths, where protoliths with low Mg# experienced higher degrees of partial melting than sources with higher Mg#. Positive Eu and Sr anomalies in bulk rocks, indicative of their origin from cumulitic crustal gabbros, are preserved mostly in high Mg# pyroxenites due to their higher melting temperatures and consequent lower partial melting degrees. The results of this study show that the genesis of UHP garnet pyroxenites in orogenic peridotites requires a new recipe for the marble cake mantle hypothesis, combining significant recycling and stirring of both oceanic and continental lower crust in the Earth's mantle. Furthermore, this study establishes a firm connection between the isotopic signatures of UHP pyroxenite heterogeneities in the mantle and the continental lower crust.
Mineralogy and Petrology, 10.1007/ s00710-018- 0577-8, 19p.
Africa
metasomatism, subduction, geobarometry
Abstract: The Kaapvaal craton and its underlying mantle is probably one of the best studied Archean entity in the world. Despite that, discussion is still vivid on important aspects. A major debate over the last few decades is the depth of melting that generated the mantle nuclei of cratons. Our new evaluation of melting parameters in peridotite residues shows that the Cr2O3/Al2O3 ratio is the most useful pressure sensitive melting barometer. It irrevocably constrains the pressure of melting (melt separation) to less than 2 GPa with olivine (ol), orthopyroxene (opx) and spinel (sp) as residual phases. Garnet (grt) grows at increasing pressure during lithosphere thickening and subduction via the reaction opx?+?sp ? grt?+?ol. The time of partial melting is constrained by Re-depletion model ages (TRD) mainly to the Archean (Pearson and Wittig 2008). However, only 3% of the ages are older than 3.1 Ga while crustal ages lie mainly between 3.1 to 2.8 Ga for the W- and 3.7 to 2.8 Ga for the E-block. Many TRD-ages are probably falsified by metasomatism and the main partial melting period was older than 3.1 Ga. Also, Nd- and Hf- model ages of peridotitic lithologies from the W-block are 3.2 to 3.6 Ga old. The corresponding very negative ?Nd (?40) and ?Hf values (?65) signal the presence of subducted crustal components in these old mantle portions. Subducted components diversify the mantle in its chemistry and thermal structure. Adjustment towards a stable configuration occurs by fluid transfer, metasomatism, partial melting and heat transfer. Ages of metasomatism from the Lu-Hf isotope system are 3.2 Ga (Lace), 2.9 Ga (Roberts Victor) and 2.62 Ga (Finsch) coinciding with the collision of cratonic blocks, the growth of diamonds, metamorphism of eclogites and of Ventersdoorp magmatism. The cratonic lithosphere was stabilized thermally by the end of the Archean and cooled since then with a rate of 0.07 °C/Ma.
Mineralogy and Petrology, June 14, DOI:10.1007/ s00710-018- 0593-8, 14p.
Africa, Botswana
deposit - Nxau
Abstract: The Nxau Nxau kimberlites in northwest Botswana belong to the Xaudum kimberlite province that also includes the Sikereti, Kaudom and Gura kimberlite clusters in north-east Namibia. The Nxau Nxau kimberlites lie on the southernmost extension of the Congo Craton, which incorporates part of the Damara Orogenic Belt on its margin. The Xaudum kimberlite province is geographically isolated from other known clusters but occurs within the limits of the NW-SE oriented, Karoo-aged Okavango Dyke Swarm and near NE-SW faults interpreted as the early stages of the East African Rift System. Petrographic, geochronological and isotopic studies were undertaken to characterise the nature of these kimberlites and the timing of their emplacement. The Nxau Nxau kimberlites exhibit groundmass textures, mineral phases and Sr-isotope compositions (87Sr/86Sri of 0.7036?±?0.0002; 2?) that are characteristic of archetypal (Group I) kimberlites. U-Pb perovskite, 40Ar/39Ar phlogopite and Rb-Sr phlogopite ages indicate that the kimberlites were emplaced in the Cretaceous, with perovskite from four samples yielding a preferred weighted average U-Pb age of 84?±?4 Ma (2?). This age is typical of many kimberlites in southern Africa, indicating that the Xaudum occurrences form part of this widespread Late Cretaceous kimberlite magmatic province. This time marks a significant period of tectonic stress reorganisation that could have provided the trigger for kimberlite magmatism. In this regard, the Nxau Nxau kimberlites may form part of a NE-SW oriented trend such as the Lucapa corridor, with implications for further undiscovered kimberlites along this corridor.
Abstract: The diamondiferous Jwaneng kimberlite cluster (~240 Ma) is located on the NW rim of the Archaean Kaapvaal Craton in central Botswana. Previous studies report eclogitic diamond formation in the late Archean (2.9 Ga) and in the Middle Proterozoic (1.5 Ga) involving different mantle and sedimentary components [1;2;3]. Here we report newly acquired Sm- Nd ages of individual eclogitic pyrope-almandine and omphacite inclusions along with their major element data and nitrogen data from the diamond hosts to re-examine Jwaneng’s diamond formation ages. The Sm-Nd isotope analyses were performed via TIMS using 1013? resistors [4]. An initial suite of three pyropealmandine and 14 omphacite inclusions yield 143Nd/144Nd from 0.51102±7 to 0.5155±5. 147Sm/144Nd vary from 0.024 to 0.469. Major element data defines two inclusion populations: (1) seven omphacites with high Mg#, high Cr# and one pyropealmandine with low-Ca define an isochron age of 1.93±0.16 Ga with ?Ndi= +3.5; (2) seven omphacites with low Mg#, low Cr# and two pyrope-almandines with low-Ca define an isochron age of 0.82±0.06 Ga with ?Ndi= +3.7. Nitrogen contents of corresponding diamond host growth zones in Group (1) are ? 50 at.ppm whereas Group (2) range between 50 to 700 at.ppm with N-aggregation > 70 %B. Additional data used to define “co-genetic” inclusion suites include Sr-isotopes and trace elements of the inclusions and carbon isotopes of the diamond hosts. Re-Os data of coexisting sulphide inclusions from the same silicate-bearing diamonds further validates the ages and indicates more periods of diamond formation at Jwaneng than previously assumed. The integrated data indicate the possibility of an extensive Paleoproterozoic diamond-forming event in southern Africa.
Geological Society, London Magmatic Rifting and Active Volcanism, Eds. Wright, T.J., Ayele, A., Ferguson, D.J., Kidane, T., Vye-Brown, X. publ. 2016, SP 420, pp. 11-30. from http://sp.lyell collection.org
Africa
tectonics, geodynamics
Abstract: Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model.
Mineralogy and Petrology, 10.1007/ s00710-018- 0589-4, 14p.
Africa, South Africa, Canada, Northwest Territories, Ontario
deposit - Venetia, Voorspoed, Gahcho Kue, Victor, Snap Lake
Abstract: De Beers kimberlite mine operations in South Africa (Venetia and Voorspoed) and Canada (Gahcho Kué, Victor, and Snap Lake) have the potential to sequester carbon dioxide (CO2) through weathering of kimberlite mine tailings, which can store carbon in secondary carbonate minerals (mineral carbonation). Carbonation of ca. 4.7 to 24.0 wt% (average?=?13.8 wt%) of annual processed kimberlite production could offset 100% of each mine site’s carbon dioxide equivalent (CO2e) emissions. Minerals of particular interest for reactivity with atmospheric or waste CO2 from energy production include serpentine minerals, olivine (forsterite), brucite, and smectite. The most abundant minerals, such as serpentine polymorphs, provide the bulk of the carbonation potential. However, the detection of minor amounts of highly reactive brucite in tailings from Victor, as well as the likely presence of brucite at Venetia, Gahcho Kué, and Snap Lake, is also important for the mineral carbonation potential of the mine sites.
Journal of African Earth Sciences, Vol. 145, pp. 274-283.
Africa, Chad
lineaments
Abstract: This work reports an analysis of the relationships existing between the structural lineaments and the Cenozoic volcanism of the Tibesti area (northern Chad). Shield volcanoes, cinder cones, structural lineaments, intersection points of lineaments and faults are mapped using the combination of Shuttle Radar Topography Mission (SRTM), Digital Elevation Models (DEMs) and Landsat satellite images of the Tibesti Volcanic Province. The interpretation of the distribution of these structural and morphological features allows constraining the structural/tectonic setting of the Tibesti. We show that the relationships between the lineaments and the volcanic centres of the Tibesti province can locally be explained as the result of the combination of two Riedel dextral tectonic systems, respectively oriented at N120°E and N30-35°E. Taking into account the geological features of the area, a geodynamical model is proposed: the emplacement of the Tibesti Volcanic Province results from the reactivation of inherited structures of the Saharan metacraton, characterized by relict rigid cratonic nuclei and metacratonic areas reworked during the Pan-African orogeny, among which is located the Tibesti. The contrasted behaviour of these rheologically different zones can explain the location and the evolution of the Tibesti swell and volcanism. The new data presented in this paper and their interpretation in terms of the emplacement of the Tibesti volcanic province in the Saharan metacraton bring a new and major information about the behaviour of the African plate within its collisional context with Europe.
Mineralogy and Petrology, 10.1007/ s00710-018- 0568-9, 15p.
Africa, South Africa
geochronology
Abstract: The West Coast of Namaqualand in South Africa hosts extensive detrital diamond deposits, but considerable debate exists as to the provenance of these diamonds. Some researchers have suggested derivation of the diamonds from Cretaceous-Jurassic kimberlites (also termed Group I kimberlites) and orangeites (also termed Group II kimberlites) located on the Kaapvaal Craton. However, others favour erosion of diamonds from the ca.300 Ma Dwyka Group sediments, with older, pre-Karoo kimberlites being the original source(s). Previous work has demonstrated that 40Ar/39Ar analyses of clinopyroxene inclusions, extracted from diamonds, yield ages approaching the time(s) of source kimberlite emplacement, which can be used to constrain the provenance of placer diamond deposits. In the current study, 40Ar/39Ar analyses were conducted on clinopyroxene inclusions from two similar batches of Namaqualand detrital diamonds, yielding (maximum) ages ranging from 117.5?±?43.6 Ma to 3684?±?191 Ma (2?) and 120.6?±?15.4 Ma to 688.8?±?4.9 Ma (2?), respectively. The vast majority of inclusions (88%) produced ages younger than 500 Ma, indicating that most Namaqualand diamonds originated from Cretaceous-Jurassic kimberlites/orangeites, with few, if any, derived from the Dwyka tillites. The provenance of the Namaqualand diamonds from ca.115-200 Ma orangeites is consistent with Late Cretaceous paleo-drainage reconstructions, as these localities could have been sampled by the ‘paleo-Karoo’ River and transported to the West Coast via an outlet close to the current Olifants River mouth. At ca.90 Ma, this drainage system appears to have been captured by the ‘paleo-Kalahari’ River, a precursor to the modern Orange River system. This latter drainage is considered to have transported diamonds eroded from both ca.80-90 Ma kimberlites and ca.115-200 Ma orangeites to the West Coast, which were subsequently reworked along the Namibian coast, forming additional placer deposits.
Mineralogy and Petrology, June 14, DOI:10.1007/ s00710-018 -0595-6, 16p.
Africa, South Africa
deposit - Boshof
Abstract: We describe the petrography and mineral chemistry of sixteen eclogite and garnet pyroxenite xenoliths from the reworked Boshof road dump (Kimberley) and define three groups that stem from different depths. Group A, the shallowest derived, has low HREE (heavy rare earth element) abundances, flat middle to heavy REE patterns and high Mg# [= 100•Mg/(Mg?+?Fe)]. Their protoliths probably were higher pressure cumulates (~ 0.7 GPa) of mainly clinopyroxene (cpx) and subordinate orthopyroxene (opx) and olivine (ol). Group B1 xenoliths, derived from the graphite/diamond boundary and below show similarities to present-day N-MORB that were modified by partial melting (higher Mg# and positively inclined MREE (middle REE) and HREE (heavy REE) patterns of calculated bulk rocks). Group B2 samples from greatest depth are unique amongst eclogites reported so far worldwide. The calculated bulk rocks have humped REE patterns with very low La and Lu and prominent maxima at Sm or Eu and anomalously high Na2O (up to 5 wt%) which makes protolith identification difficult. The complex trace element signatures of the full spectrum of Kimberley eclogites belie a multi-stage history of melt depletion and metasomatism with the introduction of new phases especially of phlogopite (phlog). Phlogopite appears to be characteristic for Kimberley eclogites and garnet peridotites. Modelling the metasomatic overprint indicates that groups A and B1 were overprinted by volatile- and potassium-rich melts probably by a process of chromatographic fractionation. Using constraints from other metasomatized Kimberley mantle rocks suggest that much of the metasomatic phlogopite in the eclogites formed during an intense episode of metasomatism that affected the mantle beneath this region 1.1 Gyr ago.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018- 0587-6 16p.
Africa, Sierra Leone
deposit - Zimmi
Abstract: Zimmi diamonds (Sierra Leone) have 500 million year mantle residency times whose origin is best explained by rapid tectonic exhumation to shallower depths in the mantle, associated with continental collision but prior to kimberlite eruption. Here we present spectroscopic data for a new suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the link between their spectroscopic features and their unusual geological history. Cathodoluminesence (CL) imaging of these diamonds revealed irregular patterns with abundant deformation lamellae, associated with the diamonds' tectonic exhumation. Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/- centres and H3 (NVN0). The brownish-yellow to greenish-yellow colours observed in Zimmi Ib-IaA diamonds result from visible absorption by a combination of isolated substitutional nitrogen ( {N}S^0 ) and deformation-related vacancy clusters. Colour-forming centres and other spectroscopic features can all be attributed to the unique geological history of Zimmi Ib-IaA diamonds and their rapid exhumation after formation.
Mineralogy and Petrology, June 14, DOI:10.1007/ s00710-018 -0588-5, 16p.
Africa, South Africa
deposit - De Beers dyke
Abstract: We present petrographic and mineral chemical data for a suite of samples derived from the De Beers dyke, a contemporaneous, composite intrusion bordering the De Beers pipe (Kimberley, South Africa). Petrographic features and mineral compositions indicate the following stages in the evolution of this dyke: (1) production of antecrystic material by kimberlite-related metasomatism in the mantle (i.e., high Cr-Ti phlogopite); (2) entrainment of wall-rock material during ascent through the lithospheric mantle, including antecrysts; (3) early magmatic crystallisation of olivine (internal zones and subsequently rims), Cr-rich spinel, rutile, and magnesian ilmenite, probably on ascent to the surface; and (4) crystallisation of groundmass phases (i.e., olivine rinds, Fe-Ti-rich spinels, perovskite, apatite, monticellite, calcite micro-phenocrysts, kinoshitalite-phlogopite, barite, and baddeleyite) and the mesostasis (calcite, dolomite, and serpentine) on emplacement in the upper crust. Groundmass and mesostasis crystallisation likely forms a continuous sequence with deuteric/hydrothermal modification. The petrographic features, mineralogy, and mineral compositions of different units within the De Beers dyke are indistinguishable from one another, indicating a common petrogenesis. The compositions of antecrysts (i.e., high Cr-Ti phlogopite) and magmatic phases (e.g., olivine rims, magnesian ilmenite, and spinel) overlap those from the root zone intrusions of the main Kimberley pipes (i.e., Wesselton, De Beers, Bultfontein). However, the composition of these magmatic phases is distinct from those in ‘evolved’ intrusions of the Kimberley cluster (e.g., Benfontein, Wesselton water tunnel sills). Although the effects of syn-emplacement flow processes are evident (e.g., alignment of phases parallel to contacts), there is no evidence that the De Beers dyke has undergone significant pre-emplacement crystal fractionation (e.g., olivine, spinel, ilmenite). This study demonstrates the requirement for detailed petrographic and mineral chemical studies to assess whether individual intrusions are in fact ‘evolved’; and that dykes are not necessarily produced by differentiated magmas.
Mineralogy and Petrology, 10.1007/ s710-018-0592 -9, 10p.
Africa, Botswana
deposit - Orapa, Damtshaa, BK09, BK12,AK01
Abstract: Twenty eclogitic diamonds from Orapa Mine (Botswana) with an unusual yellow colour are characterised for their growth structure, N systematics, and C isotope composition, and the major element composition of their silicate inclusions. The diamonds show complex luminescence with green, blue and non-luminescent zones and occasional sector zonation. All parts of the diamonds have low total N concentrations (<50 at.ppm, with one exception of <125 at.ppm) and a limited range in C isotope composition (?5.7 to ?10.6‰). Fourier Transform Infrared spectra show bands at 1334, 1332, 1282, and 1240 cm?1 typical for Ib-IaA diamonds. Relict unaggregated N defects (Nso and Ns+) are present and the preservation is likely caused by the low N concentrations and possible low mantle residence temperatures rather than young diamond formation (inclusion ages of 140, 1096, 1699 Ma; Timmerman et al. Earth Planet Sc Lett 463:178-188, 2017). Garnet and clinopyroxene inclusions extracted from 14 diamonds have an eclogitic composition with relatively low Ca contents and based on all characteristics, these diamonds form a distinct population from Orapa.
Mineralogy and Petrology, 10.1007/ s710-018- 0603-x 15p.
Africa, South Africa
deposit - Finsch, De Beers Pool, Koffiefontein
Abstract: Fibrous diamond growth zones often contain abundant high-density fluid (HDF) inclusions and these provide the most direct information on diamond-forming fluids. Noble gases are incompatible elements and particularly useful in evaluating large-scale mantle processes. This study further constrains the evolution and origin of the HDFs by combining noble gas systematics with ?¹³C, N concentrations, and fluid inclusion compositions for 21 individual growth zones in 13 diamonds from the Finsch (n = 3), DeBeers Pool (n = 7), and Koffiefontein (n = 3) mines on the Kaapvaal Craton. C isotope compositions range from ?2.8 to ?8.6‰ and N contents vary between 268 and 867 at.ppm, except for one diamond with contents of <30 at.ppm N. Nine of the thirteen studied diamonds contained saline HDF inclusions, but the other four diamonds had carbonatitic or silicic HDF inclusions. Carbonatitic and silicic HDFs yielded low He concentrations, R/Ra (³He/?Hesample/³He/?Heair) values of 3.2–6.7, and low ??Ar/³?Ar ratios of 390–1940. Noble gas characteristics of carbonatitic-silicic HDFs appear consistent with a subducted sediment origin and interaction with eclogite. Saline HDFs are characterised by high He concentrations, with R/Ra mostly between 3.9 and 5.7, and a wide range in ??Ar/³?Ar ratios (389–30,200). The saline HDFs likely originated from subducted oceanic crust with low He but moderate Ar contents. Subsequent interaction of these saline HDFs with mantle peridotite could explain the increase in He concentrations and mantle-like He isotope composition, with the range in low to high ??Ar/³?Ar ratios dependent on the initial ³?Ar content and extent of lithosphere interaction. The observed negative correlation between ?He contents and R/Ra values in saline HDFs indicates significant in situ radiogenic ?He production.
Noble gas geochemistry of fluid inclusions in South African diamonds: implications for the origin of diamond-forming fluids.
Journal of Volcanology and Geothermal Research, Vol. 358, pp. 262-272.
Africa, Tanzania
carbonatite
Abstract: Hanang volcano is the southern volcano of, the southern area of the east part of the East African Rift (the North Tanzanian Divergence) and represents volcanic activity of the first stage of continental break-up. In this study, we investigate glassy melt inclusions in nepheline phenocrysts to constrain the late stage of Mg-poor nephelinite evolution and the behaviour of volatiles (CO2, H2O, S, F, Cl) during magma storage and ascent during early stage rifting. The melt inclusions have a green silicate glass, a carbonate phase and a shrinkage bubble free of gas phase indicating that carbonatite:silicate (18:82) liquid immiscibility occurred during nephelinite magmatic evolution. The silicate glasses have trachytic composition (Na?+?K/Al?=?1.6-7.2, SiO2?=?54-65.5?wt%) with high CO2 (0.43?wt% CO2), sulfur (0.21-0.92?wt% S) and halogens (0.28-0.84?wt% Cl; 0.35-2.54?wt% F) contents and very low H2O content (<0.1?wt%). The carbonate phase is an anhydrous Ca-Na-K-S carbonate with 33?wt% CaO, 20?wt% Na2O, 3?wt% K2O, and 3?wt% S. The entrapped melt in nepheline corresponds to evolved interstitial CO2-rich phonolitic composition (Na?+?K/Al?=?6.2-6.9) with 6?±?1.5?wt% CO2 at pressure of 800?±?200?MPa after crystallization of cpx (17%), nepheline (40%) garnet (6.5%) and apatite (1.7%) from Mg-rich nephelinitic magma. During ascent, immiscibility in phonolitic melt inclusions leads to Ca-Na carbonate melt with composition within the range of carbonate melt from Oldoinyo Lengai and Kerimasi, in equilibrium with trachytic silicate melt (closed-system, P?
Mineralogy and Petrology, 10.1007/s00710-018-0608-5 8p.
Africa, Botswana
indicator minerals
Abstract: The majority of the diamond mines in Botswana were discovered as a direct consequence of soil sampling for indicator minerals such as garnet and picroilmenite. Over the past 60 years the application of soil sampling for indicator minerals as a primary exploration tool has declined while aeromagnetic surveys have increased in popularity. The rate of kimberlite discovery in Botswana has declined significantly. The obvious magnetic kimberlites have been discovered. The future of new kimberlite discoveries is once again dependent on soil sampling for kimberlite indicator minerals. It is essential to have an in depth understanding of the transport mechanism of kimberlite indicator minerals from the kimberlite to the modern day surface of the Kalahari Formation, which is solely via termite bioturbation. Field observations indicate that the concentration of indicator minerals at surface is directly dependent on the physical characteristics and capabilities as well as behavioural patterns of the particular termite species dominant in the exploration area. The discovery of future diamond mines in Botswana will be closely associated with an in depth understanding of the relationship between size and concentration of kimberlite indicator minerals in surface soils and the seasonal behaviour, depth penetration capabilities, earthmoving efficiencies and mandible size of the dominant termite species within the exploration area. Large areas in Botswana, where kimberlite indicator minerals recovered from soil samples have been described as distal from source or background, will require re-evaluation. Without detailed termite studies the rate of discovery will continue to decline.
Mineralogy and Petrology, doi.org/10.1007/s710-018-0601-z 15p.
Africa, South Africa
deposit - Cullinan
Abstract: We studied a suite of Cullinan diamonds (<0.3 ct) with mineral inclusions, which comprised 266 Type I and 75 blank Type II (<20 ppm N) diamonds, as classified by infrared spectroscopy. More than 90% (n?=?68) of Type II diamonds do not luminesce. In contrast, 51.9% (n?=?177) of Type I diamonds luminesce, with blue colors of different intensity. Carbon isotopic compositions of Type I and II diamonds are similar, with ?13CVPDB ranging from ?2.1 to ?7.7‰for Type I diamonds (n?=?25), and from ?1.3 to ?7.8- for Type II diamonds (n?=?20). The Type II diamonds are sourced from three parageneses, lithospheric lherzolitic (45%), lithospheric eclogitic (33%), and sublithospheric mafic (22%). The lherzolitic suite contains Cr-pyrope, forsterite, enstatite, clinopyroxene and Cr-spinel formed at 1090-1530 °C and P?=?4.6-7.0 GPa. Lithospheric eclogitic diamonds containing garnet, omphacite, kyanite and coesite comprise 33% of Type II diamonds. The sublithospheric mafic paragenesis is mainly represented by Cr-free majorite, various CaSiO3 phases and omphacite equilibrated at 11.6-26 GPa, in the transition zone and the lower mantle. The lherzolitic paragenesis predominates in Type II diamonds, whereas 79% Type I diamonds are sourced from eclogites. The higher incidence of sublithospheric inclusions was found in Type II diamonds, 22% against 6% in Type I diamonds. The similarity of the mineral parageneses and C isotopic compositions in the small Cullinan Type II and Type I diamonds indicate the absence of distinct mantle processes and carbon sources for formation of studied Type II diamonds. The parent rocks and the carbon sources generally vary for Type II diamonds within a kimberlite and between kimberlites.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0604-9 12p.
Africa, Botswana
deposit - Karowe
Abstract: Mineral inclusions in diamonds play a critical role in constraining the relationship between diamonds and mantle lithologies. Here we report the first major and trace element study of mineral inclusions in diamonds from the Karowe Mine in north-east Botswana, along the western edge of the Zimbabwe Craton. From a total of 107 diamonds, 134 silicate, 15 oxide, and 22 sulphide inclusions were recovered. The results reveal that 53% of Karowe inclusion-bearing diamonds derived from eclogitic sources, 44% are peridotitic, 2% have a sublithospheric origin, and 1% are websteritic. The dominant eclogitic diamond substrates sampled at Karowe are compositionally heterogeneous, as reflected in wide ranges in the CaO contents (4-16 wt%) of garnets and the Mg# (69-92) and jadeite contents (14-48 mol%) of clinopyroxenes. Calculated bulk rock REEN patterns indicate that both shallow and deep levels of the subducted slab(s) were sampled, including cumulate-like protoliths. Peridotitic garnet compositions largely derive from harzburgite/dunite substrates (~90%), with almost half the garnets having CaO contents <1.8 wt%, consistent with pyroxene-free (dunitic) sources. The highly depleted character of the peridotitic diamond substrates is further documented by the high mean and median Mg# (93.1) of olivine inclusions. One low-Ca garnet records a very high Cr2O3 content (14.7 wt%), implying that highly depleted cratonic lithosphere at the time of diamond formation extended to at least 220 km depth. Inclusion geothermobarometry indicates that the formation of peridotitic diamonds occurred along a 39-40 mW/m2 model geotherm. A sublithospheric inclusion suite is established by three eclogitic garnets containing a majorite component, a feature so far unique within the Orapa cluster. These low- and high-Ca majoritic garnets follow pyroxenitic and eclogitic trends of majoritic substitution, respectively. The origin of the majorite-bearing diamonds is estimated to be between 330 to 420 km depth, straddling the asthenosphere-transition zone boundary. This new observation of superdeep mineral inclusions in Karowe diamonds is consistent with a sublithospheric origin for the exceptionally large diamonds from this mine.
Geochemistry, Geophysics, Geosystems, Vol. 19, 5, pp. 1690-1712.
Africa
geomorphology
Abstract: West African drainage reorganization during Cretaceous opening of the Atlantic Ocean is deciphered here from geochemical provenance studies of Central Atlantic sediments. Changes in the geochemical signature of marine sediments are reflected in major and trace element concentrations and strontium?neodymium radiogenic isotopic compositions of Cretaceous sedimentary rocks from eight Deep Sea Drilling Project (DSDP) sites and one exploration well. Homogeneous major and trace element compositions over time indicate sources with average upper (continental) crust signatures. However, detailed information on the ages of these sources is revealed by neodymium isotopes (expressed as ?Nd). The ?Nd(0) values from the DSDP sites show a three?step decrease during the Late Cretaceous: (1) the Albian?Middle Cenomanian ?Nd(0) values are heterogeneous (-5.5 to ?14.9) reflecting the existence of at least three subdrainage basins with distinct sedimentary sources (Hercynian/Paleozoic, Precambrian, and mixed Precambrian/Paleozoic); (2) during the Late Cenomanian?Turonian interval, ?Nd(0) values become homogeneous in the deepwater basin (-10.3 to ?12.4), showing a negative shift of 2 epsilon units interpreted as an increasing contribution of Precambrian inputs; (3) this negative shift continues in the Campanian?Maastrichtian (?Nd(0)?=??15), indicating that Precambrian sources became dominant. These provenance changes are hypothesized to be related to the opening of the South and Equatorial Atlantic Ocean, coincident with tectonic uplift of the continental margin triggered by Africa?Europe convergence. Finally, the difference between ?Nd(0)values of Cretaceous sediments from the Senegal continental shelf and from the deepwater basins suggests that ocean currents prevented detrital material from the Mauritanides reaching deepwater areas.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0620-9 11p.
Africa, Sierra Leone
deposit - Koidu
Abstract: This paper documents the application of a microdiamond-based approach to the estimation of diamond grade in the Pipe 1 kimberlite at the Koidu mine in Sierra Leone. A geological model of Pipe 1 was constructed to represent the distribution and volume of the dominant kimberlite units within the pipe. Bulk samples, along with representative microdiamond samples, were collected from these units at surface and were used to define the ratio between microdiamond stone frequency (+212 ?m stones per kilogram) and recoverable macrodiamond grade (+1.2 mm carats per tonne; 1 carat?=?0.2 g). These ratios were applied to a comprehensive, spatially representative microdiamond sample dataset and were combined with a spatial model of country-rock xenolith dilution within the pipe to estimate +1.2 mm recoverable grades. The resource estimate was reconciled with subsequent production results in the elevation range 160 to 100 m above sea level. Production results for each of the six 10 m benches covering this elevation range were compared to the estimated average grades for these zones in the pipe. For the five cases where most of the kimberlite mass on a given bench is represented in the production data, the results show a maximum discrepancy of 6% between predicted and reported production grade with no indication of any consistent bias. This indicates that, when supported by a sound geological model and suitable microdiamond and macrodiamond data, the microdiamond-based estimation approach can provide reliable constraints on macrodiamond grade, even in the case of geologically complex bodies such as Koidu Pipe 1.
Abstract: In the mid 1880s a little-known photographer named Robert Harris produced a series of albumen prints showing the stages of body searching that black labourers in De Beers diamond mines were obliged to undergo by state ordinance enacted in 1883. The original photographs surfaced briefly in the sale-room in 2007 but have since disappeared. Two sets of copies survive. Bearing in mind the history of documentary photography in South Africa, this essay examines the historical and textual significance of this series of photographs in the context of the history of mining and discusses the imperatives and ethics of locating, researching and publishing controversial imagery in the internet age.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0624-5 12p.
Africa, Lesotho
deposit - Liqhobong
Abstract: The Cretaceous Liqhobong kimberlite cluster comprises at least six known diamondiferous Group 1 kimberlite bodies; namely the circular Main Pipe (8.5 ha), ovoid Satellite Pipe (1.6 ha), Discovery Blow (0.15 ha), Blow (0.1 ha), the Main Dike adjoining the blows and pipes, and one other recently exposed dike. The kimberlites intrude Jurassic Drakensberg lavas and outcrop at ~2650 masl in rugged Maluti Mountain terrain, and are emplaced along a strike of about 2.5km. The cluster represents at least three episodes of structurally controlled kimberlite intrusion; the first which comprised the dike(s?) and the two blows (the blows being dike enlargements emplaced 1km apart) and later the two separate emplacements of the Main and Satellite Pipes.
Geochemistry, Geophysics, Geosystems, Vol. 19, 5, pp. 1499-1518.
Africa, South Africa
craton
Abstract: Cratons, the ancient cores of continents, have an unusually thick lithosphere (the tectonic plate beneath them). At least ?200 km thick, it has a highly anomalous composition, making it less dense than the surrounding mantle. Cratonic lithosphere can thus be cooled to much lower temperatures than elsewhere. Variations in this delicate buoyancy balance probably give rise to variations in the surface elevation across the Earth's stable continents. Lithospheric thickness and composition are key parameters, but both are notoriously difficult to determine. Here we use very accurate measurements of seismic surface?wave velocities and determine deep structure beneath cratons in southern Africa. We discover an unexpectedly strong, gradual thickening of the lithosphere from the central Kaapvaal Craton to the neighboring Limpopo Belt (from 200 to 300 km thick). Curiously, surface elevation decreases monotonically with increasing lithospheric thickness. This demonstrates the effect of the deep lithosphere on topography and gives us new information on the composition of the deepest parts of lithosphere.
Abstract: In this study, we compare the major- and trace-element compositions of olivine, garnet, and clinopyroxene that occur as single crystals (142 grains), with those derived from xenoliths (51 samples) from six kimberlites in the Lucapa area, northeastern Angola: Tchiuzo, Anomaly 116, Catoca, Alto Cuilo-4, Alto Cuilo-63 and Cucumbi-79. The samples were analyzed using electron probe microanalysis (EPMA) and laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). The results suggest different paragenetic associations for these kimberlites in the Lucapa area. Compositional overlap in some of the macrocryst and mantle xenolith samples indicates a xenocrystic origin for some of those macrocrysts. The presence of mantle xenocrysts suggests the possibility of finding diamond. Geothermobarometric calculations were carried out using EPMA data from xenoliths by applying the program PTEXL.XLT. Additional well calibrated single-clinopyroxene thermobarometric calculations were also applied. Results indicate the underlying mantle experienced different equilibration conditions. Subsequent metasomatic enrichment events also support a hypothesis of different sources for the kimberlites. These findings contribute to a better understanding of the petrogenetic evolution of the kimberlites in northeastern Angola and have important implications for diamond exploration.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0613-8 19p.
Africa, South Africa
deposit - Finsch
Abstract: Microinclusion-bearing diamonds offer the opportunity to investigate relationships between mantle metasomatism, diamond formation and kimberlite eruptions in intracratonic provinces. We have analyzed a suite of 7 microinclusion-bearing diamonds from the Finsch Group II kimberlite, South Africa, and identified two diamond populations: ‘Finsch IaA’ diamonds have nitrogen solely in A-centers and contain saline high-density-fluid (HDF) microinclusions, while ‘Finsch IaAB’ diamonds have nitrogen in both A- and B-centers (25-35% B-centers) and are characterized by carbonatite HDF compositions. Based on nitrogen aggregation states and estimates for mantle residence temperatures, we conclude that ‘Finsch IaA’ diamonds formed during a young saline metasomatic event that preceded kimberlite eruption by ~50 kyr to 15 Myr. The possible timing of metasomatism and formation of ‘Finsch IaAB’ diamonds by carbonatite HDFs is less constrained, and could have taken place between ~15 Myr and 2 Gyr before eruption. Two of the diamonds encapsulated omphacite microinclusions in association with saline or low-Mg carbonatitic-like HDF. We observe compositional differences for Al2O3 vs. CaO between these metasomatised omphacites, and also compared to omphacites in mantle eclogites which were identified as metasomatised by kimberlite or high-Mg carbonatite; suggesting a possible relationship between Al2O3 and CaO in metasomatised omphacite and the type of fluid/melt it interacted with. The combined data for microinclusion-bearing diamonds from the Finsch Group II kimberlite and the neighbouring Group I kimberlites at Koffiefontein and De Beers Pool indicate that a substantial volume of the southwest Kaapvaal deep lithosphere was impacted by saline metasomatism during Cretaceous time, and a direct relationship between saline metasomatism, diamond formation and the Kaapvaal late-Mesozoic ‘kimberlite bloom’. We therefore conclude that saline HDFs play a key role in the buildup of metasomatic mantle sources leading to kimberlite eruptions.
Abstract: Carbonatites are rare igneous rocks that have a high content of carbonate minerals and nearly no silica. Carbonatitic magmas are derived from carbonated mantle sources but the origin of the carbonates (recycling of surface material or primary mantle source) is still debated. While mafic igneous rocks present a ?44/40Ca around 0.8-1.2‰ normalised to SRM, surface carbonates have ?44/40Ca ~ 0‰. Ca isotopes are therefore well suited to study the origin of Ca in carbonatites. We analysed the Ca isotopic composition of 25 carbonatites from continental and oceanic locations and from different ages (from 2 Ga to present day). The large majority of the carbonatites are isotopically light (?44/40Ca down to 0.07‰) compared to mantle derived rocks. On the other hand, the natrocarbonatite from Oldoinyo Lengai is isotopically heavier (?44/40Ca =0.82‰), similarly to basalts. Three mechanisms can explain the very light isotopic composition of the calciocarbonatites i) A very low degree of partial melting of the mantle may enrich the melt in light isotopes, yet there is no evidence of such large isotopic fractionation during partial melting. ii) The mantle source for the calciocarbonatites is enriched in light Ca likely due to recycling of surface material. iii) aqueous alteration has enriched the calciocarbonatites in the lighter isotopes. On the other hand, the natrocarbonatite from Oldoinyo Lengai have a MORB-like Ca isotopic composition. The difference of ?44/40Ca between natro- and calcio-carbonatite would then suggest that they either have different mantle sources, were formed from different degree of partial melting and/or that aqueous alteration has modified the Ca isotopic composition of calciocarbonatites.
Geochimica et Cosmochimica Acta, Vol. 213, 1, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and ?18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy ?18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu?, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like ?18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ?3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Mineral Processing and Extractive Metallurgy Review, Vol. 39, pp. 136-144.
Africa, Angola
mineral processing
Abstract: Kimberlite is the host rock of diamonds and varies widely in geological and mineralogical features as well as color, processing capability, and dewatering characteristics. This study investigated the dewatering behavior of problematic Angolan kimberlites. The presence of clay minerals in kimberlite causes difficulties in dewatering due to high flocculant demand, poor supernatant clarity, and low settling rates. Identifying critical parameters governing the settling behavior will assist in managing the settling behavior of different kimberlite slurries. The influence of particle size, pH of the kimberlite slurry, cation exchange capacity, exchangeable sodium percentage, and smectite content of the kimberlite on the settling rate were investigated for 18 different African kimberlite samples. The settling rate and slurry bed compaction during natural settling were also measured for the kimberlite slurries. Seventeen different Angolan clay-rich kimberlites and one South African clay-rich kimberlite were tested, and, except for two kimberlites, colloidal stability was experienced during natural settling. The pH values of the kimberlite slurries ranged between 9 and 11, which is similar to the pH band where colloidal stability was found during earlier research. The results indicate that colloidal stable slurries were experienced with kimberlites that had exchangeable sodium percentages as low as 0.7%. The cation exchange capacity of the various kimberlites differentiated more distinctly between colloidal stability and instability. A new model is proposed whereby clay-rich kimberlites with a cation exchange capacity of more than 10cmol/kg will experience colloidal stability if the pH of the solvent solution is within the prescribed pH range of 9-11.
Abstract: Kimberlite is the host rock of diamonds and varies widely in geological and mineralogical features as well as color, processing capability, and dewatering characteristics. This study investigated the dewatering behavior of problematic Angolan kimberlites. The presence of clay minerals in kimberlite causes difficulties in dewatering due to high flocculant demand, poor supernatant clarity, and low settling rates. Identifying critical parameters governing the settling behavior will assist in managing the settling behavior of different kimberlite slurries. The influence of particle size, pH of the kimberlite slurry, cation exchange capacity, exchangeable sodium percentage, and smectite content of the kimberlite on the settling rate were investigated for 18 different African kimberlite samples. The settling rate and slurry bed compaction during natural settling were also measured for the kimberlite slurries. Seventeen different Angolan clay-rich kimberlites and one South African clay-rich kimberlite were tested, and, except for two kimberlites, colloidal stability was experienced during natural settling. The pH values of the kimberlite slurries ranged between 9 and 11, which is similar to the pH band where colloidal stability was found during earlier research. The results indicate that colloidal stable slurries were experienced with kimberlites that had exchangeable sodium percentages as low as 0.7%. The cation exchange capacity of the various kimberlites differentiated more distinctly between colloidal stability and instability. A new model is proposed whereby clay-rich kimberlites with a cation exchange capacity of more than 10cmol/kg will experience colloidal stability if the pH of the solvent solution is within the prescribed pH range of 9-11.The Trans-Saharan Belt is one of the most important orogenic systems constitutive of the Pan-African cycle, which, at the end of the Neoproterozoic, led to the formation of the Gondwana Supercontinent. It is marked by the opening and closing of oceanic domains, collision of continental blocks and the deformation of thick synorogenic sedimentary basins. It extends from north to south over a distance of 3000?km in Africa, including the Nigerian Shield and the Tuareg Shield as well as their counterparts beneath the Phanerozoic oil-rich North- and South-Saharan sedimentary basins. In this study, we take advantage of potential field methods (magnetism and gravity) to analyze the crustal-scale structures of the Tuareg Shield terranes and to track these Pan-African structures below the sedimentary basins, offering a new, >1000?km extent. The map interpretations are based on the classical potential field transforms and two-dimensional forward modeling. We have identified geophysical units and first-order bounding lineaments essentially defined owing to magnetic and gravimetric anomaly signatures. In particular, we are able to highlight curved terminations, which in the Trans-Saharan context have been still poorly documented. We provide for the first time a rheological map showing a categorization of contrasted basement units from the south of the Tuareg Shield up to the Atlas Belt. These units highlight the contrasted rheological behavior of the Tuareg tectonostratigraphic terranes during (i) the northerly Pan-African tectonic escape characteristic of the Trans-Saharan Belt and (ii) the North Sahara basin development, especially during intraplate reworking tied to the Variscan event. The discovery of a relatively rigid E-W oriented unit to the south of the Atlas system, and on which the escaping Pan-African terranes were blocked, offers a new perspective on the structural framework of the north-Gondwana margin. It will help to understand how occurred the rendezvous of the N-S oriented Pan-African terranes and the E-W oriented Cadomian peri-Gondwanan terranes.
Abstract: Carbonates in fresh hypabyssal kimberlites worldwide have been studied to understand their origin [i.e. primary magmatic (high T) versus deuteric (‘low T’) versus hydrothermal/alteration (‘low T’)] and identify optimal strategies for petrogenetic studies of kimberlitic carbonates. The approach presented here integrates detailed textural characterisation, cathodoluminescence (CL) imaging, in situ major- and trace-element analysis, as well as in situ Sr-isotope analysis. The results reveal a wide textural diversity. Calcite occurs as fine-grained groundmass, larger laths, segregations, veins or as a late crystallising phase, replacing olivine or early carbonates. Different generations of carbonates commonly coexist in the same kimberlite, each one defined by a characteristic texture, CL response and composition (e.g., variable Sr and Ba concentrations). In situ Sr isotope analysis revealed a magmatic signature for most of the carbonates, based on comparable 87Sr/86Sr values between these carbonates and the coexisting perovskite, a robust magmatic phase. However, this study also shows that in situ Sr isotope analysis not always allow distinction between primary (i.e., magmatic) and texturally secondary carbonates within the same sample. Carbonates with a clear secondary origin (e.g., late-stage veins) occasionally show the same moderately depleted 87Sr/86Sr ratios of primary carbonates and coexisting perovskite (e.g., calcite laths-shaped crystals with 87Sr/86Sr values identical within uncertainty to those of vein calcite in the De Beers kimberlite). This complexity emphasises the necessity of integrating detailed petrography, geochemical and in situ Sr isotopic analyses for an accurate interpretation of carbonate petrogenesis in kimberlites. Therefore, the complex petrogenesis of carbonates demonstrated here not only highlights the compositional variability of kimberlites, but also raises concerns about the use of bulk-carbonate C-O isotope studies to characterise the parental melt compositions. Conversely, our integrated textural and in situ study successfully identifies the most appropriate (i.e. primary) carbonates for providing constraints on the isotopic parameters of parental kimberlite magmas.
Abstract: Natural-color blue diamonds are among the rarest and most valuable gemstones. Gray and violet diamonds are also included here, as these diamonds can coexist on a color continuum with blue diamonds. More so than most other fancy colors, many diamonds in this color range are sourced from specific locations-the Cullinan mine in South Africa and the Argyle mine in Australia. Although blue color is often associated with boron impurities, the color of diamonds in this range (including gray and violet) also originates from simple structural defects produced by radiation exposure or from more complex defects involving hydrogen. These different mechanisms can be characterized by absorption and luminescence spectroscopy. A fourth mechanism-micro-inclusions of grayish clouds or tiny graphite particles in gray diamonds-can be distinguished through microscopy. In this article, we summarize prior research as well as collected data such as color and carat weight on more than 15,000 naturally colored blue/gray/violet diamonds from the GIA database (along with an analysis of spectroscopic data on a subset of 500 randomly selected samples) to provide an unprecedented description of these beautiful gemstones.
Abstract: We report preliminary results of a systematic search for fluid/melt microinclusions in mantle minerals. “Dusty” garnets from xenolith XRV6 [1], a heavily metasomatised Type I eclogite from Roberts Victor mine, SA, carry many microinclusions (<1 ?m). FTIR analyses of "dusty" zones indicate the presence of molecular water in the inclusions and hydroxyl groups in the garnet. EPMA analysis of 136 microinclusions constrains the bulk composition of the microinclusions. Compared to the host garnet, they are enriched in TiO2, FeO, CaO, Na2O and K2O and depleted in Al2O3 and MgO. The silica contents seem to be similar to that of the host garnet. Figure 1: a. Backscatter image of the microinclusions in XRV6 garnet. b. K2O vs. MgO of the clear garnet (red) and the microinclusions (+ their surrounding garnet, blue). Most of the elements form compositional mixing arrays of microinclusion+garnet (Fig. 1b). The arrays trend away from the compositions of large melt pools or secondary minerals found in the xenolith. They point towards the array of silicic to low-Mg carbonatitic high density fluids (HDFs) trapped in diamonds, indicating the role of such fluids in mantle metasomatism.
Abstract: Roots of continental cratons keep a long record of multiple metasomatic events, but their trace is complicated due to the mixed signals left by these events in the composition of mantle silicate minerals. Simple composition helps diamonds to provide a more robust record of the latest metasomatic events which they witnessed. Growth and dissolution features on the diamond surface are sensitive to the composition of the reacting media. In this study we use mantle-derived resorption features on natural diamonds to examine the nature of metasomatic events in diamondiferous mantle lithologies. We use experiments at mantle conditions to examine how the composition of fluids and melts affect diamond resorption. We then compare these results to the features of natural diamonds to determine which of the tested compositions could have acted as metasomatic agents in Earth’s cratonic roots. Diamond dissolution experiments conducted at 6 GPa, 1200 - 1500oC using synthetic MgO-CaO-SiO2-CO2-H2O system examined the effect of CHO fluid, silica-saturated CHO fluid, aqueous and “dry” silica-carbonate and carbonate melts. Results show that the main control of diamond resorption morphology is the state of the reacting media: fluid vs. melt. We compared the experimental results to diamonds with mantle-derived resorption features from two kimberlites from the Orapa kimberlite cluster (Botswana). We identified twelve mantle-derived resorption types, none of which resembled the products of resorption in fluids. Most of the observed resorption types could be produced by dissolution in mantle melts with variable proportions of carbonate and silicate components and in the range of temperatures. The most abundant resorption type resembles the product of diamond dissolution in carbonate melts at temperatures above 1450oC. Our results suggest that fluid-metasomatism is not destructive for diamonds while melt-metasomatism is. The lower hydrous carbonated solidus of lherzolite compared to harzburgite can result in the shift the process from diamond growth in fluids to diamond dissolution in melts due to metasomatic transformation of harzburgite into lherzolite.
Abstract: Continental rifts most often nucleate within orogenic belts. However, some studies in the East African Rift System (EARS) have shown that continental rifts can also develop withincratons. This work investigated the ~1.5 Ma Eyasibasin,which propagates in a WSW direction into the Tanzanian craton. The basin is located where the Eastern Branch of the EARS transitions from a narrow rift (~70 km wide) thewider(~300 km wide) North Tanzanian Divergence. Unlike the rest of the Eastern Branch segments, the Eyasibasindoes not follow the Mozambique orogenic belt located on the eastern margin of the Tanzanian craton. This work generatedlithospheric?scale sections across the basinusing: (1) Digital Elevation Model to map surface rift?related brittle structures; (2) Aeromagnetic data to determine the depth to the Precambrian basement;and (3) World Gravity Model 2012 to estimatecrustal and lithospheric thickness by applying the two?dimensional(2D) radially?averaged power spectral analysis and 2D forward gravity modeling. These cross?sectionsshow that the Eyasibasinnucleates within a previously unidentified suture zone within the Tanzanian cratonand that this suture zone is characterized by thinner lithospherethat can be as thin as ~95 km. This zone ofthinner lithosphere is offset southeastwardfrom the surface expression of the Eyasibasinand might have facilitated the formation of other basins further south. Furthermore, the lithospheric thickness map indicates that the Tanzanian craton is heterogeneous and possibly composed of multiplesmaller cratonic fragments.
Abstract: The Republic of Liberia in West Africa is underlain mostly by Precambrian rocks of Archaean (Liberian) age in the west and of Proterozoic (Eburnean) age in the east. By analogy with similar terranes elsewhere in the world, and in West Africa in particular, the geology of Liberia is favourable for the occurrence of deposits of a wide range of metals and industrial minerals, including gold, iron ore, diamonds, base metals, bauxite, manganese, fluorspar, kyanite and phosphate. Known gold deposits, mostly orogenic in style, occur widely and are commonly associated with north-east-trending regional shear zones. Gold mining commenced at the New Liberty deposit in western Liberia in 2015, while significant gold resources have also been identified at several other sites in both Archaean and Proterozoic terranes. Liberia has large resources of itabirite-type iron ores, most of which are located in the Liberian terrane, and was the largest producer in Africa prior to the onset of civil war in 1989. Production of iron ore is currently restricted to a single mine, Yekepa, in the Nimba Range. Other important deposits, some of them previously mined, include Bong, the Western Cluster, Putu and Goe Fantro. There is a long history of alluvial diamond production in western and central Liberia, together with more than 160 known occurrences of kimberlite. Most of the known kimberlites occur in three clusters of small pipes and abundant dykes, located at Kumgbor, Mano Godua and Weasua, close to the border with Sierra Leone. Many of these are considered to be part of a single province that includes Jurassic age diamondiferous kimberlites in Sierra Leone and Guinea. Deposits and occurrences of a wide range of other metals and industrial minerals are also known. Several of these have been worked on a small scale in the past, mainly by artisanal miners, but most are poorly known in detail with sub-surface information available at only a few localities. By comparison with most other countries in West Africa, the geology of Liberia is poorly known and there has been very little systematic exploration carried out for most commodities other than gold, iron ore and diamonds since the 1960s and 1970s. Further detailed field and laboratory investigations using modern techniques are required to properly evaluate the potential for the occurrence of economic deposits of many minerals and metals in a variety of geological settings. Digital geological, geochemical, geophysical and mineral occurrence datasets, including new national airborne geophysical survey data, provide a sound basis for the identification of new exploration targets, but in almost every part of the country there is a need for new and more detailed geological surveys to underpin mineral exploration.
deposit - Argyle, De Beers Pool, Jwaneng, Orapa, Udachnaya, Venetia, Wawa, Diavik
Abstract: Earth’s mantle is by far the largest silicate-hosted reservoir of carbon. Diamonds are unrivalled in their ability to record the cycle of mantle carbon and other volatiles over a vast portion of the Earth’s history. They are the product of ascending, cooling, carbon-saturated, metasomatic fluidsmelts and/or redox reactions, predominantly within peridotitic and eclogitic domains in the mantle lithosphere. This paper reports the results of a major secondary ion mass spectrometry (SIMS) carbon isotope study, carried out on 127 diamond samples, spanning a large range of geological time. Detailed transects across the incremental growth zones within each diamond were measured for C isotopes, N abundances and, for samples with N >~200 at.ppm, N isotopes. Given that all of the samples are fragments, recovered when the original crystals were broken to liberate their inclusions, 81 of the analytical traverses have confirmed growth direction context. 98 samples are from studies that have confirmed the dates of the individual diamonds through analysis of their silicate or sulphide inclusions, from source localities including Argyle, De Beers Pool, Jwaneng, Orapa, Udachnaya & Venetia. Additional samples come from Wawa (a minimum age) and Diavik where the samples are tied via inclusion paragenesis to published ages. The peridotitic dataset covers the age range of ~3.3 - 2.0 Ga, with the eclogitic data from 2.9 - 1.0 Ga. In total, 751 carbon isotope and nitrogen concentration measurements have been obtained (425 on peridotitic diamonds, and 326 on eclogitic diamonds) with 470 nitrogen isotope measurements (190 P, 280 E). We attempt to constrain the diamond carbon isotope record through time and its implications for (i) the mantle carbon reservoir, (ii) its oxygen fugacity, (iii) the fluid / melt growth environment of diamonds, (iv) fractionation trends recorded in individual diamonds, and (v) diamond population studies using bulk combustion carbon isotope analysis.
Abstract: Polycrystalline diamond aggregates (framesites, boart, diamondite) are an understudied variety of mantle diamond, but can make up 20% of the production in some Group I kimberlites. Their polycrystalline nature indicates rapid precipitation from carbon-oversaturated fluids and individual PDAs often contain a chemically heterogeneous suite of websteritic and pyroxenitic inclusions and minerals intimately intergrown with the diamond crystals. Geochemical and microstructural evidence suggests that fluid-driven redox reactions with lithospheric material occurring episodically over millions of years play a major role in freezing carbon in the subcratonic lithosphere (Jacob et al., 2000; 2016; Mikhail et al., 2014). A suite of 39 samples from the Venetia kimberlite pipe in South Africa allows a more detailed look at the diamondforming fluids. 13C values in the diamonds measured by secondary ion mass spectrometry range from +2 to -28 and cover the entire range for PDA from the literature. Nitrogen concentrations are mostly very low (less than 100 at ppm), but reach up to 2660 at ppm in individual samples. These high nitrogen concentrations in concert with mostly positive 15N values of up to +17 and some very negative 3C values suggest crustal material as the source of the nitrogen and the carbon. However, detailed analysis of the sample provides evidence for a more complex growth history followed by alteration. Individual diamond crystals show complex growth zonations by cathodoluminescence imaging that can be related with the carbon and nitrogen isotopic compositions and points to growth incorporating several pulses of carbon-nitrogen fluid with distinct isotopic compositions. Most of these growth events show decoupled carbon and nitrogen systematics. In addition, EBSD identifies deformation and recrystallization and nitrogen aggregation states range from pure IaA to pure IaB, supporting a heterogeneous and episodic growth history.
Africa, Zimbabwe. Australia, Canada, Northwest Territories, South Africa, Botswana
deposit - Murowa, Argyle, Diavik, Venetia, Orapa
Abstract: FTIR maps of diamond plates, cut through the centre of growth, contain abundant information about changing defect concentrations from core to rim. These data can, in principle, be interpreted in terms of the variation in conditions of diamond growth and the temperatures experienced by the diamond during the period of mantle residence between growth and exhumation. Many diamonds show multiple growth zones that can be observed by cathodoluminescence. Importantly, the combination of nitrogen concentration and nitrogen aggregation measured by FTIR can be used to determine whether the growth zones are of similar or very different ages (Kohn et al., 2016). In this study, we use automated fitting of several thousand individual spectra within each FTIR map to define a model temperature for each pixel using the Python program, QUIDDIT. We then use a two-stage aggregation model to constrain potential temperature-time histories for each diamond. To take full advantage of the temperature history recorded by zoned diamonds, radiometric ages of inclusions are required. If the growth ages of each zone and the date of exhumation are well-known, then a model temperature can be calculated for each zone. The combination of zone-specific ages and improved quality and processing of FTIR spectra is able to provide unique new insights into the thermal history of diamondbearing lithospheric mantle. For the first time we will be able to use the N defects in diamonds to work out whether a particular location in the lithosphere has heated or cooled over long periods of geological time. The implications for the mechanism of formation of lithosphere will be discussed. We will illustrate the approach using examples of zoned diamonds from Murowa (Zimbabwe), Argyle (Australia), Diavik (Canada), Venetia (South Africa) and Orapa (Botswana).
Africa, South Africa, Guinea, South America, Brazil
deposit - Kankan, Jagersfontein, Juina
Abstract: Inclusions in super-deep diamonds provide a unique window to the sublithospheric mantle (e.g. [1-4]). Here we present oxygen isotopes for Kankan majoritic garnet and former bridgmanite inclusions. The clustering of Kankan majorites around a ?18O of +9‰ is nearly identical to those reported from Jagersfontein [1]. This elevated and nearly constant ?18O signal indicates homogenization of partial melts from the uppermost part of altered basaltic slabs. Conversely, ?18O values in Juina majorites are highly variable [2] due to crystallization from small, discrete melt pockets in a heterogeneous eclogitic source. While all these majorites have eclogitic/pyroxenitic Cr2O3 and CaO contents, charge-balance for Si[VI] is achieved very differently, with Jagersfontein [3], Kankan [4], and Juina [2] majorites transitioning from eclogitic Na[VIII]Si[VI] to peridotitic-pyroxenitic [5] Mg[VI]Si[VI] substitutions. We interpret this shift as the result of homogenized eclogitic partial melts infiltrating and reacting with adjacent pyrolitic mantle at Kankan and Jagersfontein. Increases in Mg# and Cr2O3 with reductions in ?18O support this reaction. This model is in agreement with recent experiments in which majorites and diamonds form from a reaction of slab-derived carbonatite with reduced pyrolite at 300-700 km depth [6]. The Kankan diamonds also provide an opportunity to establish the chemical environment of the lower mantle. Four inclusions of MgSiO3, inferred to be former bridgmanite [4], provide the first-measured ?18O values for lower mantle samples. These values suggest derivation from primitive mantle, or unaltered subducted oceanic lithospheric mantle. The Kankan super-deep inclusions thus provide a cross-section of deep mantle that highlights slab-pyrolite reactions in the asthenosphere and primitive compositions in the lower mantle.
Abstract: This study investigates the color origin of 40 natural Fancy Dark brown-black round brilliant diamonds from the Marange alluvial deposits in eastern Zimbabwe. Visual observations show that the dark appearance of the Marange diamonds is due to a combination of graphite micro-inclusions (associated with methane), graphite needles, and dark brown radiation stains that occur along internal fractures. The GR1 (V0) defect, typically formed during natural and artificial irradiation, is observed in the optical spectra of 43% of the diamonds, al- though its intensity is too low to significantly impact the bodycolor. Natural irradiation in these diamonds is likely related to their billion-year residence in the Umkondo conglomerate, which is known to contain radioac- tive minerals such as zircon. Aside from radiation staining, irradiation-damaged diamond appears non-lumines- cent in DiamondView images and shows a weaker, broader diamond peak (at 1332 cm?1) in Raman spectroscopy. Brown coloration of the radiation stains is due to heating of the diamonds during later regional metamorphism, which also facilitated the formation of the H3 (NVN0) and NiN complexes.
Abstract: Volcanic pipes, or maar-diatreme volcanoes, form during explosive eruptions of mantle-derived magmas near Earth's surface. Impressive examples are the carrot-shaped, downward tapering structures formed by kimberlite magmas. Kimberlites originate from >150 km depth within Earth's mantle beneath thick continental roots, away from tectonic plate margins. Kimberlite pipes can be significant diamond deposits, and the complex architecture revealed during exploration and mining is ascribed to repeated magma injections leading to multiple eruptions. Repeated magmatic pulses cause diatremes to widen and grow downward, forming kilometer-sized subterranean structures. However, the time-resolved evolution of kimberlite pipe systems is largely unknown. We present the first U/Pb perovskite ages for newly discovered kimberlite dikes (1139.8 ± 4.8 Ma) that cut through the volcaniclastic infill of the Premier kimberlite pipe (1153.3 ± 5.3 Ma) at Cullinan Diamond Mine, South Africa. The ages reveal that renewed kimberlite volcanic activity occurred, at a minimum, 3 m.y. after the main pipe formation. This finding suggests that the largest kimberlite pipes, and maar-diatreme volcanoes in general, may be magmatically active for several millions of years, which conflicts with this volcanism being described as 'monogenetic' at millennia time scales. Exemplified by Tier-1 diamond deposits on the Kaapvaal craton, long-lasting kimberlite volcanic activity may be an important factor in growing large diatremes, plus enabling effective transport of mantle cargo from the diamond stability field to Earth's surface.
Journal of Geophysical Research, Vol. 123, 7, pp. 5644-5660.
Africa, east Africa
geophysics - seismic
Abstract: Although the East African rift system formed in cratonic lithosphere above a large?scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small?volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake?bottom seismometers and 67 land stations in the Tanganyika?Rukwa?Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift?perpendicular strain, rift?parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N?S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS?splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large?scale asthenospheric flow or passive rifting.
Abstract: Archean supracrustal rocks (i.e. chemical sediments and metavolcanics) preserve sulfur Mass Independent Fractionations (MIF) that originate from photochemical reactions occurring in atmosphere before the great oxygenation event, 2.45 Ga. Reduced and oxidized aerosols were produced by photochemistry and respectively carry 33S enrichment (?33S > 0‰) and depletion (?33S < 0‰). The relative abundance of the minor isotope of sulfur (36S) was also affected by MIF in such a way that compact negative correlation exists between ?33S and ?36S. For much of Archean sediments, ?36S /?33S? -1, while slight variation of this slope have been attributed to minor change in the chemical composition of the atmosphere affecting global MIF source mechanism. On another hand, 36S abundance is also affected by microbial cycling and in this specific case, ?36S /?33S? -7. Accordingly, ?33S-?36S co-variations can be used to discriminate distinct sedimentary pool. This contribution aims to test the robustness of MIF array of specific exospheric sulfur pools along their journey from the surface to the mantle. We examine the ?36S in addition to ?34S and ?33S signatures measured in-situ with secondary ion mass spectrometer, in sub lithospheric peridotitic and eclogitic sulfides from Kaapvaal (Jwaneng and Kimberley Pool) and Siberian craton (Mir and Udachnaya). Unlike peridotitic sulfides, eclogitic sulfides from both localities display significant MIF attesting from the presence of surficial sulfur in their source. More interestingly, the magnitude of the anomalies as well as ?36S/?33S ratio, differ from one locality to the other. Siberian eclogites match the composition of Eoarchean sulfate (?36S/?33S=-3 and ?33S<0‰). Sample from Jwaneng follow the MIF array previously reported in Archean chemical sediment (?36S/?33S=-1) while sulfide from Kimberley pool match the composition of some meso-Archean sediments in good agreement with isochron age reported in the literature for the sulfide from this locality. This study confirms that surficial sulfur has been efficiently transferred to the lithospheric mantle. More interestingly, it shows that peculiar sedimentary pools are still preserved in the cratonic keels.
Africa, South Africa, China, United States, Canada, South America
diamond inclusions
Abstract: We present the first evidence for inclusions of ice-VII in diamonds from southern Africa, China, North- and South-America [1]. Combining synchrotron X-ray diffraction, - X-ray fluorescence and IR spectroscopy, we show the presence of ice-VII as inclusions in diamonds that have formed at depth > 410 km to about 800 km in the Earth's mantle. What is now crystalline ice-VII, a high pressure polymorph of water-ice, was component of an aqueous fluid entrapped in the diamonds that were growing in the deep mantle. Because of the confinement by the host diamonds, the inclusions retain high pressures. The same holds for inclusions of magnesian calcite, halite, and ilmenite found in the same diamond specimens. These inclusions reflect the presence of aqueous and carbonaceous fluids in the mantle transition zone and the shallow lower mantle. Using their current residual pressures and the equations of state, we can reconstruct their recovery paths [2,3]. Further, we can use the intersection of modelled recovery paths to better constrain the encapsulation pressure and temperature of these inclusions in diamonds.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0627-2 9p.
Africa, Botswana
deposit - Orapa
Abstract: This paper presents the results of an investigation into the structure of eolian kimberlite indicator minerals (KIMs) haloes present within Quaternary Kalahari Group sediments (up to 20 m thick) overlying the Late Cretaceous kimberlites in the Orapa field in North-East Botswana. A database of more than 8000 samples shows that kimberlites create a general mineralogical blanket of KIMs of various distances of transportation from primary sources in the Orapa area. Models of the reflection and dispersion patterns of KIMs derived from kimberlite pipes including AK10/ AK22/AK23 have been revealed based on 200 selected heavy mineral samples collected during diamond prospecting activities in Botswana from 2014 to 2017. Short distance eolian haloes situated close to kimberlite bodies cover gentle slopes within plains up to 500 × 1000 m in size. They have regularly have oval or conical shapes and are characterized by the presence mainly of unabraded or only slightly abraded KIMs. A sharp reduction of their concentration from hundreds and thousands of grains / 20 l immediately above kimberlites toto 10 grains/20 l at a distance of only 100-200 m from the pipes is a standard feature of these haloes. The variation of concentration, morphology and abrasion of specific KIMs with increasing distance from the primary sources has been investigated and presented herein. Sample volumes recommended for pipes present within a similar setting as those studied, with different depth of sedimentary cover are as follows: up to 10-20 m cover at 20-50 l, 20-30 m cover at 50-100 l and 30-80 m cover at 250 l. It is important to appreciate that the discovery of even single grains of unabraded or slightly abraded KIMs in eolian haloes are of high prospecting significance in this area. The results of the research can be applied to in diamond prospecting programs in various regions with similar environments.
Abstract: Mineral and fluid/melt inclusions in diamonds, which are encapsulated and isolated during a metasomatic event, offer the opportunity to constrain changes in the sub-continental lithospheric mantle that occurred during individual thermochemical events. Fibrous diamonds from the Group I De Beers Pool kimberlites, South Africa (SA), trapped incompatibleelement enriched saline high-density fluids (HDFs) and peridotitic mineral microinclusions. Their substitutional nitrogen resides almost exclusively in A-centers. With regard to the elevated thermal conditions that prevailed in the SA lithosphere during and following Karoo volcanism at ~180 Ma, this low-aggregation state of nitrogen suggests a short mantle residence time, constraining the time of saline metasomatism to be close to the eruption of the kimberlites at ~85 Ma. Thermometry of mineral microinclusions yield temperatures between 875-1080 ºC (at 5 GPa). These temperatures overlap with conditions recorded by touching inclusion pairs, which represent the mantle ambient conditions just before eruption, and are altogether lower by 150-250°C compared to P-T gradients recorded by peridotite xenoliths from the same locality. In addition, the oxygen fugacity calculated for the saline HDF compositions (?log??O2(FMQ) = -2.5 to -1.3) are higher by about a log unit compared with that recorded by xenoliths at 4-7 GPa. We conclude that enriched saline HDFs mediated the metasomatism that preceded Group I kimberlite eruptions in the southwestern Kaapvaal craton, and that their ‘cold and oxidized’ nature reflects their derivation from a deep subducting slab. To reconcile the temperature and oxygen fugacity discrepancy between inclusions in diamonds and xenoliths, we argue that xenoliths did not equilibrate during the last saline metasomatic event or kimberlite eruption. Thus the P-T-??O2 gradients they record express pre-existing lithospheric conditions that were likely established during the last major thermal event in the Kaapvaal craton (i.e. the Karoo magmatism at ca. 180 Ma).
Journal of African Earth Sciences, Vol. 147, pp. 374-382.
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: The northern Tanzanian sector of the Gregory Rift is an area of an active continental rifting, in which sedimentation processes are strongly affected by volcanism. Due to limited stratigraphic exposure, the volcanic record of the region is rather sparse, and assigning volcanic centres for the individual eruptions is difficult. This study presents new data on the tephrostratigraphy of the sedimentary sequence of Lake Emakat, Empakaai Crater, northern Tanzania. Seven volcanic ash layers are identified and described from a 1.1-m core of lake sediments. Geochemical, mineralogical, petrographic and magnetic analyses show that: (1) all ash layers are products of highly explosive eruptions of melilite-bearing magmas; (2) most of the eruptions originate from a complex magmatic system; (3) all ash horizons are very well preserved in the lake environment; and (4) there are significant fluctuations of the bulk magnetic susceptibility of the lacustrine sediments which is related to microtephra from additional eruptions, the result of detritus, washed from the shore during periods of strong lake level fluctuations or periods of high erosion rates, or simply by the contamination by the material from the ash layers. Based on geochemistry and mineralogy of the seven identified ash layers in Lake Emakat, combined with the eruption ages from ¹?C datings, we can pinpoint Oldoinyo Lengai volcano as the source of these specific layers. The combination of this new data with existing chronological data from Ryner et al. (2007), retrieved from the same core, provides precise ages of the voluminous highly explosive eruptions in this region of East Africa during the Pleistocene-Holocene transition.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0634-3 12p.
Africa, Botswana
deposit - Jwaneng
Abstract: To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.
Global and Planetary Change, doi.org/j.gloplacha.2017.10.011 24p.
Africa
geodynamics
Abstract: Modern satellite gravimetry is now considered one of the most powerful and effective instrument for regional tectono-geodynamic zonation. Satellite gravity observations clearly fit the definition of 'big data' because of their volume and variety. The Arabian - NE African region discussed in this article has intricate geodynamic features including active rift zones, high seismic activity and collision processes, a rich structural pattern made up of the mosaic block system of continental and oceanic crusts of different ages, as well as several of the greatest gravity anomalies and complex magnetic anomaly mosaics. This region also has the world's main hydrocarbon resources and a vast number of other economic deposits. A comprehensive analysis of these satellite derived gravity data were used to construct a series of new maps that localize the key properties of the lithosphere of the region. A careful examination of numerous geological sources and their combined inspection with satellite derived gravity and other geophysical data resulted in this new integrated tectonic map of the Arabian-African region. An analysis of the series of gravity map transformations and certain geological indicators document the significant geodynamic features of the region.
Abstract: Olivine phenocryst chemistry is a useful tracer of mantle source lithology as olivine is typically the first mineral to crystallize in a range of mafic to alkaline magma types and contains geochemical information about the primary parent magma composition. It is particularly useful in discriminating between pyroxenite (i.e.,recycled crustal component) and peridotite mantle source lithologies. Radiogenic isotope studies of HIMU basalts have shown the important role of a recycled crustal component in the source. However, olivine chemistry of HIMU basalts suggests a dominantly peridotitic mantle source with a subduction-derived metasomatic carbonate component. To further investigate how olivine chemistry can be used to understand the source of HIMU magmas, I present major and trace element data for olivine megacrysts related to the 89?Ma Cr-poor megacryst suite from the Monastery kimberlite (South Africa), which have previously been interpreted to crystallize at high P-T conditions of ~1400?°C and ~5?GPa from a magma sourced from a HIMU reservoir. Olivine megacrysts have high-Ni concentrations at a given Fo (forsterite) content, overlapping ocean island basalts (OIB) interpreted to have formed from pyroxenite-dominated sources but are distinct from typical olivine in HIMU basalts. However, they have low 100*Mn/Fe (0.8-1.1) and no correlation is observed between Ni and trace elements indicative of recycled components such as Co, Li, or Zn. The olivine megacryst chemistry is similar to that of aillikite olivine interpreted to be controlled by phlogopite in the source rather than pyroxenite. Comparison with olivine chemistry from orangeites/Group II kimberlites (sourced from phlogopite-rich mantle lithologies) supports a phlogopite controlon low 100*Mn/Fe in olivine. Further comparison with olivine phenocryst chemistry of HIMU melilitites (76-58?Ma) in southern Africa suggests a mineralogically heterogeneous HIMU reservoir formed by metasomatic modification of the lithospheric mantle in southern Africa. Thus, olivine megacryst chemistry supports interpretations for the recycled component of the HIMU reservoir as mineralogically complex metasomatic lithologies formed by the infiltration of subduction-derived melts into the base of the lithospheric mantle. In addition to carbonate-richHIMU mantle lithologies in southern Africa and worldwide, Monastery kimberlite olivine megacrysts and olivine in melilitites suggest that a phlogopite-richHIMU lithology is present in southern Africa. An important question arising from this study is in reconciling the homogenous isotopic ratios of HIMU basalts worldwide with an apparent heterogeneous lithological source indicated by olivine chemistry.
Abstract: The flux of elements into Earth's sub-continental lithospheric mantle is facilitated by the passage of small-fraction melts that either crystallise new phases or react with pre-existing minerals.Metasomatised peridotite records the end product of this exchange but rarely captures the process in the act due to subsolidus re-equilibration. We present the results of a systematic investigation of a metasomatic melt channel preserved in a mantle peridotite from the Late Cretaceous Bultfontein kimberlite (Kaapvaal craton) that shows rare direct evidence of the melt-rock reaction processes. We show that the metasomatic proto-kimberlite melt underwent variable crystallisation of clinopyroxene, sulfides, phlogopite, spinel and zircon together with interaction and diffusive exchange with the surrounding olivine-rich mantle. Element profiles across large olivine porphyroclasts (Fo88) show significant core-to-rim variations in Ni (NiO?=?0.18-0.32?wt%) and Cr (Cr?=?35-60?ppm), while concentrations of all other elements (e.g. Mg, Fe, Mn, Co, V)are remarkably homogeneous. Electron backscatter diffraction analysis shows that the disequilibrium of Ni and Cr is greatest where the crystal contains large components of the [100] and [010] axes. The disequilibrium is preserved in certain orientations because diffusion of Ni and Cr in olivine is more anisotropic than Fe-Mg and Mn, and slower in the [100] and [010] directions. We present the first observations of Ni and Cr decoupling from other elements in mantle olivine and suggest that this is a consequence of: (i)changing mineral-melt concentration gradients associated with the reactive percolation of a precursory kimberlite melt; and (ii) late-stage sulfide and spinel precipitation. We use the diffusion limited re-equilibration of Ni in olivine to quantify the timing of metasomatism prior to xenolith entrainment by the host kimberlite. Our modelling indicates that reactive percolation occurred on the order of 103-105?years prior to entrainment; this provides an additional line of support for the hypothesis that a period of metasomatism by proto-kimberlite melts precedes the final kimberlite ascent to the surface. The broader implication of our finding of variable rates of minor element diffusion in natural olivine is that it highlights the importance of anisotropy and the impact of changing local concentration gradients during subsolidus re-equilibration.
Journal of African Earth Sciences, Vol. 146, pp. 28-47.
Africa, Cameroon
craton
Abstract: Field, microstructural, and anisotropy of magnetic susceptibility (AMS, magnetic fabrics) studies assessed the Pan-African deformational history and strain geometry at the southern margin of the Central African Fold Belt (CAFB) against the older, cratonic basement of the Congo Shield (CS). Reflected light microscopy and thermomagnetic studies supported the identification of magnetic minerals. Data cover a low angle thrust margin (Mbengis-Sangmelima area) in the east and high angle shear zones cutting the margin (Kribi area) in the west, at the Atlantic coast. In the CS basement units, magnetic anisotropy is generally higher than in the low grade Pan-African units. In the latter, early D1/D2 shortening produced a flat-lying magnetic foliation parallel with the regional trend of the belt, a shallow magnetic lineation, and mostly oblate fabrics. Subsequent D3 deformation is only of local importance in the Mbengis-Sangmelima area. The magnetic lineation shows distinct maxima in NNE-SSW direction, parallel with the low angle tectonic transport direction. In the Kribi area, the NNE-SSW trending Kribi-Campo shear zone (KCSZ) affected both older rocks and Pan-African high grade metapelites of the Yaoundé unit together with their basal thrust. The early planar fabric (S1) was overprinted during D2 folding under relatively high T conditions, and subsequent D3 wrenching. Magnetic fabrics document a progressive change from oblate towards prolate ellipsoids towards the KCSZ. Magnetic foliations with medium to steep dips curve into the N-S to NE-SW orientation of the KCSZ, lineations follow the same trend with shallow to medium plunges. This fabric implies that the KCSZ is a Pan-African strike-slip shear zone with a subordinate component of compression. Strike-slip tectonics in the west (KCSZ) and thrusting in the east imply N-S to NE-SW convergence during Pan-African terrane assembly against the present northern margin of the CS. In addition, the KCSZ may separate the CS from the São Francisco Craton in Brazil and thus be the northern part of a link connecting the CAFB to the West Congo Belt in the south. This putative Pan-African link separated the São Francisco Craton from the Congo Shield prior to Mesozoic Gondwana break-up.
Journal of Rural Studies, Vol. 61, 1, pp. 110-122.
Africa, Sierra Leone
alluvials
Abstract: This paper reflects critically on the transformational impacts the recent Ebola epidemic has had in diamond-rich areas of rural Sierra Leone. It focuses specifically on the country's ‘diggers’, a sizable group of labourers who occupy the bottom of the country's artisanal diamond mine production pyramid. Based upon research conducted in the diamond-producing localities of Kenema and Kono, the paper argues how, in sharp contrast to the gloomy picture painted in the literature about their existences and struggles, diggers exhibited considerable resilience during the Ebola crisis. Their diversified livelihood portfolios proved to be effective survival strategies and buffers against the shocks and stresses brought about by lengthy periods of quarantine, and during times when mobility was restricted by the government in a bid to prevent the spreading of the disease. Drawing inspiration from the resilience literature, the paper captures the essence of these survival strategies, which should be viewed as latest reshuffling and expansion of diggers' rural livelihood portfolios. Policymakers and donors have yet to embrace fully these changes in a country where the Ebola recovery period promises to be lengthy and at a time when fresh, locally-informed rural development solutions are in short supply.
International Journal of Earth Sciences, Vol. 107, 7, pp. 2627-2633.
Africa, Botswana
kimberlites
Abstract: Glassy beads were found in the sand cover associated with known and suspected kimberlites on the Kalahari plateau, Botswana. Morphology and chemistry were examined by ESEM and EDAX. The polymetallic and quartzose "beads", here described for the first time and termed fulguroids, formed at very high temperatures, well in excess of those reached by the kimberlites. They solidified in free fall. We propose that they were melted in the atmosphere by lightning strikes on Kalahari overburden entrained when the kimberlites erupted.
Mineralogy and Petrology, doi:10.1007/ s00710-018-0568-9 15p.
Africa, South Africa
deposit - Group I, orangeites Group II
Abstract: The West Coast of Namaqualand in South Africa hosts extensive detrital diamond deposits, but considerable debate exists as to the provenance of these diamonds. Some researchers have suggested derivation of the diamonds from Cretaceous-Jurassic kimberlites (also termed Group I kimberlites) and orangeites (also termed Group II kimberlites) located on the Kaapvaal Craton. However, others favour erosion of diamonds from the ca.300 Ma Dwyka Group sediments, with older, pre-Karoo kimberlites being the original source(s). Previous work has demonstrated that 40Ar/39Ar analyses of clinopyroxene inclusions, extracted from diamonds, yield ages approaching the time(s) of source kimberlite emplacement, which can be used to constrain the provenance of placer diamond deposits. In the current study, 40Ar/39Ar analyses were conducted on clinopyroxene inclusions from two similar batches of Namaqualand detrital diamonds, yielding (maximum) ages ranging from 117.5?±?43.6 Ma to 3684?±?191 Ma (2s) and 120.6?±?15.4 Ma to 688.8?±?4.9 Ma (2s), respectively. The vast majority of inclusions (88%) produced ages younger than 500 Ma, indicating that most Namaqualand diamonds originated from Cretaceous-Jurassic kimberlites/orangeites, with few, if any, derived from the Dwyka tillites. The provenance of the Namaqualand diamonds from ca.115-200 Ma orangeites is consistent with Late Cretaceous paleo-drainage reconstructions, as these localities could have been sampled by the ‘paleo-Karoo’ River and transported to the West Coast via an outlet close to the current Olifants River mouth. At ca.90 Ma, this drainage system appears to have been captured by the ‘paleo-Kalahari’ River, a precursor to the modern Orange River system. This latter drainage is considered to have transported diamonds eroded from both ca.80-90 Ma kimberlites and ca.115-200 Ma orangeites to the West Coast, which were subsequently reworked along the Namibian coast, forming additional placer deposits.
History of Photography, Vol. 42, 1, pp. 4-24. doi.org/10.1080/ 03087298.2018. 1429095
Africa, South Africa
deposit - Kimberley
Abstract: In the mid 1880s a little-known photographer named Robert Harris produced a series of albumen prints showing the stages of body searching that black labourers in De Beers diamond mines were obliged to undergo by state ordinance enacted in 1883. The original photographs surfaced briefly in the saleroom in 2007 but have since disappeared. Two sets of copies survive. Bearing in mind the history of documentary photography in South Africa, this article examines the historical and textual significance of this series of photographs in the context of the history of mining and discusses the imperatives and ethics of locating, researching, and publishing controversial imagery in the Internet age.
Contributions to Mineralogy and Petrology, Vol. 173, pp. 76- doi.org/10.1007/ s00410-018-1502-1
Africa, Lesotho
deposit - Letseng
Abstract: The Letšeng-la-Terae kimberlite (Lesotho), famous for its large high-value diamonds, has five distinct phases that are mined in a Main and a Satellite pipe. These diatreme phases are heavily altered but parts of a directly adjacent kimberlite blow are exceptionally fresh. The blow groundmass consists of preserved primary olivine with Fo86?88, chromite, magnesio-ulvöspinel and magnetite, perovskite, monticellite, occasional Sr-rich carbonate, phlogopite, apatite, calcite and serpentine. The bulk composition of the groundmass, extracted by micro-drilling, yields 24-26 wt% SiO2, 20-21 wt% MgO, 16-19 wt% CaO and 1.9-2.1 wt% K2O, the latter being retained in phlogopite. Without a proper mineral host, groundmass Na2O is only 0.09-0.16 wt%. However, Na-rich K-richterite observed in orthopyroxene coronae allows to reconstruct a parent melt Na2O content of 3.5-5 wt%, an amount similar to that of highly undersaturated primitive ocean island basanites. The groundmass contains 10-12 wt% CO2, H2O is estimated to 4-5 wt%, but volatiles and alkalis were considerably reduced by degassing. Mg# of 77.9 and 530 ppm Ni are in equilibrium with olivine phenocrysts, characterize the parent melt and are not due to olivine fractionation. 87Sr/86Sr(i)?=?0.703602-0.703656, 143Nd/144Nd(i)?=?0.512660 and 176Hf/177Hf(i)?=?0.282677-0.282679 indicate that the Letšeng kimberlite originates from the convective upper mantle. U-Pb dating of groundmass perovskite reveals an emplacement age of 85.5?±?0.3 (2?) Ma, which is significantly younger than previously proposed for the Letšeng kimberlite.
The Southern African Institute of Mining and Metallurgy Diamonds - from source to use 2013, 9p. Pdf researchgate
Africa, Angola, South Africa
deposit - Mulepe, Voorspoed
Abstract: Construction of geological models for kimberlites has historically largely been based on visual volcanic textures and features, with chemical data only produced on an ad-hoc basis depending on the level of perceived complexity observed in the geological data. The focus of this manuscript is a high-level mineral resource management view of the impact which the initial absence of proper chemical data had on two kimberlite resource models as well as the increasing magnitude of the problem caused by the lack of chemical data once the resource estimation process commenced. The impact of the absence of chemical data on projects where micro-diamond estimation techniques are employed is also discussed. Metallurgical parameters have historically not been incorporated into kimberlite resource models, but recent work at De Beers Group Services has shown that geostatistical quantification of the DMS (Dense Media Separator) yield is useful for mine planning purposes, and will add value in the understanding of the kimberlite emplacement process. A case study is presented where a geostatistical analysis and conditional simulation was performed on the DMS yield in a kimberlite at a local block scale. These data were used to identify mining blocks where the DMS yield would exceed the capability of the treatment plant.
Abstract: The continental margins of East Africa and West Madagascar are a frontier for hydrocarbon exploration. However, the links between the regional tectonic history of sedimentary basins and margin evolution are relatively poorly understood. We use a plate kinematic model built by joint inversion of seafloor spreading data as a starting point to analyse the evolution of conjugate margin segments and corresponding sedimentary basins. By correlating megasequences in the basins to the plate model we produce a margin?scale tectono?stratigraphic framework comprising four phases of tectonic development. During Phase 1 (183-133 Ma) Madagascar/India/Antarctica separated from Africa, first by rifting and later, after breakup (at ca. 170-165 Ma), by seafloor spreading in the West Somali and Mozambique basins and dextral strike?slip movement on the Davie Fracture Zone. Mixed continental/marine syn?rift megasequences were deposited in rift basins followed by shallow?marine early postrift sequences. In Phase 2 (133-89 Ma) spreading ceased in the West Somali basin and Madagascar became fixed to the African plate. However, spreading continued between the African and Antarctic plates and deposition of the early postrift megasequence continued. The onset of spreading on the Mascarene Ridge separated India from Madagascar in Phase 3 (89-60 Ma). Phase 3 was characterized by the onset of deposition of the late postrift megasequence with continued deep marine sedimentation. At the onset of Phase 4 (60 Ma onward) spreading on the Mascarene ridge ceased and the Carlsberg Ridge propagated south to form the Central Indian Ridge, separating India from the Seychelles and the Mascarene Plateau. Late postrift deposition continued until a major unconformity linked to the development of the East African Rift System marked the change to deposition of the modern margin megasequence.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0628-1 14p.
Africa, Angola
kimberlites
Abstract: Based on a comprehensive analysis of kimberlite pipes of Angola, including the near surface structural setting, deep lithospheric structure, pipe morphology and emplacement, mineralogical and petrographic features, diamond characteristics and locations of secondary deposits four geographical regions have been outlined within Angola representing four types of diamond bearing potential. These areas include high diamond bearing potential pipes, possible potential, no potential, and unclear potential areas. It was found that the depth of magmatism and diamond potential of kimberlites increases from the Atlantic coast in southwestern Angola into the continent in the north-easterly direction. Areas prospective for the discovery of new primary diamond deposits have been identified.
Journal of African Earth Sciences, Vol. 149, pp. 215-234.
Africa, Cameroon
subduction
Abstract: High- to ultrahigh-pressure metamorphic assemblages consisting of garnet-omphacitic clinopyroxene bearing mafic rocks have been identified within the Paleoproterozoic Nyong Group in SW Cameroon, at the northwestern margin of the Archean Congo craton. These rocks were investigated in detail and for the first time evidence for eclogite facies metamorphism at ca 25?kbar and 850?°C is provided. A clockwise P-T path with nearly isothermal decompression (ITD) is deduced from mineral zoning and textural relationships characterized by mineral recrystallization and multi-layered coronitic overgrowths of plagioclase and clinopyroxene surrounding garnet porphyroblasts. These P-T conditions imply a burial depth greater than 90?km, at lower geothermal gradient of ca 10?°C/km. The geochemical signature of ten representative rock samples show that two groups of eclogite facies rocks genetically originate from mostly basaltic and basaltic andesite compositions, with a characteristic upper mantle-derived tholeiitic trend. Moreover, their chondrite and MORB normalized REE and trace element concentrations are characterized by nearly flat REE patterns with very little to no Eu anomaly, (La/Sm)N???1 and Zr/Nb???10, as well as a gradual depletion from LREE to HREE with also very little to no Eu anomaly, but (La/Sm)N < 1, Zr/Nb > 10 and negative anomalies in Th, K, Nb, Ta, Sr, Zr and Ti consistent with mid-ocean ridge basalt (MORB) contaminated by a subduction component or by a crustal component. Previous available geochronological data coupled with our new petrological, mineralogical and geochemical findings clearly indicate that the eclogite facies metabasites from the Eburnean Nyong Group between 2100 and 2000 Ma represent one of the oldest subducted oceanic slab or trace of a suture zone so far recorded within the West Central African Fold Belt (WCAFB). The geodynamic implications of these eclogites suggest a subduction-related process followed by a rapid exhumation of their protoliths, therefore, providing critical information corroborating that plate tectonic processes operated during the Paleoproterozoic.
Abstract: Geologists have a new window onto the early Earth: zircon crystals from South Africa that could be as much as 4.1 billion years old. Ancient crystals of zircon - a durable mineral found in rock that has been squeezed and heated - from Western Australia have revealed some of the planet’s early secrets, such as clues to the chemistry of its primordial crust. But researchers have had little in the way of other records for this period of Earth’s infancy. Now, Benjamin Byerly at Louisiana State University in Baton Rouge and his colleagues report their discovery of a second rich trove of zircons. The crystals lie east of Pretoria, in a rock formation known as the Barberton greenstone belt. The African zircons are important because they have been subject to less heating and squeezing than the Australian samples. As a result, the African crystals may have richer stories to tell about the chemistry of Earth’s early years.
Contributions to Mineralogy and Petrology, Vol. 173, 22p. Doi.org/10.1007/s00410-018-1512-z
Africa, Tanzania
magmatism
Abstract: Magmatism on Earth is most abundantly expressed by surface volcanic activity, but all volcanism has roots deep in the crust, lithosphere, and mantle. Intraplate magmatism, in particular, has remained enigmatic as the plate tectonic paradigm cannot easily explain phenomena such as large flood basalt provinces and lithospheric rupture within continental interiors. Here, I explore the role of deep crustal magmatic processes and their connection to continental rift volcanism as recorded in deep crustal xenoliths from northern Tanzania. The xenoliths are interpreted as magmatic cumulates related to Cenozoic rift volcanism, based on their undeformed, cumulate textures and whole-rock compositions distinct from melt-reacted peridotites. The cumulates define linear trends in terms of whole-rock major elements and mineralogically, can be represented as mixtures of olivine?+?clinopyroxene. AlphaMELTS modeling of geologically plausible parental melts shows that the end-member cumulates, clinopyroxenite and Fe-rich dunite, require fractionation from two distinct melts: a strongly diopside-normative melt and a fractionated picritic melt, respectively. The former can be linked to the earliest, strongly silica-undersaturated rift lavas sourced from melting of metasomatized lithosphere, whereas the latter is linked to the increasing contribution from the upwelling asthenospheric plume beneath East Africa. Thus, deep crustal cumulate systematics reflect temporal and compositional trends in rift volcanism, and show that mixing, required by the geochemistry of many rift lava suites, is also mirrored in the lavas’ cumulates.
Journal of African Earth Sciences, Vol. 137, pp. 9-21.
Africa, Botswana
deposit - Jwaneng
Abstract: Country rock at Jwaneng Diamond Mine provides a rare insight into the deformational history of the Transvaal Supergroup in southern Botswana. The ca. 235 Ma kimberlite diatremes intruded into late Archaean to Early Proterozoic, mixed, siliciclastic-carbonate sediments, that were subjected to at least three deformational events. The first deformational event (D1), caused by NW-SE directed compression, is responsible for NE-trending, open folds (F1) with associated diverging, fanning, axial planar cleavage. The second deformational event (D2) is probably progressive, involving a clockwise rotation of the principal stress to NE-SW trends. Early D2, which was N-S directed, involved left-lateral, oblique shearing along cleavage planes that developed around F1 folds, along with the development of antithetic structures. Progressive clockwise rotation of far-field forces saw the development of NW-trending folds (F2) and its associated, weak, axial planar cleavage. D3 is an extensional event in which normal faulting, along pre-existing cleavage planes, created a series of rhomboid-shaped, fault-bounded blocks. Normal faults, which bound these blocks, are the dominant structures at Jwaneng Mine. Combined with block rotation and NW-dipping bedding, a horst-like structure on the northwestern limb of a broad, gentle, NE-trending anticline is indicated. The early compressional and subsequent extensional events are consistent throughout the Jwaneng-Ramotswa-Lobatse-Thabazimbi area, suggesting that a large area records the same fault geometry and, consequently, deformational history. It is proposed that Jwaneng Mine is at or near the northernmost limit of the initial, northwards-directed compressional event.
Abstract: We have studied mineral-hosted melt, crystal and fluid inclusions from two ugandite, one mafurite and two katungite samples from the Toro-Ankole volcanic province in the East African Rift, which is the archetypal location for kamafugitic rocks. A main finding of our study is the presence of orthopyroxene as inclusions in an early generation of olivine from all three types of kamafugites, suggesting interaction of a carbonate-rich metasomatic agent with lithospheric peridotite mantle that may have caused almost complete dissolution of orthopyroxene. This process was preceded, accompanied or followed by the formation of phlogopite-clinopyroxene veins resulting from interaction of F-rich and low H2O/CO2 metasomatic fluids with the mantle rocks, which then became the source of the Toro-Ankole kamafugites. Pressure-temperature (P-T) estimates suggest that the parental kamafugitic melts last equilibrated with their source rocks at ?16?±?8?kbar and ?1160?±?130°C. This implies that they could have originated significantly below the solidus of dry, carbonated peridotite, but above the solidus of phlogopite-bearing clinopyroxenite. We conclude that the Toro-Ankole kamafugites originated by very low degrees of partial melting at moderately oxidized conditions (?FMQ = +2•2?±?0•4?atm log units, where FMQ is fayalite-magnetite-quartz buffer) under a high geothermal gradient of 60-80?mW?m?2, in response to lithospheric extension and probable association with an adjacent mantle plume. We estimate that differentiation of parental ugandite, mafurite and katungite magmas could have occurred at depths <12?km in the T range 1150-850°C. Laboratory-heated, homogenized melt inclusions trapped by a second generation of olivine and clinopyroxene are characterized by remarkable silica-undersaturation, compared with mid-ocean ridge basalt and ocean island basalt magmas, with high concentrations of alkalis, Ti, Ba, Sr and Zr, but varying to very low concentrations of Al and Ca. Such alkali-rich, strongly evolved melts might have resulted from extreme (>95%) fractional crystallization of the parental magmas, assuming their chemical compositions to be similar to those of the respective lavas. However, this estimate is about three times higher than the modal amount of phenocrysts in the lavas that could be reinforced by the presence of excess cognate crystals and/or xenocrysts in the lavas. Strong oxidation from FMQ?+?2 to FMQ?+?4 to +?5•2?atm log units of the evolved mafuritic magmas at ?900-1000°C has occurred during the final stage of magma evolution at very shallow crustal depths or possibly directly in the lava flow.
Abstract: The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2?Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66-3.20?Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2?Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane. Some previous models have related the 3.2?Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an underplating event at ca. 3.2?Ga, giving rise to melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.
Journal of Geophysical Research: Solid Earth, doi: 10.1029/2018JB016012
Africa, South Africa
diamond inclusions
Abstract: Here we report the first results from elastic geobarometry applied to a kyanite inclusion entrapped within an eclogitic diamond (from Voorspoed mine, South Africa) using micro?Raman and Fourier transform infrared spectroscopy, electron microprobe analysis, ab initio calculations, and finite element modeling. Application of elastic geobarometry to very elastically anisotropic kyanite inclusions is challenging, as current models do not allow for elastic anisotropy. In order to minimize the effects of anisotropy, we have explored the effects of deviatoric stress on Raman modes via ab initio density functional theory. The results allowed us to select the Raman mode (at ca. 638 cm?1) that is the least sensitive to deviatoric stress. The shift of this band in the inclusion while still trapped within the diamond relative to the inclusion in air (once liberated) was used under hydrostatic approximation to determine a residual pressure on the inclusion of 0.184 ± 0.045 GPa and an entrapment pressure of 5.2 ± 0.3 GPa (~160 km depth) for an FTIR N?aggregation residence temperature of 1119 ± 50 °C. This is the first geothermobarometric determination for a diamond from the Voorspoed kimberlite. It overlaps with P-T estimates obtained by traditional chemical geobarometry for diamonds from other kimberlites from the Kaapvaal craton, suggesting that the hydrostatic approximation does not introduce significant errors in the geobarometric evaluation. Our protocol of Raman peak selection can be used for geobarometry of further kyanite?bearing diamonds and may provide a guide for more robust geobarometry of other types of mineral inclusions in diamonds, both eclogitic and peridotitic.
Abstract: We present original results that contribute to the understanding of lithospheric structures modification of regions that have witnessed superimposition of multiple tectonic events throughout their geological history. We analyze satellite gravity data through two-dimensional radially-averaged power spectral analysis as well as passive seismic data through thermal modeling to image the depth to the Moho and the lithosphere - asthenosphere boundary (LAB beneath the Central Africa Orogenic Belt (CAOB). The CAOB is an ENE-trending deformation belt extending from Cameroon in the west to Sudan in the east. In Cameroon, it is found on the northern edge of the Congo craton represented by the Oubanguides orogenic belt (the Western Cameroon, the Adamawa - Yade, and the Yaoundé domains). It coincides with the Adamawa plateau and the Benue Trough, and it is spotted by the Cenozoic Cameroon Volcanic Line (CVL). The CAOB was formed during the Precambrian Greater Gondwana assembly but was reactivated during the Mesozoic as a result of Gondwana breakup. We find deeper Moho and LAB) beneath Congo craton and the Yaoundé domain reaching ~50?km and ~200?km, respectively. We map shallower Moho and LAB beneath the CAOB (together with the Adamawa plateau and the Benue trough) reaching ~25?km and ~70?km, respectively. We interpret the shallower LAB beneath the CAOB as due to zonal sub-continental lithospheric mantle (SCLM) delamination along the northern edge of the Congo craton that occurred in association with collisional assembly of Greater Gondwana. This allowed for channelization of mantle flow during the Cenozoic resulting in the formation of the CVL and the uplift of the Adamawa plateau. Our approach can be used to understand the modification of lithospheric structures beneath other terrains that have long tectonic history.
Abstract: Mobilistic plate-tectonic interpretation of Precambrian orogens requires that two conjoined crustal blocks may derive from distant portions of the globe. Nonetheless, many proposed Precambrian cratonic juxtapositions are broadly similar to those of younger times (so-called “strange attractors”), raising the specter of bias in their construction. We evaluated the possibility that the Congo and Kalahari cratons (Africa) were joined together prior to their amalgamation along the Damara-Lufilian-Zambezi orogen in Cambrian time by studying diabase dikes of the Huila-Epembe swarm and sills in the southern part of the Congo craton in Angola and in Namibia. We present geologic, U-Pb geochronologic, and paleomagnetic evidence showing that these two cratons were directly juxtaposed at ca. 1.1 Ga, but in a slightly modified relative orientation compared to today. Recurring persistence in cratonic connections, with slight variations from one supercontinent to the next, may signify a style of supercontinental transition similar to the northward motion of Gondwana fragments across the Tethys-Indian oceanic tract, reuniting in Eurasia.
Abstract: The Archean Yilgarn Craton in Western Australia hosts at least five generations of Proterozoic mafic dykes, the oldest previously identified dykes belonging to the ca. 2408-2401?Ma Widgiemooltha Supersuite. We report here the first known Archean mafic dyke dated at 2615?±?6?Ma by the ID-TIMS U-Pb method on baddeleyite and at 2610?±?25?Ma using in situ SHRIMP U-Pb dating of baddeleyite. Aeromagnetic data suggest that the dyke is part of a series of NE-trending intrusions that potentially extend hundreds of kilometres in the southwestern part of the craton, here named the Yandinilling dyke swarm. Mafic magmatism at 2615?Ma was possibly related to delamination of the lower crust during the final stages of assembly and cratonisation, and was coeval with the formation of late-stage gold deposit at Boddington. Paleogeographic reconstructions suggest that the Yilgarn and Zimbabwe cratons may have been neighbours from ca. 2690?Ma to 2401?Ma and if the Zimbabwe and Kaapvaal cratons amalgamated at 2660-2610?Ma, the 2615?Ma mafic magmatism in the southwestern Yilgarn Craton may be associated with the same tectonic event that produced the ca. 2607-2604?Ma Stockford dykes in the Central Zone of the Limpopo Belt. Paleomagnetic evidence and a similar tectonothermal evolution, including coeval low-pressure high-temperature metamorphism, voluminous magmatism, and emplacement of mafic dykes, support a configuration where the northern part of the Zimbabwe Craton was adjacent to the western margin of the Yilgarn Craton during the Neoarchean. Worldwide, reliably dated mafic dykes of this age have so far been reported from the Yilgarn Craton, the Limpopo Belt and the São Francisco Craton.
Earth and Planetary Science Letters, Vol. 503, pp. 108-117.
Africa, South Africa, Zimbabwe
geophysics - seismic
Abstract: The depths of the 410 km (d410) and 660 km (d660) discontinuities beneath southern Africa, which is presumably underlain by the lower-mantle African Superswell, are imaged in 1? radius consecutive circular bins using over 6400 P-to-S receiver functions (RFs) recorded by 130 seismic stations over a 27 yr period. When the IASP91 standard Earth model is utilized for moveout correction and time-depth conversion, a normal mantle transition zone (MTZ) thickness of 246 ± 7 km is observed, suggesting that the Superswell has no discernible effect on mantle transition zone temperature. Based on the negligible disparity of the mean MTZ thicknesses between on (246 ± 6 km) and off (246 ± 8 km) cratonic regions, we conclude that the deep Archean cratonic keels possess limited influence on MTZ thermal structure. The apparently shallower-than-normal MTZ discontinuities and the parallelism between the d410 and d660 are mostly the results of upper mantle high wave speed anomalies probably corresponding to a thick lithosphere with a mean thickness of about 245 km beneath the Kaapvaal and 215 km beneath the Zimbabwe cratons. In contradiction to conclusions from some of the previous studies, the resulting spatial distribution of the stacking amplitudes of the P-to-S converted phases at the discontinuities is inconsistent with the presence of an excessive amount of water in the MTZ and atop the d410.
Abstract: A complex mineral sequence in a kimberlite from the Banankoro Cluster (Guinea Conakry) has been interpreted as the result of magma mixing processes. The composition of the early generations of phlogopite and spinel suggest direct crystallisation of a kimberlitic magma. However, the compositional trends found in the late generations of phlogopite and spinels could suggest magma mixing. In this context, four ilmenite generations formed. The first generations (types 1 and 2) are geikielitic and are associated with spinel and phlogopite which follow the kimberlitic compositional trends. They are interpreted as produced by crystallization from the kimberlite magma. A third generation of euhedral tabular Mg-rich ilmenite (type 3) formed during the interval between two generations of serpentine. Finally, a late generation of Mn-rich ilmenite (type 4) replaces all the Ti-rich minerals and is contemporaneous with the last generation of serpophitic non-replacing serpentine. Therefore, the formation of type 3 and type 4 ilmenite took place after the crystallization of the groundmass, during late hydrothermal process. Our results suggest a detailed textural study is necessary when use Mg-rich and Mn-rich ilmenites as KIMs.
Abstract: The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5?m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7?±?4.4?Ma and 86.5?±?2.6?Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster. The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections. The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration. Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn?/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.
2018 Yellowknife Geoscience Forum , p. 91-92. abstract
Africa, South Africa
deposit - Roberts Victor
Abstract: It is well known that mechanical disaggregation, such as jaw crushing, can cause irreversible damage to valuable gemstones hosted in crystalline rocks. The SELFRAG Lab device uses electrical pulses at high voltages - typically between 150 and 200 kV - to separate material into individual grains along natural boundaries. The purpose of this research is to assess the viability of the SELFRAG as a tool to disaggregate diamond-bearing eclogites, and to assess if this method preserves grains that would otherwise be damaged through mechanical disaggregation. In order to test the applicability of the SELFRAG to diamond recovery from mechanically strong diamond-bearing lithologies, we studied its effects on a diamondiferous eclogite, RV09, from Roberts Victor mine. The Roberts Victor mine is located in South Africa and is renowned for its unusually high abundance of mantle-derived eclogite xenoliths1. Before the eclogite was disaggregated, we bisected the sample and used a CT scan to determine its constituent minerals and the spatial distribution of diamond. One half of the sample was then placed into the SELFRAG, where it was subjected to ~100 shots of 200 kV electrical discharges that segregated the sample into individual grains of similar sizes. The other half was jaw crushed, using a steel jaw crusher which produced non-uniform composite grains and abundant fine material. The varying sizes and aggregate pieces made it difficult to pick diamonds, and after no diamonds were found, the jaw-crushed portion underwent further disaggregation in the SELFRAG. After exerting the same time and effort picking through both portions of the RV09 sample, ten diamonds were recovered from the electronically disaggregated portion, while no diamonds were found in the conventionally disaggregated sample. The diamonds released from the SELFRAG were then imaged with a scanning electron microscope (SEM) to determine the extent to which the diamonds were damaged. Most of the released diamonds showed no evidence of breakage, but a few showed signs of damage that may have occurred prior to kimberlite eruption. The dramatic disparity between the number of diamonds recovered with the SELFRAG and the lack of diamonds in the jaw crushed portion indicates that electrical disaggregation is a superior method compared to the conventional mechanical comminution technique. There are little to no signs of breakage in the SELFRAG-liberated diamonds, whereas, the damage caused by jaw crushing was extensive enough to produce small fragments not readily visible via optical microscopy. The SELFRAG is a promising alternative to conventional disaggregation and offers a practical solution for lessening damage to valuable stones in rocks such as eclogites and kimberlites.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 425-
Abstract: A detailed electron microprobe study has been carried out on the compositional variations of mica and apatite from carbonatites, phoscorites and associated pyroxenites (and fenites) of the Loolekop deposit, Palabora Carbonatite Complex (South Africa). Mica in pyroxenites and fenites is Mg-rich biotite, whilst micas in carbonatites and phoscorites are compositionally diverse including phlogopite, Ba-rich phlogopite (up to 30% kinoshitalite component), IVAl-rich phlogopite (up to 30% eastonite component) and tetraferriphlogopite. The various types of phlogopites are interpreted as orthomagmatic phases, whereas tetraferriphlogopite precipitation was a late-magmatic to hydrothermal process that additionally introduced REE into the system. Orthomagmatic apatite is generally REE- and Sr-poor fluorapatite and does not show large compositional differences between rock types. Apatite associated with the late-stage tetraferriphlogopite mineralization reaches higher levels of REE (up to 4.9?wt%), Si (up to 1.5?wt% SiO2), Sr (up to 2.6?wt% SrO) and Na (up to 1.0?wt% Na2O). The compositional variation of micas and apatites, which is affiliated with distinct rock types, reflects the multi-stage evolution of the Loolekop deposit and provides detailed insight into the relationships of the carbonatite-phoscorite assemblage. The obtained data support the separation of phoscorite and carbonatite by immiscibility from a common parental magma, which may happen due to a decrease of temperature and/or pressure during the ascent of the magma. This results in a density contrast between the carbonatitic and phoscoritic components that will lead to descending phoscorite accumulations at the outer zones of the magma channel and a jet-like ascent (further promoted by its extremely low viscosity) of the carbonatite magma. The genetic model deduced here explains the peculiar association of carbonatites, phoscorites and silicate rocks in many alkaline complexes worldwide.
Abstract: We present original results that contribute to the understanding of lithospheric structures modification of regions that have witnessed superimposition of multiple tectonic events throughout their geological history. We analyze satellite gravity data through two-dimensional radially-averaged power spectral analysis as well as passive seismic data through thermal modeling to image the depth to the Moho and the lithosphere - asthenosphere boundary (LAB beneath the Central Africa Orogenic Belt (CAOB). The CAOB is an ENE-trending deformation belt extending from Cameroon in the west to Sudan in the east. In Cameroon, it is found on the northern edge of the Congo craton represented by the Oubanguides orogenic belt (the Western Cameroon, the Adamawa - Yade, and the Yaoundé domains). It coincides with the Adamawa plateau and the Benue Trough, and it is spotted by the Cenozoic Cameroon Volcanic Line (CVL). The CAOB was formed during the Precambrian Greater Gondwana assembly but was reactivated during the Mesozoic as a result of Gondwana breakup. We find deeper Moho and LAB) beneath Congo craton and the Yaoundé domain reaching ~50?km and ~200?km, respectively. We map shallower Moho and LAB beneath the CAOB (together with the Adamawa plateau and the Benue trough) reaching ~25?km and ~70?km, respectively. We interpret the shallower LAB beneath the CAOB as due to zonal sub-continental lithospheric mantle (SCLM) delamination along the northern edge of the Congo craton that occurred in association with collisional assembly of Greater Gondwana. This allowed for channelization of mantle flow during the Cenozoic resulting in the formation of the CVL and the uplift of the Adamawa plateau. Our approach can be used to understand the modification of lithospheric structures beneath other terrains that have long tectonic history.
Abstract: The flux of elements into Earth’s sub-continental lithospheric mantle is facilitated by the passage of small-fraction melts that either crystallise new phases or react with pre-existing minerals. Metasomatised peridotite records the end product of this exchange but rarely captures the process in the act due to subsolidus re-equilibration. We present the results of a systematic investigation of a metasomatic melt channel preserved in a mantle peridotite from the Late Cretaceous Bultfontein kimberlite (Kaapvaal craton) that shows rare direct evidence of the melt-rock reaction processes. We show that the metasomatic proto-kimberlite melt underwent variable crystallisation of clinopyroxene, sulfides, phlogopite, spinel and zircon together with interaction and diffusive exchange with the surrounding olivine-rich mantle. Element profiles across large olivine porphyroclasts (Fo88) show significant core-to-rim variations in Ni (NiO = 0.18-0.32 wt.%) and Cr (Cr = 35-60 ppm), while concentrations of all other elements (e.g. Mg, Fe, Mn, Co, V) are remarkably homogeneous. Electron backscatter diffraction analysis shows that the disequilibrium of Ni and Cr is greatest where the crystal contains large components of the [100] and [010] axes. The disequilibrium is preserved in certain orientations because diffusion of Ni and Cr in olivine is more anisotropic than Fe-Mg and Mn, and slower in the [100] and [010] directions. We present the first observations of Ni and Cr decoupling from other elements in mantle olivine and suggest that this is a consequence of: (i) changing mineral-melt concentration gradients associated with the reactive percolation of a precursory kimberlite melt; and (ii) late-stage sulfide and spinel precipitation. We use the diffusion limited re-equilibration of Ni in olivine to quantify the timing of metasomatism prior to xenolith entrainment by the host kimberlite. Our modelling indicates that reactive percolation occurred on the order of 103-105 years prior to entrainment; this provides an additional line of support for the hypothesis that a period of metasomatism by proto-kimberlite melts precedes the final kimberlite ascent to the surface. The broader implication of our finding of variable rates of minor element diffusion in natural olivine is that it highlights the importance of anisotropy and the impact of changing local concentration gradients during subsolidus re-equilibration.
Abstract: The breakup of Africa from South America is associated with the emplacement of the Paraná-Etendeka flood basalt province from around 134 Ma and the Tristan da Cunha plume. Yet many additional volcanic events occur that are younger than the main pulse of the Paraná-Etendeka and straddle the rift to drift phases of the main breakup. This contribution reports on new geochronological constraints from the Angolan part of the African Margin. Three coastal and one inland section have been sampled stretching across some 400 Km, with 39Ar/40Ar, U-Pb and Palaeontology used to provide age constraints. Ages from the new data range from ~100 to 81 Ma, with three main events (cr. 100, 91 and 82-81 Ma). Volcanic events are occurring within the Early to Late Cretaceous, along this part of the margin with a general younging towards Namibia. With the constraints of additional age information both onshore and offshore Angola, a clear younging trend at the early stages of rift to drift is recorded in the volcanic events that unzip from North to South. Similar age volcanic events are reported from the Brazilian side of the conjugate margin, and highlight the need to fully incorporate these relatively low volume volcanic pulses into the plate tectonic breakup models of the South Atlantic Margin.
Canada, Ontario, Attawapiskat, Africa, South Africa
deposit - Victor, Finsch, Newlands
Abstract: Quantitative trace element data from high-purity gem diamonds from the Victor Mine, Ontario, Canada as well as near-gem diamonds from peridotite and eclogite xenoliths from the Finsch and Newlands mines, South Africa, acquired using an off-line laser ablation method show that we see the same spectrum of fluids in both high-purity gem and near-gem diamonds that was previously documented in fibrous diamonds. “Planed” and “ribbed” trace element patterns characterize not only the high-density fluid (HDF) inclusions in fibrous diamonds but also in gem diamonds. Two diamonds from two Finsch harzburgite xenoliths show trace element patterns similar to those of saline fluids, documenting the involvement of saline fluids in the precipitation of gem diamonds, further strengthening the link between the parental fluids of both gem and fibrous diamonds. Differences in trace element characteristics are evident between Victor diamonds containing silicate inclusions compared with Victor diamonds containing sulphide inclusions. The sulphide-bearing diamonds show lower levels of inter-element fractionation and more widely varying siderophile element concentrations - indicating that the silicate and sulphide-bearing diamonds likely formed by gradations of the same processes, via melt-rock reaction or from a subtly different fluid source. The shallow negative LREEN-HREEN slopes displayed by the Victor diamonds establish a signature indicative of original derivation of the diamond forming agent during major melting (~10% melt). Consequently, this signature must have been passed on to HDFs separating from such silicate melts.
Geodynamics and Tectonphysics in RUS, Vol. 9, 3, pp. 827-854. only 1 p. english abstract
Africa, Angola
deposit - Catoca
Abstract: This paper presents the first results of the geostructural and tectonophysical studies of the crustal stress state in the Catoca kimberlite pipe area at the southwestern flank of the Kasai Shield in the northeasternAngola. In the evolution of the crustal stress state, six main stages are distinguished by analyzing the displacements of markers, fold hinges, long axes of boudins, granite dikes of various intrusion phases and kimberlites, as well as fractures with striations. For each of these stages, a dominating horizontal tectonic stress and its orientation is identified. During stage 1 (NW extension and shearing) and at the beginning of stage 2 (NW compression), structures formed in the host rocks in brittle-plastic conditions. The replacement of plastic deformation by faulting could occur about 530-510 Ma ago, when the continental crust ofAfricahad completely formed. Stage 3 (radial, mainly NW extension) and stage 4 (shearing, NW extension, and NE compression) were the most important for kimberlite occurrence: in the Early Cretaceous, radial extension was replaced by shearing. Both stages are related to opening of the central segment of theSouth Atlantic. The main kimberlite magmas occurred during the break-up of the Angola-Brazilian segment of Gondwana. In the course of all the four stages, stress was mainly released by the NE- and E-NE-striking faults and, to a lesser extent, by the NW-striking and latitudinal faults. The initial stage of kimberlite magmatism is associated with the NE- and E-NE-striking faults due to the presence of the Precambrian zones of flow and schistosity, which facilitated the NW-trending subhorizontal extension. Stage 5 (NE compression) began in the second half of the Cretaceous and possibly lasted until the end of the Paleogene, and compression occurred mainly along the NW-striking faults. Regionally, it corresponds to two stages of inversion movements in the southern regions of Africa, during which theAngoladome-shaped uplift emerged and the shoulders of the East African rifts began to take shape. Stage 6 (horizontal extension, mainly in the N-NE direction) is related to the processes that took place in the southern segment of theTanganyikarift and the eastern coast of theAtlantic. Based on the results of our studies, it became for the first time possible to get an idea of the main stages in the evolution of the studied region. Further geostructural measurements and dating of the host rocks will provide for a more precise definition of the proposed stages.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 379-402.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 403-424.
Regier, M.E., Pearson, D.G., Stachel, T., Stern, R.A., Harris, J.
Tracing the formation and abundance of superdeep diamonds.
2018 Yellowknife Geoscience Forum , p. 63. abstract
Africa, Guinea
deposit - Kankan
Abstract: Super-deep diamonds from the transition zone and lower mantle are valuable targets for mining, as they are often large, gem-quality1 or ultra-valuable type IIb stones2. Hence, in mine prospects, it may become important to determine the various populations of sub-lithospheric diamonds. Unambiguously identifying a diamond’s depth of formation is difficult as some minerals can be indicative of various depth regimes (e.g., ferropericlase, Ca-walstromite, enstatite, clinopyroxene, coesite). Here, we use the oxygen isotope compositions of inclusions in Kankan diamonds from Guinea to distinguish between the various diamond-forming processes that happen at lithospheric, asthenospheric to transition zone, and lower mantle depths. In this way, we hope to establish a process by which isotope geochemistry can better constrain the populations of superdeep diamonds in kimberlites, and can assist in estimating a pipe’s propensity for large, valuable stones. Oxygen isotopic analysis by secondary ion mass spectrometry (SIMS) is a high-precision technique that can track hydrothermal alteration that occurred at or close below the ocean floor. Our analyses of inclusions from Kankan diamonds demonstrate that garnets with 3-3.03 Si cations (pfu) have ?18O that are well-constrained within the normal values expected for peridotitic and eclogitic inclusions, but that garnets with ?3.04 Si cations (pfu) have consistently high ?18O (median: 10‰) that slightly decreases with increasing Cr2O3. We interpret this signal as the reaction between a melted carbonate-rich oceanic slab and normal convecting asthenosphere3. In contrast, retrogressed, or former, bridgmanite has ?18O values similar to primitive mantle, suggesting little involvement of slab melts. In contrast to the worldwide suite of lithospheric inclusions of eclogitic paragenesis (median ?18O of 7.03‰)4,5, diamonds derived from ~250 to 500 km have inclusions with consistent, extremely high oxygen isotopes (median: 9.32‰)6,7, due to the melting of extremely enriched carbonated oceanic crust. Diamonds from the lower mantle, however, have inclusions with primitive mantle oxygen isotopes, suggesting a different formation process. The clear distinction in inclusion ?18O between lithospheric, asthenospheric to transition zone, and lower mantle diamond populations is useful in informing the depth regime of a suite of stones, especially those with inclusions of ambiguous depths (e.g., clinopyroxene, coesite, Ca-walstromite, enstatite, ferropericlase, etc.). For instance, we are currently searching for exotic oxygen isotopes in ferropericlase that indicate asthenospheric diamond growth, rather than the primitive mantle values expected for lower mantle ferropericlase. In conclusion, oxygen isotopic analyses of diamond inclusions can identify various sublithsopheric diamond populations, and may benefit the assessment of a mine’s potential for large gem-quality, or type IIb diamonds.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 359-378.
Abstract: Garnet chemistry provides a well-established tool in the discrimination and interpretation of sediment provenance. Current discrimination approaches, however, (i) suffer from using less variables than available, (ii) subjective determination of discrimination fields with strict boundaries suggesting clear separations where in fact probabilities are converging, and (iii) significant overlap of compositional fields of garnet from different host-rock groups. The new multivariate discrimination scheme is based on a large database, a hierarchical discrimination approach involving three steps, linear discriminant analysis at each step, and the five major host-rock groups to be discriminated: eclogite- (A), amphibolite- (B) and granulite- (C) facies metamorphic rocks as well as ultramafic (D) and igneous rocks (E). The successful application of statistical discrimination approaches requires consideration of the a priori knowledge of the respective geologic setting. This is accounted for by the use of prior probabilities. Three sets of prior probabilities (priors) are introduced and their advantages and disadvantages are discussed. The user is free to choose among these priors, which can be further modified according to the specific geologic problem and the level of a priori knowledge. The discrimination results are provided as integrated probabilities of belonging to the five major host-rock groups. For performing calculations and results a supplementary Excel® spreadsheet is provided. The discrimination scheme has been tested for a large variety of examples of crystalline rocks covering all of the five major groups and several subgroups from various geologic settings. In most cases, garnets are assigned correctly to the respective group. Exceptions typically reflect the peculiarities of the regional geologic situation. Evaluation of detrital garnets from modern and ancient sedimentary settings of the Western Gneiss Region (Norway), Eastern Alps (Austria) and Albertine Rift (Uganda) demonstrates the power to reflect the respective geologic situations and corroborates previous results. As most garnet is derived from metamorphic rocks and many provenance studies aim at reconstructing the tectonic and geodynamic evolution in the source area, the approach and the examples emphasize discrimination of metamorphic facies (i.e., temperature-pressure conditions) rather than protolith composition.
Abstract: A study of the morphology, mineral inclusions, nitrogen content, and nitrogen aggregation state of diamonds from the Voorspoed kimberlite, was conducted. The diamonds are characterised by a highly unusual inclusion mineral paragenesis dominated by eclogitic and transitional websteritic-lherzolitic inclusions along with related websteritic inclusions, while a comparatively minor harzburgitic diamond inclusion suite account for the remainder. This differs substantially from many of the diamond populations in kimberlites on the Kaapvaal Craton where a harzburgitic inclusion paragenesis predominates. Only in the case of the Orapa kimberlite in Botswana has a similar diamond inclusion suite been encountered before, although in that instance an eclogitic and websteritic inclusion suite predominate. Calculated garnet-clinopyroxene equilibration temperatures, at an assumed pressure of 50?kbar, range from 1040 °C to 1296?°C. Temperatures of 1114?°C to 1348?°C, at 50?kbar, are calculated for lamellar orthopyroxene-clinopyroxene intergrowths. ‘Reconstituted’ pyroxenes mathematically reconstructed from the lamellar orthopyroxene-clinopyroxene intergrowths produce temperatures of 1238?°C to 1416?°C, suggesting that the lithosphere cooled by at least 100?°C after diamond crystallisation. Nitrogen contents and nitrogen aggregation states of whole diamonds range from below the detection limit (~10?ppm) to 1442 atomic ppm (average 167 atomic ppm), and up to 97% of the highly aggregated ‘B' component (average 65%), respectively. The elevated aggregation state of the nitrogen in the diamonds from Voorspoed, coupled with a high level of platelet degradation in many of the diamonds analysed, relative to a world-wide database, is unusual, but broadly comparable to diamonds from kimberlites occurring in craton margin settings, such as Argyle and Venetia. Diamond inclusion thermobarometry, as well as the elevated nitrogen aggregation states of the diamonds (coupled with the evidence for platelet degradation in the diamonds) are indicative of a diamond crystallisation event associated with a transient thermal pulse, as well as associated deformation of the mantle containing the diamonds. The websteritic and the transitional websteritic-lherzolitic mantle source rocks of the Voorspoed diamonds likely formed through melt infiltration into mantle peridotite, possibly in the reaction envelope surrounding an ascending mantle plume. In order to account for the unusually low abundance of harzburgitic diamonds at Voorspoed, it is postulated that a primary, pre-existing suite of harzburgitic diamonds which have crystallised in the Mesoarchaean, was destroyed through heating and oxidation during the passage of magmas associated with the Ventersdorp large igneous province at 2.72?Ga, and which was subsequently followed by the crystallisation of a younger eclogitic and transitional websteritic-lherzolitic suite of diamonds.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 273-4.
Africa, Botswana
deposit - Karowe
Abstract: In November 2015, Lucara Diamond’s operation at the Karowe mine in Botswana gained notoriety due to the extraction of a series of large colorless diamonds, including the 1,109 ct Lesedi La Rona and the 812 ct Constellation. The Lesedi La Rona marks the largest gem diamond recovered since the Cullinan (3,106 ct) in 1905. The Constellation, considered to be the seventh-largest recorded diamond, attained the highest price ever paid for a rough, selling for $63.1 million ($77,649 per carat). Additionally, three other significant colorless diamonds were recovered during the same period, weighing 374, 296, and 183 ct. Due to the similarity in their external characteristics— which include cleavage faces—as well as their extraction locations and dates, it was suspected that these stones might have originated from a larger rough that had broken. Lucara demonstrated that the 374 ct diamond and the Lesedi La Rona fit together, yet a large cleavage plane is still unaccounted for. GIA was able to study several rough and/or faceted pieces of these five diamonds using a range of spectroscopic and imaging techniques to gain insight into the presence and distribution of point defects in these diamonds. Diamonds are commonly classified according to their nitrogen content measured by Fourier-transform infrared (FTIR) spectroscopy: Type I diamonds contain nitrogen in either isolated (Ib) or aggregated (IaAB) forms, while type II diamonds do not contain detectable nitrogen concentrations (IIa) but may contain boron (IIb). Analysis of faceted stones cut from the Lesedi La Rona indicates that the rough is a mixed-type diamond, containing both type IIa and pure type IaB regions. These types of diamonds, though exceedingly unusual, have been observed at GIA and reported by Delaunay and Fritsch (2017). The Constellation and the 374, 296, and 183 ct diamonds were determined to be type IaB, containing 20 ± 4 ppm B-aggregates (N4V), in agreement with the concentration for the type IaB pieces of the Lesedi La Rona. Pure type IaB diamonds such as these are actually quite rare, accounting for only 1.2% of a random suite of 5,060 large (>10 ct) D-to-Z diamonds submitted to GIA, whereas 24.6% were type II. Photoluminescence spectra further confirmed analogous defect content for the five large Karowe diamonds, with emissions from H4 (N4V2 0, 496 nm), H3 (NVN0, 503 nm), 505 nm, NV– (637 nm), and GR1 (V0, 741 nm) defects showing similar relative intensities and peak widths. Even for diamonds of the same type, parallel defect content and characteristics across such a variety of defects is unlikely for unrelated stones. The external morphologies of the diamonds showed primary octahedral, resorbed, and fractured faces, with the Constellation and the 296 ct diamond featuring fractures containing metallic inclusions and secondary iron oxide staining. Deep UV fluorescence (< 230 nm) imaging elucidated the internal growth structures of the samples. For the Constellation and the 374, 296, and 183 ct diamonds, at least two growth zones with differing blue fluorescence intensities were observed within single pieces. Combined with the spectroscopic data, these results provide compelling evidence that the Lesedi La Rona, the Constellation, and the 374, 296, and 183 ct diamonds from Karowe had comparable growth histories and likely originated from the same rough, with a combined weight of at least 2,774 ct.
Conflict, Security and Develoment, Vol. 18, 6, pp. 463-492.
Africa, Sierra Leone
economics
Abstract: Shortcomings in natural resource governance leading to economic mismanagement, political clientelism, underdevelopment and civil conflict, have caused an increase in global norms of ‘good governance’ of natural resource sectors. As a result, a growing number of global governance initiatives are targeting conflict-prone natural resource sectors. Whether these regulatory efforts stand a chance of being successful depends on their implementation in producer countries. As the transnational regulatory framework aimed at curbing the trade in conflict minerals is expanding, this article investigates the local translation of global norms of resource governance. Drawing on the ‘local-to-global’ research perspective developed in this special issue and norm diffusion theories, the article examines one of the most prominent cases of governance reform targeting conflict-affected natural resource sectors: The Sierra Leonean diamond market. Based on extensive field research, the article analyses the implementation of KPCS requirements on the national and subnational level of governance institutions. It evaluates the accomplishments, the challenges and the local adaption to and (formal and informal) interpretation of KPCS norms.
South African Journal of Geology, Vol. 121, pp. 271-286.
Africa, Mozambique
geodynamics
Abstract: Major, trace, radiogenic isotope and stable isotope data from lavas along the northeastern coast of Mozambique are described. The whole rock composition data demonstrate that the rocks are dominantly andesitic with compositions typical of calc-alkaline volcanic rocks from arc environments. SHRIMP U/Pb data from zircons indicate that the zircons are xenocrystic, having ages of between 500 Ma and 660 Ma, with the age of the lava constrained by Rb/Sr data at ~184 Ma. Strontium, Nd and Pb radiogenic isotope data support an interpretation of extensive mixing between a Karoo age basaltic magma (dolerite) from Antarctica and continental crust similar in composition to the Mozambique basement. Oxygen isotope data also imply a significant crustal contribution to the lavas. Possible tectonic settings for the lavas are at the margin of a plume or from a locally restricted compressional setting during Gondwana breakup processes.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, Fall 1p. Abstract p. 258
Africa, Namibia, South America, Brazil
photoluminescence
Abstract: Laser- and ultraviolet-excited luminescence spectroscopy and imaging are important techniques for gemstone testing, as they are among the most sensitive spectroscopic methods (see Hainschwang et al., 2013). They are able to identify optically active crystallographic defects such as vacancies and substitutions that are present in such small amounts that they cannot be detected by any other analytical method. Photoluminescence (PL) analysis became particularly important in the last decade for the separation of natural from synthetic diamonds and the detection of treatments. Today the availability of specially designed and reasonably priced portable equipment enables the rapid in situ identification of mounted and unmounted natural diamonds. Although PL spectroscopy is most commonly used for diamond identification, it can also be applied to colored stones. Some stones exhibit unique luminescence patterns, which can be used to identify the material. Other examples are the separation of natural from synthetic spinel and the detection of heat-treated spinel. Since chromium is a typical PL-causing trace element, it is also possible to separate chromium-colored gems such as ruby and jadeite from their artificially colored counterparts. The color authenticity of specific types of corals and pearls can also be determined. The rare earth elements (REE) are among the main substituting luminescence centers in Ca2+-bearing minerals (Gaft et al., 2005). Recently, REE photoluminescence has been observed in cuprian liddicoatite tourmalines from Mozambique (Milisenda and Müller, 2017). When excited by a 785 nm laser, the stones showed a series of bands at 861, 869, 878, 894, and 1053 nm, consistent with the PL spectra of other calcium-rich minerals (Chen and Stimets, 2014). LA-ICP-MS analysis confirmed the REE enrichment in this type of tourmaline compared to cuprian elbaites from Brazil and Nigeria. As a result, photoluminescence can be used as a further criterion for origin determination of Paraíba-type tourmalines. We have extended our research on other calcium-rich gems, including various grossular garnet varieties such as hessonite and tsavorite (figure 1), uvarovite garnet, apatite, titanite, and scheelite, as well as a number of high-refractive-index glasses and colorchange glasses, respectively.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 272-3.
Africa, Sierra Leone
geochronology
Abstract: Diamond ages are obtained from radiogenic isotopic analysis (Rb-Sr, Sm-Nd, Re-Os, and Ar-Ar) of mineral inclusions (garnet, pyroxene, and sulfide). As diamonds are xenocrysts that cannot be dated directly, the ages obtained on mineral inclusions provide a unique set of interpretive challenges to assure accuracy and account for preexisting history. A primary source of geological/mineralogical uncertainty on diamond ages is any process affecting protogenetic mineral inclusions before encapsulation in the diamond, especially if it occurred long before diamond formation. In practical application, the isotopic systems discussed above also carry with them inherent systemic uncertainties. Isotopic equilibrium is the essential condition required for the generation of a statistically robust isochron. Thus, isochron ages from multiple diamonds will record a valid and accurate age when the diamond-forming fluid promotes a large degree of isotopic equilibrium across grain scales, even for preexisting (“protogenetic”) minerals. This clearly can and does occur. Furthermore, it can be analytically tested for, and has multiple analogues in the field of dating metamorphic rocks. In cases where an age might be suspect, an age will be valid if its regression uncertainties can encompass a known and plausible geological event (especially one for which an association exists between that event and the source of diamond-forming fluids) and petrogenetic links can be established between inclusions on the isochron. Diamonds can be dated in six basic ways: 1. model ages 2. radiogenic daughter Os ages (common-Os-free) 3. single-diamond mineral isochrons 4. core to rim ages 5. multiple single-diamond isochron/array ages 6. composite isochron/array ages Model ages (1) are produced by the intersection between the evolution line for the inclusion and a reference reservoir such as the mantle. The most accurate single-diamond age is determined on a diamond with multiple inclusions (3). In this case an internal isochron can be obtained that not only establishes equilibrium among the multiple grains but also unequivocally dates the time of diamond growth. With extreme luck in obtaining the right diamond, concentric diamond growth zones visible in UV fluorescence or cathodoluminescence can sometimes be shown to constrain inclusions to occur in the core of the diamond and in the exterior at the rim. These single grains can be extracted to give a minimum growth time (4) for the diamond. In optimal situations, multiple inclusions are present within single growth zones, in single diamonds, allowing internal isochrons to be constructed for individual growth zones in single diamonds. If enough diamonds with inclusions can be obtained for study, valid ages for diamond populations can be obtained on multiple single-diamond ages that agree (5) or on composited, mineralogically similar inclusions to give an average age (6).
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 307.
Africa, Sierra Leone
deposit - zimmi
Abstract: Type Ib diamonds from Zimmi, Sierra Leone, have 500 My mantle residency times whose origin is best explained by rapid tectonic exhumation after continental collision to shallower depths in the mantle prior to kimberlite eruption (Smit et al., 2016). Here we present spectroscopic data for a new suite of Zimmi sulfide-bearing type Ib diamonds that allow us to evaluate the link between their rare Fancy yellow colors, the distribution of their spectroscopic features, and their unusual geological history. Cathodoluminesence (CL) imaging revealed irregular patterns with abundant deformation lamellae, associated with the diamonds’ tectonic exhumation (Smit et al., 2018). Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/? centers, and H3 (NVN0). The brownish yellow to greenish yellow colors observed in Zimmi type Ib diamonds result from visible absorption by a combination of isolated nitrogen and deformation-related vacancy clusters (Smit et al., 2018). Color-forming centers and other spectroscopic features can all be attributed to the unique geological history of Zimmi type Ib diamonds and their rapid exhumation after formation.
South African Journal of Geology, Vol. 121, pp. 227-236.
Africa, South Africa
geomorphology
Abstract: Rocky reaches of the southeast African coastline are characterized mainly by log-spiral and headland-bound bays. Extensive fieldwork was carried out to investigate both documented and new exposures of Cretaceous beds on the southern KwaZulu-Natal and upper Eastern Cape (Transkei) coasts. Our results suggest that geological inheritance plays an important role in the contemporary rocky coast geomorphology. We offer evidence that post-Gondwana break-up mass flow channels play an important role in the present southeast African coastline morphology. Mass flow channels contain fills of incompetent Cretaceous rocks which are being preferentially eroded by prevailing marine and fluvial processes to form headland-bound embayment landforms. This study has identified an important geomorphic process for the development of the current southeast African coastline.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 274.
Africa, South Africa, Angola
deposit - Cullinan, Lulo
Abstract: Many of the world’s largest and most valuable gem diamonds exhibit an unusual set of physical characteristics. For example, in addition to their conspicuously low nitrogen concentrations, diamonds such as the 3,106 ct Cullinan (type IIa) and the Hope (type IIb, boron bearing) tend to have very few or no inclusions, and in their rough state they are found as irregular shapes rather than as sharp octahedral crystals. It has long been suspected that type IIa and IIb diamonds form in a different way than most other diamonds. Over the past two years, systematic investigation of both type IIa and IIb diamonds at GIA has revealed that they sometimes contain rare inclusions from unique geological origins. Examination of more than 130 inclusion-bearing samples has established recurring sets of inclusions that clearly show many of these diamonds originate in the sublithospheric mantle, much deeper in the earth than more common diamonds from the cratonic lithosphere. We now recognize that type IIa diamonds, or more specifically, diamonds with characteristics akin to the historic Cullinan diamond (dubbed CLIPPIR diamonds), are distinguished by the occurrence of ironrich metallic inclusions. Less frequently, CLIPPIR diamonds also contain inclusions of majoritic garnet and former CaSiO3perovskite that constrain the depth of formation to within 360–750 km. The inclusions suggest that CLIPPIR diamonds belong to a unique paragenesis with an intimate link to metallic iron in the deep mantle (Smith et al., 2016, 2017). Similarly, findings from type IIb diamonds also place them in a “superdeep” sublithospheric mantle setting, with inclusions of former CaSiO3 perovskite and other high-pressure minerals, although the iron-rich metallic inclusions are generally absent (Smith et al., 2018). Altogether, these findings show that high-quality type II gem diamonds are predominantly sourced from the sublithospheric mantle, a surprising result that has refuted the notion that all superdeep diamonds are small and nongem quality. Valuable information about the composition and behavior of the deep mantle is cryptically recorded in these diamonds. CLIPPIR diamonds (figure 1) confirm that the deep mantle contains metallic iron, while type IIb diamonds suggest that boron and perhaps water can be carried from the earth’s surface down into the lower mantle by plate tectonic processes. In addition to being gemstones of great beauty, diamonds carry tremendous scientific value in their unique ability to convey information about the interior of our planet.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, Fall 1p. Abstract p. 262-3
Africa, Sierra Leone
deposit - Zimmi
Abstract: Isolated nitrogen is one of the major defects in producing yellow color in natural diamonds. In regular type Ib yellow diamonds, isolated nitrogen is normally the dominant form, with limited aggregations in A centers (nitrogen pairs). Type Ib diamonds normally experienced strong plastic deformations. In addition to vacancy clusters, many other optic centers were introduced during annealing over their long geological history, such as GR1, NV, and H3 centers. Diamonds from the Zimmi area of West Africa are a typical example (Smit et al., 2016). As a result, clear brownish and greenish hues are common among these diamonds, so most do not possess true “canary” yellow color. Here we studied more than 2,000 diamonds with real canary yellow color. Their color origin and relationship with type Ib diamonds were explored. Sizes of the studied diamonds ranged from 0.01 to about 1.0 ct. They showed pure yellow color, with grades of Fancy Intense or Fancy Vivid yellow. Infrared absorption analysis showed that they were all type IaA with very high nitrogen concentrations, but a very weak absorption from isolated nitrogen at 1344 cm–1 was detected in all samples. Concentration of isolated nitrogen was estimated at ~2–3 ppm. This isolated nitrogen created smooth absorption in the ultraviolet-visible (UV-Vis) region, increasing gradually to the high-energy side. No other defects were detected using UV-Vis absorption spectroscopy, which explained the pure yellow color we observed. Fluorescence imaging revealed multiple nucleation centers with dominant green color, which was attributed to the S3 defects confirmed through photoluminescence analysis. Compared with natural type Ib diamonds, an outstanding feature of the studied samples is the absence of plastic deformation. For this reason, other vacancy-related defects were not introduced to these diamond lattices over the geological period after their formation. Sulfide inclusions are common in type Ib diamonds, but they were not observed in these canary stones. Instead, some calcite inclusions were observed. All the observations from this study indicated that the canary diamond samples were formed in a different geological environment than type Ib diamonds.
Abstract: The late Mesoproterozoic Ngualla carbonatite complex in southwest Tanzania comprises a central magnesiocarbonatite plug surrounded sequentially by an annular calcite carbonatite intrusion and fenitised felsic igneous country rocks. The calcite carbonatite contains phlogopite-rich (glimmerite) enclaves interpreted as fenitised wallrock xenoliths that have contributed silicate minerals, apatite and magnetite through dispersal and interaction, mainly within the calcite carbonatite magma. Ultramafic magmas were emplaced into the magnesiocarbonatite magma chamber before complete solidification of the magnesiocarbonatite. Contemporaneity allowed the two magmas to mingle. Rounded enclaves of hematite-barite in the magnesiocarbonatite are tentatively attributed to magma immiscibility. Following complete solidification of the calcite carbonatite, and overlapping late crystallization of the magnesiocarbonatite plug, late magnesiocarbonatite dikes and ultramafic dikes were emplaced, some of the latter as diatremes. Crystallization of ferroan dolomite in the magnesiocarbonatite plug resulted in residual magmatic concentration of Si, Ba, F and rare earth elements (REE), and crystallization of barite, quartz, calcite, fluorite and REE fluorocarbonates in miarolitic cavities. Concentrations of (total) rare earth oxides (TREO) in the unweathered magnesiocarbonatite are 1 to 2%. REE ore with 3 to 6% TREO resulted from weathering, during which CaCO3 and MgCO3 were leached from ferroan dolomite leaving a porous goethite-rich residue containing barite and bastnaesite, the latter having replaced primary synchesite. Other commodities with potential economic significance include phosphate and niobium, both of which were enriched by residual accumulation over the calcite carbonatite as a result of karstic weathering. Although weathering was a critical factor in the formation of REE ore at Ngualla, the primary proto-ore resulted mainly from in situ igneous processes. This genetic model is different from that used to account for many carbonatite-hosted ore bodies, which result from late-stage hydrothermal processes. Examples of hydrothermal rare earth deposits include those of the late Jurassic to early Cretaceous Chilwa Province, located 800?km south of Ngualla. The differences in ore-forming processes may reflect the relative ages of the carbonatites and a deeper level of erosion at Ngualla.
Abstract: The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5?m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7?±?4.4?Ma and 86.5?±?2.6?Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster. The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections. The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel - magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration. Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn?/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.
Contributions to Mineralogy and Petrology, Vol. 174, 8 22p.
Africa, South Africa, Russia, Canada, Northwest Territories
deposit - Bultfontein, Roberts Victor, Udachnaya-East, Obnazhennaya, Vtorogodnitsa, Koala, Leslie
Abstract: Djerfisherite (K6(Fe,Ni,Cu)25S26Cl) occurs as an accessory phase in the groundmass of many kimberlites, kimberlite-hosted mantle xenoliths, and as a daughter inclusion phase in diamonds and kimberlitic minerals. Djerfisherite typically occurs as replacement of pre-existing Fe-Ni-Cu sulphides (i.e. pyrrhotite, pentlandite and chalcopyrite), but can also occur as individual grains, or as poikilitic phase in the groundmass of kimberlites. In this study, we present new constraints on the origin and genesis of djerfisherite in kimberlites and their entrained xenoliths. Djerfisherite has extremely heterogeneous compositions in terms of Fe, Ni and Cu ratios. However, there appears to be no distinct compositional range of djerfisherite indicative of a particular setting (i.e. kimberlites, xenoliths or diamonds), rather this compositional diversity reflects the composition of the host kimberlite melt and/or interacting metasomatic medium. In addition, djerfisherite may contain K and Cl contents less than the ideal formula unit. Raman spectroscopy and electron backscatter diffraction (EBSD) revealed that these K-Cl poor sulphides still maintain the same djerfisherite crystal structure. Two potential mechanisms for djerfisherite formation are considered: (1) replacement of pre-existing Fe-Ni-Cu sulphides by djerfisherite, which is attributed to precursor sulphides reacting with metasomatic K-Cl bearing melts/fluids in the mantle or the transporting kimberlite melt; (2) direct crystallisation of djerfisherite from the kimberlite melt in groundmass or due to kimberlite melt infiltration into xenoliths. The occurrence of djerfisherite in kimberlites and its mantle cargo from localities worldwide provides strong evidence that the metasomatising/infiltrating kimberlite melt/fluid was enriched in K and Cl. We suggest that kimberlites originated from melts that were more enriched in alkalis and halogens relative to their whole-rock compositions.
Abstract: Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (3-6?ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb?=?13.57-13.52) and low 238U/204Pb (1.0-1.5) give single-stage model Pb dates of 2.90-2.84?Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18?±?0.45?Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20?±?0.54?Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin. Gabbroic eclogites (Eu/Eu*?>?1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model ? (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0?Ga addition of a sedimentary component, possibly derived from reworking of a high-? basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.
A geochemical and Nd, Sr and stable Ca isotopic study of carbonatites and associated silicate rocks from the ~65 Ma old Ambadongar carbonatite complex and the Phenai Mata igneous complex, Gujarat, India: implications for crustal contamination, carbonate r
Dyke Swarms of the World: a modern perspective, Srivastava et al. eds. Springer , pp. 263-314.
Africa, West Africa, South America
geochronology
Abstract: Eight different generations of dolerite dykes crosscutting the Paleoproterozoic basement in West Africa and one in South America were dated using the high precision U-Pb TIMS method on baddeleyite. Some of the individual dykes reach over 300 km in length and they are considered parts of much larger systems of mafic dyke swarms representing the plumbing systems for large igneous provinces (LIPs). The new U-Pb ages obtained for the investigated swarms in the southern West African Craton (WAC) are the following (oldest to youngest): 1791?±?3 Ma for the N010° Libiri swarm, 1764?±?4 Ma for the N035° Kédougou swarm, 1575?±?5 for the N100° Korsimoro swarm, ~1525-1529 Ma for the N130° Essakane swarm, 1521?±?3 Ma for the N90° Sambarabougou swarm, 915?±?7 Ma for the N070° Oda swarm, 867?±?16 Ma for the N355° Manso swarm, 202?±?5 Ma and 198?±?16 Ma for the N040° Hounde swarm, and 200?±?3 Ma for the sills in the Taoudeni basin. The last ones are related to the Central Atlantic Magmatic Province (CAMP) event. The Hounde swarm is oblique to the dominant radiating CAMP swarm and may be linked with the similar-trending elongate Kakoulima intrusion in Guinea. In addition, the N150° Käyser swarm (Amazonian craton, South America) is dated at 1528?±?2 Ma, providing a robust match with the Essakane swarm in a standard Amazonia-West African craton reconstruction, and resulting in a combined linear swarm >1500 km by >1500 km in extent. The Precambrian LIP barcode ages of c. 1790, 1765-1750, 1575, 1520, 915. 870 Ma for the WAC are compared with the global LIP record to identify possible matches on other crustal blocks, with reconstruction implications. These results contribute to the refinement of the magmatic ‘barcode’ for the West African and Amazonian cratons, representing the first steps towards plausible global paleogeographic reconstructions involving the West African and Amazonian cratons.
Abstract: Extensive partial melting of the middle to lower crustal parts of orogens, such as of the current Himalaya-Tibet orogen, significantly alters their rheology and imposes first-order control on their tectonic and topographic evolution. We interpret the late Proterozoic Araçuaí orogen, formed by the collision between the São Francisco (Brazil) and Congo (Africa) cratons, as a deep section through such a hot orogen based on U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon ages and Ti-in-zircon and Zr-in-rutile temperatures from the Carlos Chagas anatectic domain. This domain is composed of peraluminous anatexites and leucogranites that typically exhibit interconnected networks of garnet-rich leucosomes or a magmatic foliation. Zirconium-in-rutile temperatures range from 745 to 820 °C, and the average Ti-in-zircon temperature ranges from 712 to 737 °C. The geochronologic and thermometry data suggest that from 597 to 572 Ma this domain was partially molten and remained so for at least 25 m.y., slowly crystallizing between temperatures of ?815 and >700 °C. Significant crustal thickening must have occurred prior to 600 Ma, with initial continental collision likely before 620 Ma, a time period long enough to heat the crust to temperatures required for widespread partial melting at middle crustal levels and to favor a "channel flow" tectonic behavior.
Earth and Planetary Science Letters, Vol. 506, pp. 15-26.
Africa, South Africa
deposit - Newlands, Kimberley, Bultfontein
Abstract: MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks occur as mantle-derived xenoliths in kimberlites and other alkaline volcanic rocks. Both rock types are alkaline and ultramafic in composition. The H2O and alkali metal enrichments in MARID and PIC rocks, reflected in abundant phlogopite, have been suggested to be caused by extreme mantle metasomatism. Radiogenic (Sr-Nd-Hf-Pb) isotope and trace element compositions for mineral separates from MARID (clinopyroxene and amphibole) and PIC (clinopyroxene only) samples derived from Cretaceous kimberlites (Kimberley) and orangeites (Newlands) from South Africa are used here to examine the source(s) of mantle metasomatism. PIC clinopyroxene is relatively homogeneous, with narrow ranges in initial isotopic composition (calculated to the emplacement age of the host Bultfontein kimberlite; 87Sr/86Sri: 0.7037-0.7041; ?Ndi: +3.0 to +3.6; ?Hfi: +2.2 to +2.5; 206Pb/204Pbi: 19.72-19.94) similar to kimberlite values. This is consistent with PIC rocks representing peridotites modified by intense metasomatic interaction with kimberlite melts. The MARID clinopyroxene and amphibole separates () studied here display broader ranges in isotope composition (e.g., 87Sr/86Sri: 0.705-0.711; ?Ndi: ?11.0 to ?1.0; ?Hfi: ?17.9 to ?8.5; 206Pb/204Pbi: 17.33-18.72) than observed in previous studies of MARID rocks. The Nd-Hf isotope compositions of kimberlite-derived MARID samples fall below the mantle array (??Hfi between ?13.0 and ?2.4), a feature reported widely for kimberlites and other alkaline magmas. We propose that such displacements in MARID minerals result from metasomatic alteration of an initial “enriched mantle” MARID composition (i.e., 87Sr/86Sri = 0.711; ?Ndi = ?11.0; ?Hfi = ?17.9; and 206Pb/204Pbi = 17.3) by the entraining kimberlite magma (87Sr/86Sr; ?Nd; ?Hf; 206Pb/204Pb). A model simulating the flow of kimberlite magma through a mantle column, thereby gradually equilibrating the isotopic and chemical compositions of the MARID wall-rock with those of the kimberlite magma, broadly reproduces the Sr-Nd-Hf-Pb isotope compositions of the MARID minerals analysed here. This model also suggests that assimilation of MARID components could be responsible for negative ??Hfi values in kimberlites. The isotopic composition of the inferred initial MARID end-member, with high 87Sr/86Sr and low ?Nd, ?Hf, and 206Pb/204Pb, resembles those found in orangeites, supporting previous inferences of a genetic link between MARID-veined mantle and orangeites. The metasomatic agent that produced such compositions in MARID rocks must be more extreme than the EM-II mantle component and may relate to recycled material that experienced long-term storage in the lithospheric mantle.
Abstract: The oxidation state of iron in Earth’s mantle is well known to depths of approximately 200?km, but has not been characterized in samples from the lowermost upper mantle (200-410?km depth) or the transition zone (410-660?km depth). Natural samples from the deep (>200?km) mantle are extremely rare, and are usually only found as inclusions in diamonds. Here we use synchrotron Mössbauer source spectroscopy complemented by single-crystal X-ray diffraction to measure the oxidation state of Fe in inclusions of ultra-high pressure majoritic garnet in diamond. The garnets show a pronounced increase in oxidation state with depth, with Fe3+/(Fe3++ Fe2+) increasing from 0.08 at approximately 240?km depth to 0.30 at approximately 500?km depth. The latter majorites, which come from pyroxenitic bulk compositions, are twice as rich in Fe3+ as the most oxidized garnets from the shallow mantle. Corresponding oxygen fugacities are above the upper stability limit of Fe metal. This implies that the increase in oxidation state is unconnected to disproportionation of Fe2+ to Fe3+ plus Fe0. Instead, the Fe3+ increase with depth is consistent with the hypothesis that carbonated fluids or melts are the oxidizing agents responsible for the high Fe3+ contents of the inclusions.
Abstract: Quantitative trace element data from high-purity gem diamonds from the Victor Mine, Ontario, Canada as well as near-gem diamonds from peridotite and eclogite xenoliths from the Finsch and Newlands mines, South Africa, acquired using an off-line laser ablation method show that we see the same spectrum of fluids in both high-purity gem and near-gem diamonds that was previously documented in fibrous diamonds. "Planed" and "ribbed" trace element patterns characterize not only the high-density fluid (HDF) inclusions in fibrous diamonds but also in gem diamonds. Two diamonds from two Finsch harzburgite xenoliths show trace element patterns similar to those of saline fluids, documenting the involvement of saline fluids in the precipitation of gem diamonds, further strengthening the link between the parental fluids of both gem and fibrous diamonds. Differences in trace element characteristics are evident between Victor diamonds containing silicate inclusions compared with Victor diamonds containing sulphide inclusions. The sulphide-bearing diamonds show lower levels of inter-element fractionation and more widely varying siderophile element concentrations - indicating that the silicate and sulphide-bearing diamonds likely formed by gradations of the same processes, via melt-rock reaction or from a subtly different fluid source. The shallow negative LREEN-HREEN slopes displayed by the Victor diamonds establish a signature indicative of original derivation of the diamond forming agent during major melting (~10% melt). Consequently, this signature must have been passed on to HDFs separating from such silicate melts.
Abstract: Mobilistic plate-tectonic interpretation of Precambrian orogens requires that two conjoined crustal blocks may derive from distant portions of the globe. Nonetheless, many proposed Precambrian cratonic juxtapositions are broadly similar to those of younger times (so-called “strange attractors”), raising the specter of bias in their construction. We evaluated the possibility that the Congo and Kalahari cratons (Africa) were joined together prior to their amalgamation along the Damara-Lufilian-Zambezi orogen in Cambrian time by studying diabase dikes of the Huila-Epembe swarm and sills in the southern part of the Congo craton in Angola and in Namibia. We present geologic, U-Pb geochronologic, and paleomagnetic evidence showing that these two cratons were directly juxtaposed at ca. 1.1 Ga, but in a slightly modified relative orientation compared to today. Recurring persistence in cratonic connections, with slight variations from one supercontinent to the next, may signify a style of supercontinental transition similar to the northward motion of Gondwana fragments across the Tethys-Indian oceanic tract, reuniting in Eurasia.
International Journal of Earth Sciences, Vol. 108, pp. 89-113.
Africa, Angola
thermochronology
Abstract: There are two main points of view regarding how continental margins evolve. The first one argues that the present-day margins have been developed by long-term denudation since a major exhumation episode, probably driven by rifting or another relevant tectonic event. The second one argues that continental margins underwent alternating burial and exhumation episodes related to crustal tectonic and surface uplift and subsidence. To demonstrate that the proximal domain of the southwestern Angolan margin has evolved in a polycyclic pattern, we present a review of geological and thermochronological information and integrate it with new combined apatite fission-track and (U-Th)/He data from Early Cretaceous volcanic and Precambrian basement samples. We also provide hypotheses on the possible mechanisms able to support the vertical crustal movements of this margin segment, which are also discussed based on some modern rifting models proposed for Central South Atlantic. The central apatite fission-track ages range from 120.6?±?8.9 to 272.9?±?21.6 Ma, with the mean track lengths of approximately 12 µm. The single-grain apatite (U-Th)/He ages vary between 52.2?±?1 and 177.2?±?2.6 Ma. The integration of the thermochronological data set with published geological constraints supports the following time-temperature evolution: (1) heating since the Carboniferous-Permian, (2) cooling onset in the Early Jurassic, (3) heating onset in the Early Cretaceous, (4) cooling onset in the Mid- to Late Cretaceous, (5) heating onset in the Late Cretaceous, and (6) cooling onset in the Oligocene-Miocene. The thermochronological data and the geological constraints, support that the proximal domain of the southwestern Angolan margin was covered in the past by pre-, syn-, and post-rift sediments, which were eroded during succeeding exhumation events. For this margin segment, we show that a development based on long-term denudation is less realistic than one based on burial and exhumation episodes during the last 130 Myr.
Geochemistry, Geophysics, Geosystems, Vol. 20, 1, pp. 120-147.
Africa
tomography
Abstract: We use advanced seismic imaging techniques (full?waveform tomography), constrained by data from background (ambient) seismic noise to image the upper mantle beneath the African continent and search for low?velocity structures (hot spots) that might coincide with regions of volcanism, surface uplift, and continental rifting, particularly along the East African Rift. We also searched for high?velocity structures (old, rigid blocks) that could influence how warm, buoyant material flows within the Earth's upper mantle. Our seismic tomography method allowed us to obtain a clear image of structure beneath parts of Africa where no or very few seismometers are located (such as the Sahara Desert and the Congo Basin). Our results provide indications for segmented secondary (or shallow) upwellings in the upper mantle beneath East Africa, as opposed to earlier models suggesting one large, continuous plume within the upper mantle. Our results also suggest that the one large, rigid, cratonic block previously imaged beneath the Congo region may instead be composed of smaller, distinct blocks. These results provide insight into the factors that control continental rifting along East Africa and provide new testable models that help us to understand the relationships between upper mantle flow, rifting, volcanism, surface uplift, and sedimentation records.
Abstract: A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60-70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume?fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.
Abstract: Diamonds originate deep in the Earth's mantle since billions of years ago. Through their long history diamonds accumulate information about the Earth's evolution, and preserve it owing to their extreme chemical and mechanical stability. The surface of natural diamonds shows a variety of growth and dissolution features, which reflect the diversity of conditions in the mantle and in kimberlite magma, providing an important clue for understanding the deep regions of subcratonic mantle. However, such studies are hampered by an absence of a systematic approach for studying diamond surface features and morphology. This review integrates studies of natural diamonds with the results of diamond dissolution experiments to explore the origin of the most typical resorption features of diamonds and the information they provide. It uses detailed studies of over ~ 3500 diamonds from eight kimberlite bodies in the Northwest Territories in Canada and Orapa kimberlite cluster in Botswana, and the data from diamond dissolution experiments covering a pressure range of between 0.1?MPa - 7.5?GPa, temperature range of between 900?°C - 1750?°C, and over 12 log units of oxygen fugacity values. Examining the effects of these parameters on diamond resorption morphology shows that the shape and size of the etch pits depends on the temperature and H2O:CO2 ratio in the fluid, whereas pressure affects the efficiency of diamond crystal shape transformation from octahedral into rounded resorbed forms. The effect of pressure on the physical properties of the reacting fluid / melt controls the character of diamond etching. A comparison between the experimentally-induced and naturally occurring diamond resorption demonstrates a clear difference between the features developed in kimberlite magma and features inherited from the mantle source. Kimberlite-induced resorption on diamonds shows a strong correlation with the geology and emplacement mode of the hosting kimberlite unit. Low-relief surfaces develop on diamonds from pyroclastic kimberlites in all kimberlite classes, whereas surface features on diamonds from coherent kimberlites differ between kimberlite localities and often show corrosive character. Diamond resorption morphology can offer a robust method to better understand emplacement processes in different kimberlite localities, which are a matter of significant debate. The proposed here classification scheme for diamond resorption features is based on the features observable under a stereomicroscope. It helps differentiating resorption produced in the mantle source from that in the kimberlite magma and assigning diamond resorption to a particular mode of kimberlite emplacement, or a mantle metasomatic event.
Journal of the Southern African Insitiute of Mining and Metallurgy, Vol. 118, 8, pp. 845- 852.
Africa, South Africa
artisanal, alluvial
Abstract: The number of people involved in artisanal and small-scale mining (ASM) has grown quickly to about 40.5 million, compared to 7 million in industrial mining. Furthermore, the ASM sector is contributing significantly to global mineral supply and new opportunities are arising for ASM in an evolving mining ecosystem. Given this growth trend, it is important to ask whether ASM is likely to be successful in the mining of all types of orebodies. The history of early South African diamond mining suggests that the mining of a massive ore deposit by numerous artisanal and small-scale miners is likely to result in poor safety conditions as the depth of mining increases. Early photographs taken at the Kimberley mine showed a very uneven pit floor with leads-lags between the claims. This raises the question of why neighbouring miners did not ensure safer working conditions for each other. Two models described in the paper illustrate why there is likely to be a lack of cooperation and coordination between miners to address this and other safety-related problems. The dynamics of multiple claim holders mining next to one another at increasing depths are analysed, and it is shown that a consolidation of claims into a single firm per kimberlite pipe was required for improved planning, coordination, safety, efficiency, and sustainability.
Earth and Planetary Science Letters, Vol. 510, pp. 219-230.
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: Historic eruptions of Earth's only active carbonatite volcano, Oldoinyo Lengai (Tanzania), have repeatedly switched from low energy carbonatite lava extrusion to highly energetic explosive silicate volcanism, most recently in 1966-67 and 2007-08. The explosive eruptions produce strongly Si-undersaturated peralkaline silicate ashes with unusually high (Na + K)/Al of 3.4-6.3 when compared to the average peralkalinity of ?0.8 in the East African Rift System. A series of experiments in the carbonatite-clinopyroxene system at 750-1150 °C, 0.1 GPa, reveal that augitic clinopyroxene breaks down peritectically at >900 °C yielding strongly peralkaline conjugated silicate- and carbonatite melts. The clinopyroxene-derived silicate melt dissolves (Na,K)2O from the (Na,K)2CO3-component of the carbonatite leading to high peralkalinities and to liberation of excess CO2, since the solubility of carbon dioxide in silicate liquids is ?1 wt.% at subvolcanic pressures. Carbonatite injection into subvolcanic clinopyroxene-rich crystal mushes hence explains the occurrence of strongly peralkaline silicate melts and provides a mechanism for CO2-driven explosive eruptions. The silicate melt compositions mostly depend on the (Na + K)/Ca ratio of the intruding carbonatite, the silicate ashes erupted in 1966-67 and 2007-08 require an interaction of a clinopyroxene-rich crystal mush with a slightly less evolved alkali-carbonatite than presently erupted at Oldoinyo Lengai. The mechanism identified here, where mineral breakdown induced melt hybridization triggers volatile saturation and highly explosive volcanism is generally applicable to igneous systems that involve carbonatites or other low-viscosity CO2-bearing alkaline silicate melts.
Abstract: The Archean-Proterozoic transition was among the most important in geological history for it includes evidence of establishment of ‘modern-style’ plate tectonics, unprecedented paleoclimatic upheavals, and oxygenation of the atmosphere. The early Paleoproterozoic sedimentary record includes evidence of the world's first widespread glacial episodes, which have come to be known as the ‘Huronian Glacial Event’. None of these important changes coincides precisely with the accepted date of 2.5?Ga for the ‘boundary’ between the two great Precambrian eons. Rather, the geological record contains evidence of gradual transitions over many millions of years. For example the Archean sedimentary record in areas such as South Africa includes evidence of stable conditions (e.g. the Pongola Supergroup) that were not achieved in the Laurentian craton until much later during the Paleoproterozoic Era. The Pongola Supergroup in South Africa contains some of the world's oldest (c. 2.9?Ga) but locally developed glacial deposits. Many of these important changes are now considered to have been gradual and oscillatory in nature, including evidence of ‘whiffs of oxygen’ in Archean rocks, long before the Paleoproterozoic Great Oxidation Event. Oxidation of the oceans was also a long and extremely complex process, the details of which are still poorly understood. Glaciations near the beginning of the Paleoproterozoic Era have been considered by some to have been world-spanning ‘snowball Earth’ events. Repeated Huronian glaciations were probably brought about by weathering of Lauroscandia, the world's first ‘supercraton’, controlled by episodic rift-related uplifts during its disintegration. Among these glaciations only the third, represented by the Gowganda Formation and equivalents, was widespread throughout Lauroscandia. Because the two older glaciogenic units are known from only two locations in North America, their formation and preservation were probably controlled by local tectonic events. In like fashion, Paleoproterozoic glaciogenic units in South Africa and Western Australia appear to be local deposits from mountain glaciers formed during periods of tectonically generated (compressional?) uplift. The restricted distribution and diachronous nature of such tectonic events, and associated glaciogenic deposits, cast doubt on the viability of attempts at global correlations of individual Paleoproterozoic glaciogenic formations and on the existence of a Paleoproterozoic (or Neoproterozoic?) snowball Earth.
Journal of the Geological Society of London, in press available 25p.
Africa, Madagascar
thermochronology
Abstract: Madagascar occupied an important place in the amalgamation of Gondwana, and preserves a record of several Neoproterozoic events that can be linked to orogenesis of the East African Orogen. We integrate remote sensing and field data to unravel complex deformation in the Ikalamavony and Itremo domains of central Madagascar. The deformation sequence comprises a gneissic foliation (S1), followed by south to south-west directed, tight to isoclinal, recumbent folding (D2). These are overprinted by north-trending upright folds that formed during a ~E-W shortening event. Together these produced type 1 and type 2 fold interference patterns throughout the Itremo and Ikalamavony domains. Apatite U-Pb and muscovite and biotite Rb-Sr thermochronometers indicate that much of central Madagascar was thermally reset to at least ~500oC at c. 500 Ma. Deformation in west-central Madagascar occurred between c. 750 Ma and c. 550 Ma, and we suggest this deformation formed in response to the c. 650 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, which formed in response to the final closure of the Mozambique Ocean along the Betsimisaraka Suture that amalgamated Madagascar with the Dharwar Craton of India.
Contributions to Mineralogy and Petrology, Vol 174, https://doi.org/ 10.1007/s00410-019-1552-z
Africa, South Africa
deposit - Roberts Victor, Jagersfontein
Abstract: Mantle eclogites are commonly accepted as evidence for ancient altered subducted oceanic crust preserved in the subcontinental lithospheric mantle (SCLM), yet the mechanism and extent of crustal recycling in the Archaean remains poorly constrained. In this study, we focus on the petrological and geochemical characteristics of 58 eclogite xenoliths from the Roberts Victor and Jagersfontein kimberlites, South Africa. Non-metasomatized samples preserved in the cratonic root have variable textures and comprise bimineralic (garnet (gt)-omphacite (cpx)), as well as kyanite (ky)- and corundum (cor)-bearing eclogites. The bimineralic samples were derived from a high-Mg variety, corresponding to depths of ~ 100-180 km, and a low-Mg variety corresponding to depths of ~ 180-250 km. The high-Al (ky-, cor-bearing) eclogites originated from the lowermost part of the cratonic root, and have the lowest REE abundances, and the most pronounced positive Eu and Sr anomalies. On the basis of the strong positive correlation between gt and cpx ?18O values (r2 = 0.98), we argue that ?18O values are unaffected by mantle processes or exhumation. The cpx and gt are in oxygen isotope equilibrium over a wide range in ?18O values (e.g., 1.1-7.6‰ in garnet) with a bi-modal distribution (peaks at ~ 3.6 and ~ 6.4‰) with respect to mantle garnet values (5.1 ± 0.3‰). Reconstructed whole-rock major and trace element compositions (e.g., MgO variation with respect to Mg#, Al2O3, LREE/HREE) of bimineralic eclogites are consistent with their protolith being oceanic crust that crystallized from a picritic liquid, marked by variable degrees of partial melt extraction. Kyanite and corundum-bearing eclogites, however, have compositions consistent with a gabbroic and pyroxene-dominated protolith, respectively. The wide range in reconstructed whole-rock ?18O values is consistent with a broadly picritic to pyroxene-rich cumulative sequence of depleted oceanic crust, which underwent hydrothermal alteration at variable temperatures. The range in ?18O values extends significantly lower than that of present-day oceanic crust and Cretaceous ophiolites, and this might be due to a combination of lower ?18O values of seawater in the Archaean or a higher temperature of seawater-oceanic crust interaction.
Abstract: The Upper Ruvubu Alkaline Plutonic Complex (URAPC) in Burundi consists of three separate intrusions, each with a specific emplacement age and petrological composition. Three main units are recognized: an outer unit with silica-saturated plutonic rocks (from gabbro to granite), an inner unit with silica-undersaturated plutonic rocks (feldspathoidal syenite with subordinate feldspathoidal monzonite and ijolite) and a carbonatitic body in the subsoil, known by drilling. The URAPC is quite large in size (?24?km long and up to 10?km wide). It is considered to have been intruded syntectonically in an overall extensional context, thanks to the kilometric shear zones that accommodated its emplacement. Radiometric ages from literature range from 748 to 705?Ma and point to structurally-controlled magmatic differentiation followed by long-lived circulations of late-stage fluids postdating the emplacement of a part of the undersaturated rocks and the carbonatites. In the north-western part of the outer unit, gabbro likely has been emplaced at a deeper structural level than the granite, which represents a more apical structural level of emplacement. This petrological, geochemical and isotopic (Sr-Nd-Hf) study concentrates on the processes that generated the URAPC: (i) fractional crystallization, evidenced by the chemical evolution trends of the major and trace elements, and by marked P, Ti and Ba anomalies in the trace element patterns; (ii) crustal assimilation/contamination, as shown by the wide range of Nd isotope compositions and the general increase of the Sr isotope ratios with increasing SiO2 contents, and (iii) late-magmatic/hydrothermal alteration inducing an increase of the Sr isotope composition without changing significantly the Nd isotope composition. The isotopic data are consistent with an asthenospheric mantle source, though less depleted than the Depleted Mantle (DM), contaminated by the Subcontinental Lithospheric Mantle (SCLM). The silicate and carbonate magmatic series are cogenetic. The outer unit is clearly more contaminated than the inner unit, whereas the carbonatitic body could have evolved by liquid immiscibility. The URAPC lies within East Africa’s Western Rift Valley, which is marked by 23 alkaline plutonic complexes. Their emplacement has been ascribed to reactivation of Proterozoic lithospheric weakness zones resulting from the breakup of the Neoproterozoic supercontinent Rodinia supercontinent.
Abstract: A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60-70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume?fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.
Geochimica et Cosmochimica Acta, in press available 21p.
Africa, South Africa
deposit - Bultfontein
Abstract: Current understanding of the fate of subducted material (and related fluids) in the deep Earth can be improved by combining major and trace element geochemistry with stable isotopic compositions of mantle rocks or minerals. Limited isotopic fractionation during high temperature processes means that significant deviations from mantle-like isotope ratios in mantle rocks probably result from recycling of surficial material. To determine the effects and origins of mantle metasomatic fluids/melts, new ?15N and ?18O data have been collected for thirteen mantle xenoliths - harzburgites, wehrlites, lherzolites, and MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) rocks - from the Bultfontein kimberlite (Kimberley, South Africa), which show varying degrees of metasomatism. The ?18O values of olivine and orthopyroxene in phlogopite-free harzburgites match the mantle composition (?18Oolivine?=?+5.2?±?0.3‰; ?18Oorthopyroxene?=?+5.7?±?0.3‰; 2?s.d.), consistent with previous inferences that harzburgites were formed by interaction with ancient silica-rich melts unrelated to subduction processes. Wehrlite samples display mineral compositional characteristics (e.g., low La/Zr in clinopyroxene) resembling those of other products of kimberlite melt metasomatism, such as PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks. The inferred interaction with kimberlite melts may be responsible for O isotopic disequilibrium between clinopyroxene and olivine (?18O?=?+0.2‰) in the wehrlites of this study. In contrast with broadly mantle-like ?18O values, the ?15N value of phlogopite in a wehrlite sample (+5.9‰) differs from the mantle composition (?15N?=??5?±?2‰). This unusual N isotopic composition in kimberlite-related mantle products might indicate that a recycled crustal component occurred in the source of the Kimberley kimberlites, or was assimilated during interaction with the lithospheric mantle. Similar major and trace element characteristics in clinopyroxene from phlogopite-lherzolite and MARID samples suggest metasomatism by fluids of similar composition. Lherzolite and MARID clinopyroxene ?18O values (as low as +4.4‰) extend below those reported in mantle peridotites (i.e. ?18Oclinopyroxene?=?+5.6?±?0.3‰; 2?s.d.), and strong negative correlations are found between mineral ?18O values and major element compositions (e.g., Na2O contents in clinopyroxene). Furthermore, phlogopite ?15N values (+4 to +7‰) in the studied lherzolite and MARID samples are higher than mantle values. Combined, the low ?18O-high ?15N isotopic signatures of MARID and lherzolite samples suggest progressive mantle metasomatism by a melt containing a recycled oceanic crust (eclogitic) component. This study demonstrates that progressive enrichment of the subcontinental lithospheric mantle may be inextricably linked to plate tectonics via recycling of subducted crustal material into the deep mantle.
Abstract: Subduction-Transform Edge Propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, NW Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with modal variations. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790-1165?°C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second phase particles. In the coarse-grained peridotites, well-developed olivine crystal preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high-temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010]-fiber and the [010]- and [001]-axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg# melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite), and with a low-Mg# evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt-peridotite reaction - promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing - resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and played a key effect on grain size reduction during the operation of the Rif-Tell STEP fault. Melt-rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.
Abstract: The evaluation of primary kimberlite compositions is hindered by significant melt modifications during ascent through the lithosphere by entrainment of xenolithic material, volatile degassing, and near surface alteration. Consequently, hypabyssal kimberlite emplaced in the upper crust may not provide a true reflection of the primary kimberlite magma. This contribution places new constraints on kimberlite melt composition by providing an assessment of quenched glass-rich polymineralic melt inclusions hosted in olivine megacrysts from the Monastery kimberlite, South Africa. Melt inclusions predominantly contain variable proportions of euhedral or skeletal grains of calcite, phlogopite, spinel, perovskite, serpentine, and fresh to devitrified glass. Estimates of the bulk compositions of melt inclusions, and the compositions of crystalline phases present therein, are compatible to those of hypabyssal kimberlites worldwide and show a volatile-rich (CO2?+?H2O ~10-17?wt%) carbonated silicate (SiO2 ~27-41?wt%) composition. The glass component has a Si-Mg-Fe-rich and largely CaO-, K2O- and TiO2-free major element composition and is REE-depleted. It also contains approximately 10?wt% H2O but is CO2?free. The glass represents a residual melt that existed after crystallization of the observed mineral assemblage. From some, but not all melt inclusions, apophyses radiate outwards. These fractures typically contain partially devitrified glass that is compositionally identical to the fresh residual glass within the melt inclusions, indicating fracture formation during decompression of the hosting megacryst and at a stage after the melt had evolved. These features are consistent with a trapping of the melt inclusions at high pressure, prior to kimberlite ascent to the surface, in the SCLM at a depth corresponding to 4.5-6?GPa. Textures and compositions of phases within the melt inclusions represent stages of the kimberlite melt and magma evolution. They provide evidence in support of high-pressure experimental studies suggesting a carbonated silicate primary melt rather than a carbonatite. Furthermore, the composition of fresh glass in the melt inclusions, which is compositionally similar to serpentine, suggests that much groundmass serpentine in hypabyssal kimberlites may have formed from similar silicate melt or devitrified glass.
Abstract: The breakup of Africa from South America is associated with the emplacement of the Paraná-Etendeka flood basalt province from around 134?Ma and the Tristan da Cunha plume. Yet many additional volcanic events occur that are younger than the main pulse of the Paraná-Etendeka and straddle the rift to drift phases of the main breakup. This contribution reports on new geochronological constraints from the Angolan part of the African Margin. Three coastal and one inland section have been sampled stretching across some 400?Km, with 39Ar/40Ar, U-Pb and Palaeontology used to provide age constraints. Ages from the new data range from ~100 to 81?Ma, with three main events (cr. 100, 91 and 82-81?Ma). Volcanic events are occurring within the Early to Late Cretaceous, along this part of the margin with a general younging towards Namibia. With the constraints of additional age information both onshore and offshore Angola, a clear younging trend at the early stages of rift to drift is recorded in the volcanic events that unzip from North to South. Similar age volcanic events are reported from the Brazilian side of the conjugate margin, and highlight the need to fully incorporate these relatively low volume volcanic pulses into the plate tectonic breakup models of the South Atlantic Margin.
The Southern African Institute of Mining and Metallurgy, Vol. 119, pp. 155-164.
Africa, Botswana
deposit - Jwaneng
Abstract: Despite the pre-eminence of the Jwaneng Diamond Mine as the world's richest diamond mine, the discovery story has long been clouded in mystery. This is the 45-year old untold story of the Jwaneng discovery and contemporaneous Bechuanaland/Botswana political and socioeconomic history.
Abstract: The Tanzania?North Mozambique continental margin is a transform segment associated with Davie Fracture Zone (DFZ). The DFZ is described as an elongated linear oceanic fracture zone, commonly linked with the breakup between Eastern and Western Gondwana. We conducted a synthesized study using gravity, magnetic and seismic data presenting the crustal architecture, geometry and the kinematic nature of continental breakup along a transform margin. The Crustal nature of DFZ, its role in forming kinematic linkage between two extensional margins during continental breakup processes is focus of our study. The two extensional margins, Somalia?Majunga and North Mozambique?Antarctica were linked via a 2600 km long dextral transform segment, partially overlapping with DFZ. Absence of classical rift indicators, weak signs of hyperextension, abrupt ocean?continent boundary (OCB) suggests transform margin architecture. We redefined this feature as the Davie Transform System (DTS). The nature of deformation varies form transtensional pull?apart in Tanzania to almost pure strike?slip in North Mozambique. The southern transform segment exhibits abrupt change in ocean continent transition with a narrow zone of continental extension. This variation is recognized through the newly interpreted OCB along this entire transform segment. Notably, within large pull?apart systems in the north, presence of fossilized incipient spreading center suggest that the extension had reached at quite advanced stages, characterized by significant thermal weakening as a consequence of strong magmatic activity. Through a series of reconstruction snapshots, we show the geodynamic evolution along the Tanzania?North Mozambique margin explaining the role of DTS in the southward movement of Madagascar.
Abstract: Southern and western Madagascar is comprised of five tectonic provinces that, from northeast to southwest, are defined by the: (i) Ikalamavony, (ii) Anosyen, (iii) Androyen, (iv) Graphite and (v) Vohibory Domains. The Ikalamavony, Graphite and Vohibory Domains all have intermediate and felsic igneous protoliths of tonalite-trondhjemite-granodiorite-granite composition, with positive ?Nd, and low Sr and Pb isotopic ratios. All three domains are interpreted to be the products of intra-oceanic island arc magmatism. The protoliths of the Ikalamavony and Graphite Domains formed repectively between c. 1080-980?Ma and 1000-920?Ma, whereas those of the Vohibory Domain are younger and date to between c. 670-630?Ma. Different post-formation geologic histories tie the Vohibory-Graphite and Ikalamavony Domains to opposite sides of the pre-Gondwana Mozambique Ocean. By contrast, the Androyen and Anosyen Domains record long crustal histories. Intermediate to felsic igneous protoliths in the Androyen Domain are of Palaeoproterozoic age (c. 2200-1800?Ma), of tonalite-trondhjemite-granodiorite-granite composition, and show negative ?Nd, moderate to high 87Sr/86Sr and variable Pb isotopic compositions. The felsic igneous protoliths of the Anosyen Domain are of granitic composition and, when compared to felsic gneisses of the Androyen Domain, show consistently lower Sr/Y and markedly higher Sr and Pb isotope ratios. Like the Vohibory and Graphite Domains, the Androyen Domain can be linked to the western side of the Mozambique Ocean, while the Anosyen Domain shares magmatic and detrital zircon commonalities with the Ikalamavony Domain. It is consequently linked to the opposing eastern side of this ocean. The first common event observed in all domains dates to c. 580-520?Ma and marks the closure of the Mozambique Ocean. The trace of this suture lies along the boundary between the Androyen and Anosyen Domains and is defined by the Beraketa high-strain zone.
Chemical Geology, doi.org/10,1016/j.chem geo.2019.04.014 37p.
Africa, Sierra Leone
deposit - Zimmi
Abstract: Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low ?13C values below ?15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. ?13C values between ?6.7 and ?8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low ?13C values) and carbonates (high ?13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H2O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing ?13C (?23.4 to ?24.5‰) and decreasing [N] is indicative of formation from reduced CH4-bearing fluids. Unlike mixed CH4-CO2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in ?13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking.
Geochimica et Cosmochimica Acta, in press available, 19p.
Africa, South Africa, Canada, South America, Brazil
deposit - Lac de Gras, Paranaiba
Abstract: Kimberlites are the deepest melts produced on Earth that are erupted at the surface and can therefore provide unique insights into the composition and evolution of the mantle. Radiogenic isotopes provide ambiguous evidence for the occurrence of recycled crustal material in kimberlite sources. Oxygen isotopes can fractionate significantly only in the shallow crust, and thus represent a powerful tracer of subducted material in the sources of kimberlite. To constrain the oxygen isotope composition of kimberlite melts, we have examined olivine grains in eleven Cretaceous to Eocene archetypal kimberlites from southern Africa, Lac de Gras (Canada) and Alto Paranaiba (Brazil), which exhibit radiogenic isotope evidence for recycled crustal material in their sources including highly radiogenic Pb isotopes and Nd-Hf isotope compositions deviating below the mantle array. Olivine grains are commonly zoned between a mantle-derived xenocrystic core and one or more magmatic overgrowths, i.e. occasional internal zones, ubiquitous rims and rare rinds (moving outward from the core). The oxygen isotope composition of different olivine zones was determined in situ within separated olivine grains by secondary ion mass spectrometry (SIMS) after point selection using back-scattered electron (BSE) images combined with major and minor element analyses. With the exception of a few cores, the ?18O values of different olivine zones do not deviate from typical mantle olivine values of 5.18?±?0.28‰ (Mattey et al., 1994). There are no correlations between oxygen isotopes and major/minor element compositions for internal zones and rims from individual localities or in the entire dataset. This indicates that the oxygen isotope composition of kimberlite melts is not affected by melt differentiation to the point of olivine rim crystallisation. However, olivine rinds from the Koala kimberlite (Canada) display an inverse correlation between ?18O and Mn-Ca concentrations, with ?18O values extending below the mantle range, which is probably due to carbonate fractionation, CO2 degassing and/or assimilation of serpentine-rich material after kimberlite emplacement in the upper crust. The mantle-like ?18O composition of olivine internal zones and rims suggests that assimilation of mantle material and liberation of a CO2-rich phase during ascent in the mantle do not significantly modify the original ?18O signature of kimberlite melts. Modelling of oxygen isotope fractionation shows that up to 15 wt% of CO2 can be lost by kimberlites en route to the upper crust. Our results combined with mass balance calculations indicate that only a limited amount (<5-10 wt%) of recycled crustal material could occur in the source of kimberlites from southern Africa, Lac de Gras and Alto Paranaiba, or that the recycled material had an oxygen isotope composition similar to the mantle.
Abstract: Earth’s only active natrocarbonatite volcanism, occurring at Oldoinyo Lengai (OL), Tanzania, suggests that natrocarbonatite melts are formed through a unique geological process. In the East African Rift, the extinct Kerimasi (KER) volcano is a neighbor of OL and also contains nephelinites hosting melt and fluid inclusions that preserve the igneous processes associated with formation of natrocarbonatite melts. Here, we present evidence for the presence of coexisting nephelinite melt, fluorine-rich carbonate melt, and alkali carbonate fluid. The compositions of these phases differ from the composition of OL natrocarbonatites; therefore, it is unlikely that natrocarbonatites formed directly from one of these phases. Instead, mixing of the outgassing alkali carbonate fluid and the fluorine-rich carbonate melt can yield natrocarbonatite compositions at temperatures close to subsolidus temperatures of nephelinite (<630-650 °C). Moreover, the high halogen content (6-16 wt%) in the carbonate melt precludes saturation of calcite (i.e., formation of calciocarbonatite) and maintains the carbonate melt in the liquid state with 28-41 wt% CaO at temperatures ?600 °C. Our study suggests that alkali carbonate fluids and melts could have commonly formed in the geological past, but it is unlikely they precipitated calcite that facilitates fossilization. Instead, alkali carbonates likely precipitated that were not preserved in the fossil nephelinite rocks. Thus, alkali carbonate fluids and melts have been so far overlooked in the geological record because of the lack of previous detailed inclusion studies.
Abstract: Together with Mauritius, Botswana is often categorized as one of two growth miracles in sub-Saharan Africa. Due to its spectacular long-run economic performance and impressive social development, it has been termed both an economic success story and a developmental state. While there is uniqueness in the Botswana experience, several aspects of the country’s opportunities and challenges are of a more general nature. Throughout its history, Botswana has been both blessed and hindered by its natural resource abundance and dependency, which have influenced growth periods, opportunities for economic diversification, strategies for sustainable economic and social development, and the distribution of incomes and opportunities. Through a political economy framework, Hillbom and Bolt provide an updated understanding of an African success story, covering the period from the mid-19th century, when the Tswana groups settled, to the present day. Understanding the interaction over time between geography and factor endowments on the one hand, and the development of economic and political institutions on the other, offers principle lessons from Botswana’s experience to other natural resource rich developing countries.
Geochimica et Cosmochimica Acta, in press available, 16p.
Africa, South Africa
deposit - Kimberly region
Abstract: Carbonate-bearing assemblages in the mantle have been interpreted to be the source for Si-undersaturated, CO2-rich magmas, including kimberlites. However, direct evidence for carbonate in the mantle is rare in the contemporary literature. Here we present petrography, trace element, and C-O-Sr-Nd-Pb isotope composition for a suite of carbonate xenoliths from the Kimberley region kimberlites to ascertain their mantle or crustal origin and gain insight to the potential for the occurrence of carbonate in the mantle. Carbonate xenoliths were found in large kimberlite blocks from the Bultfontein kimberlite and Big Hole region. The xenoliths are characterised by pale green alteration margins made of fine-grained microlites of an unknown mineral as well as spherules surrounded by glassy material. They are generally 1–4?cm in size, coarse-grained (1–2?mm), and comprised entirely of calcite. Carbonate xenoliths from the Bultfontein kimberlite have low total REE concentrations (0.2–4.9?ppm), constant 87Sr/86Sri (0.7047–0.7049) combined with variable ?Ndi (?0.1 to ?26.2) and 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi of 16.7–18.8, 15.3–15.6, 36.5–38.4, respectively. Xenoliths from the Big Hole sample have higher 87Sr/86Sri (0.7088–0.7095), lower ?Ndi (?24.5 to ?3.8), and 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi of 18.9–19.9, 15.7–15.8, 38.4–38.8, respectively. The ?13C values for both Bultfontein (?5.7 to ?6.6‰) and Big Hole (?4.7 to ?5.4‰) carbonates are within the typical range expected for mantle-derived carbonate. The ?18O values (15.5–17.5‰) are higher than those of mantle silicate rocks, indicative of late-stage low-temperature interaction with fluids; a common feature of groundmass calcite in the Kimberley kimberlites. The Sr- and C- isotope composition of the Bultfontein xenoliths indicates a mantle origin whereas the Big Hole xenolith Sr- and C-isotopes are more ambiguous. Isotope mixing models are inconsistent with interaction between the host kimberlite and carbonate xenoliths. Correlation between ?Ndi and ?18O values for the Bultfontein xenoliths indicates late-stage interaction with low-temperature fluids, which may also be responsible for the large range in ?Ndi. This in turn indicates that the highest ?Ndi of ?0.1 represents the primary carbonate xenolith signature, and this value overlaps typical Group I kimberlites. We discuss two possible origins for the carbonate xenoliths. (1) Carbonate xenoliths from the sub-continental lithospheric mantle (SCLM), where quenched margins and the large range of ?Ndi are related to formation in the mantle. (2) Carbonate xenoliths from an earlier phase of carbonatite magmatism. The similarity of isotope signatures of the Bultfontein carbonates to Group I kimberlite may further suggest a link between kimberlite and carbonatite volcanism such as observed elsewhere in the world.
Abstract: Unusually deep earthquakes occur beneath rift segments with and without surface expressions of magmatism in the East African Rift system. The Tanganyika rift is part of the Western rift and has no surface evidence of magmatism. The TANG14 array was deployed in the southern Tanganyika rift, where earthquakes of magnitude up to 7.4 have occurred, to probe crust and upper mantle structure and evaluate fault kinematics. Four hundred seventy?four earthquakes detected between June 2014 and September 2015 are located using a new regional velocity model. The precise locations, magnitudes, and source mechanisms of local and teleseismic earthquakes are used to determine seismogenic layer thickness, delineate active faults, evaluate regional extension direction, and evaluate kinematics of border faults. The active faults span more than 350 km with deep normal faults transecting the thick Bangweulu craton, indicating a wide plate boundary zone. The seismogenic layer thickness is 42 km, spanning the entire crust beneath the rift basins and their uplifted flanks. Earthquakes in the upper mantle are also detected. Deep earthquakes with steep nodal planes occur along subsurface projections of Tanganyika and Rukwa border faults, indicating that large offset (?5 km) faults penetrate to the base of the crust, and are the current locus of strain. The focal mechanisms, continuous depth distribution, and correlation with mapped structures indicate that steep, deep border faults maintain a half?graben morphology over at least 12 Myr of basin evolution. Fault scaling based on our results suggests that M > 7 earthquakes along Tanganyika border faults are possible.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0588-5 17p.
Africa, South Africa
deposit - De Beers dyke
Abstract: We present petrographic and mineral chemical data for a suite of samples derived from the De Beers dyke, a contemporaneous, composite intrusion bordering the De Beers pipe (Kimberley, South Africa). Petrographic features and mineral compositions indicate the following stages in the evolution of this dyke: (1) production of antecrystic material by kimberlite-related metasomatism in the mantle (i.e., high Cr-Ti phlogopite); (2) entrainment of wall-rock material during ascent through the lithospheric mantle, including antecrysts; (3) early magmatic crystallisation of olivine (internal zones and subsequently rims), Cr-rich spinel, rutile, and magnesian ilmenite, probably on ascent to the surface; and (4) crystallisation of groundmass phases (i.e., olivine rinds, Fe-Ti-rich spinels, perovskite, apatite, monticellite, calcite micro-phenocrysts, kinoshitalite-phlogopite, barite, and baddeleyite) and the mesostasis (calcite, dolomite, and serpentine) on emplacement in the upper crust. Groundmass and mesostasis crystallisation likely forms a continuous sequence with deuteric/hydrothermal modification. The petrographic features, mineralogy, and mineral compositions of different units within the De Beers dyke are indistinguishable from one another, indicating a common petrogenesis. The compositions of antecrysts (i.e., high Cr-Ti phlogopite) and magmatic phases (e.g., olivine rims, magnesian ilmenite, and spinel) overlap those from the root zone intrusions of the main Kimberley pipes (i.e., Wesselton, De Beers, Bultfontein). However, the composition of these magmatic phases is distinct from those in ‘evolved’ intrusions of the Kimberley cluster (e.g., Benfontein, Wesselton water tunnel sills). Although the effects of syn-emplacement flow processes are evident (e.g., alignment of phases parallel to contacts), there is no evidence that the De Beers dyke has undergone significant pre-emplacement crystal fractionation (e.g., olivine, spinel, ilmenite). This study demonstrates the requirement for detailed petrographic and mineral chemical studies to assess whether individual intrusions are in fact ‘evolved’; and that dykes are not necessarily produced by differentiated magmas.
Abstract: Six diamonds with a fibrous core, intermediate zone and monocrystalline outer zone (“cloudy diamonds”) from the Koffiefontein mine, South Africa, were investigated for N concentrations, carbon isotope compositions and micro-inclusion compositions along core to rim traverses. This study evaluates the nature of the change from fibrous to gem diamond growth and the relation between major element composition of high density fluid inclusions and N ? ?¹³C in fibrous growth zones. Three diamonds contain saline to carbonatitic fluid micro-inclusions with constant or increasing carbon isotope values which are inferred to have formed by varying amounts of Rayleigh fractionation in a closed system of a carbonate-bearing fluid. Continuous N ? ?¹³C fractionation trends from the fibrous to gem growth zone in two of the diamonds and equally low nitrogen aggregation states indicate formation of diamond shortly before kimberlite eruption from a single fluid without a time gap between fibrous and gem diamond growth. High major element/CO32- ratios in the growth media resulted in a constant major element composition of the fluid inclusions found in the studied fibrous diamonds. The transition from fibrous to gem diamond growth is likely caused by the precipitation of diamond reducing the degree of oversaturation of carbon in the fluid and hence decreasing the rate of diamond growth. Two other diamonds have inclusions that change from silicate minerals in the inner fibrous growth zones towards pure saline fluid compositions in the outer fibrous growth zones. This decrease in Si, Mg and Ca and increase in K and Cl in the inclusions is accompanied by a decrease in ?¹³C values and N contents. These trends are suggested to be the result from gradually mixing in more saline fluids with lower ?¹³C values. One diamond with silicic inclusions has significant N aggregation into B-centres, suggesting this fluid is different and that diamond formation occurred significantly (e.g. 1250 °C gives ?10 Ma) before the kimberlite eruption.
Contributions to Mineralogy and Petrology, Vol. 174, 5, 27p.
Africa, Ethiopia
xenoliths
Abstract: The behavior of sub-continental lithospheric mantle (SCLM) in extensional settings, up to successful rifting, plays an important role in geodynamics and in the global carbon cycle, yet the underlying processes and rates of lithosphere destruction remain poorly constrained. We determined platinum-group element (PGE: Os, Ir, Ru, Pt, and Pd) abundances and Re-Os-isotope systematics for well-characterized mantle xenoliths hosted in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia to provide new insights on the nature and timing of processes leading to the formation and transformation of the off-cratonic lithospheric mantle beneath the East Africa rift system (EARS). The whole-rock PGE concentrations are highly variable, with total PGE abundances ranging from 6.6 to 12.6 ppb for Gundeweyn, 11.5 to 23.3 ppb for Dillo, and 9.9 to 19.4 ppb for Megado mantle xenoliths. The 187Os/188Os ratios of the whole-rock mantle xenoliths vary from 0.1180 to 0.1287 for Gundeweyn, 0.1238 to 0.1410 for Dillo and 0.1165 to 0.1277 for Megado, compared to 0.130 for the Afar plume and???0.14 for the Kenya plume, with Re depletion ages up to 1.45 Ga for Gundeweyn, 0.64 Ga for Dillo, and 1.65 Ga for Megado mantle xenoliths. The regional differences between refertilizing agents recorded in mantle xenoliths from the plateau area and the rift systems reflect distinct tectonomagmatic settings: (1) low PGE abundances, with some retention of low 187Os/188Os in Gundeweyn peridotites, are ascribed to scavenging by early small-volume oxidizing melts, generated in the convecting mantle ahead of the arrival of the Afar plume. (2) Percolation of late-stage silicate/basaltic melts, associated with the arrival of hot mantle plume and lithosphere thinning in the rift setting, locally led to refertilization and sulfide precipitation and partial replenishment of the PGE (Dillo), with convecting mantle-like 187Os/188Os. Local enclaves of older, cryptically metasomatised mantle with unradiogenic Os (Megado) attest to the heterogeneous nature of melt-peridotite interaction at this stage (pervasive vs. focused melt flow). Highly depleted abundances of the compatible PGE are characteristic of SCLM affected by incipient rifting and percolation of oxidizing melts, here associated with the Afar and Kenya plume beneath the East Africa rift, and may be precursors to advanced degrees of lithosphere destruction/transformation.
Abstract: Bimineralic eclogites and pyroxenites (n?=?75?±?accessory rutile, ilmenite, sulphide, apatite) from the Koidu kimberlite (West African Craton) were investigated for mineral major and trace elements and mineral Sr-Nd isotope compositions to constrain (1) the nature and age of their basaltic to picritic protoliths, and (2) the effect, timing and source of mantle metasomatism. Consistent with published work, samples are grouped into low-Mg eclogites with Mg# from 0•49 to 0•73 (median 0•59; n?=?40) and high-Mg eclogites with Mg# from 0•60 to 0•88 (median 0•75; n?=?14), plus pyroxenites [clinopyroxene Na/(Na + Ca) <0•2; n?=?8] and gabbroic eclogites and pyroxenite (Eu/Eu* of reconstructed bulk-rocks >1•05; n?=?8), with five unclassifiable samples. Reconstructed low-Mg and gabbroic eclogites have major and trace element systematics (Eu/Eu*-heavy rare earth elements-Y) indicating crustal protolith crystallisation, confirming an origin as subducted oceanic crust. Their high FeO contents at MgO >?10?wt % require an Fe-rich source, the high melt productivity of which led to the formation of thicker crust, perhaps in a plateau-like setting. This is consistent with SiO2-MgO relationships indicating differentiation at ?0•5?GPa. Unradiogenic Sr in some clinopyroxene (87Sr/86Sr of 0•7010-0•7015), combined with light rare earth element (LREE) depletion relative to normal mid-ocean ridge basalt (N-MORB) for the majority of samples (average N-MORB-normalised Nd/Yb of unmetasomatised samples = 0•51), suggests eclogitisation and partial melt loss in the Neoarchaean, possibly coeval with and parental to 2•7?Ga overlying continental crust. Most reconstructed high-Mg eclogites and some pyroxenites formed by metasomatic overprinting of low-Mg eclogites and gabbroic eclogites, as indicated by the preservation of positive Eu anomalies in some samples, and by the Mg-poorer composition of included versus matrix minerals. Coupled enrichment in MgO, SiO2 and Cr2O3 and in incompatible elements (Sr, LREE, Pb, Th and U) is ascribed to metasomatism by a kimberlite-like, small-volume, carbonated ultramafic melt, mediated by addition of clinopyroxene from the melt (i.e. stealth metasomatism). Strontium-Nd isotope systematics suggest a Neoproterozoic age for this metasomatic event, possibly linked to Rodinia break-up, which facilitated intrusion of asthenospheric carbonated melts with an ocean island basalt-like 87Sr/86Sri of ?0•7035. Cretaceous kimberlite magmatism (including Koidu), with more radiogenic 87Sr/86Sr (?0•7065, intermediate between Kaapvaal kimberlites and orangeites), may have been partially sourced from associated Neoproterozoic metasomes. The presence of diamonds in low-Mg eclogites, but absence in high-Mg eclogites, indicates the diamond-destructive nature of this event. Nevertheless, the moderate proportion of affected eclogites (?35%) suggests preservation of a sizeable diamond-friendly mantle eclogite reservoir beneath Koidu.
Abstract: The Gakara Rare Earth Elements (REE) deposit is one of the world’s highest grade REE deposits, likely linked to a carbonatitic magmatic-hydrothermal activity. It is located near Lake Tanganyika in Burundi, along the western branch of the East African Rift. Field observations suggest that the mineralized veins formed in the upper crust. Previous structures inherited from the Kibaran orogeny may have been reused during the mineralizing event. The paragenetic sequence and the geochronological data show that the Gakara mineralization occurred in successive stages in a continuous hydrothermal history. The primary mineralization in bastnaesite was followed by an alteration stage into monazite. The U-Th-Pb ages obtained on bastnaesite (602 ± 7 Ma) and on monazite (589 ± 8 Ma) belong to the Pan-African cycle. The emplacement of the Gakara REE mineralization most likely took place during a pre-collisional event in the Pan-African belt, probably in an extensional context.
Geochemistry, Geophysics, Geosystems, Vol. 20, 4, pp. 2091-2109.
Africa
geophysics
Abstract: In Ethiopia, the African Continent is rifting apart to slowly form a new ocean basin, which will expand the Red Sea and the Gulf of Aden. How and why this rifting is occurring remains an important unanswered question in earth science. We know tectonic forces are partly responsible, but magmatism also seems a key ingredient for breaking up Africa. Here we use seismic images obtained from signals pulled out of noise, to understand the crustal structure of the region; In particular, how and where magma is stored in the crust, and its relationship to the different stages of continental breakup visible in the region. We find evidence for long?term melt storage in places where rifting is just beginning in southern Ethiopia; whereas in regions where the crust is thinner due to extensive rifting, magma erupts more regularly. The long?term storage of magma in unrifted crust may help to heat and weaken it, allowing rifting to accelerate and propagate further south. We are also able to image regions with hydrothermal fluids in the shallow parts of the crust in inactive fault zones. These results provide insight into the breakup process and the role magma plays at different stages of rifting.
Abstract: The ca. 1153?Ma Premier kimberlite pipe on the Kaapvaal craton has been intruded by late-stage kimberlite and carbonatite magmas forming discrete 0.5 to 5?m wide dykes within the lower diatreme. On the basis of petrography and geochemistry, the fresh kimberlite dykes represent archetypal monticellite phlogopite kimberlite of Group-1 affinity. Their mineral compositions, however, show marked deviations from trends that are typically considered as diagnostic for Group-1 kimberlite in mineralogical-genetic classification schemes for volatile-rich ultramafic rocks. Groundmass spinel compositions are transitional between magnesian ulvöspinel (a Group-1 kimberlite hallmark feature) and titanomagnetite trends, the latter being more diagnostic for lamproite, orangeite (formerly Group-2 kimberlite), and aillikite. The Premier kimberlite dykes contain groundmass phlogopite that evolves by Al- and Ba-depletion to tetraferriphlogopite, a compositional trend that is more typical for orangeite and aillikite. Although high-pressure cognate and groundmass ilmenites from the Premier hypabyssal kimberlites are characteristically Mg-rich (up to 15?wt% MgO), they contain up to 5?wt% MnO, which is more typical for carbonate-rich magmatic systems such as aillikite and carbonatite. Manganese-rich groundmass ilmenite also occurs in the Premier carbonatite dykes, which are largely devoid of mantle-derived crystal cargo, suggesting a link to the kimberlite dykes by fractionation processes involving development of residual carbonate-rich melts and fluids. Although mineralogical-genetic classification schemes for kimberlites and related rocks may provide an elegant approach to circumvent common issues such as mantle debris entrainment, many of the key mineral compositional trends are not as robust for magma type identification as previously thought. Utilizing an experimentally constrained CO2-degassing model, it is suggested that the Premier kimberlite dykes have lost between 10 and 20?wt% CO2 during magma ascent through the cratonic lithosphere, prior to emplacement near the Earth's surface. Comparatively low fO2 values down to ?5.6 ?NNO are obtained for the kimberlite dykes when applying monticellite and perovskite oxybarometry, which probably reflects significant CO2 degassing during magma ascent rather than the original magma redox conditions and those of the deep upper mantle source. Thus, groundmass mineral oxybarometry may have little value for the prediction of the diamond preservation potential of ascending kimberlite magmas. After correction for olivine fractionation and CO2-loss, there remains a wide gap between the primitive kimberlite and carbonatite melt compositions at Premier, which suggests that these magma types cannot be linked by variably low degrees of partial melting of the same carbonated peridotite source in the deep upper mantle. Instead, fractionation processes produced carbonate-rich residual melts/fluids from ascending kimberlite magma, which led to the carbonatite dykes within Premier pipe.
Abstract:
Earth’s only active natrocarbonatite volcanism, occurring at Oldoinyo Lengai (OL), Tanzania, suggests that natrocarbonatite melts are formed through a unique geological process. In the East African Rift, the extinct Kerimasi (KER) volcano is a neighbor of OL and also contains nephelinites hosting melt and fluid inclusions that preserve the igneous processes associated with formation of natrocarbonatite melts. Here, we present evidence for the presence of coexisting nephelinite melt, fluorine-rich carbonate melt, and alkali carbonate fluid. The compositions of these phases differ from the composition of OL natrocarbonatites; therefore, it is unlikely that natrocarbonatites formed directly from one of these phases. Instead, mixing of the outgassing alkali carbonate fluid and the fluorine-rich carbonate melt can yield natrocarbonatite compositions at temperatures close to subsolidus temperatures of nephelinite (<630-650 °C). Moreover, the high halogen content (6-16 wt%) in the carbonate melt precludes saturation of calcite (i.e., formation of calciocarbonatite) and maintains the carbonate melt in the liquid state with 28-41 wt% CaO at temperatures ?600 °C. Our study suggests that alkali carbonate fluids and melts could have commonly formed in the geological past, but it is unlikely they precipitated calcite that facilitates fossilization. Instead, alkali carbonates likely precipitated that were not preserved in the fossil nephelinite rocks. Thus, alkali carbonate fluids and melts have been so far overlooked in the geological record because of the lack of previous detailed inclusion studies.
Abstract: The tectonic setting of northeastern Angola was influenced by the opening of the South Atlantic Ocean, which reactivated deep NE-SW-trending faults during the early Cretaceous. The new interpretation of a kimberlitic pulse during the middle of the Aptian and the Albian, which provides precise data on the age of a significant diamond-bearing kimberlite pulse in Angola, will be an important guide in future diamond exploration. These findings contribute to a better understanding of the petrogenetic evolution of the kimberlites in northeastern Angola and have important implications for diamond exploration. Six kimberlite pipes within the Lucapa structure in northeastern Angola have been investigated using major and trace element geochemistry of mantle xenoliths, macro- and megacrysts. Geothermobarometric calculations were carried out using xenoliths and well-calibrated single crystals of clinopyroxene. Geochronological and isotopic studies were also performed where there were samples available of sufficient quality. Results indicate that the underlying mantle experienced variable conditions of equilibration among the six sites. Subsequent metasomatic enrichment events also support a hypothesis of different sources for these kimberlites. The U/Th values suggest at least two different sources of zircon crystals from the Catoca suite. These different populations may reflect different sources of kimberlitic magma, with some of the grains produced in U- and Th-enriched metasomatized mantle units, an idea consistent with the two populations of zircon identified on the basis of their trace element compositions. This research shows the absence of fresh Mg-rich ilmenite in the Catoca kimberlite (one of the largest bodies of kimberlite in the world), as well as the occurrence of Fe3+-rich ilmenite, do not exclude the presence of diamond in the kimberlite. This is a new insight into the concept of ilmenite and diamond exploration and leads to the conclusion that compositional attributes must be evaluated in light of textural attributes.
Geochimica et Cosmochimica Acta, in press available 29p.
Africa, South Africa, Botswana
deposit - Koffiefontein, Letlhakane, Orapa, Finsch, De Beers Pool
Abstract: Ten individual gem-quality monocrystalline diamonds of known peridotite/eclogite paragenesis from Southern Africa (Koffiefontein, Letlhakane, Orapa) were studied for trace element concentrations and He and Ar abundances and isotopic compositions. In addition, two samples, consisting of pooled fragments of gem-quality peridotitic diamonds from Finsch and DeBeers Pool respectively, were analysed for noble gases. Previous studies (Richardson et al., 1984; Pearson et al., 1998; Gress et al., 2017; Timmerman et al., 2017) provided age constraints of 0.09, 1.0-1.1, 1.7, 2.3, and 3.2-3.4?Ga on mineral inclusions in the studied diamonds, allowing us to study trace elements and noble gases over 3 Gyr of geological time. Concentrations of trace elements in the diamonds are very low - a few hundred ppt to several tens of ppbs - and are likely dependent on the amount of sub-micron inclusions present. Trace element patterns and trace element/3He ratios of the studied monocrystalline diamonds are similar to those in fibrous diamonds, suggesting that trace elements and stable noble gas isotopes reside within the same locations in diamond and track the same processes that are reflected in the trace element patterns. We cannot discern any temporal differences in these geochemical tracers, suggesting that the processes generating them have been occurring over at least the past 2.3?Ga. 3He/4He ratios decrease and 4He and 40Ar* contents increase with increasing age of peridotitic and some eclogitic diamonds, showing the importance of in-situ radiogenic 4He and 40Ar ingrowth by the decay of U-Th-Sm and K respectively. For most gem-quality monocrystalline diamonds, uncertainties in the 3He/4He evolution of the continental lithospheric mantle combined with large analytical uncertainties and possible spatial variability in U-Th-Sm concentrations limit our ability to provide estimates of diamond formation ages using 4He ingrowth. However, the limited observed 4He ingrowth (low U?+?Th/3He) together with a R/Ra value of 5.3 for peridotitic diamond K306 is comparable to the present-day sub-continental lithospheric mantle value and supports the young diamond formation age found by Re-Os dating of sulphides in the same diamond by Pearson et al. (1998). After correction for in-situ radiogenic 4He produced since diamond formation a large variation in 3He/4He remains in ?1?Ga old eclogitic diamonds that is suggested to result from the variable influence of subducted altered oceanic crust that has low 3He/4He. Hence, the 3He/4He isotope tracer supports an origin of the diamond-forming fluids from recycled oceanic crust for eclogitic diamonds, as indicated by other geochemical proxies.
Abstract: The physical characteristics and impermeability of diamonds allow them to retain radiogenic 4He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic 4He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped 4He, the in-situ produced radiogenic 4He, ?-implantation, ?-ejection, diffusion, and cosmogenic 3He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic 3He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the 4He and U-Th concentrations approximates the initial 4He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the 4He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71?Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.
American Economic Review, Vol. 107, 6, pp. 1564-1610. pdf
Africa
legal
Abstract: We combine georeferenced data on mining extraction of 14 minerals with information on conflict events at spatial resolution of 0.5 degree x 0.5 degree for all of Africa between 1997 and 2010. Exploiting exogenous variations in world prices, we find a positive impact of mining on conflict at the local level. Quantitatively, our estimates suggest that the historical rise in mineral prices (commodity super-cycle) might explain up to one-fourth of the average level of violence across African countries over the period. We then document how a fighting group's control of a mining area contributes to escalation from local to global violence. Finally, we analyze the impact of corporate practices and transparency initiatives in the mining industry.
Journal of African Earth Sciences, Vol. 158, available 14p. pdf
Africa, South Africa
magmatism
Abstract: Field and sub-surface data from the Victoria West sill complex in the Karoo Large Igneous Province (ca. 180 Ma) of South Africa are used to constrain the emplacement controls of the regional-scale sill complexes in the central Karoo basin. Cross-cutting relationships point to the presence of five distinct and successively emplaced saucer-shaped sills. Growth of the sill complex was achieved through magmatic underaccretion of magma batches below earlier sills and associated uplift of the overlying strata. The magmatic underaccretion suggests that earlier sills were fully crystallized during the emplacement of later magma pulses and that the rigid (high E) dolerites, in particular, acted as stress barriers that impeded further upward propagation of steep feeder sheets. The resulting nested structure of sills-in-sills within a confined area of less than 2000 km2 also suggests the reutilization of the same or similar feeder system even after full crystallization thereof. The emplacement controls of sills in the central Karoo through stress barriers implies that sill emplacement occurred under very low deviatoric stresses or in a mildly compressional stress regime prior to the break-up of Gondwana. The swap from earlier (184-180?Ma), mainly sill complexes to later (182-174?Ma) dykes and dyke swarms is indicative of a switch in the stress field during the early stages of Gondwana break-up. We speculate that loading, thermal subsidence and lithospheric flexure associated with the emplacement of the earlier, stacked and voluminous sill complexes in the Karoo basins may have determined the formation of the large Karoo dyke swarms, particularly when coinciding with deeper crustal structures. The original and inherited basin geometry and lithospheric structure is pivotal in the development of later Karoo magmatism.
Muller, D., Zahirovic, S., Williams, S.E., Cannon, J., Seton, M., Bower, D.J., Tetley, M., Heine, C., Le Breton, E., Liu, S., Russell, S.H.J., Yang, T., Leonard, J., Gurnis, M.
Abstract: Global deep?time plate motion models have traditionally followed a classical rigid plate approach, even though plate deformation is known to be significant. Here we present a global Mesozoic-Cenozoic deforming plate motion model that captures the progressive extension of all continental margins since the initiation of rifting within Pangea at ~240 Ma. The model also includes major failed continental rifts and compressional deformation along collision zones. The outlines and timing of regional deformation episodes are reconstructed from a wealth of published regional tectonic models and associated geological and geophysical data. We reconstruct absolute plate motions in a mantle reference frame with a joint global inversion using hot spot tracks for the last 80 million years and minimizing global trench migration velocities and net lithospheric rotation. In our optimized model, net rotation is consistently below 0.2°/Myr, and trench migration scatter is substantially reduced. Distributed plate deformation reaches a Mesozoic peak of 30 × 106 km2 in the Late Jurassic (~160-155 Ma), driven by a vast network of rift systems. After a mid?Cretaceous drop in deformation, it reaches a high of 48 x 106 km2 in the Late Eocene (~35 Ma), driven by the progressive growth of plate collisions and the formation of new rift systems. About a third of the continental crustal area has been deformed since 240 Ma, partitioned roughly into 65% extension and 35% compression. This community plate model provides a framework for building detailed regional deforming plate networks and form a constraint for models of basin evolution and the plate?mantle system.
Africa, Sierra Leone, Democratic Republic of Congo, Angola
legal
Abstract: This article investigates the impact of the world price of a “lootable,” labor-intensive natural resource on intensity of violent conflict. Results suggest that a price increase can have opposite effects at different geographical levels of analysis: a decrease in conflict intensity overall in resource-rich countries, but an increase in conflict intensity in resource-rich subnational regions. The article argues that intensity of violence decreases overall due to rising opportunity costs of rebellion but that violence concentrates in resource-rich areas as returns to looting rise. The article introduces a new measure of diamond propensity based on geological characteristics, which is arguably exogenous to conflict and can capture small-scale labor-intensive production better than existing measures. The stated effects are found for secondary diamonds, which are lootable and related to opportunity costs of fighting, but not for primary diamonds, which are neither.
Abstract: Stable isotope (?18O, ?13C) analyses were performed on well preserved belemnites, oysters, and rhynchonellid brachiopods from the Middle to Upper Jurassic of the Morondava Basin in southern Madagascar. Both brachiopods and oysters indicate similar average temperatures of 18.7 to 19.3?°C in the Early Callovian, followed by a temperature decrease towards the Middle Oxfordian (13.9?°C) and a minimum in the Early Kimmeridgian (12.3?°C). In contrast, belemnites from the Oxfordian show lower average temperatures of 10.0?°C, which is likely caused by specific conditions for these organisms (e.g., different fractionation or life habits). Additionally, three oysters from the Upper Oxfordian and Lower Kimmeridgian were used for high-resolution stable isotope analyses. The data show seasonal fluctuations of >6?°C around averages between 14.4 and 14.7?°C. Latitudinal temperature gradients for the Callovian and Kimmeridgian are similar to today at the examined low latitudes of the southern hemisphere. The observed cooling of around 5?°C from the Callovian to the Oxfordian/Kimmeridgian can be attributed to a concurrent southward drift of Madagascar during the break-up of Gondwana. Thus, the study underlines the importance of considering palaeogeography in interpreting stable isotope data as well as the potential of detecting and timing palaeogeographic events by using stable isotope analyses.
Abstract: The Southern Irumide Belt (SIB) of Zambia consists of predominantly Mesoproterozoic terranes that record a pervasive tectono-metamorphic overprint from collision between the Congo and Kalahari cratons in the final stages of Gondwana amalgamation. This study applies multi-method thermochronology to samples throughout southern Zambia to constrain the post-collisional, Phanerozoic thermo-tectonic evolution of the region. U-Pb apatite and 40Ar/39Ar muscovite data are used to constrain the cooling history of the region following Congo-Kalahari collision, and reveal ages of c. 550-450?Ma. Variations in the recorded cooling ages are interpreted to relate to localised post-tectonic magmatism and the proximity of analysed samples to the Congo-Kalahari suture. Apatite fission track data are used to constrain the low-temperature thermo-tectonic evolution of the region and identify mean central ages of c. 320-300, 210-200 and 120-110?Ma. Thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the Phanerozoic. Carboniferous to Permian-Triassic heating is suggested to relate to the development of Karoo rift basins found throughout central Africa and constrain the timing of sedimentation in the basin. Permian to Jurassic cooling is identified in a number of samples, reflecting exhumation as a result of the Mauritanian-Variscan and Gondwanide orogenies. Subsequent cooling of the majority of samples occurs from the Cretaceous and persists until present, reflecting exhumation in response to larger scale rifting associated with the break-up of Gondwana. Each model reveals a later phase of enhanced cooling beginning at c. 30?Ma that, if not an artefact of modelling, corresponds to the development of the East African Rift System. The obtained thermochronological data elucidate the previously unconstrained thermal evolution of the SIB, and provides a refined regional framework for constraining the tectonic history of central Africa throughout the Phanerozoic.
Geochimica et Cosmochimica Acta, in press available doi.org/10.1016 / j.gca.2019.06.041 36p.
Africa, Tanzania, Canada, East Africa, Europe, Germany, Greenland
deposit - Oldoinyo Lengai
Abstract: To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar ?137/134Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in ?137/134Ba values from ?0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average ?137/134Ba value of their mantle sources is estimated to be +0.04?±?0.06‰ (2SD, n?=?16), which is similar to the average value of +0.05?±?0.06‰ for mid-ocean ridge basalts. The lower ?137/134Ba value of ?0.08‰ in a Canadian sample and higher ?137/134Ba values of +0.14‰ and?+?0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.
Abstract: A 5.6-km-long line of refraction and reflection seismic data spanning the Pliocene-Pleistocene fill of the Olduvai Basin, Tanzania is presented. The line is oriented along a northwest-southeast profile through the position of Olduvai Gorge Coring Project (OGCP) Borehole 2A. Our aims are to (1) delineate the geometry of the basin floor by tracing bedrock topography of the metaquartzitic and gneissic basement, (2) map synsedimentary normal faults and trace individual strata at depth, and (3) provide context for the sequence observed in OGCP cores. Results with refraction tomography and poststack migration show that the maximum basin depth is around 405?m (±25?m) in the deepest portion, which quadruples the thickness of the basin-fill previously known from outcrops. Variations in seismic velocities show the positions of lower density lake claystones and higher density well-cemented sedimentary sequences. The Bed I Basalt lava is a prominent marker in the refraction seismic results. Bottom-most sediments are dated to >2.2?Ma near where Borehole 2A bottoms out at the depth of 245?m. However, the seismic line shows that the basin-fill reaches a maximum stratigraphic thickness of around 380?m deep at Borehole 2A, in the western basin where the subsidence was greatest. This further suggests that potential hominin palaeoenvironments were available and preserved within the basin-fill possibly as far back as around 4?Ma, applying a temporal extrapolation using the average sediment accretion rate.
Gondwana Research, doi.org/10.1016 /j.gr.2013.03.013 21p. Pdf
Africa, Mozambique
tectonics
Abstract: The birth of modern life on Earth can be linked to the adequate supply of nutrients into the oceans. In this paper, we evaluate the relative supply of nutrients into the ocean. These nutrients entered the ocean through myriad passageways, but primarily through accelerated erosion due to uplift. In the ‘second ecosystem’, uplift is associated with plume-generation during the breakup of the Rodinia supercontinent. Although the evidence is somewhat cryptic, it appears that the second ecosystem included the demospongia back into the Cryogenian (~ 750 Ma). During the Ediacaran-Cambrian interval, convergent margin magmatism, arc volcanism and the closure of ocean basins provided a second pulse of nutrient delivery into the marine environment. A major radiation of life forms begins around 580 Ma and is represented by the diverse and somewhat enigmatic Ediacaran fauna followed by the Cambrian Explosion of modern phyla during the 540-520 Ma interval. Tectonically, the Ediacaran-Cambrian time interval is dominated by the formation of ultra-high pressure (UHP), high pressure (HP) and ultra-high temperature (UHT) orogenic belts during Gondwana orogenesis. Erosion of this extensive mountainous region delivered vast nutrients into the ocean and enhanced the explosiveness of the Cambrian radiation. The timing of final collisional orogeny and construction of the mountain belts in many of the Gondwana-forming orogens, particularly some of those in the central and eastern belts, post-date the first appearance of modern life forms. We therefore postulate that a more effective nutrient supply for the Cambrian radiation was facilitated by plume-driven uplift of TTG crust, subsequent rifting, and subduction-related nutrient systems prior to the assembly of Gondwana. In the outlined scenario, we propose that the birth of the ‘second ecosystem’ on our planet is plume-driven.
Abstract: Accurately dating the formation and modification of Earth’s sub-cratonic mantle still faces many challenges, primarily due to the long and complex history of depletion and subsequent metasomatism of this reservoir. In an attempt to improve this, we carried out the first study on peridotites from the Kaapvaal craton (Finsch Mine) that integrates results from Re-Os, Lu-Hf, Sm-Nd and Sr-isotope systems together with analyses of major-, trace- and platinum-group elements. The Finsch peridotites are well-suited for such a study because certain compositional features reflect they were highly depleted residues of shallow melting (1.5?GPa) at ambient Archean mantle temperatures. Yet, many of them have overabundant orthopyroxene, garnet and clinopyroxene compared to expected modal amounts for residues from partial melting. Finsch peridotites exhibit a wide range of rhenium depletion ages (TRD) from present day to 2.7?Ga, with a prominent mode at 2.5?Ga. This age overlaps well with a Lu-Hf isochron of 2.64?Ga (?Hf (t)?=?+26) which records silico-carbonatitic metasomatism of the refractory residues. This late Archean metasomatism is manifested by positive correlations of Pt/Ir and Pd/Ir with 187Os/188Os ratios and good correlations of modal amounts of silicates, especially garnet, with Os isotope ratios. These correlations suggest that the Highly Siderophile Elements (HSE) and incompatible element reenrichment and modal metasomatism result from one single major metasomatic event at late Archean. Our detailed study of Finsch peridotites highlights the importance of using multiple isotopic systems, to constrain the ages of events defining the evolution of lithospheric mantle. The Re-Os isotope system is very effective in documenting the presence of Archean lithosphere, but only the oldest TRD ages may accurately date or closely approach the age of the last major partial melting event. For a meaningful interpretation of the Re-Os isotope systematics the data must be combined with HSE patterns, trace-element compositions and ideally other isotopic systems, e.g. Lu-Hf. This is highlighted by the widespread evidence in Finsch peridotites of Pt, Pd and Re enrichment through significant Base Metal Sulfide (BMS) addition (mainly in the range of 0.002-0.08?wt%) that systematically shifts the mode of TRD model ages to younger ages.
Abstract: The powerpoint of the presentation made by Wiehan Smit at the SAIMM July 2019 Grade Control School has been made available by the author as a pdf file.
Abstract: Recent exploration work in South Morocco revealed the occurrence of several carbonatite bodies, including the Paleoproterozoic Gleibat Lafhouda magnesiocarbonatite and its associated iron oxide mineralization, recognized here as iron-oxide-apatite (IOA) deposit type. The Gleibat Lafhouda intrusion is hosted by Archean gneiss and schist and not visibly associated with alkaline rocks. Metasomatized micaceous rocks occur locally at the margins of the carbonatite outcrop and were identified as glimmerite fenite type. Rare earth element (REE) and Nb mineralization is mainly linked to the associated IOA mineralization and is represented by monazite-(Ce) and columbite-(Fe) as major ore minerals. The IOA mineralization mainly consists of magnetite and hematite that usually contain large apatite crystals, quartz and some dolomite. Monazite-(Ce) is closely associated with fluorapatite and occurs as inclusions within the altered parts of apatite and along cracks or as separate phases near apatite. Monazite shows no zonation patterns and very low Th contents (<0.4?wt%), which would be beneficial for commercial extraction of the REE and which indicates monazite formation from apatite as a result of hydrothermal volatile-rich fluids. Similar monazite-apatite mineralization and chemistry also occurs at depth within the carbonatite, although the outcropping carbonatite is barren, suggesting an irregular REE ore distribution within the carbonatite body. The barren carbonatite contains some tiny unidentified secondary Nb-Ta-U phases, synchysite and monazite. Niobium mineralization is commonly represented by anhedral minerals of columbite-(Fe) which occur closely associated with magnetite-hematite and host up to 78?wt% Nb2O5, 7?wt% Ta2O5 and 1.6?wt% Sc2O3. This association may suggest that columbite-(Fe) precipitated by an interaction of Nb-rich fluids with pre-existing Fe-rich minerals or as pseudomorphs after pre-existing Nb minerals like pyrochlore. Our results most strongly suggest that the studied mineralization is economically important and warrants both, further research and exploration with the ultimate goal of mineral extraction.
Abstract: A large book entitled Layered Intrusions (edited by Bernard Charlier, Olivier Namur, Rais Latypov and Christian Tegner, Springer) has been published recently. This book (almost 750 pages) has 15 contributions by 36 experts in the field. While Part I deals with subjects such as geochronology, igneous layering, textures, silicate liquid immiscibility and behaviour of precious metals in these intrusions, Part II examines six examples that are reviewed by experts: Panzhihua (China), Sept Iles (Canada), Bushveld (South Africa), Kiglapait (Labrador), Ilímaussaq (Greenland) and ophiolitic magma chambers in the Canadian Appalachians. The publication of this book has led me to consider the geology of the most famous of them all-the Skaergaard Intrusion of Greenland-and my long history of studying it.
Mineralogical Magazine, in press available 54p. Pdf
Africa, Namibia
carbonatite - Okorusu
Abstract: The Cretaceous Okorusu carbonatite, Namibia, includes diopside-bearing and pegmatitic calcite carbonatites, both exhibiting hydrothermally altered mineral assemblages. In unaltered carbonatite, REE, Sr and Ba are largely hosted by calcite and fluorapatite. However, in hydrothermally altered carbonatites, small (< 50 ?m) parisite-(Ce) grains are the dominant REE host, while Ba and Sr are hosted in baryte, celestine, strontianite and witherite. Hydrothermal calcite has a much lower trace element content than the original, magmatic calcite. Despite the low REE contents of the hydrothermal calcite, the REE patterns are similar to those of parisite-(Ce), and magmatic minerals and mafic rocks associated with the carbonatites. These similarities suggest that hydrothermal alteration remobilised REE from magmatic minerals, predominantly calcite, without significant fractionation or addition from an external source. Ba and Sr released during alteration were mainly reprecipitated as sulfates. The breakdown of magmatic pyrite into Fe-hydroxide is inferred to be the main source of sulfate. The behaviour of sulfur suggests that the hydrothermal fluid was somewhat oxidising and it may have been part of a geothermal circulation system. Late hydrothermal massive fluorite replaced the calcite carbonatites at Okorusu and resulted in extensive chemical change, suggesting continued magmatic contributions to the fluid system.
Abstract: Carbonatites and syenites from Ihouhaouene (2 Ga; In Ouzzal terrane, Hoggar, South of Algeria) have close spatial relationships. Their analogous mineral assemblages with diopside/hedenbergite (cpx), apatite, wollastonite +/- calcite and alkali-feldspar suggest that they were emplaced from a common igneous parental event. Carbonatites from In Ouzzal terrane are calciocarbonatites and form a continuous range of whole-rock major and trace element composition from Sipoor carbonatite (<20 wt.% SiO2; 24-36 wt.% CO2) to Si-rich carbonatite (20-35 wt.% SiO2; 11-24 wt.% CO2) then white syenite (52-58 wt.% SiO2; 0.1-6.5 wt.% CO2) and red syenite (57-65 wt.% SiO2; 0.1-0.4 wt.% CO2). Equilibrium calculations reveal that apatite (Ce/Lu= 1690-6182; Nb/Ta >50) and cpx (Ce/Lu= 49-234; Nb/Ta<10) from Si-rich carbonatites and white syenites crystallized from a REEenriched carbonate melt and an evolved silicate melt, respectively. Likewise, Si-poor carbonatites have a higher REE contents in calculated apatite equilibrium melts than in their cpx and a wide range of Nb/Ta ratios with a majority of subchondritic value (<10) that reflects the segregation of the carbonate fraction from an evolved parental melt. Otherwise, red syenites have similar REE contents in apatite and clinopyroxene equilibrium melts (Nb/Ta>10) suggesting an origin from homogeneous evolved melt batches. Both mineralogical and geochemical features reveal the intimate link between carbonatites and syenites and their cogenetic signature. Immiscibility and fractional crystallization processes modelling explain the trace element contents and low Nb/Ta ratio in minerals. These processes were partly counterbalanced by intermingling of partially crystallized melt fractions and hybridization of segregated minerals during the progressive cooling of a silico-carbonated mantle melt.
Abstract: The physical manifestations of plate tectonics on the modern Earth are relatively well-understood, but the nature and timing of its onset remains enigmatic, with the geodynamic regime(s) that operated during the Archaean hotly debated. This absence of a consistent geodynamic framework within which regional-scale observations can be placed limits our understanding of Archaean assemblages and associated mineral deposits. To engage with the Archaean geodynamics discussion, this thesis focuses on ultramafic-mafic complexes in the Lewisian Gneiss Complex (LGC) of the North Atlantic Craton and Johannesburg Dome of the Kaapvaal Craton. Globally, such complexes have been the subject of wide-ranging interpretations that have disparate implications for Archaean geodynamic regimes. Throughout this thesis, it is demonstrated that confidently constraining element mobility is of paramount importance when aiming to constrain the origin of Archaean ultramafic rocks, with a variety of geochemical proxies shown to be susceptible to element mobility. Notably, high field strength element anomalies - a geochemical proxy commonly used to fingerprint subduction-related magmatism - are here shown to be highly susceptible to element mobility, with the role of subduction as an Archaean geodynamic process potentially overestimated as a result. Such mobility can, however, be constrained and a primary geochemistry obtained using the integrated approach utilised here, whereby detailed petrography, bulk-rock geochemistry and mineral chemistry are examined using the context provided by rigorous field geology. Using this approach, the ultramafic-mafic complexes in the LGC are here interpreted as recording two temporally and petrogenetically distinct phases of Archaean magmatism. One group of complexes likely represents an early ultramafic-mafic crust that pre-dates the tonalite-trondhjemite-granodiorite (TTG) magmas, while a second group of complexes are interpreted as representing several layered intrusions that were emplaced into TTG. The ultramafic-mafic complexes in the Johannesburg Dome are considered to represent intrusive and extrusive remnants of an Archaean greenstone belt, contradicting a recently proposed hypothesis whereby the complexes are interpreted as fragments of an Archaean ophiolite. When combined with similar opposition to other proposed Archaean ophiolite occurrences in other cratons, this contradiction is potentially significant to the Archaean geodynamics debate, raising questions as to the validity of a > 3.6 Ga onset for modern-style plate tectonics.
Abstract: The Earth’s only active natrocarbonatite volcanism at Oldoinyo Lengai suggests that natrocarbonatite melts are formed through a unique geological process. In the East African Rift, the Kerimasi is a neighbor of Oldoinyo Lengai and also contains nephelinites hosting melt and fluid inclusions, which preserve the magmatic processes associated with formation of natrocarbonatite melts. In this study, we present evidence for the presence of coexisting nephelinite melt, fluorine-rich carbonate melt and alkali carbonate fluid. Compositions of these phases differ from the composition of Oldoinyo Lengai natrocarbonatites, therefore, it is not likely that natrocarbonatites formed directly from one of these phases. However, mixing of the outgassing alkali carbonate fluid and the fluorine-rich carbonate melt can yield natrocarbonatite compositions at temperatures close to subsolidus temperatures of the nephelinite (~630 oC). Moreover, the high halogen (F+Cl) content (6-16 wt%) in the carbonate melt precludes saturation of calcite (i.e., formation of calciocarbonatite) and maintains the carbonate melt in the liquid state with 28-41 wt% CaO at temperatures ?600 oC. Our study suggests that alkali carbonate fluids and melts could have commonly formed in the geological past, but it is unlikely they precipitated calcite that facilitates fossilization. Instead, alkali carbonates precipitated which were not preserved in the fossil nephelinite rocks. Thus, alkali carbonate fluids and melts have been so far unaccounted in the geological record because of the lack of former detailed inclusion studies. In addition, the observed low H2O (<4 wt%) content of the alkali carbonate fluid phase shows that a H2O-rich environment is not required for the generation of natrocarbonatites as suggested by previous models. Our model [1] is consistent with the observation that natrocarbonatites are associated with nephelinite rocks [2], as occurs at Oldoinyo Lengai, rather than with calciocarbonatites.
Geochimica et Cosmochimica Acta, in press available 49p. Pdf
Africa, South Africa
deposit - Benfontein
Abstract: Olivine chemistry has been widely used to track the petrogenesis of mafic and ultramafic magmas from their mantle source to eruption at the surface. A major challenge in these studies is deciphering crystal growth versus diffusion controlled zoning. Here we report a multi-element approach using high-precision electron microprobe techniques to evaluate crystal growth versus diffusion in kimberlitic olivine from the Benfontein kimberlite, South Africa. These results have implications for both the petrogenesis of kimberlite magmas and the understanding of crystal growth and diffusion-based zoning in igneous olivine in general. The Benfontein olivine contain multiple phosphorous (P)-rich and P-poor zones. Core zones are characterized by homogenous low-P (<78?ppm) concentrations, consistent with xenocrystic origins. Gradational changes in Fo, Ni, Cr and other minor/trace elements at core-margins are similarly characterized by constant low-P concentrations that are indistinguishable from the central regions of the core. Olivine P-maps effectively outline the original xenocryst core, whereas gradational margins are interpreted as diffusion controlled zones related to early-stage equilibration of xenocrystic olivine with proto-/kimberlite melt. Multiple P-poor (100-150?ppm) and P-rich (200-450?ppm) concentric, oscillatory zones with inclusions of kimberlitic oxide phases are observed surrounding the low-P xenocrystic cores. Oxide phases change from chromite in the inner zones to ilmenite in the intermediate zones to magnetite-rich spinel in the outer zones of the olivine. The P-zoning corresponds with changes in Fo content implying that stages of crystal growth was preserved by both fast and slow diffusing elements rather than diffusion processes. Elements compatible with olivine (±chromite) crystallization (i.e., Ni and Cr) display a constant decrease across all zones, suggesting that magma mixing is unlikely a controlling process for P-zoning. We interpret P-rich zones to result from stages of solute trapping related of rapid disequilibrium growth driven by extrinsic factors such as changes in pressure-temperature during kimberlite evolution. In contrast, P-poor zones represent stages of equilibrium crystal growth. The outer olivine zones are characterized by an increase in Fo contents up to Fo96, and in conjunction with a change to more Fe3+-rich oxides, suggest late stage increase in fO2. Correlated Fo and P changes in the Benfontein olivine suggest that major element zonation represents an example where crystal growth-induced Fo zoning has been preserved in olivine. Furthermore, P-rich olivine zones preserve evidence for concentric growth rather than common dendritic structures seen in other occurrences. These results have implications for understanding the effect of magma dynamics and changes in pressure-temperature-fO2 conditions on olivine growth in igneous rocks.
Abstract: Madagascar has become one of the world’s top sources of fine blue sapphire in recent times. In addition to beautiful untreated material, increasing numbers of treated stones have appeared in the market. Some have been heated to relatively low temperatures, below 1350°C, to lighten their color. To help separate unheated and heated Madagascar sapphire, the authors performed experiments to document the changes they undergo with low-temperature heat treatment in air, which is an oxidizing atmosphere.
Abstract: Carbonatites are rare magmas containing almost no silica; their igneous counterparts represent the main rare earth element deposits inoperation. No consensus exists on their origin, genesis and evolution. Oldoinyo Lengai (Tanzania) is the only active carbonatite volcano, but the alkali-rich natrocarbonatites it erupts are unique among the >500 reported fossil carbonatite occurrences. Here, we use threephase melt inclusions hosted in minerals from cognate cumulates (clinopyroxene, nepheline, Ti-garnet, interstitial melt)— which sampled the active Oldoinyo Lengai magma chamber during the 2007-08 sub-Plinian explosive eruption—to track the carbonatite presence within the plumbing system, and to eventually quantify its composition at depth. We show that although natrocarbonatites are emitted at the Oldoinyo Lengai summit, more classical calciocarbonatites are present at magma chamber depth (~3.5 kbar, 1050 to 900°C), which is consistent with the model of rare natrocarbonatites deriving from calciocarbonatites by further magma differentiation. We also show that those calciocarbonatites are not isolated but rather conjugated with alkali-rich silica melt suggesting a joint evolution. We eventually present the first direct measurements of major and trace element partition coefficients between natural coexisting carbonate and silicate melts. Partitioning behaviour and recent experiments support our conclusion that natrocarbonatites derive from calciocarbonatites by fractionating Ca-rich, Na-poor phases. As natrocarbonatites are highly unstable at surface conditions, they were likely erupted but not preserved in association with fossil calciocarbonatites worldwide. Oldoinyo Lengai can then be considered as representative of other carbonatite systems, and provide us with the unique opportunity to observe the plumbing system architecture, and to constrain the protracted differentiation path of a carbonatite system.
Phd. Thesis University of Alberta, 147p. See note on front page - see U of A search strategy on cover page
Africa, Botswana
deposit - Karowe
Abstract: This study presents the first comprehensive data on the recently developed Karowe diamond mine from the Orapa kimberlite cluster, which hosts mines such as Orapa, Damtshaa, and Letlhakane. The objectives of the study were to establish the compositional characteristics of the lithospheric mantle beneath the Karowe mine at the time of kimberlite eruption by studying the major- and trace-element characteristics of 24 mantle xenoliths and 106 clinopyroxene xenocrysts. In addition, the possible diamond sources beneath Karowe were evaluated through examination of major- and trace-element compositions of mineral inclusions in 120 diamonds. Finally, the physical characteristics of the diamonds themselves as well as their total nitrogen contents and carbon isotope compositions were investigated.The mantle xenoliths are predominantly melt-depleted garnet-free spinel peridotites (n = 14), with a small proportion of pervasively refertilised garnet-spinel lherzolites (n = 4), and variably metasomatised garnet-bearing pyroxenites (n = 3). The remaining three xenoliths are a garnet lherzolite, an eclogite, and a megacrystic olivine. The mineralogical evidence indicates that all these come from relatively low-pressure regions of the mantle outside of the diamond stability field. The predominant spinel lherzolites are characterised by coarse-grained and granolublastic textures, exsolution lamellae in clinopyroxenes and orthopyroxenes, and low equilibration temperatures (630 to 800 oC). They have high Mg# in olivines (median: 92.1) and orthopyroxenes (median: 92.4) as well as variable Cr# in spinels (13 to 47; median: 37). Clinopyroxenes show variable LREEN enrichment (LaN/SmN=0.24 to 3.8) which is consistent with secondary re-enrichement processes. Trace element compositions of the garnets and clinopyroxenes in the garnet-bearing xenoliths indicate cryptic melt metasomatism as well as modal metasomatism associated with the introduction of phlogopite. Clinopyroxene-based geothermobarometry on clinopyroxene xenocrysts (25 out of the106) and one garnet lherzolite xenolith indicate a 39 to 40 mW/m2 model paleogeotherm, which overlaps at greater depths with two non-touching garnet-orthopyroxene inclusion pairs recovered from Karowe diamonds. These data imply that the lithospheric mantle beneath Karowe is 210 km thick with a diamond window of approximately 90 km thickness, which is consistent with other estimates of lithospheric depths in this region of the Zimbabwe Craton derived from petrology and geophysics. The major- and trace-element compositions of mineral inclusions in the diamonds indicate that 53 % are derived from eclogitic sources, 44 % are peridotitic, 2% have a sublithospheric origin, and 1 % are websteritic. The sublithospheric inclusion suite consists of three eclogitic garnets containing a majorite component (>6.12 to 6.46 apfu Si; with [O] = 24). This new observation of superdeep mineral inclusions in Karowe diamonds is unique within the Orapa cluster and may provide a key link to the presence of exceptionally large diamonds from this mine.From the present diamond suite, Karowe diamonds have variable morphologies with a dominance of octahedra (30 %), macles (20 %), and moderately resorbed morphologies (transitional between octahedra and dodecahedra; 18 %). The remaining 32 % are made up of 10 % rounded dodecahedra, 4 % cubo-octahedra, 2 % pseudo-hemimorphic and 16 % aggregated crystals. Overall, diamonds have FTIR nitrogen contents that range from below the limit of detection (?15 at.ppm) to 1217 at.ppm with variable nitrogen aggregation states (0 to 100 %B) and ?13C values from -34.5 to -2.0 ‰. Eclogitic diamonds contain 24 to 1217 at.ppm nitrogen with a median of 513 at.ppm and their carbon isotope compositions range between -21.5 and -2.5 ‰ with a median at -4.9 ‰. Peridotitic diamonds contain up to 937 at.ppm nitrogen with a median of 129 at.ppm. The majority of peridotitic diamonds fall into a typical mantle-like ?13C range (-8.6 to -2.0 ‰), with the exception of two diamonds that display unusual strongly 13C-depleted isotope compositions (-34.5 and -14.9 ‰). The mode in ?13C values for peridotitic diamonds (in class -4.0 to -3.5 ‰) is offset by about +1 ‰ relative to eclogitic diamonds (mode in class -5.0 to -4.5 ‰). These results could reflect derivation of mantle-like carbon from different sources through time for the two main parageneses or relate to the speciation of carbon fluids with constant ?13C (e.g., a minor increase in the CO2/CH4 ratio in the diamond-forming fluid from peridotitic to eclogitic diamonds).
Diamonds and Related Materials, in press available 36p. Pdf
Africa, Ghana
deposit - Akwatia
Abstract: Magnetic mineral inclusions, as iron oxides or sulfides, occur quite rarely in natural diamonds. Nonetheless, they represent a key tool not only to unveil the conditions of formation of host diamonds, but also to get hints about the paleointensity of the geomagnetic field present at times of the Earth's history otherwise not accessible. This possibility is related to their capability to carry a remanent magnetization dependent on their magnetic history. However, comprehensive experimental studies on magnetic inclusions in diamonds have been rarely reported so far. Here we exploit X-ray diffraction, Synchrotron-based X-ray Tomographic Microscopy and Alternating Field Magnetometry to determine the crystallographic, morphological and magnetic properties of ferrimagnetic Fe-oxides entrapped in diamonds coming from Akwatia (Ghana). We exploit the methodology to estimate the natural remanence of the inclusions, associated to the Earth's magnetic field they experienced, and to get insights on the relative time of formation between host and inclusion systems. Furthermore, from the hysteresis loops and First Order Reversal Curves we determine qualitatively the anisotropy, size and domain state configuration of the magnetic grains constituting the inclusions.
Abstract: The late evolution of kimberlite magmas, i.e., during and-following emplacement into the upper crust, remains a-controversial aspect of kimberlite petrogenesis. Likewise, it is-unclear whether or not there is a link between melt composition-and the emplacement mechanism of intrusive kimberlites (i.e.,-planar dykes/sills vs. irregular intrusions in the root zone of-pipes). Resolving these issues is hampered by the absence of-comparative studies of late-magmatic kimberlite phases (e.g.,-apatite, monticellite, mica) in dykes, sills, and root-zone-intrusions from the same locality.-Here we report petrographic and mineral-chemical results-for groundmass phases in samples of dykes, sills, and root zone-intrusions from the Kimberley kimberlites (South Africa).-Early crystalised phases (e.g., olivine, spinel) in dykes/sills and-root-zone intrusions have indistingushable compositions, and-hence crystallised from similar primitive melts. Conversely,-apatite major element compositions are variable and can-discriminate dykes/sills (i.e., low and constant Sr, with-increasing Si) from root zone intrusions (high but variable Sr,-low and constant Si). The Sr depletion in root zone apatite is-interpreted to represent fractional crystallisation of earlier-apatite, perovskite, and calcite from a broadly similar parental-melt. Silica enrichment of apatite from dykes/sills may be-attributed to the coupled incorporation of CO32- and Si into the-apatite structure, reflecting higher CO2 contents in the melts-that formed dykes/sills. CO2 enrichment in the dykes/sills is-consistent with petrographic obervations. Dykes/sills are-enriched in carbonates, may contain dolomite, and are depleted-in mica and monticellite compared to the groundmass of rootzone-kimberlites. This suggests the melts parental to-dykes/sills have a higher CO2/H2O ratio compared to those-parental to root zone intrusions. These two distinct melt-evolution paths cannot be due to crustal contamination before-emplacement because the Sr-isotope compositions of latecrystallised-carbonates are indistinguishable in dykes/sills and-root-zone intrusions. We speculate that CO2 is better retained-in dykes/sills due to a higher confining pressure (i.e., lack of-breakthrough to the surface).-
Abstract: We present geochemical and isotopic (Nd, Sr) data for a picrite lava suite from the Luenha River and adjacent areas in Mozambique. The Luenha picrites represent a previously unknown type of picrites related to the Karoo large igneous province (LIP) and are distinguished by their notably low TiO2 contents (0.3-1.0?wt%) and coupling of high Nb/Y with low Zr/Y and Sm/Yb. Relatively high CaO and low Zn/Fe point to a peridotitic mantle source. Contamination-sensitive incompatible element ratios show that one lava flow is likely to be uncontaminated by the crust and its composition suggests a mantle source with primitive mantle-like incompatible element ratios and mildly depleted isotopic ratios (initial 87Sr/86Sr?=?0.7041 and ?Nd?=?+1.4 at 180?Ma). The primary melts of the Luenha picrites had MgO contents in the range of 13-21?wt%. Our preferred estimate for a primary melt composition (MgO?=?18?wt%) resembles experimental melts of fertile mantle peridotite at 3-4?GPa and indicates liquidus temperature of 1445-1582?°C. Geochemical similarities suggest the Luenha picrites were generated from the same overall primitive mantle-like reservoir that produced the main volume of Karoo flood basalts in the Karoo, Kalahari, and Zambezi basins, whereas the previously identified enriched and depleted (upper) mantle sources of Karoo picrite suites (Mwenezi, Antarctica) were subordinate sources for flood basalts. We propose that the Luenha picrites record melting of a hot, chemically primitive mantle plume source that may have been rooted in the sub-African large low shear velocity province boundary and that such a source might have been the most significant magma source in the Karoo LIP.
Abstract: In less than a decade, Mozambique has become the world’s most productive source for gem-quality ruby. Since the discovery in 2009, GIA has followed these deposits from the front lines, collecting data in the field and in the laboratory. The development of the deposit in Montepuez has been extremely interesting, with different players involved and different types of material unearthed. This article provides a summary and overview of the current knowledge about Mozambican ruby, including the history of mining and the market impact, as well as a comprehensive gemological characterization and discussion of the most common treatments applied to the stones. Much of the information in this article is based on the authors’ observations in the field and market as well as several publications (Pardieu et al., 2009, 2013, 2015; Saeseaw et al., 2018).
Abstract: Jebel Dumbier is the first-identified carbonatite-bearing alkaline complex in Sudan. It is located on the northeastern margin of the Nuba Mountains in the south part of Sudan. The complex exposed as small elliptical hills with outcrops of around 8 km2. It is composed of dominant orthoclasite and ditroite and subdominant carbonatite and fluorite dykes. The fluorite dykes are mined and together with the carbonatite dykes are controlled by a NNE strike-slip fault system. Orthoclasite is the dominant rock type, comprising of orthoclase, kalsilite, few interstitial biotite and calcium carbonate and accesserary minerals of fluorite, apatite and zircon. Ditroite consists of perthite, aegirine-augite, nepheline, sodalite, and minor annite-phlogopite and richterite, with common accessories of fluorite, titanite, apatite and zircon. Zircon U-Pb dating reveals that both orthoclasite and ditroite emplaced at around 600 Ma. Relative to orthoclasites, ditroites display higher FeOtotal and MgO and lower Al2O3 contents, contain higher volatiles (F, Cl, Br, S), and are more depleted in LILEs (Rb, Sr, Ba) and enriched in HFSEs (Nb, Ta, Zr, Hf, Th, U) and REEs. Isotopic data imply that the ditoite, orthoclasite, fluorite and carbonatite dyke originated from a common source of depleted mantle affinities, with identical low initial 87Sr/86Sr ratios (0.7033-0.7037) and high ?Nd (t) values (1.6-2.7). The carbonatites display ?13C(V-PDB) of -5.8 to -6.7‰ and ?18O(SMOW) of 9.1 to 11.3‰, typical of primary igneous carbonatite worldwide. We propose that the orthoclasite, ditroite, carbonatite, and fluorite association in Jebel Dumbier is product of variable degrees of fractional crystallization of mantlederived volatile-rich magma. Magma immiscibility among silicates, carbonates and fluorates may proceed. The Jebel Dumbier alkaline-carbonatite complex represents the postorogenic alkaline magmatism during the end evolution of Pan-African orogen (650-550 Ma) at Arabian-Nubian Shield.
Geochemistry, Geophysics, Geosystems, Vol. 20, 7, pp. 3311-3327.
Africa, South Africa
geophysics
Abstract: We conduct a joint inversion of teleseismic receiver functions and Rayleigh wave phase velocity dispersion from both ambient noise and earthquakes using data from 79 seismic stations in southern Africa, which is home to some of the world's oldest cratons and orogenic belts. The area has experienced two of the largest igneous activities in the world (the Okavango dyke swarm and Bushveld mafic intrusion) and thus is an ideal locale for investigating continental formation and evolution. The resulting 3?D shear wave velocities for the depth range of 0-100 km and crustal thickness measurements show a clear spatial correspondence with known geological features observed on the surface. Higher than normal mantle velocities found beneath the southern part of the Kaapvaal craton are consistent with the basalt removal model for the formation of cratonic lithosphere. In contrast, the Bushveld complex situated within the northern part of the craton is characterized by a thicker crust and higher crustal Vp/Vs but lower mantle velocities, which are indicative of crustal underplating of mafic materials and lithospheric refertilization by the world's largest layered mafic igneous intrusion. The thickened crust and relatively low elevation observed in the Limpopo belt, which is a late Archean collisional zone between the Kaapvaal and Zimbabwe cratons, can be explained by eclogitization of the basaltic lower crust. The study also finds evidence for the presence of a stalled segment of oceanic lithosphere beneath the southern margin of the Proterozoic Namaqua?Natal mobile belt.
Abstract: A new Massadou kimberlite field, was discovered in southeastern Guinea, near the town of Macenta. It consists of 16 poorly diamondiferous kimberlite dikes, ~1 m thick on average. The ore-controlling zone has a width of around 600 m, its orientation corresponds to the K-4 trend after S. Haggerty, and it is quite well detectable in satellite images. A thick laterite weathering profile has developed on the kimberlites. The main indicator minerals are pyrope, chromite, and ilmenite. Ilmenite grains have a zoned structure with a high-Fe core (hemoilmenite) overgrown by a parallel-columnar aggregate of Mg-ilmente rim resulting from interaction of the core phase with kimberlitic melt. The age of kimberlites is estimated as 140-145 Ma by analogy with those in adjacent areas. Dikes occur as an independent form of kimberlite magmatism in the Guinean-Liberian shield, rather than being roots of kimberlite pipes; therefore, the erosion cutout is relatively small and large-scale diamond placers should not be expected.
Abstract: In the Archaean, global surface heat flow was substantially higher than today because of greater internal radiogenic heat production and primordial heat content within the Earth. Nonetheless, the lithospheric roots of Archaean cratons were apparently surprisingly cool, recording similarly low ambient temperatures to those inferred today, allowing e.g. for the stabilisation of diamond. This finding is seemingly in conflict with a generally ‘hotter’ Archaean mantle, as is widely postulated, but the paradox could be explained if the sub-cratonic lithospheric mantle was substantially thicker in the Archaean than today. Here, we report a re-investigation of the thermal structure of the Archaean Kalahari lithosphere using published and unpublished petrological data of diamond inclusions indicated to be of Archaean age. Our thermobarometric calculations agree with earlier findings that the Archaean cratonic mantle root was surprisingly cool. Importantly, the shape of the inclusion-derived P-T array deviates from the modern geotherm recorded by peridotite xenoliths. Specifically, diamond inclusions define a systematically steeper geothermal gradient than is observed in cratonic xenoliths. We find that Archaean diamond inclusion and modern xenolith P-T data cannot be reconciled by a single steady-state geotherm. The P-T conditions recorded in diamond inclusions are incompatible with the current characteristically low present-day heat-production of the overlying crust. Instead, the steeper geotherm implies high heat production in the crust during diamond formation and the distinctive geothermal gradient recorded in the studied diamond inclusions could reflect ancient mantle conditions. We modelled a suite of ‘fossil’ geotherms, with increased radiogenic heat production within the crust during the Archaean. Solutions providing very good fits with the diamond inclusion geotherm all require that the Archaean lithosphere must have extended to far greater depths than is preserved today. The required depth ranges from ~ 300 km to ~ 450 km depth, for a modern (~ 1350°C) and a significantly hotter (~ 1600°C) mantle potential temperature, respectively. In either case, it is clear that the Kalahari lithosphere must have experienced significant (at least 100 km) basal erosion subsequent to its formation.
The Journal of the Southern African Institute of Mining and Metallurgy, Vol. 119, pp. 97-103.
Africa, South Africa
deposit - Finsch
Abstract: Accurate delineation of the contact between a kimberlite pipe and country rock at production level depths is a challenge due to limited geological data. Geological information is obtained from widely spaced diamond core boreholes which are drilled either from surface or from higher mining levels within the pipe. Kimberlite pipe/country rock contacts are notoriously irregular and variable, further reducing the confidence in contact positions defined by the drill-holes. At Finsch Diamond Mine (FDM), the opportunity arose to further improve the confidence in the contact positions relative to the planned slot (end) positions of each sublevel cave tunnel during the development stage of these tunnels. As a result, the accuracy of the 3D geological model has improved.
The use of diamond drill core for this purpose is expensive due to site establishment requirements. The lengthy time taken during site establishment also delays the development of tunnels and support cycles, thereby extending the completion dates. FDM has reduced delays during development by adopting percussion drilling, in conjunction with gamma ray logging. The S36 drill rig is mounted on a moveable platform and does not require a costly and lengthy site establishment. The holes are generally drilled (0°/flat) on grade elevation, and these holes could also be drilled from the rim tunnels (developed in waste) into the kimberlite pipe. A single-boom production drill rig is normally used to drill holes about 20 m in length. On completion of the contact delineation drilling, gamma logging of the holes is conducted using the GeoVista geophysical sonde (or probe) to log the natural gamma signature of the dolomite/ kimberlite contact. The advantage of this tool is that the readings are continuous within centimetre intervals, and due to contrasting characteristics between kimberlite (rich in clay minerals) and dolomite, the contact position can be determined accurately. The better definition of contact positions also adds value to tunnel stopping distance in terms of developing the tunnel's slot at the optimum distance from the contact (easier blasting of longhole rings, avoidance of contact overbreak and premature waste ingress, and other matters relating to extraction of ore from these tunnels). This method is highly successful and has reduced development costs (on-time completion), improved definition of the pipe's contact position for geological modelling, improved blast design, and mitigated early waste ingress by maintaining the contact's integrity.
Abstract: The recent discovery of alluvial diamonds in Marange, Zimbabwe, has rekindled the interest of environmental scholars in critiquing the political economy of artisanal mining. The increasing recurrence of this 'illegal' small-scale mining has partly been attributed to its 'lucrative' nature, but more importantly as a safety net to the deepening crises rooted in the country's adverse economic climate in the period under review. The economic structural adjustments during the 1990s, the hefty off-budget gratuities awarded to restive war veterans in 1997, the country's ill-fated intervention in the DRC war in 1988 and the violent land seizures of the early 2000s have contributed to this prolonged setback. This paper first assesses the sustainability of artisanal mining as a livelihood option mostly for the unemployed. It appears that diamond mining produced positive outcomes for some, but by no means all, artisanal miners who accumulated considerable wealth in cattle and real estate. Others failed to break through altogether, suffering heavy losses, including deaths under mining pits. The paper then explores the effects of artisanal mining on the physical environment, including river denudation and soil erosion, deforestation, creation of wastelands and pollution of water bodies. The overriding argument of this study is that artisanal mining has continued to be a sustainable livelihood avenue in spite of its well-known negative impacts. The study is based on semi-structured interviews conducted between 2015 and 2017 with artisanal miners, security personnel, rural district councillors, environmental authorities and former employees of defunct mining firms in Marange. Other sources of data included community-based organisations and civil society groups, as well as newspapers that reported on the unfolding events in Marange at the time.
Abstract: Goldschmidtite is a new perovskite-group mineral (IMA No. 2018-034) with the ideal formula (K,REE,Sr)(Nb,Cr)O3. A single grain of goldschmidtite with a maximum dimension of ?100 ?m was found as an inclusion in a diamond from the Koffiefontein pipe in South Africa. In addition to the dark green and opaque goldschmidtite, the diamond contained a Cr-rich augite (websteritic paragenesis) and an intergrowth of chromite, Mg-silicate, and unidentified K-Sr-REE-Nb-oxide. Geothermobarometry of the augite indicates that the depth of formation was ?170 km. The chemical composition of gold-schmidtite determined by electron microprobe analysis (n = 11, WDS, wt%) is: Nb2O5 44.82, TiO2 0.44, ThO2 0.10, Al2O3 0.35, Cr2O3 7.07, La2O3 11.85, Ce2O3 6.18, Fe2O3 1.96, MgO 0.70, CaO 0.04, SrO 6.67, BaO 6.82, K2O 11.53, total 98.53. The empirical formula (expressed to two decimal places) is (K0.50La0.15Sr0.13Ba0.09Ce0.08)?0.95(Nb0.70Cr0.19Fe0.05Al0.01Mg0.04Ti0.01)?1.00O3. Goldschmidtite is cubic, space group Pm3m, with unit-cell parameters: a = 3.9876(1) Å, V = 63.404(6) Å3, Z = 1, resulting in a calculated density of 5.32(3) g/cm3. Goldschmidtite is the K-analog of isolueshite, (Na,La)NbO3. Raman spectra of goldschmidtite exhibit many second-order broad bands at 100 to 700 cm-1 as well as a pronounced peak at 815 cm-1, which is possibly a result of local ordering of Nb and Cr at the B site. The name goldschmidtite is in honor of the eminent geochemist Victor Moritz Goldschmidt (1888-1947), who formalized perovskite crystal chemistry and identified KNbO3 as a perovskite-structured compound.
The Journal of the Southern African Institute of Mining and Metallurgy, Vol. 199, pp. 613-620.
Africa, Lesotho
deposit - Liqhobong
Abstract: Liqhobong Mining Development Company (LMDC) has been experiencing problems with boulders after blasting where the fragment sizes exceed the maximum of 800 mm as per mine standard. As a result, the mine has employed various methods to improve the fragmentation. The goal is to produce a run-of-mine (ROM) feed that does not choke the crusher and cause delays in production. In order to achieve this goal, fragmentation distribution within the fines and coarse envelope must be optimized through effective planning of blasting activities and accurate execution. The mine determined the fines-coarse envelope within which the entire crushing system can handle fragments using Split Desktop software. It is expected that both the predicted and actual fragmentation curves lie within that envelope for optimal fragmentation. The Kuz-Ram model with blast design parameters of 2.6 m for burden, 2.8 m for spacing, and 127 mm hole diameter was used to predict the fragmentation. The results show that the blast design parameters may need altering to achieve optimum fragmentation. Furthermore, the execution of the drilling and blasting may be the cause of the fragmentation problems. The mean fragmentation size (X50) differs greatly, unlike the uniformity index (n)s values which are relatively close to each other (0.6 to 2.2). The mean squared error (MSE) values have a large range. A proposed solution is a modified burden, spacing, and hole diameter. It is concluded that blast design parameters need to be reviewed in order to obtain correct predictions.
Abstract: A group of geochemists suggests they have found the smoking gun in the Karoo magma province. Their new article reports the discovery of primitive picrite lavas that may provide the first direct sample of a hot mantle plume underneath southern Africa in the Jurassic period.
Abstract: Magnetic mineral inclusions, as iron oxides or sulfides, occur quite rarely in natural diamonds. Nonetheless, they represent a key tool not only to unveil the conditions of formation of host diamonds, but also to get hints about the paleointensity of the geomagnetic field present at times of the Earth's history otherwise not accessible. This possibility is related to their capability to carry a remanent magnetization dependent on their magnetic history. However, comprehensive experimental studies on magnetic inclusions in diamonds have been rarely reported so far. Here we exploit X-ray diffraction, Synchrotron-based X-ray Tomographic Microscopy and Alternating Field Magnetometry to determine the crystallographic, morphological and magnetic properties of ferrimagnetic Fe-oxides entrapped in diamonds coming from Akwatia (Ghana). We exploit the methodology to estimate the natural remanence of the inclusions, associated to the Earth's magnetic field they experienced, and to get insights on the relative time of formation between host and inclusion systems. Furthermore, from the hysteresis loops and First Order Reversal Curves we determine qualitatively the anisotropy, size and domain state configuration of the magnetic grains constituting the inclusions.
Geochemistry International, Vol. 57, 9, pp. 1015-1023.
South America, Brazil, Africa, Central African Republic, Russia
carbonado
Abstract: Structural peculiarities of several types of cryptocrystalline diamond varieties: carbonado, impact-related yakutite and cryptocrystalline diamond aggregates from kimberlite were studied using Infrared spectroscopy, X-ray diffraction contrast (DCT—Diffraction Contrast Tomography) and phase contrast tomography (PCT). It is shown that the porosity of the carbonado and kimberlitic cryptocrystalline aggregates is similar being in range of 5-10 vol %, possibly indicating similar formation mechanism(s), whereas that of yakutite is essentially zero. Crystallographic texture is observed for some carbonado samples. It is suggested that at least partially the texture is explained by deformation-related bands. Infrared spectroscopy reveals presence of hydrous and, probably, of hydrocarbon species in carbonado.
Abstract: We present geochemical and isotopic (Nd, Sr) data for a picrite lava suite from the Luenha River and adjacent areas in Mozambique. The Luenha picrites represent a previously unknown type of picrites related to the Karoo large igneous province (LIP) and are distinguished by their notably low TiO2 contents (0.3-1.0?wt%) and coupling of high Nb/Y with low Zr/Y and Sm/Yb. Relatively high CaO and low Zn/Fe point to a peridotitic mantle source. Contamination-sensitive incompatible element ratios show that one lava flow is likely to be uncontaminated by the crust and its composition suggests a mantle source with primitive mantle-like incompatible element ratios and mildly depleted isotopic ratios (initial 87Sr/86Sr?=?0.7041 and ?Nd?=?+1.4 at 180?Ma). The primary melts of the Luenha picrites had MgO contents in the range of 13-21?wt%. Our preferred estimate for a primary melt composition (MgO?=?18?wt%) resembles experimental melts of fertile mantle peridotite at 3-4?GPa and indicates liquidus temperature of 1445-1582?°C. Geochemical similarities suggest the Luenha picrites were generated from the same overall primitive mantle-like reservoir that produced the main volume of Karoo flood basalts in the Karoo, Kalahari, and Zambezi basins, whereas the previously identified enriched and depleted (upper) mantle sources of Karoo picrite suites (Mwenezi, Antarctica) were subordinate sources for flood basalts. We propose that the Luenha picrites record melting of a hot, chemically primitive mantle plume source that may have been rooted in the sub-African large low shear velocity province boundary and that such a source might have been the most significant magma source in the Karoo LIP.
Geochimica et Cosmochimica Acta, Vol. 266, pp. 307-311.
United States, California, Africa, Morocco
metasomatism
Abstract: We investigate the potential to use concentrations and zoning patterns of phosphorus (P) in clinopyroxene as indicators of the rates of igneous and metasomatic processes, comparable to recent applications of P in olivine but applicable to more evolved rocks and lower temperatures of crystallization. Few high-P pyroxenes have been previously reported, and none have been analyzed in detail for the mechanism of P enrichment or the implications for mineral growth kinetics. Here, we report the discovery and characteristics of exotic phosphorus-rich secondary clinopyroxene in glassy pockets and veins in composite mantle xenoliths from the Cima Volcanic Field (California, USA) and the Middle Atlas Mountains (Morocco, West Africa). These glass-bearing xenoliths preserve evidence of melt infiltration events and the contrasting behavior of P in their pyroxene crystals constrains the different rates of reaction and extents of equilibration that characterized infiltration in each setting. We report optical petrography and chemical analysis of glasses and minerals for major elements by electron microprobe microanalyzer and trace elements by laser-ablation Inductively Coupled Plasma Mass Spectrometry. The Cima Volcanic Field specimen shows one end-member behavior, with unzoned P-rich clinopyroxene in a melt pocket. We attribute this occurrence to a slow crystallization process that occurred after the melt temperature reached near-equilibrium with the host rock and during which the P concentration in the melt was buffered by apatite saturation. In the Morocco xenolith, by contrast, clinopyroxene exhibits zonation with P increasing all the way to the rim, in contact with the glass. We ascribe this feature to a rapid growth process in which excess P was incorporated into the growing clinopyroxene from a diffusive boundary layer. We demonstrate quantitative agreement between the enrichment of P and other trace elements and their expected diffusion and partitioning behavior during rapid growth. We suggest that P has not been widely reported in clinopyroxene in large part because it has rarely been looked for and that its analysis offers considerable promise as a kinetic indicator both in xenoliths and volcanic rocks.
Abstract: Recent exploration work in South Morocco revealed the occurrence of several carbonatite bodies, including the Paleoproterozoic Gleibat Lafhouda magnesiocarbonatite and its associated iron oxide mineralization, recognized here as iron-oxide-apatite (IOA) deposit type. The Gleibat Lafhouda intrusion is hosted by Archean gneiss and schist and not visibly associated with alkaline rocks. Metasomatized micaceous rocks occur locally at the margins of the carbonatite outcrop and were identified as glimmerite fenite type. Rare earth element (REE) and Nb mineralization is mainly linked to the associated IOA mineralization and is represented by monazite-(Ce) and columbite-(Fe) as major ore minerals. The IOA mineralization mainly consists of magnetite and hematite that usually contain large apatite crystals, quartz and some dolomite. Monazite-(Ce) is closely associated with fluorapatite and occurs as inclusions within the altered parts of apatite and along cracks or as separate phases near apatite. Monazite shows no zonation patterns and very low Th contents (<0.4?wt%), which would be beneficial for commercial extraction of the REE and which indicates monazite formation from apatite as a result of hydrothermal volatile-rich fluids. Similar monazite-apatite mineralization and chemistry also occurs at depth within the carbonatite, although the outcropping carbonatite is barren, suggesting an irregular REE ore distribution within the carbonatite body. The barren carbonatite contains some tiny unidentified secondary Nb-Ta-U phases, synchysite and monazite. Niobium mineralization is commonly represented by anhedral minerals of columbite-(Fe) which occur closely associated with magnetite-hematite and host up to 78?wt% Nb2O5, 7?wt% Ta2O5 and 1.6?wt% Sc2O3. This association may suggest that columbite-(Fe) precipitated by an interaction of Nb-rich fluids with pre-existing Fe-rich minerals or as pseudomorphs after pre-existing Nb minerals like pyrochlore. Our results most strongly suggest that the studied mineralization is economically important and warrants both, further research and exploration with the ultimate goal of mineral extraction.
Review of African Political Economy, Vol. 45, no. 158, pp. 522-540.
Africa, Sierra Leone
history
Abstract: This article explores the relationship between the 2014-2016 Ebola outbreak and the political economy of diamond mining in Kono District, Sierra Leone. The authors argue that foreign companies have recycled colonial strategies of indirect rule to facilitate the illicit flow of resources out of Sierra Leone. Drawing on field research conducted during the outbreak and in its aftermath, they show how this ‘indirect rule redux’ undermines democratic governance and the development of revenue-generation institutions. Finally, they consider the linkages between indirect rule and the Ebola outbreak, vis-à-vis the consequences of the region’s intentionally underdeveloped health care infrastructure and the scaffolding of outbreak containment onto the paramount chieftaincy system.
Review of African Political Economy, Vol. 46, pp. 317-335.
Africa, Russia
News item - economics
Abstract: Despite the historical legacy of the Soviet Union, the Russian Federation’s economic presence in Africa today is minuscule in comparison to that of the West or China. The aim of this Briefing is to provide a framework for the trajectory of African-Russian economic ties in the changing international environment. Although the economic, trade and investment affairs could develop more complementarity, it is still an open question whether African countries benefit from the deepening economic ties or whether these inhibit local socio-economic development.
Geochimica et Cosmochimica Acta, in press available. 13p.
Africa, Cameroon
peridotite
Abstract: Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9-91.2) and low spinel Cr# (8.4-19.3), together with moderate to high whole-rock Al2O3 contents (2.0-3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01?±?0.18?Ga (1?), and when olivine and spinel are considered, is 1.82?±?0.14?Ga (1?). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2-2.4?Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0?Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ?2.0?Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ?2.0?Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts.
Abstract: For the past two decades, Zimbabwe has experienced a pervasive economic collapse. Most of the challenges were caused by policy inconsistencies, bad policy choices, economic mismanagement and political instability. This led to deindustrialization with a sharp decline in manufacturing and agriculture productivity and output, which consequently caused a sharp increase in unemployment and poverty. Although it is not fully developed, the mining industry in Zimbabwe presents an opportunity for economic stimulation that may lead to economic recovery, but requires broad-based economic reforms. This paper presents the findings of a review, and benchmarking of Zimbabwe's policies, which affect mining investment, inclusive economic growth and human development. The policies were benchmarked and compared to similar policies of Botswana, Namibia and South Africa using the Natural Resources Benchmarking Charter Framework. The outcomes of the review and benchmarking process were taken into consideration when coming up with policy suggestions that are meant to economically transform Zimbabwe, which at the same time brings sustained human development. The work reported in this paper is part of an MSc research study in the School of Mining Engineering at the University of the Witwatersrand.
Geophysical Research Letters, Vol. 46, 16, pp. 9509-9518.
Africa, South Africa
geophysics - seismics
Abstract: P and S waves travel times from large, distant earthquakes recorded on seismic stations in Botswana and South Africa have been combined with existing data from the region to construct velocity models of the upper mantle beneath southern Africa. The models show a region of higher velocities beneath the Rehoboth Province and parts of the northern Okwa Terrane and the Magondi Belt, which can be attributed to thicker cratonic lithosphere, and a region of lower velocities beneath the Damara?Ghanzi?Chobe Belt and Okavango Rift, which can be attributed a region of thinner off?craton lithosphere. This finding suggests that the spatial extent of thick cratonic lithosphere in southern Africa is greater than previously known. In addition, within the cratonic lithosphere an area of lower velocities is imaged, revealing parts of the cratonic lithosphere that may have been modified by younger magmatic events.
Geochimica et Cosmochimica Acta, Vol. 267, pp. 179-195.
Africa, Tanzania
peridotite
Abstract: Hydrogen concentration profiles through olivine and pyroxene in peridotite xenoliths carried in rift basalts from northern Tanzania (Lashaine, Eledoi, and Kisite localities) show bell-shaped distributions, indicating that diffusive hydrogen loss has occurred in all minerals. Homogeneous major element concentrations and equilibration of hydrogen between the cores of olivine and coexisting pyroxene suggest that hydrogen loss resulted from diffusive degassing during host magma emplacement. For these samples, hydrogen diffusivities in olivine and coexisting pyroxene must be within the same order of magnitude, similar to experimentally determined diffusivities, but in contrast to previous observations made on xenolithic peridotites. We demonstrate here, for the first time using natural samples, that significant differences in activation energy is likely the primary parameter that causes the discrepancy between hydrogen diffusion in olivine and pyroxene observed in different suites of mantle xenoliths. Because hydrogen diffuses faster in olivine than in pyroxene as temperature increases, hydrogen loss in the Tanzanian mantle xenoliths must have occurred at relatively low temperatures (?750 - ?900 °C), whereas hydrogen loss observed in previous xenolith studies likely occurred at higher temperatures (?950 to > 1200 °C). Thus, the diffusive loss of hydrogen in the Tanzanian mantle xenoliths may have occurred at shallow depths or at the Earth’s surface.
Journal of Southern African Studies, Vol. 45, 5, pp. 805-820.
Africa, South Africa
economics
Abstract: Existing studies on mining in Angola are mostly concerned with its social and military underpinnings and tend to analyse the diamond sector as empirically distinct from the state. In addition, little attention has been paid to how they are bound together and what these interconnections mean for the nature of politics in Angola. This gap in the literature is significant because diamond companies produce far more than revenue and profits: for some 100?years, the diamond sector has governed, policed, defended and controlled the strategic, diamond-rich provinces of Lunda Sul and Lunda Norte. In order to fill this lacuna, this article offers a case study on the role of the diamond industry for the state in Angola from the creation of the first diamond company in 1917 to the end of the Angolan civil war in 2002. Drawing on a wide range of untapped official documents and on interviews, it argues that the diamond sector has functioned historically as the conduit through which the state projects its power and secures its interests in strategic but hostile territory.
Contributions to Mineralogy and Petrology, Vol. 174, 28p. Pdf
Africa, Ghana
deposit - Akwatia
Abstract: Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al??300 µg/g Ca or?>?60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m2 conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt?=?1115-1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P-T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the refertilisation processes that have affected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting first. Trivalent cations appear to behave differently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na-M3+ exchange. In contrast, Al partitioning in harzburgites is not significantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al-Al exchange.
Abstract: Southern Africa is characterised by unusually elevated topography and abnormal heat flow. This can be explained by thermal perturbation of the mantle, but the origin of this is unclear. Geophysics has not detected a thermal anomaly in the upper mantle and there is no geochemical evidence of an asthenosphere mantle contribution to the Cenozoic volcanic record of the region. Here we show that natural CO2 seeps along the Ntlakwe-Bongwan fault within KwaZulu-Natal, South Africa, have C-He isotope systematics that support an origin from degassing mantle melts. Neon isotopes indicate that the melts originate from a deep mantle source that is similar to the mantle plume beneath Réunion, rather than the convecting upper mantle or sub-continental lithosphere. This confirms the existence of the Quathlamba mantle plume and importantly provides the first evidence in support of upwelling deep mantle beneath Southern Africa, helping to explain the regions elevation and abnormal heat flow.
Geostandards and Geoanalltical Research, Vol. 43, 3, pp. 543-565.
China, Europe, Sweden, Asia, Mongolia, United States, Africa, Malawi, Madagascar
REE
Abstract: Bastnäsite is the end member of a large group of carbonate-fluoride minerals with the common formula (REE) CO3F•CaCO3. This group is generally widespread and, despite never occurring in large quantities, represents the major economic light rare earth element (LREE) mineral in deposits related to carbonatite and alkaline intrusions. Since bastnäsite is easily altered and commonly contains inclusions of earlier?crystallised minerals, in situ analysis is considered the most suitable method to measure its U?Th?Pb and Sr?Nd isotopic compositions. Electron probe microanalysis and laser ablation (multi?collector) inductively coupled plasma?mass spectrometry of forty?six bastnäsite samples from LREE deposits in China, Pakistan, Sweden, Mongolia, USA, Malawi and Madagascar indicate that this mineral typically has high Th and LREE and moderate U and Sr contents. Analysis of an in?house bastnäsite reference material (K?9) demonstrated that precise and accurate U?Th?Pb ages could be obtained after common Pb correction. Moreover, the Th?Pb age with its high precision is preferable to the U?Pb age because most bastnäsites have relatively high Th rather than U contents. These results will have significant implications for understanding the genesis of endogenous ore deposits and formation processes related to metallogenic geochronology research.
2019 Twelth International Conference Oct 1-3. Moscow, IEEE DOI 11.09/MLSD .2019.8911014
Africa, Angola, Russia, Yakutia
geophysics
Abstract: We show how to increase the effectiveness of the prognoses of kimberlite bodies by using airborne geophysical technologies. We show the advantages of electromagnetic and magnetic methods for predicting kimberlite pipes. You will see examples of a regional diamond survey in Angola and Siberia.
Abstract: Numerous mafic dykes, sills and laccoliths crop out in the southern part of the Tafilalt basin (Eastern Anti-Atlas, Morocco). These rocks intrude the mildly folded Ordovician to Early Carboniferous formations, consisting mainly of lamprophyric dolerites and camptonites with minor gabbros and syenodiorites. Previous geochemical studies have shown that the Tafilalt magmatism of sodic-alkaline affinity has been produced by low degrees of partial melting from an enriched deep mantle source within the garnet stability field. However, the age and the geodynamic context of these rocks were presently unknown since no isotopic dating had so far been made of the Tafilalt dolerites. To resolve this issue, we present here the first 40Ar/39Ar biotite and U-Pb zircon dating from the Tafilalt alkaline magmatism. Three samples (biotite separates) yielded well-defined 40Ar/39Ar plateau ages of 264.2?±?2.7 Ma, 259.0?±?6.3 Ma and 262.6?±?4.5 Ma whereas 206Pb/238U dating of zircon from one of these samples yielded an age of 255?±?3 Ma. These ages coincide within the dating error, and indicate that this magmatism occurred in the late Permian. Considering geochronological and geochemical data, we propose that the Tafilalt magmatism reflects an early-rift magmatic activity that preceded the Triassic rifting heralded by the Central Atlantic Magmatic Province. This magmatic activity is recorded in both sides of the future Atlantic Ocean by small-volume alkaline magmatism that started in the late Permian and extends into the Triassic. The alkaline magmas are probably generated in response to an increase in the mantle potential temperature underneath the Pangea supercontinent.
Journal of Geochemical Exploration, Vol. 208, 106394
Africa, Democratic Republic of Congo
alluvials
Abstract: The Democratic Republic of the Congo (DRC) has a long history of mining activities. However, environmental and socio-economic problems associated with abandoned mines as well as activities including Artisanal and Small-Scale Mining (ASM) remain very concerning issues throughout the country. In this paper, we review abandoned mines, Artisanal and Small-Scale Mining (ASM), environmental and human impact assessment, conflict-minerals, and perspectives for future researches on mining activities in DRC, by selecting 26 significant and relevant peer-reviewed papers. With the recent mining boom in DRC, the issue of social and environmental impacts of abandoned mines and ASM has become a matter of concern for Congolese people. This actual development of several mining projects has driven the development of regulation, monitoring and impact assessment, and scientific publications of these activities. However, the findings from multiple studies regarding the impacts of abandoned mines and artisanal and small-scale mining on the environment and human health across the country has not yet been sufficiently brought together in a reliable way. Focusing on health, social and environmental issues, this mini-review aims to increase awareness of negative impacts of mining operations and promote further collaborative between citizens, mining companies, government institutions and other stakeholders with the hope to reduce negative impacts associated with minerals and metals mining and to extend the benefits to the surrounding Congolese communities. The overarching objective of this paper is to characterize the existing studies in the field of the impact assessment of abandoned mines and ASM on public health and environment with a view to stimulate research, guide development of policies and implementation of appropriate interventions. This report draws from material published and available in the public domain over the past years. It also explores the response of the state and the mining companies to these issues, exploring responses such as regulatory changes and implementations of existing legislation. According to the results, additional studies that examine the relationship between minerals mining, bioremediation processes, training and dialogue aspects and the associated adverse environmental and health outcomes are needed to more adequately characterize the impact of mining activities on health and environment, in order to increase formalization of artisanal and small-scale mining sector while strengthening their social and environmental responsibility, and promoting sustainable mining practice standards that improve people lives and respect the environment in DRC.
Contributions to Mineralogy and Petrology, Vol. 174, (12) doi: 10.1007/s00410-019-1634-y
Africa, Ghana
deposit - Akwatia
Abstract: Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al??300 µg/g Ca or?>?60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m2 conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt?=?1115-1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P-T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the refertilisation processes that have affected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting first. Trivalent cations appear to behave differently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na-M3+ exchange. In contrast, Al partitioning in harzburgites is not significantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al-Al exchange.
Abstract: Sulfur-bearing monazite-(Ce) occurs in silicified carbonatite at Eureka, Namibia, forming rims up to ~0.5 mm thick on earlier-formed monazite-(Ce) megacrysts. We present X-ray photoelectron spectroscopy data demonstrating that sulfur is accommodated predominantly in monazite-(Ce) as sulfate, via a clino-anhydrite-type coupled substitution mechanism. Minor sulfide and sulfite peaks in the X-ray photoelectron spectra, however, also indicate that more complex substitution mechanisms incorporating S2 and S4+ are possible. Incorporation of S6+ through clino-anhydrite-type substitution results in an excess of M2+ cations, which previous workers have suggested is accommodated by auxiliary substitution of OH for O2. However, Raman data show no indication of OH, and instead we suggest charge imbalance is accommodated through F substituting for O2. The accommodation of S in the monazite-(Ce) results in considerable structural distortion that may account for relatively high contents of ions with radii beyond those normally found in monazite-(Ce), such as the heavy rare earth elements, Mo, Zr and V. In contrast to S-bearing monazite-(Ce) in other carbonatites, S-bearing monazite-(Ce) at Eureka formed via a dissolutionprecipitation mechanism during prolonged weathering, with S derived from an aeolian source. While large S-bearing monazite-(Ce) grains are likely to be rare in the geological record, formation of secondary S-bearing monazite-(Ce) in these conditions may be a feasible mineral for dating palaeo-weathering horizons.
Abstract: Eclogite mantle xenoliths from various kimberlite occurrences on the Kaapvaal craton show evidence for depth- and redox-dependent metasomatic events that led to variable base metal sulphide and incompatible element enrichments. Eclogite xenoliths from the Roberts Victor, Jagersfontein, Kimberley (Kamfersdam) and Premier kimberlites were investigated for their silicate and base metal sulphide geochemistry, stable oxygen isotope compositions and oxybarometry. The variably metasomatised eclogites had basaltic, picritic and gabbroic protolith compositions and have garnet ?18O values that range from +3.3 to +7.9‰, which, when coupled with the trace element characteristics, indicate oceanic lithosphere protoliths that had undergone variable degrees of seawater alteration. The deepest equilibrated eclogites (175220?km depth) from near the base of the Kaapvaal craton lithosphere are the most refractory and feature significant light rare earth element (LREE) depletions. They show the most oxidised redox compositions with ?logƒO2 values of FMQ-3.9 to FMQ-1.5. Subtle metasomatic overprinting of these eclogites resulted in base metal sulphide formation with relatively depleted and highly fractionated HSE compositions. These deepest eclogites and their included base metal sulphides suggest interaction with relatively oxidised melts or fluids, which, based on their HSE characteristics, could be related to precursor kimberlite metasomatism that was widespread within the Kaapvaal craton mantle lithosphere. In contrast, eclogites that reside at shallower, “mid-lithospheric” depths (140180?km) have been enriched in LREE and secondary diopside/phlogopite. Importantly, they host abundant metasomatic base metal sulphides, which have higher HSE contents than those in the deeper eclogites at the lithosphere base. The mid-lithospheric eclogites have more reducing redox compositions (?logfO2?=?FMQ-5.3 ? FMQ-3.3) than the eclogites from the lowermost Kaapvaal lithosphere. The compositional overprint of the shallower mantle eclogites resembles basaltic rather than kimberlitic/carbonatitic metasomatism, which is also supported by their relatively reducing redox state. Base metal sulphides from the mid-lithospheric eclogites have HSE abundances and distributions that are similar to Karoo flood basalts from southern Africa, suggesting a link between the identified shallow mantle metasomatism of the Kaapvaal cratonic lithosphere and the Karoo large igneous event during the Mesozoic. The sulphide-hosted platinum group element abundances of the mid-lithospheric eclogites are higher compared with their analogues from the deeper lithospheric eclogites, which in combination with their contrasting oxidation states, may imply redox-controlled HSE mobility during sulphur-rich metasomatism of continental mantle lithosphere.
Abstract: The most dominant features in the present-day lower mantle are the two antipodal African and Pacific large low-shear-velocity provinces (LLSVPs). How and when these two structures formed, and whether they are fixed and long lived through Earth history or dynamic and linked to the supercontinent cycles, remain first-order geodynamic questions. Hotspots and large igneous provinces (LIPs) are mostly generated above LLSVPs, and it is widely accepted that the African LLSVP existed by at least ca. 200 Ma beneath the supercontinent Pangea. Whereas the continental LIP record has been used to decipher the spatial and temporal variations of plume activity under the continents, plume records of the oceanic realm before ca. 170 Ma are mostly missing due to oceanic subduction. Here, we present the first compilation of an Oceanic Large Igneous Provinces database (O-LIPdb), which represents the preserved oceanic LIP and oceanic island basalt occurrences preserved in ophiolites. Using this database, we are able to reconstruct and compare the record of mantle plume activity in both the continental and oceanic realms for the past 2 b.y., spanning three supercontinent cycles. Time-series analysis reveals hints of similar cyclicity of the plume activity in the continent and oceanic realms, both exhibiting a periodicity of ?500 m.y. that is comparable to the supercontinent cycle, albeit with a slight phase delay. Our results argue for dynamic LLSVPs where the supercontinent cycle and global subduction geometry control the formation and locations of the plumes.
Africa, Madagascar, Zambia, Asia, Sri Lanka, South America, Colombia
emerald
Abstract: The great challenge of geographic origin determination is to connect the properties and features of individual gems to the geology of their deposits. Similar geologic environments can produce gems with similar gemological properties, making it difficult to find unique identifiers. Over the last two decades, our knowledge of corundum and emerald deposit formation has improved significantly. The mineral deposits are classically separated into primary and secondary deposits. Primary corundum deposits are subdivided into two types based on their geological environment of formation: (1) magmatic and (2) metamorphic. Magmatic deposits include gem corundum in alkali basalts as in eastern Australia, and sapphire in lamprophyre and syenite as in Montana (United States) and Garba Tula (Kenya), respectively. Metamorphic deposits are divided into two subtypes (1) metamorphic deposits sensu stricto (in marble; mafic and ultramafic rocks, or M-UMR), and (2) metamorphic-metasomatic deposits characterized by high fluid-rock interaction and metasomatism (i.e., plumasite or desilicated pegmatites in M-UMR and marble, skarn deposits, and shear zonerelated deposits in different substrata, mainly corundum-bearing Mg-Cr-biotite schist). Examples of the first subtype include the ruby deposits in marble from the Mogok Stone Tract or those in M-UMR from Montepuez (Mozambique) and Aappaluttoq (Greenland). The second subtype concerns the sapphire from Kashmir hosted by plumasites in M-UMR. Secondary corundum deposits (i.e., present-day placers) result from the erosion of primary corundum deposits. Here, corundum is found in the following types of deposits: eluvial (derived by in situ weathering or weathering plus gravitational movement), diluvial (scree or talus), colluvial (deposited at the base of slopes by rainwash, sheetwash, slow continuous downslope creep, or a combination of these processes), and alluvial (deposited by rivers). Today, most sapphires are produced from gem placers related to alkali basalts, as in eastern Australia or southern Vietnam, while placers in metamorphic environments, such as in Sri Lanka (Ratnapura, Elahera) and Madagascar (Ilakaka), produce the highest-quality sapphires. The colluvial Montepuez deposit in Mozambique provides a huge and stable supply of clean and very high-quality rubies. Primary emerald deposits are subdivided into two types based on their geological environment of formation: (1) tectonic-magmatic-related (Type I) and (2) tectonic-metamorphic-related (Type II). Several subtypes are defined and especially Type IA, hosted in M-UMR, which accounts for about 70% of worldwide production (Brazil, Zambia, Russia, and others). It is characterized by the intrusion of pegmatites or quartz veins in M-UMR accompanied by huge hydrothermal fluid circulation and metasomatism with the formation of emerald-bearing desilicated pegmatite (plumasite) and biotite schist. Type IB in sedimentary rocks (China, Canada, Norway, Kazakhstan, and Australia) and Type IC in granitic rocks (Nigeria) are of minor importance. The subtype Type IIA of metamorphic deposits is related to hydrothermal fluid circulation at high temperature, in thrust fault and/or shear zones within M-UMR of volcano-sedimentary series, such as at the Santa Terezinha de Goiás deposit in Brazil. The subtype Type IIB is showcased by the Colombian emerald deposits located in the Lower Cretaceous black shales of the Eastern Cordillera Basin. These are related to the circulation of hydrothermal basinal fluids in black shales, at 300330°C, that dissolved evaporites in (1) thrust and tear faults for the deposits of the western emerald zone (Yacopi, Coscuez, Muzo, Peñas Blancas, Cunas, and La Pita mines) and (2) a regional evaporite level intercalated in the black shales or the deposits of the eastern emerald zone (Gachalá, Chivor, and Macanal mining districts). Secondary emerald deposits are unknown because emerald is too fragile to survive erosion and transport in rivers.
Contributions to Mineralogy and Petrology, Vol. 175, 9p. Pdf
Africa, Lesotho
olivine
Abstract: This paper highlights published and new field and petrographic observations for late-stage (crustal level) deformation associated with the emplacement of kimberlites and other mantle-derived magmas. Thus, radial and tangential joint sets in the competent 183 Ma Karoo basalt wall rocks to the 5 ha. Lemphane kimberlite blow in northern Lesotho have been ascribed to stresses linked to eruption of the kimberlite magma. Further examples of emplacement-related stresses in kimberlites are brittle fractures and close-spaced parallel shears which disrupt olivine macrocrysts. In each of these examples, there is no evidence of post-kimberlite regional tectonism which might explain these features, indicating that they reflect auto-deformation in the kimberlite during or immediately post-emplacement. On a microscopic scale, these inferred late-stage stresses are reflected by fractures and domains of undulose extinction which traverse core and margins of some euhedral and anhedral olivines in kimberlites and olivine melilitites. Undulose extinction and kink bands have also been documented in olivines in cumulates from layered igneous intrusions. Our observations thus indicate that these deformation features can form at shallow levels (crustal pressures), which is supported by experimental evidence. Undulose extinction and kink bands have previously been presented as conclusive evidence for a mantle provenance of the olivines—i.e. that they are xenocrysts. The observation that these deformation textures can form in both mantle and crustal environments implies that they do not provide reliable constraints on the provenance of the olivines. An understanding of the processes responsible for crustal deformation of kimberlites could potentially refine our understanding of kimberlite emplacement processes.
Abstract: Field evidence from the Pilbara craton (Australia) and Kaapvaal craton (South Africa) indicate that modern tectonic processes may have been operating at ca. 3.2 Ga, a time also associated with a high density of preserved Archaean impact indicators. Recent work has suggested a causative association between large impacts and tectonic processes for the Hadean. However, impact flux estimates and spherule bed characteristics suggest impactor diameters of <100 km at ca. 3.5 Ga, and it is unclear whether such impacts could perturb the global tectonic system. In this work, we develop numerical simulations of global tectonism with impacting effects, and simulate the evolution of these models throughout the Archaean for given impact fluxes. We demonstrate that moderate-size (?70 km diameter) impactors are capable of initiating short-lived subduction, and that the system response is sensitive to impactor size, proximity to other impacts, and also lithospheric thickness gradients. Large lithospheric thickness gradients may have first appeared at ca. 3.5-3.2 Ga as cratonic roots, and we postulate an association between Earth’s thermal maturation, cratonic root stability, and the onset of widespread sporadic tectonism driven by the impact flux at this time.
Abstract: Over the last several decades, geographic origin determination for fine rubies has become increasingly important in the gem trade. In the gemological laboratory, rubies are generally broken down into two groups based on their trace element chemistry: marble-hosted (low-iron) rubies and high-iron rubies. High-iron rubies are usually a straightforward identification based on their inclusions and trace element profiles. Marble-hosted rubies can be more challenging, with some deposits showing overlap in some of their inclusion scenes. But many marblehosted rubies, especially Burmese stones from Mogok and Mong Hsu, can be accurately identified based on their internal features and trace element profiles. This contribution will outline the methods and criteria used at GIA for geographic origin determination for ruby.
South America, Colombia, China, Europe, Afghanistan, Africa, Zambia
emerald
Abstract: The gem trade has grown to rely on gemological laboratories to provide origin determination services for emeralds and other fine colored stones. In the laboratory, this is mostly accomplished by careful observations of inclusion characteristics, spectroscopic analysis, and trace element profile measurements by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). Inclusions and spectroscopy can often separate Colombian emeralds from other sources (although there is some overlap between Colombian, Afghan, and Chinese [Davdar] emeralds). For non-Colombian emeralds, trace element analysis by LA-ICP-MS is needed in addition to information from the stone’s inclusions. The relative chemical diversity of emeralds from worldwide deposits allows confidence in origin determination in most cases. This contribution outlines the methods and criteria used at GIA for geographic origin determination for emerald.
Abstract: Topographic uplift of the southern African Plateau is commonly attributed to mantle causes, but the links between mantle processes, uplift, and erosion patterns are not necessarily straightforward. We acquired apatite (U-Th)/He (AHe) dates from eight kimberlite and basement samples from the lower reaches of the large westward-draining Orange River system with the goal of evaluating the roles of lithospheric modification and river incision on the erosion history here. Average AHe dates range from 79 to 118 Ma and thermal history models suggest that most samples are consistent with a main erosion phase at ca. 120-100 Ma, with some variability across the region indicating a complex erosion history. Major erosion overlaps with the timing of strong lithospheric thermochemical modification as recorded in xenoliths from the studied kimberlites, but the denudation pattern does not mimic the northward progression of lithospheric alteration across the study region. We attribute this area’s denudation history to a combination of mantle effects, rifting, establishment of the Orange River outlet at its current location, and later faulting. When considering these results with other kimberlite-derived surface histories from an ?1000-km-long E-W transect across the plateau, an eastward-younging trend in denudation is evident. The interplay of mantle processes and the shape of the large, west-draining Orange River basin likely control this first order-pattern.
Russia, South America, Brazil, Africa, Tanzania, Zimbabwe, India, Asia, Sri Lanka
alexandrite
Abstract: The gem and jewelry trade has come to place increasing importance on the geographic origin of alexandrite, as it can have a significant impact on value. Alexandrites from Russia and Brazil are usually more highly valued than those from other countries. In 2016, GIA began researching geographic origin of alexandrite with the intent of offering origin determination as a laboratory service. Unfortunately, collecting reliable samples with known provenance can be very difficult. Alexandrite is often recovered as a byproduct of mining for other gemstones (e.g., emerald and corundum), so it can be difficult to secure reliable parcels of samples because production is typically erratic and unpredictable. The reference materials studied here were examined thoroughly for their trace element chemistry profiles, characteristic color-change ranges under daylight-equivalent and incandescent illumination, and inclusion scenes. The data obtained so far allow us to accurately determine geographic origin for alexandrites from Russia, Brazil, Sri Lanka, Tanzania, and India. Future work may help to differentiate alexandrites from other localities.
Abstract: We addressed when plate-tectonic processes first started on Earth by examining the ca. 2.0 Ga Limpopo orogenic belt in southern Africa. We show through palinspastic reconstruction that the Limpopo orogen originated from >600 km of west-directed thrusting, and the thrust sheet was subsequently folded by north-south compression. The common 2.7-2.6 Ga felsic plutons in the Limpopo thrust sheet and the absence of an arc immediately predating the 2.0 Ga Limpopo thrusting require the Limpopo belt to be an intracontinental structure. The similar duration (?40 m.y.), slip magnitude (>600 km), slip rate (>15 mm/yr), tectonic setting (intracontinental), and widespread anatexis to those of the Himalayan orogen lead us to propose the Limpopo belt to have developed by continent-continent collision. Specifically, the combined Zimbabwe-Kaapvaal craton (ZKC, named in this study) in the west (present coordinates) was subducting eastward below an outboard craton (OC), which carried an arc equivalent to the Gangdese batholith in southern Tibet prior to the India-Asia collision. The ZKC-OC collision at ca. 2.0 Ga triggered a westward jump in the plate convergence boundary, from the initial suture zone to the Limpopo thrust within the ZKC. Subsequent thrusting accommodated >600 km of plate convergence, possibly driven by ridge push from the west side of the ZKC. As intracontinental plate convergence is a key modern plate-tectonic process, the development of the Limpopo belt implies that the operation of plate tectonics, at least at a local scale, was ongoing by ca. 2.0 Ga on Earth.
Arabian Journal of Geosciences, Vol. 13, , 209 orchid.org/ 0000-002-3287-9537
Africa, Mauritania
craton
Abstract: We used remote sensing, geographical information systems, Google Earth™ images, and regional geology in order to (i) improve the mapping of linear structures and understand the chronology of different mafic dyke swarms in the Ahmeyim area that belongs to the Archean Tasiast-Tijirit Terrane of the Reguibat Shield, West African craton, NW Mauritania. The spatial and temporal distributions with the trends of the dyke swarms provide important information about geodynamics. The analysis of the mafic dyke swarms map and statistical data allow us to distinguish four mafic dyke swarm sets: a major swarm trending NE-SW to NNE-SSW (80%) and three minor swarms trending EW to ENE-WSW (9.33%), NW-SE to WNW-ESE (9.06%), and NS (1.3%). The major swarms extend over 35 km while the minor swarms do not exceed 13 km. The Google Earth™ images reveal relative ages through crossover relationships. The major NE-SW to NNE-SSW and the minor NS swarms are the oldest generations emplaced in the Ahemyim area. The NW-SE-oriented swarm dykes which are cutting the two former swarms are emplaced later. The minor E-W to WSW-ENE swarms are probably the youngest. A precise U-Pb baddeleyite age of 2733?±?2 Ma has been obtained for the NNE-SSW Ahmeyim Great Dyke. This dyke is approximately 1500 m wide in some zone and extends for more than 150 km. The distinct mafic dyke swarms being identified in this study can potentially be linked with coeval magmatic events on other cratons around the globe to identify reconstructed LIPs and constrain continental reconstructions.
Abstract: Diamondiferous rock types worldwide are broadly divided into kimberlite and lamproite, the latter of which have unique characteristics in different regions and include carbonate-rich varieties (formerly orangeites/Group II kimberlites). Diamondiferous rocks in West Africa are typically micaceous and share petrographic, mineralogical, and geochemical characteristics with both kimberlites and lamproites. To further constrain the classification and petrogenesis of diamondiferous rocks worldwide and their variability between different cratonic regions, in this study we combine detailed petrographic observations with olivine, phlogopite, and spinel chemistry for hypabyssal samples from the Jurassic Tongo dike (Sierra Leone) and the Neoproterozoic Weasua cluster (Liberia). The Tongo dike contains macrocrysts of olivine and phlogopite in a groundmass of olivine, abundant phlogopite, spinel, perovskite, and apatite with a base of calcite, dolomite, and lesser serpentine. The phlogopite is characterised by concurrent FeO and Al2O3 enrichment, which is typical of kimberlites and unlike lamproites. These features and the kimberlite-like spinel compositions allow us to classify the Tongo samples as micaceous kimberlites. The Weasua rocks comprise macrocrysts of olivine in a groundmass of olivine, phlogopite, diopside (zoned towards aegirine-rich rims), spinel, perovskite, and apatite with a base of serpentine and less common calcite. The composition of Weasua phlogopite trends to significant FeO enrichment and Al2O3 depletion, i.e. towards tetraferriphlogopite. The enrichment in mica, phlogopite chemistry and presence of magmatic diopside indicates that these rocks are olivine lamproites. The populations of olivine macrocrysts and microcrysts at Tongo and Weasua are similar and characterised by distinct core and rim zones. Two distinct olivine core populations are observed. 1) forsterite-rich (Fo?>?90) olivine interpreted to reflect xenocrysts from typical mantle peridotites. Al-in-olivine thermometry suggests that these cores have P-T equilibration within diamond stability at Weasua and Tongo. 2) Al-, Ca- and Na- rich cores with P-T formation conditions extending beyond the mantle adiabat. These cores are interpreted to reflect metasomatic and thermal perturbation linked with the infiltration of kimberlite/lamproite melts in the deep lithosphere shortly before entrainment in the ascending magma. The olivine rims at Tongo and Weasua show limited variations in Fo contents at similar values of 88.9?±?0.8 for Tongo and 89.6?±?1.2 for Weasua, as well as similar minor and trace element concentrations. Thus, whereas the Tongo and Weasua rock types are classified as kimberlite and olivine lamproite, respectively, the olivine chemistry suggests a similar petrogenetic evolution.
Abstract: The ~545 Ma-old syn-collisional Otjimbingwe alkaline complex is composed of pyroxene-amphibole-biotite-bearing, mildly nepheline-normative to quartz-normative rocks ranging in composition from monzogabbro to monzonite, syenite and granite. The alkaline rocks have moderate to high SiO2 (50.5-73.0 wt%) and Na2O + K2O (5.1-11.5 wt%) and moderate to low MgO (6.6-0.2 wt%) concentrations. All samples have high large ion lithophile element (LILE: Ba up to 4600 ppm) and high-field-strength element contents (HFSE; Zr: 155-1328 ppm; Nb: 16-110 ppm; Ta: 1.4-7.1 ppm and Hf: 4-24 ppm) and have strongly fractionated LREE patterns ((La/Yb)N = 14-51). The most primitive members lack significant negative Eu anomalies. Mantle-normalized multi-element diagrams show depletion in Ba, Rb, Nb (Ta), P and Ti. The alkaline rocks have moderate radiogenic initial 87Sr/86Sr ratios (0.7061-0.7087) and unradiogenic initial ?Nd values (?3.9 to ?6.1). This isotope signature, associated with high LREE/HFSE ratios indicates that the parental melts were generated in enriched portions of the shallow lithospheric mantle, which was probably affected by previous subduction zone processes. In addition, correlations between Sr and Nd isotopes indicate that some of these variations result from combined crustal assimilation and fractional crystallization (AFC) processes. A new model of flat subduction is presented that explains most of the unsolved problems in the orogenic evolution of the Damara orogen, namely (i) the absence of early intrusive rocks with a clear subduction zone setting, (ii) the absence of high-pressure rocks such as blueschists and eclogites, (iii) the unusual distribution of igneous rocks with a clear predominance of granite and granodiorite and (iv) the need for a asthenospheric window during a classical subduction to explain the high T/moderate P granulite facies conditions in the overriding plate.
Abstract: By virtue of their exceptionally deep origin and unique geological histories, diamonds have been found to contain a variety of minerals that are otherwise rare or absent at Earth’s surface. During examination of a type IIa diamond sample from the Letseng mine, Lesotho, a new nickel sulfide mineral was found as part of a fine-grained multiphase inclusion. This nickel sulfide is the natural analogue of the synthetic nickel monosulfide known as ?-NiS, with a NiAs-type structure. This new mineral has been named crowningshieldite, after G.R. Crowningshield (1919 - 2006), whose many years of innovation at the Gemological Institute of America produced countless advances in the understanding and identification of natural, treated and synthetic diamonds and other gem materials. Crowningshieldite is the high-temperature polymorph of millerite, with an inversion temperature of 379 °C from previous experiments on pure NiS. The observed sample of crowningshieldite has an empirical formula of (Ni0.90Fe0.10)S. The host diamond is gemmy, colorless, and relatively pure (type IIa, containing less than ~5 ppm nitrogen). It belongs to a variety of sublithospheric/superdeep diamonds named CLIPPIR diamonds that are notable for their metallic Fe-Ni-C-S melt inclusions. In this case, the inclusion is interpreted to be an alteration assemblage produced when a primary metallic Fe-Ni-C-S inclusion with a surface-reaching fracture reacted with hot fluids, likely associated with kimberlite emplacement. Other phases identified in association with crowningshieldite in this fine-grained alteration assemblage are magnetite-magnesioferrite, hematite, and graphite. Unexposed inclusions within the same diamond are of original, unaltered Fe-Ni-C-S composition. The new mineral crowningshieldite is a good example of the complexity and breadth of geological processes recorded within diamonds and their inclusions.
Abstract: Felsic to intermediate igneous rocks from the Cuchilla Dionisio (or Punta del Este) Terrane (CDT) in Uruguay and the Várzea do Capivarita Complex (VCC) in southern Brazil were emplaced in the Tonian and experienced high-grade metamorphism towards the end of the Cryogenian. Geological and geochemical data indicate an S-type origin and formation in a continental within-plate setting by recycling of lower crustal material that was initially extracted from the mantle in the Palaeoproterozoic. Similar felsic igneous rocks of Tonian age occur in the Richtersveld Igneous Complex and the Vredefontein and Rosh Pinah formations in westernmost South Africa and southern Namibia and have been correlated with their supposed equivalents in Uruguay and Brazil. Geochemical and isotope data of the largely unmetamorphosed felsic igneous rocks in southwestern Africa imply a within-plate origin and formation by partial melting or fractional crystallization of mafic rocks that were extracted from the mantle in the Proterozoic. The parental melts of all of these Tonian igneous rocks from South America and southwestern Africa formed in an anorogenic continental setting at the western margin of the Kalahari Craton and were emplaced in, and/or contaminated by, Namaqua Province-type basement after separation from their source region. However, the source regions and the time of extractions thereof are different and, moreover, occurred at different palaeogeographical latitudes. New petrological data of CDT high-grade gneiss indicate a geothermal gradient of c. 20-25 °C/km, implying continental collisional tectonics following subduction and ocean basin closure at an active continental margin at the eastern edge of present-day South America in the late Cryogenian to early Ediacaran. The associated suture may be traced by the high-grade gneiss and amphibolite-facies mafic rocks in the CDT and probably continues northwards to the Arroio Grande Complex and the VCC in southern Brazil.
Abstract: Preexisting crustal heterogeneities are shown to influence rift process at a variety of scales. However, our understanding of how crustal inheritance influences rift?scale spatiotemporal kinematics of faulting in magma?poor rift environments is still very limited. Studies of active continental rifts can provide high?fidelity assessments of extensional processes and structures that are not possible through examination of ancient rifts that have undergone subsequent deformation events or are buried deeply beneath passive margins. We examine the influence of crustal inheritance on active rifting through balancing and restoring a series of regional cross sections across the Lake Tanganyika Rift in the Western Branch of the East African Rift System. The cross sections are produced using legacy seismic reflection data, reprocessed through prestack depth migration. This type example of a young, magma?poor continental rift transects several different basement terranes, including an Archean/Paleoproterozoic craton, and Proterozoic mobile belts. The Lake Tanganyika Rift exhibits two classic bell?shaped profiles of extension along strike, reaching a maximum of 7.15 km. A spatiotemporal integration of the extension data, and comparison with the various basement terranes the rift transects, reveals that extension in cratonic blocks is more widely distributed compared to mobile belt terranes, where strain rapidly localizes onto border faults by later rift stages. These results reveal how crustal inheritance exerts a fundamental control on the evolution of extension localization, ultimately impacting the geometry and structural architecture of rift basins.
Journal of African Earth Sciences, Vol. 165, 18p. Pdf
Africa, Egypt
Boninite
Abstract: Peridotites of Abu Dahr represent the main litho-unit of a Neoproterozoic dismembered ophiolite sequence and are among the best-preserved and well-exposed mantle rocks in South Eastern Desert of Egypt. Here, we present new geochemical and mineral chemical data for peridotites and associated pyroxenites and for chromitites and their platinum-group minerals to constrain their petrogenesis and geotectonic setting. The Abu Dahr ophiolite mantle section consists mainly of harzburgites, cut by pyroxenite dykes and containing dunite-chromitite lenses. The harzburgites are composed of olivine, orthopyroxene, spinel and minor clinopyroxene (?1.0 vol %) and amphibole. Olivine from harzburgites is highly magnesian (Fo 91-93) and Cr-spinel shows a wide-range of Cr2O3 and Al2O3 contents. The enstatite component of orthopyroxene decreases from harzburgite (En = 90-91) to orthopyroxenite (En = 84-87). Amphiboles are represented by magnesiohornblende and tschermakite. The chromitites are massive to disseminated and composed of magnesiochromite with high Cr# (83-93) and Mg# (66-79), and low TiO2 (<0.1 wt%) content. Solid inclusions in chromite include olivine, orthopyroxene and hornblende. Laurite (RuS2) is the most common PGM detected in the investigated chromitite samples and forms micrometer-size inclusions in fresh chromite. Various Ni-sulfides are found both in fresh chromite and along serpentine veinlets. Harzburgites have a refractory composition with a very low Al2O3 (0.4-0.8 wt%) and CaO (0.2-1.6 wt%) contents and high bulk-rock Mg# (89-92). Geochemical data suggest that the Abu Dahr peridotites are highly depleted SSZ peridotites formed in a forearc mantle wedge setting by high degrees of hydrous partial melting and emplaced as a result of the collision of the intra-oceanic arc with the Beitan gneisses. The podiform chromitites and orthopyroxenites were formed due to impregnation of mantle wedge harzburgites by boninitic melt. The highly depleted nature of the harzburgite is responsible for the small reserves of chromite ore at Abu Dahr and in the South Eastern Desert in general.
Contributions to Mineralogy and Petrology, in press available, 21p. Pdf
Africa, South Africa
deposit - Benfontein
Abstract: Primary carbonates in kimberlites are the main CO2 carriers in kimberlites and thus can be used to constrain the original carbon and oxygen-isotope composition of kimberlite melts and their deep mantle sources. However, the contribution of syn- and post-emplacement processes to the modification of the C-O-isotope composition of kimberlites is yet to be fully constrained. This study aims to shed new light on this topic through a detailed textural, compositional (major and trace elements), and in situ C-O-Sr isotopic characterisation of carbonates in the Benfontein kimberlite sills (Kimberley, South Africa). Our multi-technique approach not only reveals the petrographic and geochemical complexity of carbonates in kimberlites in unprecedented detail, but also allows identification of the processes that led to their formation, including: (1) magmatic crystallisation of Sr-rich calcite laths and groundmass; (2) crystallisation of late groundmass calcite from hydrothermal fluids; and (3) variable degrees of crustal contamination in carbonate-rich diapirs and secondary veins. These diapirs most likely resulted from a residual C-O-H fluid or carbonate melt with contributions from methane-rich fluids from the Dwyka shale wall rock, leading to higher 87Sr/86Sr and ?18O, but lower ?13C values than in pristine magmatic calcite. Before coalescing into the diapiric segregations, these fluids/melts also variably entrained early formed calcite laths and groundmass phases. Comparison between in situ and bulk-carbonate analyses confirms that O isotopic analyses of bulk carbonates from kimberlite rocks are not representative of the original isotopic signature of the kimberlite magma, whereas bulk C-isotope compositions are similar to those of the pristine magmatic carbonates. Calcite laths and most groundmass grains at Benfontein preserve isotopic values (?18O?=?6-8‰ and ?13C?=???4 to ??6‰), similar to those of unaltered carbonatites worldwide, which, therefore, probably correspond to those of their parental melts. This narrow range suggests kimberlite derivation from a mantle source with little contribution from recycled crustal material unless the recycled material had isotopic composition indistinguishable from typical mantle values.
Contributions to Mineralogy and Petrology, Vol. 175, 34 31p. Pdf
Africa, South Africa
carbonatite
Abstract: The Phalaborwa world-class phosphate deposit (South Africa) is hosted by a Paleoproterozoic alkaline complex mainly composed of phoscorite, carbonatite, pyroxenitic rocks, and subordinate fenite. In addition, syenite and trachyte occur in numerous satellite bodies. New petrological and in-situ geochemical data along with O and Sr isotope data obtained on apatite demonstrate that apatite is in the principal host rocks (pyroxenitic rocks, phoscorite and carbonatite) formed primarily by igneous processes from mantle-derived carbonatitic magmas. Early-formed magmatic apatite is particularly enriched in light rare earth elements (LREE), with a decrease in the REE content ascribed to magma differentiation and early apatite fractionation in isolated interstitial melt pockets. Rayleigh fractionation favored a slight increase in ?18O (below 1%) at a constant Sr isotopic composition. Intrusion of fresh carbonatitic magma into earlier-formed carbonatite bodies locally induced re-equilibration of early apatite with REE enrichment but at constant O and Sr isotopic compositions. In fenite, syenite and trachyte, apatite displays alteration textures and LREE depletion, reflecting interaction with fluids. A marked decrease in ?18O in apatite from syenite and trachyte indicates a contribution from ?18O-depleted meteoric fluids. This is consistent with the epizonal emplacement of the satellite bodies. The general increase of the Sr isotope ratios in apatite in these rocks reflects progressive interaction with the country rocks over time. This study made it possible to decipher, with unmatched precision, the succession of geological processes that led to one of the most important phosphate deposits worldwide.
Geochemical Perspectives Letters, Vol. 11, pp. 39-43. pdf
Africa, Botswana, South America, French Guiana
deposit - Dachine, Orapa
Abstract: Fluid inclusions trapped in fast-growing diamonds provide a unique opportunity to examine the origin of diamonds, and the conditions under which they formed. Eclogitic to websteritic diamondites from southern Africa show 13C-depletion and 15N-enrichment relative to mantle values (?13C = -4.3 to -22.2 ‰ and ?15N = -4.9 to +23.2 ‰). In contrast the 3He/4He of the trapped fluids have a strong mantle signature, one sample has the highest value so far recorded for African diamonds (8.5 ± 0.4 Ra). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between 3He/4He ratios and 3He concentration suggests that the low 3He/4He are largely the result of ingrowth of radiogenic 4He in the trapped fluids since diamond formation. The He-C-N isotope systematics can be best described by mixing between fluid released from subducted altered oceanic crust and mantle volatiles. The high 3He/4He of low ?13C diamondites reflects the high 3He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation 4He in the fluids means that all 3He/4He are minima, which in turn implies that the slab-derived carbon has a sedimentary organic origin. In short, although carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond-formation, helium isotopes reveal an unambiguous mantle component hidden within a strongly 13C-depleted system.
Abstract: Mineral major and trace elements combined with Sr isotopes of clinopyroxene are used to unravel the origins and evolution of mantle eclogite and pyroxenite xenoliths from the Doornkloof-Sover orangeite dike (western Kaapvaal craton), and to investigate the generation and destruction of diamond in these rocks. Two different eclogite types are present: (1) MgO-poor eclogites (MgO?=?7.3 to 14.5?wt%; n?=?43) with accessory diamond ± corundum and kyanite; garnet grossular content (median Ca#?=?0.25) and clinopyroxene jadeite content (0.39). Reconstructed bulk rocks are LREE-depleted (median La 0.29?ppm) and have low median Cr2O3 (0.06?wt%) and incompatible trace-element contents (e.g. Sr, Zr, Ba, Pb, Th), and high Li and transition metal abundances. Some are characterised by stepped REE patterns or steep slopes in the MREE, similar to eclogites affected by interaction with dehydration fluids generated in subduction zones. These fluids may also have deposited diamond in typically reducing eclogite assemblages at diamond-stable pressures. (2) MgO-rich eclogites and pyroxenites (MgO?=?14.0 to 20.0?wt%; n?=?29), which are barren and enriched in LREE (median La 1.39?ppm), Cr2O3 (0.25?wt%) and incompatible trace elements, with lower Li and transition metal abundances than the MgO-poor group. These are typical signatures of carbonated ultramafic melt metasomatism in the mantle lithosphere. Strontium isotopic compositions vary widely in both groups, but high Cr2O3 and Ba contents are dominantly associated with 87Sr/86Sr?>?0.7055. This reflects interaction with metasomatic agents remobilised from ancient lithospheric metasomes, which eventually gave rise to regional orangeite magmatism. The presence of strong positive Eu anomalies in both groups, including two pyroxenites, requires low-pressure igneous protoliths, presumably derived from a ca. 3?Ga spreading ridge, as reported for other eclogite materials from the western Kaapvaal craton. Based on the proportions of MgO-poor and -rich eclogites and pyroxenites, approximately 40% of the diamond inventory were destroyed by mantle metasomatism centred at ~135?±?15?km depth, overlapping a low-velocity anomaly (mid-lithospheric discontinuity). Two diamondiferous orangeites ?20?km from Doornkloof-Sover contain significantly different eclogite xenolith populations: At Newlands, MgO-poor diamondiferous eclogites are present in addition to barren MgO-rich ones and pyroxenite, suggesting that the host orangeite sampled a source region equally affected by diamond-destructive mantle metasomatism, whereas at Bellsbank, all eclogites are MgO-poor and LREE-depleted. This may explain higher diamond grades reported for this locality compared to Newlands or Doornkloof-Sover.
Contributions to Mineralogy and Petrology, Vol. 175, 15p. Pdf
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: Trace element partitioning between minerals and liquids provides crucial constraints on igneous processes. We quantified trace element concentrations in clinopyroxene (Cpx) phenocrysts and their phonolite melt inclusions from the 2007-08 eruption of Oldoinyo Lengai (Tanzania), and report Cpx-melt partition coefficients (D) and corresponding partitioning equations for rare earth elements (REE) and high field strength elements (HFSE) in alkaline magmas. Heavy REE (HREE: Er, Tm, Yb, Lu) are enriched relative to middle REE in alkaline Cpx and display a specific partitioning behavior that is characteristic of alkaline systems. HFSE (Ti, Zr, Hf) and HREE have similar D values (DHf?=?0.25; DLu?=?0.4) that are significantly higher than MREE (DSm?=?0.06). High DHREE/DMREE are strongly correlated with the high values of DZr and DHf relative to the low DMREE values. In this study, REE partitioning between phonolite melt and Cpx is not consistent with standard models assuming incorporation of all REE in the Cpx M2 site, but rather highlights HREE substitution in both the M1 and M2 sites. Here we highlight the preferential incorporation of HREE in the VI-coordinated M1 site, whereas light REE and MREE remain mostly distributed in the VIII-coordinated M2 site. REE partitioning is strongly dependent on Cpx chemistry: the ideal ionic radius and HREE incorporation in the M1 site increase with increasing Fe3+ content and decrease with increasing Mg2+ and AlVI content. In our study, we focus on alkaline evolved magmas, and update existing models to obtain adequate DHREE for alkaline evolved melts. We provide equations to quantify REE and HFSE partitioning, and HREE enrichment in Cpx that are based on Cpx major element composition and temperature. We propose a new model based on the lattice strain approach that predicts HREE partitioning between Cpx and alkaline magmas. The knowledge of the melt composition or of the trace element contents is not required to obtain DREE from the new model. An improved parameterization of HFSE partitioning between Cpx and phonolite and trachy-phonolite melts is also provided herein. We discuss the potential implications of the new data on our understanding of REE deposits that are commonly associated with igneous alkaline complexes.
GSL SP 420 ( Lyell release May 11, 2020), Vol. 420, pp. 11-30. pdf
Africa
Tectonics
Abstract: Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model.
Abstract: To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar ?137/134Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in ?137/134Ba values from ?0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average ?137/134Ba value of their mantle sources is estimated to be +0.04?±?0.06‰ (2SD, n?=?16), which is similar to the average value of +0.05?±?0.06‰ for mid-ocean ridge basalts. The lower ?137/134Ba value of ?0.08‰ in a Canadian sample and higher ?137/134Ba values of +0.14‰ and?+?0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.
Geochimica et Cosmochimica Acta, Vol. 278, pp. 177-198.
Africa, Cameroon
peridotites
Abstract: Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9-91.2) and low spinel Cr# (8.4-19.3), together with moderate to high whole-rock Al2O3 contents (2.0-3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01?±?0.18?Ga (1?), and when olivine and spinel are considered, is 1.82?±?0.14?Ga (1?). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2-2.4?Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0?Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ?2.0?Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ?2.0?Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts.
Abstract: The Natal Valley, offshore Mozambique, is a key area for understanding the evolution of East Gondwana. Within the scope of the integrated multidisciplinary PAMELA project, we present new wide?angle seismic data and interpretations, which considerably alter Geoscience paradigms. These data reveal the presence of a 30?km?thick crust that we argue to be of continental nature. This falsifies all the most recent palaeo?reconstructions of the Gondwana. This 30?km?thick continental crust 1,000 m below sea level implies a complex history with probable intrusions of mantle?derived melts in the lower crust, connected to several occurrences of magmatism, which seems to evidence the crucial role of the lower continental crust in passive margin genesis.
Abstract: The application of Raman spectroscopy for the identification and characterization of minerals and related materials has increased appreciably during recent years. Raman spectroscopy has proven to be a most valuable and versatile analytical tool. Successful applications cover virtually all the mineralogical sub-disciplines, and have become more numerous in geochemistry. We present a general summary of present applications, illustrated by selected examples. In addition, we briefly point out several aspects of spectral acquisition, data reduction, and interpretation of Raman results that are important for the application of Raman spectroscopy as a reliable analytical tool.
Abstract: The metamorphic core of the Betic-Rif orogenic chain (Alboran Domain) is made up of lower crustal rocks forming the envelope of the Ronda (Spain) and Beni Bousera (Morocco) peridotites. The deepest sections of the crustal envelopes are made of migmatitic granulites associated with diffuse acidic magmatic products, making these exposure and ideal site to investigate the textural and petrological connection between crustal anatexis and granite magmatism in the contintental crust. However, still debated is the timing of intracrustal emplacement of the peridotite bodies, with models proposing either Alpine (early Miocene) or Hercynian ages, and still uncertain is the linkage between peridotite emplacement and crustal anatexis. In this study, by combining rock textures with whole-rock geochemistry, metamorphic thermobarometry, the U-Pb zircon geochronology and the analysis of the garnet and zircon REE chemistry, we document the P-T-t evolution of the granulite facies migmatites that form the immediate envelope of the Beni Bousera peridotites of the Rif belt. A main episode of Permo-Carboniferous (ca. 300-290?Ma) deep crustal anatexis, melt extraction and migration is documented that we link to the crustal emplacement of the Beni Bousera peridotites during collapse of the Hercynian orogen. Correlation at a regional scale suggests that the Beni-Bousera section can be tentatively correlated with the pre-Alpine (Permo-Carboniferous) basement units tectonically interleaved within the orogenic structure of the Alpine chain. The results of this study provide ultimate constraints to reconstruct the tectono-metamorphic evolution of the Alboran Domain in the Western Mediterranean and impose re-assessment of the modes and rates through which Alpine orogenic construction and collapse occurred and operated in the region.
Contributions to Mineralogy and Petrology, Vol. 175, 21p. Pdf
Africa, South Africa
deposit - Kimberley
Abstract: Kimberlites are pipe-like igneous bodies, consisting of a pyroclastic crater and diatreme, commonly underlain by coherent root-zone rocks, and with associated dyke/sill complexes. The processes that control the different modes of coherent kimberlite emplacement remain uncertain. In addition, late evolution of kimberlite melts during emplacement into the upper crust remains poorly constrained. Therefore, it is unclear whether there is a link between melt composition/evolution and the emplacement mechanism of coherent kimberlites (i.e. planar dykes/sills vs. irregular bodies in the root zone). An absence of comparative studies on late-stage magmatic phases across the different emplacement modes of coherent kimberlite from the same locality hamper resolution of these issues. Therefore, we report petrographic and mineral chemical data for groundmass apatite in samples of dyke, sill, and root-zone kimberlites from the Kimberley cluster (South Africa). Early crystallised phases (olivine, spinel, Mg-ilmenite) in dyke/sill and root-zone kimberlites have indistinguishable compositions, and hence crystallised from similar primitive melts. Conversely, apatite compositions are generally distinct in dyke/sill (low Sr, high and variable Si) and root-zone kimberlites (high and variable Sr, low Si). The Si enrichment of apatite in dykes/sills is attributed to the coupled incorporation of CO32? and SiO44? for PO43?, reflecting higher CO2 contents in their parental melts, and potentially higher Si contents due to the preferential crystallisation of carbonates over mica/monticellite. The low Sr contents of apatite in dyke/sill kimberlites reflect equilibrium with a (kimberlite) melt (i.e. DSr is close to unity for carbonate and silicate melts), whereas the higher Sr contents of apatite in root-zone kimberlites require crystallisation from, or overprinting by a H2O?±?CO2 fluid (significantly higher DSr). The relative enrichment of CO2 in kimberlite dykes/sills is evident from the abundance of carbonates, the presence of mesostasis dolomite and calcite phenocrysts in some samples, and concomitant reduced proportions of other groundmass phases (e.g. serpentine, mica, monticellite). During late alteration of kimberlite dykes/sills, monticellite is typically replaced by carbonates, whereas olivine and pleonaste are relatively stable, indicating the melts which form dykes/sills evolve to higher CO2/H2O ratios. It is unlikely that these two distinct evolutionary paths were caused by crustal contamination before or during near surface magma emplacement, because crustal assimilation is not recorded in the O and Sr isotopic composition of late crystallising olivine rinds or carbonates, respectively. We suggest that higher concentrations of CO2 are retained in kimberlite dykes/sills due to higher confining pressures (i.e. lack of breakthrough to the surface). In contrast, exsolution of CO2 from root-zone kimberlites increased melt H2O/CO2 ratios and promoted the crystallisation of mica and monticellite at the expense of dolomite and calcite. Apatite compositions have the potential to aid in the discrimination of kimberlites from lamproites (higher LREE, Sr, F, and S, lower Si contents) and carbonatites (higher LREE, F, Cl and S, lower Fe contents). However, the compositions of kimberlitic apatite overlap those from aillikites, probably due to similar late-stage melt compositions.
Journal of the Southern African Institute of Mining and Metallurgy, 8p. Pdf
Africa, South Africa
legal
Abstract: Despite the importance that barrier identification has for policy-making and industry stakeholders alike; little guidance exists on consistent processes to systematically identify barriers that are hindering the different sectors of a value chain’s expansion and growth. This article describes the development of a framework that supports the identification of barriers to growth in mineral value chains. The resultant process was applied to the case of the manganese value chain in South Africa, and revealed 31 barriers within this industry. The results were validated by a panel of experts and the feedback was used to rework and improve the framework.
Abstract: Highly carbonated mantle xenoliths have been found in rift-related alkaline basalts at the Wadi Natash area in the southern part of the Eastern Desert of Egypt. Although all olivine and most orthopyroxene was replaced by carbonate and/or quartz, textural and mineral chemical features show that they are plagioclase-free spinel peridotites (lherzolite to harzburgite). Cr and Mg numbers (Cr#, Mg#) of Cr-spinel vary from 0.06 to 0.45 and 0.73 to 0.81, respectively. The correlation between Cr# and Mg# of the Cr-spinel in the studied xenoliths is weakly negative and its TiO2 content is slightly higher than in abyssal peridotite that was not affected by melt injection. The chemistry of ortho- and clinopyroxene suggests enstatite and chromian diopside compositions, respectively, with distinct signatures of a sub-continental mantle source. In particular, the Na2O contents (>1.0?wt%) and AlVI/AlIV ratios (1.2-2.6) of chromian diopside suggest such an origin. Two-pyroxene geothermometry indicates a temperature of about 900?°C, which is slightly lower than that of ordinary spinel peridotite xenoliths from other rift zones. It is evident that the studied peridotite xenoliths had experienced mantle processes (e.g. decompression melting, magma upwelling and metasomatism) at higher pressure than abyssal peridotites. The trace-element chemistry of clinopyroxene, e.g. high LREE/HREE ratios {(Ce/Yb)n?=?7}, high LREE contents (>3.6?ppm and up to 30.0?ppm Ce) and high Sr between >85.6?ppm and 466?ppm, indicates metasomatic alteration of the peridotite. Clinopyroxene in one sample has very low Ti/Eu and high LREE/HREE ratios. Clinopyroxene with (Ce/Yb)n higher than 3-4 and Ti/Eu ratio lower than 1500 may have experienced carbonatite or carbonate-rich melt metasomatism prior to their incorporation into the host basalt. The basalt itself is almost devoid of any carbonatization and hence the studied mantle peridotites were carbonatized before the generation of the basaltic magma but following an earlier event of K-metasomatism as indicated by the presence of phlogopite. The studied peridotites from the Wadi Natash area were altered by a carbonate-rich melt during a rifting stage. The results of the present paper indicate that the Natash basalts with their peridotite xenoliths extruded along transversal fractures of the NW-trending Nuqra-Kom Ombo-Kharit continental rift on its western shoulder in the south Eastern Desert of Egypt.
Abstract: Oldoinyo Lengai, located in the Gregory Rift in Tanzania, is a world-famous volcano owing to its uniqueness in producing natrocarbonatite melts and because of its extremely high CO2 flux. The volcano is constructed of highly peralkaline [PI = molar (Na2O + K2O)/Al2O3 > 2-3] nephelinite and phonolites, both of which likely coexisted with carbonate melt and a CO2-rich fluid before eruption. Results of a detailed melt inclusion study of the Oldoinyo Lengai nephelinite provide insights into the important role of degassing of CO2-rich vapor in the formation of natrocarbonatite and highly peralkaline nephelinites. Nepheline phenocrysts trapped primary melt inclusions at 750-800 °C, representing an evolved state of the magmas beneath Oldoinyo Lengai. Raman spectroscopy, heating-quenching experiments, low current EDS and EPMA analyses of quenched melt inclusions suggest that at this temperature, a dominantly natritess-normative, F-rich (7-14 wt%) carbonate melt and an extremely peralkaline (PI = 3.2-7.9), iron-rich nephelinite melt coexisted following degassing of a CO2 + H2O-vapor. We furthermore hypothesize that the degassing led to re-equilibration between the melt and liquid phases that remained and involved 1/ mixing between the residual (after degassing) alkali carbonate liquid and an F-rich carbonate melt and 2/ enrichment of the coexisting nephelinite melt in alkalis. We suggest that in the geological past similar processes were responsible for generating highly peralkaline silicate melts in continental rift tectonic settings worldwide.
Abstract: Botswana experienced a Mw 6.5 earthquake on 3rd April 2017, the second largest earthquake event in Botswana's recorded history. This earthquake occurred within the Limpopo-Shashe Belt, ~350 km southeast of the seismically active Okavango Rift Zone. The region has no historical record of large magnitude earthquakes or active faults. The occurrence of this earthquake was unexpected and underscores our limited understanding of the crustal configuration of Botswana and highlight that neotectonic activity is not only confined to the Okavango Rift Zone. To address this knowledge gap, we applied a regularized inversion algorithm to the Bouguer gravity data to construct a high-resolution crustal thickness map of Botswana. The produced crustal thickness map shows a thinner crust (35-40 km) underlying the Okavango Rift Zone and sedimentary basins, whereas thicker crust (41-46 km) underlies the cratonic regions and orogenic belts. Our results also show localized zone of relatively thinner crust (~40 km), one of which is located along the edge of the Kaapvaal Craton within the MW 6.5 Botswana earthquake region. Based on our result, we propose a mechanism of the Botswana Earthquake that integrates crustal thickness information with elevated heat flow as the result of the thermal fluid from East African Rift System, and extensional forces predicted by the local stress regime. The epicentral region is therefore suggested to be a possible area of tectonic reactivation, which is caused by multiple factors that could lead to future intraplate earthquakes in this region.
Abstract: Satellite gravimetry is recognized now as powerful and reliable tool for regional tectono-geodynamic zonation. The studied region contains intricate geodynamical features (high seismological indicators, active rift systems and collision processes), richest structural arrangement (existence of mosaic blocks of oceanic and continental Earth’s crust of various age), and a number of high-amplitude gravity anomalies and complex geomagnetic pattern. The most hydrocarbon reserves and diamonds, gold, platinum and deposits occur in this region. Comprehensive analysis of satellite derived gravity data by different methodologies were used to develop a sequence of maps specifying crucial properties of the region deep structure. Combined analysis of the compiled gravity map and its transformations with obtained geological data allowed to detecting significant geotectonic features of lithosphere of the region. For instance, Zagros-Makran terrane was classified as a separately developing structural segment (element) of the Arabian craton. Detailed examination of numerous geological sources and their combined examination with the GPS pattern, paleomagnetic, tectonic, geoid isoline map, seismic and other data revealed some sophisticated tectono-geophysical feature apparently located in middle-lower mantle below the Arabian-African region.
Abstract: The trace element and radiogenic isotope systematics of clinopyroxene have frequently been used to characterise mantle metasomatic processes, because it is the main host of most lithophile elements in the lithospheric mantle. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100-200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40-75 ppm Zr; La/Zr < 0.04), and restricted isotopic compositions (e.g., 87Sr/86Sri = 0.70369-0.70383; ?Ndi = +3.1 to +3.6) resembling those of their host kimberlite magmas. Available trace element partition coefficients can be used to show that Type 1 clinopyroxenes are close to being in equilibrium with kimberlite melt compositions, supporting a genetic link between kimberlites and these metasomatised lithologies. Thermobarometric estimates for Type 1 samples in this study indicate equilibration depths of 135-160 km within the lithosphere, thus showing that kimberlite melt metasomatism is prevalent in the deeper part of the lithosphere beneath Kimberley. In contrast, Type 2 clinopyroxenes occur in MARID rocks and coarse granular lherzolites in this study, which derive from shallower depths (<135 km), and have higher Sr (~350-1000 ppm) and LREE contents, corresponding to higher La/Zr of > ~ 0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the “enriched mantle” towards those of Type 1 rocks (e.g., ?Ndi = ?12.7 to ?4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable 87Sr/86Sri values (0.70526-0.71177), which are correlated with Sr contents and La/Zr ratios, and which might be explained by the interaction between peridotite and melts from different enriched sources within the lithospheric mantle. Most notably, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that a pervasive, alkaline metasomatic event caused MARID to be generated and harzburgites to be converted to lherzolite in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.
Abstract: Pyroclastic deposits of the Holocene Igwisi Hills kimberlite volcanoes, Tanzania, preserve unequivocal evidence for rapid, syn-eruptive agglutination. The unusual pyroclasts are composed of ash-sized particles agglutinated to each other by thin necks. The textures suggest the magma was disrupted into droplets during ascent. Collisions between particles occurred within a volcanic plume and on deposition within the conduit to form a weakly agglutinated, porous pyroclastic deposit. Theoretical considerations indicate that agglutination occurred over short timescales. Agglutinated clasts were entrained into weak volcanic plumes and deposited around the craters. Our results support the notion that agglutination can occur during kimberlite eruptions, and that some coherent, dense rocks in ancient kimberlite pipes interpreted as intrusive rocks could instead represent agglutinated pyroclastic rocks. Differentiating between agglutinated pyroclastic rocks and effusive or intrusive rocks in kimberlite pipes is important because of the potential effects that pyroclastic processes might have on diamond concentrations in deposits.
Abstract: The ~545 Ma-old syn-collisional Otjimbingwe alkaline complex is composed of pyroxene-amphibole-biotite-bearing, mildly nepheline-normative to quartz-normative rocks ranging in composition from monzogabbro to monzonite, syenite and granite. The alkaline rocks have moderate to high SiO2 (50.5-73.0 wt%) and Na2O + K2O (5.1-11.5 wt%) and moderate to low MgO (6.6-0.2 wt%) concentrations. All samples have high large ion lithophile element (LILE: Ba up to 4600 ppm) and high-field-strength element contents (HFSE; Zr: 155-1328 ppm; Nb: 16-110 ppm; Ta: 1.4-7.1 ppm and Hf: 4-24 ppm) and have strongly fractionated LREE patterns ((La/Yb)N = 14-51). The most primitive members lack significant negative Eu anomalies. Mantle-normalized multi-element diagrams show depletion in Ba, Rb, Nb (Ta), P and Ti. The alkaline rocks have moderate radiogenic initial 87Sr/86Sr ratios (0.7061-0.7087) and unradiogenic initial ?Nd values (?3.9 to ?6.1). This isotope signature, associated with high LREE/HFSE ratios indicates that the parental melts were generated in enriched portions of the shallow lithospheric mantle, which was probably affected by previous subduction zone processes. In addition, correlations between Sr and Nd isotopes indicate that some of these variations result from combined crustal assimilation and fractional crystallization (AFC) processes. A new model of flat subduction is presented that explains most of the unsolved problems in the orogenic evolution of the Damara orogen, namely (i) the absence of early intrusive rocks with a clear subduction zone setting, (ii) the absence of high-pressure rocks such as blueschists and eclogites, (iii) the unusual distribution of igneous rocks with a clear predominance of granite and granodiorite and (iv) the need for a asthenospheric window during a classical subduction to explain the high T/moderate P granulite facies conditions in the overriding plate.
Graphical abstract
International Journal of Earth Science, Vol. 109, 4, pp. 1487-1500.
Africa, Madagascar
tectonics
Abstract: This study uses gravimetric data integrated with recent seismic data published on south Madagascar to investigate geometry of crust-mantle interface. The regional tectonic framework of Madagascar is characterised by anastomosing network of up to 15-km-wide, 600-km-long and north-oriented high-strain zones, which originated during Neoproterozoic convergence. The studied Bouguer anomalies obtained from the International Gravimetric Bureau were high-pass filtered to emphasise short-wavelength gravimetric variations (shorter than 200 km). The Pan-African high-strain zones coincide with the positive gravimetric anomalies suggesting a link with deep seated high-density material. Considering the present-day thickness of the crust (35 km) and its seismic velocity record, the gravimetric anomalies can be visualised as narrow vertical tabular bodies located at the base of the Moho. Modelling further confirmed that such narrow vertical bodies could be stable over geologic time scale since these structures are relatively small (10 to 30 km wide). The vertical tabular bodies possibly reflect material transfer such as vertical motion of sub-crustal weak and possibly partially molten mantle along vertical deformation zones. It is proposed that these structures were initiated by folding of weak mantle-crust interface characterised by low-viscosity contrast between weak mantle and stronger granulitized lower crust during bulk pure shear-dominated horizontal shortening. It is proposed that the cuspate-lobate "mullion-type" geometry mimics rheological inversions of mafic and felsic rocks and shape of folds of variable scale observed in southern Madagascar. The formation of such mega-mullion structures is possibly an expression of "crème brulée" rheological model, where the deformation of the lithosphere is governed by stronger granulitic lower crust and weaker partially molten and/or hydrated mantle.
Journal of Petrology, 10.1093/petrology /egaa062/5857610 90p. Pdf
Africa, South Africa
deposit - Kimberley
Abstract: Olivine is the most abundant phase in kimberlites and is stable throughout most of the crystallisation sequence, thus providing an extensive record of kimberlite petrogenesis. To better constrain the composition, evolution, and source of kimberlites we present a detailed petrographic and geochemical study of olivine from multiple dyke, sill, and root zone kimberlites in the Kimberley cluster (South Africa). Olivine grains in these kimberlites are zoned, with a central core, a rim overgrowth, and occasionally an external rind. Additional ‘internal’ and ‘transitional’ zones may occur between the core and rim, and some samples of root zone kimberlites contain a late generation of high-Mg olivine in cross-cutting veins. Olivine records widespread pre-ascent (proto-)kimberlite metasomatism in the mantle including: (a) Relatively Fe-rich (Mg# <89) olivine cores interpreted to derive from the disaggregation of kimberlite-related megacrysts (20% of cores); (b) Mg-Ca-rich olivine cores (Mg# >89; >0.05?wt.% CaO) suggested to be sourced from neoblasts in sheared peridotites (25% of cores); (c) transitional zones between cores and rims probably formed by partial re-equilibration of xenocrysts (now cores) with a previous pulse of kimberlite melt (i.e., compositionally heterogeneous xenocrysts); and (d) olivine from the Wesselton water tunnel sills, internal zones (I), and low-Mg# rims, that crystallised from a kimberlite melt that underwent olivine fractionation within the shallow lithospheric mantle. Magmatic crystallisation begins with internal olivine zones (II), which are common but not ubiquitous in the Kimberley olivine. These zones are euhedral, contain rare inclusions of chromite, and have a higher Mg# (90.0 ± 0.5), NiO, and Cr2O3 contents, but are depleted in CaO compared to the rims. Internal olivine zones (II) are interpreted to crystallise from a primitive kimberlite melt during its ascent and transport of olivine toward the surface. Their compositions suggest assimilation of peridotitic material (particularly orthopyroxene) and potentially sulfides prior to or during crystallisation. Comparison of internal zones (II) with liquidus olivine from other mantle-derived carbonate-bearing magmas (i.e., orangeites, ultramafic lamprophyres, melilitites) show that low (100×) Mn/Fe (?1.2), very low Ca/Fe (?0.6), and moderate Ni/Mg ratios (?1.1) appear to be the hallmarks of olivine in melts derived from carbonate-bearing garnet-peridotite sources. Olivine rims display features indicative of magmatic crystallisation, which are typical of olivine rims in kimberlites worldwide - i.e. primary inclusions of chromite, Mg-ilmenite and rutile, homogeneous Mg# (88.8 ± 0.3), decreasing Ni and Cr, increasing Ca and Mn. Rinds and high-Mg olivine are characterised by extreme Mg-Ca-Mn enrichment and Ni depletion, and textural relationships indicate these zones represent replacement of pre-existing olivine, with some new crystallisation of rinds. These zones likely precipitated from evolved, oxidised, and relatively low-temperature kimberlite fluids after crustal emplacement. In summary, this study demonstrates the utility of combined petrography and olivine geochemistry to trace the evolution of kimberlite magmatic systems from early metasomatism of the lithospheric mantle by (proto-)kimberlite melts, to crystallisation at different depths en route to surface, and finally late-stage deuteric/hydrothermal fluid alteration processes after crustal emplacement.
Contributions to Mineralogy and Petrology, Vol. 175, 49 dor.org./10.1007 /s00410-020-01686-0
Africa, South Africa
deposit - Kimberley
Abstract: Kimberlites are pipe-like igneous bodies, consisting of a pyroclastic crater and diatreme, commonly underlain by coherent root-zone rocks, and with associated dyke/sill complexes. The processes that control the different modes of coherent kimberlite emplacement remain uncertain. In addition, late evolution of kimberlite melts during emplacement into the upper crust remains poorly constrained. Therefore, it is unclear whether there is a link between melt composition/evolution and the emplacement mechanism of coherent kimberlites (i.e. planar dykes/sills vs. irregular bodies in the root zone). An absence of comparative studies on late-stage magmatic phases across the different emplacement modes of coherent kimberlite from the same locality hamper resolution of these issues. Therefore, we report petrographic and mineral chemical data for groundmass apatite in samples of dyke, sill, and root-zone kimberlites from the Kimberley cluster (South Africa). Early crystallised phases (olivine, spinel, Mg-ilmenite) in dyke/sill and root-zone kimberlites have indistinguishable compositions, and hence crystallised from similar primitive melts. Conversely, apatite compositions are generally distinct in dyke/sill (low Sr, high and variable Si) and root-zone kimberlites (high and variable Sr, low Si). The Si enrichment of apatite in dykes/sills is attributed to the coupled incorporation of CO32? and SiO44? for PO43?, reflecting higher CO2 contents in their parental melts, and potentially higher Si contents due to the preferential crystallisation of carbonates over mica/monticellite. The low Sr contents of apatite in dyke/sill kimberlites reflect equilibrium with a (kimberlite) melt (i.e. DSr is close to unity for carbonate and silicate melts), whereas the higher Sr contents of apatite in root-zone kimberlites require crystallisation from, or overprinting by a H2O?±?CO2 fluid (significantly higher DSr). The relative enrichment of CO2 in kimberlite dykes/sills is evident from the abundance of carbonates, the presence of mesostasis dolomite and calcite phenocrysts in some samples, and concomitant reduced proportions of other groundmass phases (e.g. serpentine, mica, monticellite). During late alteration of kimberlite dykes/sills, monticellite is typically replaced by carbonates, whereas olivine and pleonaste are relatively stable, indicating the melts which form dykes/sills evolve to higher CO2/H2O ratios. It is unlikely that these two distinct evolutionary paths were caused by crustal contamination before or during near surface magma emplacement, because crustal assimilation is not recorded in the O and Sr isotopic composition of late crystallising olivine rinds or carbonates, respectively. We suggest that higher concentrations of CO2 are retained in kimberlite dykes/sills due to higher confining pressures (i.e. lack of breakthrough to the surface). In contrast, exsolution of CO2 from root-zone kimberlites increased melt H2O/CO2 ratios and promoted the crystallisation of mica and monticellite at the expense of dolomite and calcite. Apatite compositions have the potential to aid in the discrimination of kimberlites from lamproites (higher LREE, Sr, F, and S, lower Si contents) and carbonatites (higher LREE, F, Cl and S, lower Fe contents). However, the compositions of kimberlitic apatite overlap those from aillikites, probably due to similar late-stage melt compositions.
South America, Brazil, Africa, South Africa, Guinea, Canada, Northwest Territories
deposit - Sao Luis, Juina
Abstract: Bridgmanite (Mg,Fe)SiO3, a high pressure silicate with a perovskite structure, is dominant material in the lower mantle at the depths from 660 to 2700 km and therefore is probably the most abundant mineral in the Earth. Although synthetic analogues of this mineral have been well studied, no naturally occurring samples had ever been found in a rock on the planet’s surface except in some shocked meteorites. Due to its unstable nature under ambient conditions, this phase undergoes retrograde transformation to a pyroxene-type structure. The identification of the retrograde phase as ‘bridgmanite’ in so-called superdeep diamonds was based on the association with ferropericlase (Mg,Fe)O and other high-pressure (supposedly lower-mantle) minerals predicted from theoretical models and HP-HT experiments. In this study pyroxene inclusions in diamond grains from Juina (Brazil), one single-phase (Sample SL-14) and two composite inclusions of (Mg,Fe)SiO3 coexisting with (Mg,Fe)3Al2Si3O12 (Sample SL-13), and with (Mg,Fe)3Al2Si3O12 and (Mg,Fe)2SiO4 (Sample SL-80) have been analyzed to identify retrograde phases of former bridgmanite. XRD and Raman spectroscopy have revealed that these are orthopyroxene (Opx). (Mg,Fe)2SiO4 and (Mg,Fe)3Al2Si3O12 in these inclusions are identified as olivine and jeffbenite (TAPP). These inclusions are associated with inclusions of (Mg,Fe)O (SL-14), CaSiO3 (SL-80) and composite inclusion of CaSiO3+CaTiO3 (SL-13). XRD patterns of (Mg,Fe)SiO3 inclusions indicate that they consist of polycrystals. This polycrystalline textures together with high lattice strain of host diamond around these inclusions observed from EBSD may be an evidence for the retrograde phase transition of former bridgmanite. Single-phase inclusions of (Mg,Fe)SiO3 in superdeep diamonds are suggested to represent a retrograde phase of bridgmanite and fully inherit its initial chemical composition, including a high Al and low Ni contents [Harte, Hudson, 2013; Kaminsky, 2017]. The composite inclusions of (Mg,Fe)SiO3 with jeffbenite and other silicate and oxide phases may be interpreted as exolusion products from originally homogeneous bridgmanite [Walter et al., 2011]. The bulk compositions of these composite inclusions are rich in Al, Ti, and Fe which are similar to Al-rich bridgmanite produced in experiments on the MORB composition. However, the retrograde origin of composite inclusions due to decomposition of Al-rich bridgmanite may be doubtful because each of observed phases may represent single-phase inclusions, i.e. bridgmanite and high pressure garnet (majoritic garnet), with similar compositional features.
Abstract: The recently discovered REE and Nb mineralization in the Twihinat area in the western part of the Oulad Dlim Massif (Adrar Souttouf) in South Morocco is linked to a Cretaceous calciocarbonatite intrusion which was likely formed in an intracontinental rift setting and crops out locally within a ring structure that mainly consists of massive Fe-oxide mineralization and silica breccia. The carbonatite shows intensively metasomatized zones, which contain bastnaesite and pyrochlore-group minerals as the main REE and Nb ore minerals. They are usually associated with apatite, quartz and Fe-oxides, or trapped in calcite voids, suggesting a secondary ore formation. Within the associated Fe-oxide mineralization, pyrochlore and monazite-(Ce) are the main ore minerals occurring closely associated with quartz and magnetite or hematite. The silica breccia also shows significant subsequent infill of barite, bastnaesite-(Ce) and hydrated ceriopyrochlore, which was identified by EPMA and Raman spectroscopy. Bastnaesite commonly forms prismatic aggregates whereas pyrochlore and ceriopyrochlore usually display subhedral grains along tiny fractures. Structural and textural relationships clearly indicate epigenetic ore formation induced by multiple stages of hydrothermal fluid flow and fracturing. Ore precipitation likely resulted from interaction between low-pH mineralizing hydrothermal fluids and the wall-rock. The latter efficiently buffered the acidity of the fluids and allowed significant amounts of REE and Nb ore minerals to precipitate. Trace element ICP-MS analyses show very high REE and Nb concentrations of up to 0.76 wt% ?REE and 0.21 wt% Nb in carbonatite and up to 3 wt% ?REE and 1.3 wt% Nb in the associated silica and Fe-oxide mineralization. The results clearly demonstrate that the Twihinat REE-Nb deposits are significant and represent a potential new high-grade resource for these critical metals.
Botswana Journal of Earth Sciences, Vol. 7, pp. 43-57. pdf
Africa, Botswana
prospecting, markets
Abstract: In this paper I describe the different components that make up the Botswana Diamond Pipeline today, which means the supply chain of diamonds, that ranges from diamond prospecting to mining, to diamond processing and recovery, to rough diamond sorting, valuation, sales and marketing, to diamond polishing and cutting, and finally to diamond jewellery manufacturing and retail. In Botswana, we can now truly witness the journey of the diamond from “Rough to Finger” or from “Mine to Store” (Fig. 1). Today, Botswana is the world’s second largest producer of diamonds by value and volume after Russia, and there are currently twelve known kimberlite fields (Fig. 2) and eight operating diamond mines. Botswana’s diamonds are cut and polished into beautiful diamond jewellery locally and across the globe.
deposit - Roberts Victor, Jagersfontein, Kimberley, Kamfersdam, Premier
Abstract: Eclogite mantle xenoliths from various kimberlite occurrences on the Kaapvaal craton show evidence for depth- and redox-dependent metasomatic events that led to variable base metal sulphide and incompatible element enrichments. Eclogite xenoliths from the Roberts Victor, Jagersfontein, Kimberley (Kamfersdam) and Premier kimberlites were investigated for their silicate and base metal sulphide geochemistry, stable oxygen isotope compositions and oxybarometry. The variably metasomatised eclogites had basaltic, picritic and gabbroic protolith compositions and have garnet ?18O values that range from +3.3 to +7.9‰, which, when coupled with the trace element characteristics, indicate oceanic lithosphere protoliths that had undergone variable degrees of seawater alteration. The deepest equilibrated eclogites (175-220 km depth) from near the base of the Kaapvaal craton lithosphere are the most refractory and feature significant light rare earth element (LREE) depletions. They show the most oxidised redox compositions with ?logƒO2 values of FMQ-3.9 to FMQ-1.5. Subtle metasomatic overprinting of these eclogites resulted in base metal sulphide formation with relatively depleted and highly fractionated HSE compositions. These deepest eclogites and their included base metal sulphides suggest interaction with relatively oxidised melts or fluids, which, based on their HSE characteristics, could be related to precursor kimberlite metasomatism that was widespread within the Kaapvaal craton mantle lithosphere. In contrast, eclogites that reside at shallower, “mid-lithospheric” depths (140-180 km) have been enriched in LREE and secondary diopside/phlogopite. Importantly, they host abundant metasomatic base metal sulphides, which have higher HSE contents than those in the deeper eclogites at the lithosphere base. The mid-lithospheric eclogites have more reducing redox compositions (?logfO2 = FMQ-5.3 ? FMQ-3.3) than the eclogites from the lowermost Kaapvaal lithosphere. The compositional overprint of the shallower mantle eclogites resembles basaltic rather than kimberlitic/carbonatitic metasomatism, which is also supported by their relatively reducing redox state. Base metal sulphides from the mid-lithospheric eclogites have HSE abundances and distributions that are similar to Karoo flood basalts from southern Africa, suggesting a link between the identified shallow mantle metasomatism of the Kaapvaal cratonic lithosphere and the Karoo large igneous event during the Mesozoic. The sulphide-hosted platinum group element abundances of the mid-lithospheric eclogites are higher compared with their analogues from the deeper lithospheric eclogites, which in combination with their contrasting oxidation states, may imply redox-controlled HSE mobility during sulphur-rich metasomatism of continental mantle lithosphere.
Abstract: The rare earth element (REE) mineralization of Gakara (Burundi) has first been discovered in 1936 and has periodically been the subject of geological studies, at times when the exploitation of bastnäsite-(Ce) and monazite-(Ce) was economically interesting. This study focuses on the establishment of a mineral paragenesis for Gakara, with special attention to the REE-bearing phases, to understand the formation history of the deposit. The paragenesis can be subdivided into 3 stages: primary ore deposition, brecciation stage and supergene alteration. Evidence for fenitization processes (i.e. pinkish-red cathodoluminescence of K-feldpar, brecciation stage) and the strong enrichment of light REEs in bastnäsite and monazite substantiate the hypothesis of a structurally controlled hydrothermal mineralization with a strong carbonatitic affinity. This likely confirms the association of the Gakara REE deposit with the Neoproterozoic alignment of alkaline complexes and carbonatites along the present-day Western Rift. It suggests a direct link with a - currently unidentified - carbonatitic body at depth, possibly derived from a predominantly metasomatized lithospheric mantle.
Botswana Journal of Earth Sciences, Vol. 7, pp. 13-28. pdf
Africa, Botswana
deposit - AK6
Abstract: The AK6 kimberlite in north-eastern Botswana, better known as Karowe, is today one of the world?s top diamond producers by value. Its potential, however, was not recognised when AK6 was first discovered some fifty years ago. This paper traces the history of Karowe from the discovery of AK6 through to evaluation and production, reflecting on the interplay of economic, technical and corporate elements and highlighting some of the lessons learnt along this journey. Karowe Mine has been operating since 2012 and is fully owned by Lucara Diamond Corporation. In 2015, Karowe yielded the second largest diamond ever found, the 1,109ct Lesedi La Rona (Fig. 1).
Abstract: In the Arabian-Northern African region, interaction of the Nubian, Arabian and Eurasian plates and many small tectonic units is conspicuous. In order to better understand this interaction, we use satellite derived gravity data (retracked to the Earth’s surface) recognized now as a powerful tool for tectono-geodynamic zonation. We applied the polynomial approximation to the gravity data which indicated the presence of a large, deep ring structure in the eastern Mediterranean centered below the Island of Cyprus. Quantitative analysis of residual gravity anomaly provides an estimate of the deep anomalous body’s upper edge at a depth of about 1700 km. Computations of the residual gravity anomalies for the lower mantle also indicate presence of anomalous sources. The GPS vector pattern coinciding with the gravity trend implies counter clockwise rotation of this structure. Independent analyses of the geoid isolines map and seismic tomography data support the existence of a deep anomaly. Paleomagnetic data analysis from the surrounding regions confirms a counter clockwise rotation. Numerous petrological, mineralogical, geodynamical and tectonic data suggest a relation between this deep structure and near-surface processes. This anomaly sheds light on a number of phenomena including the Cyprus gravity anomaly, counter clockwise rotation of the Mesozoic terrane belt and asymmetry of basins along continental transform faults.
Abstract: Experiments on diamond crystallization in kimberlite melt were performed for 40 h at 6.3 GPa in the temperature range of 1300-1570 °C and at 7.5 GPa in the temperature range of 1450-1570 °C, using a multianvil high-pressure apparatus of split-sphere type. Group I kimberlite from the Udachnaya-East pipe and a synthetic multicomponent mixture modeling the average composition of group II kimberlites were used as starting materials. The experiments have shown that diamond growth on seed crystals in the kimberlite melt in equilibrium with olivine, pyroxene, and garnet starts from 1400 °C at 7.5 GPa and from 1520 °C at 6.3 GPa. Diamond nucleation requires higher temperature and pressure, 1570 °C and 7.5 GPa. The alkali-enriched and silicate-depleted derivates of kimberlite melts ensure the growth and nucleation of diamond at lower P and T values: 1400 °C at 7.5 GPa and 1520 °C at 6.3 GPa. The results obtained evidence that temperature, pressure, and the composition of crystallization medium are the main factors controlling diamond formation processes in the kimberlite melts and their derivates.
Abstract: The petrology and bulk-rock sulphur isotopic compositions of kimberlite samples from four localities (Bultfontein, De Beers, Kimberley, Wesselton) of the archetypal Kimberley cluster, South Africa, were used to investigate the origin(s) of S in kimberlites and gain insights into the occurrence of recycled crustal material in the source of Mesozoic kimberlites. The samples, which show variable degrees of alteration, are all hypabyssal and were derived from coherent root-zones as well as dykes and sills. Typical sulphide minerals are Cu-Fe-Ni-sulphides with less common pyrite, galena, sphalerite, and djerfisherite. They occur in a variety of textural associations, for example as groundmass phases, secondary inclusions in olivine, inclusions in matrix phases (e.g., phlogopite), or in carbonate-serpentine segregations. Barite is the most commonly observed sulphate phase. Bulk-sample ?34SVCDT values of sulphides in fresh kimberlites, which mostly do not contain barite, vary from - 2.0 to -5.7 ‰. Slightly altered kimberlite samples, in which sulphides were generally associated with serpentine, returned somewhat higher bulk-sulphide ?34SVCDT (-3.8 to +1.1 ‰). One sample from the Wesselton Water Tunnel Sills complex contains abundant barite and pyrite in its groundmass, with the latter having ?34SVCDT (+0.2 to +1.9 ‰) similar to altered kimberlites. Two further altered samples returned ?34SVCDT values (-10.1 to -13.0 ‰) that suggest a contribution from the local country rocks (Dwyka shale: ?34SVCDT from -10.2 to -10.5 ‰). All samples have near-zero ?33S values, suggesting that material displaying mass-independent fractionation has not played an important role. The negative ?34SVCDT values of fresh kimberlites from Kimberley suggest the involvement of recycled crustal material in their source, which is consistent with radiogenic isotope compositions. Overall, it appears that most kimberlitic sulphide S isotopic compositions can be explained by the action of a few typical magmatic/hydrothermal processes.
The Journal of the Southern African Insitute of Mining and Metallurgy, Vol. 119, Feb. 6p. Pdf
Africa, South Africa
DMC
Abstract: The petrology and bulk-rock sulphur isotopic compositions of kimberlite samples from four localities (Bultfontein, De Beers, Kimberley, Wesselton) of the archetypal Kimberley cluster, South Africa, were used to investigate the origin(s) of S in kimberlites and gain insights into the occurrence of recycled crustal material in the source of Mesozoic kimberlites. The samples, which show variable degrees of alteration, are all hypabyssal and were derived from coherent root-zones as well as dykes and sills. Typical sulphide minerals are Cu-Fe-Ni-sulphides with less common pyrite, galena, sphalerite, and djerfisherite. They occur in a variety of textural associations, for example as groundmass phases, secondary inclusions in olivine, inclusions in matrix phases (e.g., phlogopite), or in carbonate-serpentine segregations. Barite is the most commonly observed sulphate phase. Bulk-sample ?34SVCDT values of sulphides in fresh kimberlites, which mostly do not contain barite, vary from - 2.0 to -5.7 ‰. Slightly altered kimberlite samples, in which sulphides were generally associated with serpentine, returned somewhat higher bulk-sulphide ?34SVCDT (-3.8 to +1.1 ‰). One sample from the Wesselton Water Tunnel Sills complex contains abundant barite and pyrite in its groundmass, with the latter having ?34SVCDT (+0.2 to +1.9 ‰) similar to altered kimberlites. Two further altered samples returned ?34SVCDT values (-10.1 to -13.0 ‰) that suggest a contribution from the local country rocks (Dwyka shale: ?34SVCDT from -10.2 to -10.5 ‰). All samples have near-zero ?33S values, suggesting that material displaying mass-independent fractionation has not played an important role. The negative ?34SVCDT values of fresh kimberlites from Kimberley suggest the involvement of recycled crustal material in their source, which is consistent with radiogenic isotope compositions. Overall, it appears that most kimberlitic sulphide S isotopic compositions can be explained by the action of a few typical magmatic/hydrothermal processes. One of the most important performance indicators of a dense medium cyclone (DMC) circuit is the Tromp curve, and by extension the separation density and Ecart Probable (Ep) values. The densimetric profiles of DMC product streams have been traditionally acquired using heavy liquid sinkfloat analysis, which has certain disadvantages, such as the associated safety and health risks. More recently, non-toxic media such as lithium hetero-polytungstates (LST) have been used, with the desired densities being achieved by maintaining the solutions at specific temperatures. However, the high costs of these liquids can be prohibitive. The long turnaround time of the sink-float analysis is a further disadvantage for timeous interventions to the operating set-points of the DMC process. The RhoVol technology can generate the density distribution of a batch of particles in a rapid, accurate, repeatable, and safe manner. Additional data of interest, such as particle size and shape, are also measured and reported on a per-particle basis. Furthermore, samples can be sorted into discrete sorting bins based on any of the measured parameters of the particle, making further analyses of the material possible. This technology has applications across all commodities that use the DMC, particularly in the size fractions 25 +8 mm and 8 +3 mm. To date, laboratory results have proved very encouraging separation densities are within 5% of traditional sink-float results, and the technology is being introduced to diamond DMC plants.
Journal of Contemporary African Studies, Vol. 38, 1, pp. 39-54.
Africa, Zimbabwe
deposit - Chiadzwa
Abstract: The Chiadzwa diamonds attracted widespread attention due to human rights violations and illegal smuggling. When diamonds were discovered in 2006, thousands of artisanal miners descended on the diamond fields. In response, the government unleashed the army and police in brutal crackdowns to drive artisanal miners off the diamond fields. This militarisation of diamond fields and extraction was followed by forced displacement of the Chiadzwa people. This article examines the lived, everyday experiences of the displaced Chiadzwa people. Findings reveal that displacements dislocated the livelihoods and socialities of the people. Displacements also exacerbated people's vulnerability to livelihood shocks, insecurity, and poverty. In relocating people the government adopted a ‘top-down’ approach which triggered contestations and conflicts with the people who felt alienated from their ancestral land and excluded from diamond wealth. Consequently, sabotage, resistance and subversion were commonplace in the relocation process. These socio-political ‘tactics’ should be viewed as ‘weapons of the weak’.
The Journal of the Southern African Insitute of Mining and Metallurgy, Vol. 119, Feb. 8p. Pdf
Africa, Namibia
deposit - Southern Coastal
Abstract: The mining operation at Namdeb's Southern Coastal Mine (SCM) is unique. It targets gravel layers up to 30 m below sea level, which continue to dip deeper, further west, under the Atlantic Ocean. On this storm-dominated coastline, severe water seepage into mining areas, rugged orebody footwall characteristics, and highly variable resource grades all contribute to a challenging operational environment. Namdeb has a proud history of innovation, and as the mine progresses further westwards and associated technical and economic challenges increase, this innovative culture has become essential to the future of the mine. The Theory of Constraints (ToC) has been widely used at SCM, and across the mining discipline, to focus efforts on improving overall business profitability. Through analysis of the mining processes, opportunities were identified, solutions developed, and initiatives implemented with staggering results across all three mining disciplines, i.e. stripping, load and haul, and bedrock bulking and cleaning. This paper outlines the solutions adopted and the results of the ToC analysis.
Journal of Petrology, https://doi.org/ 10.1093/petrology /egaa040 50p. Pdf
Africa, South Africa
kimberlites
Abstract: Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe–Mg exchange equilibration temperatures for the eclogites range from 815 to 1000?°C, at pressures of 1·7?±?0·4?GPa as determined by REE partitioning, indicating that they were sampled from depths of 50–55?km; i.e. within the lower crust of the Namaqua–Natal Belt. The garnet pyroxenites show slightly lower temperatures (686–835?°C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua–Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua–Natal Belt with the Kaapvaal craton at 1–1·2?Ga.
Botswana Journal of Earth Sciences, Vol. 7, pp. 29-34. pdf
Africa, Botswana
deposit - KX36
Abstract: The KX36 kimberlite pipe is situated in the southeastern part of Central Kalahari Game Reserve (CKGR), Botswana, approximately 60 km from the known Gope and Kikao kimberlite fields (see figure 1).The kimberlite is covered by 75m of Kalahari sand, has a surface area of 3.6 Ha at the base of the sand cover and was discovered by Petra Diamonds Botswana (Pty) Ltd in 2008. Application of modern geophysical techniques (Ultra hi-resolution low level flying Xcalibur magnetics) and improved sampling method led to the discovery of KX36.The kimberlite was emplaced into the Karoo Supergroup, which comprised the older sedimentary rocks (300 - 185 Ma) overlain by the flood basalts (185Ma). The Karoo Supergroup rocks are overlain by approximately 80m of Kalahari Group sediments.
The Journal of the Southern African Insitute of Mining and Metallurgy, Vol. 119, Feb. 10p. Pdf
Africa, Tanzania
financing
Abstract: Government's equity role in the minerals sector is one of the nationalist measures implemented in order to ensure greater control and management of a country's mineral resources. This paper evaluates the Tanzanian government's equity participation in the minerals sector from 1996 to 2015. The research methodology included determination of the number of mineral rights, minimum allowable exploration expenditures in prospecting licences (PLs), and forms of equity role of the government. Data was collected and analysed for PLs, mining licences (MLs), and special mining licences (SMLs). The study revealed a number of challenges faced by the Tanzanian government as regards its equity strategy in the mineral sector. One of the major challenges was the secrecy surrounding agreements and contracts entered into between the government and private sector investors, which were concluded via various business ownership and mineral development projects. This secrecy resulted in non-transparency and lack of accountability in the mining industry. The financial benefits accruing to the government were inadequately realized, evident through inconsistent payments of corporate income tax and mining royalties by the mining companies. Furthermore, the government does not have solid mechanisms and frameworks for assessing non-financial benefits, thus it is difficult to measure the impact of these factors. It is recommended that the Tanzanian government review the Mining Act and Regulations of 2010 to include the provision of solid mechanisms and frameworks for all forms of government equity role.
Botswana Journal of Earth Sciences, Vol. 7, pp. 1-2. pdf
Africa, Botswana
deposit - Orapa
Abstract: I feel tremendously honoured and privileged to be invited to attend the 50thanniversary celebrations of the Orapa discovery. Thank you, Leon Daniels and Debswana. This will be a very emotional and exciting experience for me to return to Orapa after 50 years, where early in the morning of the 21st of April 1967 my team of twelve Motswana explorers and I stepped onto a low calcrete hill littered with kimberlitic indicator minerals. This was not entirely unexpected, as some months earlier Dr Gavin Lamont (the exploration manager) and Jim Gibson (the senior geologist) had detected such minerals during a road sampling survey into this isolated region south of the Makgadigadi Pans. However, what was unexpected was the enormous size of this kimberlite pipe that was destined to become the world-class Orapa diamond mine.
The Journal of the Southern African Insitute of Mining and Metallurgy, Vol. 119, Feb, 9p. Pdf
Africa, South Africa, Zimbabwe, Namibia, Angola, DRC, Lesotho, Botswana, eSwatini
production
Abstract: Southern Africa is generally thought to be well explored, with only limited potential for major new diamond discoveries. However, Chiadzwa in Zimbabwe and reports of a significant new kimberlite find in Angola are testimony to the dangers attached to an attitude that 'there is nothing left to find'.
Since the major discoveries in the central interior of South Africa in the 1870s, diamond exploration in the region has been led by market and political factors that influence the key exploration drivers of opportunity and value proposition. Unexpected new discoveries by new players always impact on existing producers and, from time to time, denial of opportunity through political or protectionist policies has inhibited investment in exploration. Entrepreneurial exploration appetite in southern Africa will be tempered by the potential value equation and security of investment. Overlaid on this, developments in diamond recovery technologies provide opportunity to reinvigorate current mines and old prospects previously considered too difficult or costly to exploit. Position on the cost curve will remain a key factor for survival in an increasingly competitive environment.
Abstract: Because of their robust nature, diamonds survive mantle processes and protect occluded minerals since the time of diamond formation. For the Kaapvaal Craton - the archetype for craton formation and evolution - the geochemical signatures of inclusions in Koffiefontein diamonds tell a story from craton formation to evolution and from lithospheric (below about 160 km) to lower mantle (>660 km) environs. We analysed a suite of 94 lithospheric to lower mantle diamonds and their silicate and oxide inclusions. Geochemical results confirm that the diamond substrates are very depleted, with Mg#OL of 91.5-95.0 and a dominance of low-Ca (<1.8 wt% CaO), presumably dunite-derived garnet. The Si-rich nature and preserved high Mg# of the peridotitic diamond substrates beneath Koffiefontein and the formation of KNbO3 (goldschmidtite) from an extremely fractionated melt/fluid indicate that potentially both mantle- and subduction-related fluids are the cause of metasomatism in the Kaapvaal cratonic root. Mantle-like, restricted carbon isotopic compositions of both P- and E-type diamonds (avg. ?13C -5.7 ‰ and -6.6 ‰, respectively) indicate that an abundant, mantle-derived CHO fluid is responsible for diamond formation. Diamonds have a large range in nitrogen concentrations and isotopic compositions, suggesting decoupling from carbon and heterogeneous sources. ?18O of former bridgmanite and ?13C of its host diamond document a purely mantle-derived lower mantle component. Combined, these results reveal a complex and multistage evolution of the Kaapvaal Craton whereby multiple episodes of fluid and melt metasomatism re-enriched the craton already, prior to diamond formation, followed by diamond entrainment in a kimberlite possibly derived from the lower mantle.
Botswana Journal of Earth Sciences, Vol. 7, pp. 35-41. pdf
Africa, Botswana
Deposit - Gope
Abstract: The Gope (Go25) kimberlite was discovered by Falconbridge Explorations Limited (Botswana) (FELB) in 1981, following a helicopter-supported sampling programme within Reconnaissance Permits RP8/79 and RP1/80, covering approximately 78 500 km2, centred on the Central Kalahari area of Botswana (Fig. 1). The majority of this area is covered by sands of the Kalahari Group, with thicknesses up to 100m. Unfortunately, most original company files, including mineral distribution maps and mineral analyses were not available to the author. This paper draws on the excellent summary of the discovery of the Gope (Go-25) kimberlite by Lee et al. (2009), together with the author’s personal recollections.
The Journal of the Southern African Insitute of Mining and Metallurgy, Vol. 119, Feb. 10p. Pdf
Africa, South Africa
deposit - Cullinan
Abstract: In 2017, Petra Diamonds completed the construction and commissioning of a modern, fit-for-purpose diamond processing plant at Cullinan Diamond Mine (CDM). The design of CDM's milling circuit is unconventional in that it comprises an autogenous (AG) mill with a grate discharge with large ports, low-revolution jaw crushers, and high-pressure grinding roll crushers with large operating gaps. In this paper we review the design to provide guidance on what is expected from the milling circuit and to demonstrate how the design aims to address challenges experienced in the old plant, which was based on staged crushing technology. We assessed the performance of the CDM AG milling circuit from commissioning and early production stages to examine its impact along multiple dimensions. In the assessment we sought to understand the lessons from our milling circuit regarding diamond liberation, energy consumption, and the future of diamond processing as a whole.
Abstract: The thermobarometric analysis of inclusions in lithospheric diamonds indicates that they originated from a wide range of depths, with a global mode at ca. 170±15 km [1]. Studies based on diamond depth distribution at global scale, however, cannot clarify if this mode reflects a real concentration of diamonds, preferential sampling of materials from this level by rising kimberlites, or even a statistical distribution within the hard limits imposed by diamond stability, lithosphere thickness, and mantle adiabat under typical cratonic thermal regimes. We addressed this problem by comparing depth distributions for peridotitic diamonds from the three localities that have been the most prolific for diamond geobarometry (Cullinan, Kimberley and Voorspoed, South Africa) with those of mantle xenocrysts from the same kimberlite sources. P-T estimates indicate that the diamonds were formed at T higher, equal or lower than the ambient geotherm. They may record old mantle thermal regimes or local thermal perturbations related to infiltration of parent fluids or melts. Nonetheless, the diamonds show similar depth distributions for different localities, with a distinct mode at ?175 ?? 10 km. The similarity of these distributions with that calculated for peridotitic diamonds worldwide, as well as the lack of systematic correlation with kimberlite sampling efficiency as recorded by mantle xenocrysts, suggests that this mode has genetic significance. Based on observed depth distributions at both local and global scale and on thermodynamic modeling of COH fluids, diamond-forming processes are predicted to become less efficient with decreasing depth from at least ?160 km. In addition, diamond endowment near the base of the lithosphere may be negatively affected by infiltration of carbon-undersaturated melts. Considering the poor correlation between diamond and xenocryst depth distributions in single kimberlites or kimberlite clusters, even limited xenocryst records from diamond favorable depths (especially the 160-190 km interval) may correspond to significant diamond potential.
Abstract: Marange diamonds (Zimbabwe) contain both fluid-poor (gem-quality) and fluid-bearing growth zones with abundant CH4. As such, they provide the unique opportunity to compare trace element compositions of CH4-bearing diamonds with those of carbonatitic and saline high density fluid (HDF)-bearing diamonds (gem-quality and fibrous) to obtain an overview of mantle source fluids for diamond growth. HDF’s in fibrous diamonds and some gem-quality diamonds have been linked to subduction of surficial material, consistent with the global link between diamond age and collisional tectonic events. Even though Marange diamonds have +?15N indicative of surficial recycling, they do not display the expected Eu or Sr anomalies. Fibrous diamonds have the most fractionated REE patterns, with negligible HREE and high (La/Yb)N ? 100- 10000. Gem-quality diamonds have highly variable (La/Yb)N; the most unfractionated HDF’s are in Victor and Cullinan diamonds with low (La/Yb)N <76. HDF’s in Marange diamonds are intermediate between these two extremes, with (La/Yb)N = 23-240. Differences in (La/Yb)N between different diamond suites relate either to varying initial compositions (where low (La/Yb)N reflects derivation during higher degrees of melting) or to the increasing interaction of HDF’s in fibrous diamonds with mantle rocks during fluid infiltration. Marange diamonds have rare +Ce anomalies, that have so far only been reported for Victor and Brazil (sub-lithospheric) gem-quality diamonds. The oxidation state of Ce (Ce4+ vs Ce3+) and development of Ce anomalies could be attributed to ƒO2, melt/fluid composition, and PT conditions. In Marange, Victor and Brazil diamonds, Ce4+ substitution for Zr4+ does not appear to be a factor since we find no correlation between Zr content and Ce anomalies. However, in Marange diamonds, CH4-bearing zones have less variable Ce anomalies compared to the CH4-free zones, which may suggest Ce anomalies are indicative of fluid oxidation state.
South America, Brazil, Africa, South Africa, Canada, Northwest Territories
geochronology
Abstract: Kimberlites emplaced since ~2 Ga show Nd and Hf isotopic compositions that follow a remarkably consistent linear evolution [1]. However, kimberlites emplaced <200 Ma within a few thousand kilometers of the western paleo-margin of Pangea (i.e. Brazil, southern Africa, and Lac de Gras in western Canada) deviate towards more enriched Nd and Hf isotopic compositions possibly due to contribution by recycled crustal material, introduced to the deep kimberlite source via subduction [1]. To address this anomaly further we have compared new and existing geochronological and Nd isotopic data for 28 kimberlites from Lac de Gras (LDG; ca. 47 - 75 Ma) with their olivine and spinel mineral chemistries. Olivine grains typically include mantle-derived xenocrystic cores (Mg# = 83.5-94.2) overgrown by magmatic rims with relatively constant Mg# values. Olivine rims and chromite are the first magmatic phases to crystallise from kimberlite and can be used as proxies for primitive melt compositions. The average Mg# of olivine cores from each kimberlite is positively correlated with average olivine rim Mg#, suggesting that assimilation of heterogeneous lithospheric mantle contributed to the primitive melt compositions. The ?Nd(i) values from whole-rock and perovskite from LDG kimberlites vary between -3.4 and -0.4 that are negatively correlated with their emplacement ages. This correlation is indicative of an evolving kimberlite source which may have resulted from a progressively lower contribution of recycled material. No systematic relationships were observed between olivine rim or chromite compositions and age or Nd isotopic composition. This observation highlights decoupling between kimberlite source evolution and primitive melt compositions due to the combined effects of crustal recycling in the kimberlite source and lithospheric mantle assimilation during kimberlite ascent.
Botswana Journal of Earth Sciences, Vol. 7, pp. 3-12. pdf
Africa, Botswana
deposit - Jwaneng
Abstract: When Leon Daniels first contacted us, and others, about the Botswana Diamond Exploration Conference 2017, he wrote: ‘The future of diamond exploration lies in us "old-timers" who have retired or are about to retire to inspire the next generation to go out there and make the next discoveries so vitally important for Botswana.’ Sadly, in presenting on behalf of both Stuart and I, I had to own to being one of those old-timers!
Africa, South Africa, Guinea, Australia,South America, Brazil, Canada, Northwest Territories
deposit - Koffiefontein, Kankan, Lac de Gras, Juina, Machado, Orroroo
Abstract: (Mg,Fe)SiO3 bridgmanite is the dominant phase in the lower mantle; however no naturally occurring samples had ever been found in terrestrial samples as it undergoes retrograde transformation to a pyroxene-type structure. To identify retrograde phases of former bridgmanite single-phase and composite inclusions of (Mg,Fe)SiO3 in a series of superdeep diamonds have been examined with electron microscopy, electron microprobe, Raman spectroscopy and X-ray diffraction techniques. Our study revealed that (Mg,Fe)SiO3 inclusions are represented by orthopyroxene. Orthopyroxenes in single-phase and composite inclusions inherit initial chemical composition of bridgmanites, including a high Al and low Ni contents. In composite inclusions they coexist with jeffbenite (ex-TAPP) and olivine. The bulk compositions of these composite inclusions are rich in Al, Ti, and Fe, which are similar but not fully resembling Al-rich bridgmanite produced in experiments on the MORB composition. The retrograde origin of composite inclusions due to decomposition of Al-rich bridgmanite may be doubtful because each of observed minerals may represent coexisting HP phases, i.e. bridgmanite or ringwoodite.
Abstract: Mineral carbonation is a form of carbon capture, utilization and storage (CCUS) that aims to transform excess CO2 into environmentally benign carbonate minerals which are geologically stable. Here, we investigated the reactivity of processed kimberlite and kimberlite ore from the Venetia Diamond Mine (South Africa). Highly reactive phases, such as brucite [Mg(OH)2], are uncommon in the samples collected from Venetia necessitating the development of new strategies for mineral carbonation. Kimberlite ore and tailings from this mine consist of a clay-rich mineral assemblage that is dominated by lizardite (a serpentine mineral) and smectites. Smectites are swelling clays that can act as a source of Mg and Ca for carbonation reactions via cation exchange, dissolution and/or direct replacement of smectites to form carbonate phases. Although carbonation of serpentine and brucite has long been a focus of CCUS in mine wastes [1], smectite carbonation has not been explored in this setting. Quantitative X-ray diffraction using Rietveld refinements coupled with Fourier-transform infrared spectroscopy indicate that smectites of stevensite-saponite composition are abundant in the Venetia samples (1.3-15.4 wt.%). Synchrotron-based X-ray fluorescence mapping correlated with scanning and transmission electron microscopy show that smectites are distributed as altered, smooth regions measuring from 1 to 20 ?m in breadth. These phases are rich in Mg and Ca and Al-poor. To better understand the behaviour/reactivity of smectites during the cation exchange process, we have used batch experiments with pure endmembers of Ca-, Mg- and Na-montmorillonite under different treatment conditions (NH4-citrate, NH4-O-acetate, NH4-Cl and Na3-citrate). After 24 hours of reaction, ICP-MS analyses reveal that the four treatments have the same efficiency for Ca and Mg exchange, while NH4-Cl and NH4- O-acetate treatments minimize calcite dissolution. Our end goals are to optimize settling time and to maximize extraction of Ca and Mg for carbonation reactions during ore processing.
Science Advances, Vol. 6, eaba3269 June 3, 7p. Pdf
Global, Africa, Tanzania
carbonatites
Abstract: The origin of carbonatites-igneous rocks with more than 50% of carbonate minerals-and whether they originate from a primary mantle source or from recycling of surface materials are still debated. Calcium isotopes have the potential to resolve the origin of carbonatites, since marine carbonates are enriched in the lighter isotopes of Ca compared to the mantle. Here, we report the Ca isotopic compositions for 74 carbonatites and associated silicate rocks from continental and oceanic settings, spanning from 3 billion years ago to the present day, together with O and C isotopic ratios for 37 samples. Calcium-, Mg-, and Fe-rich carbonatites have isotopically lighter Ca than mantle-derived rocks such as basalts and fall within the range of isotopically light Ca from ancient marine carbonates. This signature reflects the composition of the source, which is isotopically light and is consistent with recycling of surface carbonate materials into the mantle.
Marine and Petroleum Geology, Vol. 28, pp. 1385-1401.
Africa, South Africa
geomorphology
Abstract: The application of the onset of supercontinentality, the “Great Oxidation Event” (GOE) and the first global scale glaciation in the Neoarchaean-Palaeoproterozoic as panacea-like events providing a framework or even chronological piercing points in Earth’s history at this time, is questioned. There is no solid evidence that the Kaapvaal craton was part of a larger amalgamation at this time, and its glacigenic record is dominated by deposits supporting the operation of an active hydrological cycle in parallel with glaciation, thereby arguing against the “Snowball Earth Hypothesis”. While the Palaeoproterozoic geological record of Kaapvaal does broadly support the GOE, this postulate itself is being questioned on the basis of isotopic data used as oxygen-proxies, and sedimentological data from extant river systems on the craton argue for a prolongation of the greenhouse palaeo-atmosphere (possibly in parallel with a relative elevation of oxygen levels) which presumably preceded the GOE. The possibility that these widespread events may have been diachronous at the global scale is debated.
Abstract: Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe-Mg exchange equilibration temperatures for the eclogites range from 815 to 1000?°C, at pressures of 1•7?±?0•4?GPa as determined by REE partitioning, indicating that they were sampled from depths of 50-55?km; i.e. within the lower crust of the Namaqua-Natal Belt. The garnet pyroxenites show slightly lower temperatures (686-835?°C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua-Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua-Natal Belt with the Kaapvaal craton at 1-1•2?Ga.
Nature Communications, doi:.org/10.1038/ s41467-020-17442 -8 11p. Pdf
Africa, South Africa, Russia, Siberia
water
Abstract: Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle. Water concentrations and the ratios of hydrogen isotopes in the mantle give insight into these processes, as well as into the origin of terrestrial water. Here we show the presence of molecular H2 in minerals (omphacites) from eclogites from the Kaapvaal and Siberian cratons. These omphacites contain both high amounts of H2 (70 to 460 wt. ppm) and OH. Furthermore, their ?D values increase with dehydration, suggesting a positive H isotope fractionation factor between minerals and H2-bearing fluid, contrary to what is expected in case of isotopic exchange between minerals and H2O-fluids. The possibility of incorporation of large quantities of H as H2 in nominally anhydrous minerals implies that the storage capacity of H in the mantle may have been underestimated, and sheds new light on H isotope variations in mantle magmas and minerals.
Geochimica et Cosmochimica Acta, Vol. 282, pp. 297-323.
Africa, East Africa
carbonatites
Abstract: Rare earth element (REE) enrichments in carbonatites are often described as resulting from late magmatic-hydrothermal or supergene processes. However, magmatic pre-enrichment linked to the igneous processes at the origin of carbonatites are likely to contribute to the REE fertilisation. Experimental constraints reveals that immiscibility processes between carbonate and silicate melts can lead to both REE enrichments and depletions in carbonatites making the magmatic processes controlling REE enrichments unclear.
We link REE contents of carbonatites to the magmatic stage at which carbonatites are separated from silicate magma in their course of differentiation. We present results of experiments made at pressure and temperature conditions of alkaline magmas and associated carbonatites differentiation (0.2-1.5 GPa; 725-975?°C; FMQ to FMQ?+?2.5), simultaneously addressing crystal fractionation of alkaline magmas and immiscibility between carbonate (calcio-carbonate type) and silicate melts (nephelinite to phonolite type). The experimental data shows that the degree of differentiation, controlling the chemical composition of alkaline melts, is a key factor ruling the REE concentration of the coexisting immiscible carbonate melts. In order to predict carbonate melt REE enrichments during alkaline magma differentiation, we performed a parameterisation of experimental data on immiscible silicate and carbonate melts, based exclusively on the silica content, the alumina saturation index and the alkali/alkaline-earth elements ratio of silicate melts. This parameterisation is applied to more than 1600 geochemical data of silicate magmas from various alkaline provinces (East African Rift, Canary and Cape Verde Islands) and show that REE concentrations of their potential coeval carbonatite melts can reach concentration ranges similar to those of highly REE enriched carbonatites (?REE?>?30 000?ppm) by immiscibility with phonolitic/phono-trachytic melt compositions, while more primitive alkaline magmas can only be immiscible with carbonatites that are not significantly enriched in REE.
Abstract: The thermobarometric analysis of inclusions in lithospheric diamonds has shown that these diamonds may originate from a wide range of depths, with a global mode at ~175 ± 15 km. Studies based on diamond depth distribution at global scale, however, cannot clarify if this mode reflects a real concentration of diamonds, preferential sampling of materials from this level by ascending kimberlites, or simply a statistical distribution within the hard limits imposed by diamond stability, lithosphere thickness and mantle adiabat under typical cratonic thermal regimes. We addressed this problem by comparing depth distributions for peridotitic diamonds from the three localities that have been the most prolific for diamond geobarometry (Cullinan, Kimberley and Voorspoed, South Africa) with those of mantle xenocrysts from the same kimberlite sources. The revised P-T estimates indicate that the diamonds were formed at T higher, equal or lower than the ambient geotherm recorded by the xenocrysts. These conditions may represent old mantle thermal regimes or local thermal perturbations related to infiltration of parental fluids or melts. Nonetheless, the studied diamonds show similar depth distributions for the different localities, with a distinct mode at ?180 ± 10 km. The similarity of these distributions with that calculated for peridotitic diamonds worldwide, as well as the lack of systematic correlation with kimberlite sampling efficiency as recorded by mantle xenocrysts, suggests that this mode has genetic significance. Based on observed depth distributions and thermodynamic modeling of COH fluids, diamond-forming processes are predicted to become less efficient with decreasing depth from at least ?165 km. In addition, diamond endowment near the base of the lithosphere may be negatively affected by infiltration of carbon-undersaturated melts or fluids after diamond formation. Considering the poor correlation between diamond and xenocryst depth distributions in single kimberlites or kimberlite clusters, even limited xenocryst records from diamond favorable depths (especially from the 160-190 km interval) may correspond to significant diamond potential.
Communicacoes Geologicas *** in PORT, researchgate 7p. Pdf
Africa, Angola
deposit - Bailundo
Abstract: Pyrochlore group minerals are common accessory phases in alkaline-carbonatitic complexes, and the most important Nb ore worldwide. Its capacity to embody a wide range of compositions can often provide useful insights in Nb occurrence and concentration processes. In the Bailundo Carbonatitic Complex (BCC, Angola), two major sets of pyrochlore can be distinguished: (1) magmatic/metasomatic pyrochlore in deep carbonatitic rocks, often displaying diffuse textures obliterating primary zoning, with slightly low contents in Nb2O5 (average 50 wt%), and in other chemical components (Ta, U, Na); and (2) supergene pyrochlore in the weathering profile, displaying corroded and bleached patches along microfractures (in some cases, with relics of magmatic pyrochlore), that show higher contents in Nb2O5 (up to 73 wt%), Ba and Pb. Compositional and textural variations recorded in pyrochlore crystals illustrate the geodynamic events that took place in the BCC and contributed to high Nb concentration in the weathering profile, thus being quite useful to distinguish different mineralization types and as Nb-exploration tools.
Journal of Petrology, in press available, 90p. Pdf
Africa, South Africa
deposit - Kimberley
Abstract: Olivine is the most abundant phase in kimberlites and is stable throughout most of the crystallization sequence, thus providing an extensive record of kimberlite petrogenesis. To better constrain the composition, evolution, and source of kimberlites we present a detailed petrographic and geochemical study of olivine from multiple dyke, sill, and root zone kimberlites in the Kimberley cluster (South Africa). Olivine grains in these kimberlites are zoned, with a central core, a rim overgrowth, and occasionally an external rind. Additional ‘internal’ and ‘transitional’ zones may occur between the core and rim, and some samples of root zone kimberlites contain a late generation of high-Mg olivine in cross-cutting veins. Olivine records widespread pre-ascent (proto-)kimberlite metasomatism in the mantle including the following features: (1) relatively Fe-rich (Mg# <89) olivine cores interpreted to derive from the disaggregation of kimberlite-related megacrysts (20?% of cores); (2) Mg-Ca-rich olivine cores (Mg# >89; >0•05?wt% CaO) suggested to be sourced from neoblasts in sheared peridotites (25?% of cores); (3) transitional zones between cores and rims probably formed by partial re-equilibration of xenocrysts (now cores) with a previous pulse of kimberlite melt (i.e. compositionally heterogeneous xenocrysts); (4) olivine from the Wesselton water tunnel sills, internal zones (I), and low-Mg# rims, which crystallized from a kimberlite melt that underwent olivine fractionation and stalled within the shallow lithospheric mantle. Magmatic crystallization begins with internal olivine zones (II), which are common but not ubiquitous in the Kimberley olivine. These zones are euhedral, contain rare inclusions of chromite, and have a higher Mg# (90•0 ± 0•5), NiO, and Cr2O3 contents, but are depleted in CaO compared with the rims. Internal olivine zones (II) are interpreted to crystallize from a primitive kimberlite melt during its ascent and transport of olivine toward the surface. Their compositions suggest assimilation of peridotitic material (particularly orthopyroxene) and potentially sulfides prior to or during crystallization. Comparison of internal zones (II) with liquidus olivine from other mantle-derived carbonate-bearing magmas (i.e. orangeites, ultramafic lamprophyres, melilitites) shows that low (100×) Mn/Fe (?1•2), very low Ca/Fe (?0•6), and moderate Ni/Mg ratios (?1•1) appear to be the hallmarks of olivine in melts derived from carbonate-bearing garnet-peridotite sources. Olivine rims display features indicative of magmatic crystallization, which are typical of olivine rims in kimberlites worldwide; that is, primary inclusions of chromite, Mg-ilmenite and rutile, homogeneous Mg# (88•8 ± 0•3), decreasing Ni and Cr, and increasing Ca and Mn. Rinds and high-Mg olivine are characterized by extreme Mg-Ca-Mn enrichment and Ni depletion, and textural relationships indicate that these zones represent replacement of pre-existing olivine, with some new crystallization of rinds. These zones probably precipitated from evolved, oxidized, and relatively low-temperature kimberlite fluids after crustal emplacement. In summary, this study demonstrates the utility of combined petrography and olivine geochemistry to trace the evolution of kimberlite magmatic systems from early metasomatism of the lithospheric mantle by (proto-)kimberlite melts, to crystallization at different depths en route to surface, and finally late-stage deuteric or hydrothermal fluid alteration after crustal emplacement.
Earth and Planetary Science Letters, Vol. 547, 14p. Pdf
Africa
LLSVP, superplume
Abstract: Many volcanic hotspots are connected via ‘plume’ conduits to thermochemical structures with anomalously low seismic velocities at the core-mantle boundary. Basaltic lavas from some of these hotspots show anomalous daughter isotope abundances for the short-lived 129I-129Xe, 146Sm-142Nd, and 182Hf-182W radioactive decay systems, suggesting that their lower mantle sources contain material that dates back to Earth-forming events during the first 100 million years in solar system history. Survival of such ‘primordial’ remnants in Earth's mantle places important constraints on the evolution and inner workings of terrestrial planets. Here we report high-precision 182W/184W measurements for a large suite of kimberlite volcanic rocks from across the African tectonic plate, which for the past 250 million years has drifted over the most prominent thermochemical seismic anomaly at the core-mantle boundary. This so-called African LLSVP, or ‘large low shear-wave velocity province’, is widely suspected to store early Earth remnants and is implicated as the ultimate source of global Phanerozoic kimberlite magmatism. Our results show, however, that kimberlites from above the African LLSVP, including localities with lower mantle diamonds such as Letseng and Karowe Orapa A/K6, lack anomalous 182W signatures, with an average W value of 0.0 ± 4.1 (2SD) for the 18 occurrences studied. If kimberlites are indeed sourced from the African LLSVP or superplume, then the extensive 182W evidence suggests that primordial or core-equilibrated mantle materials, which may contribute resolvable W excesses or deficits, are only minor or locally concentrated components in the lowermost mantle, for example in the much smaller ‘ultra-low velocity zones’ or ULVZs. However, the lack of anomalous 182W may simply suggest that low-volume kimberlite magmas are not derived from hot lower mantle plumes. In this alternative scenario, kimberlite magmas originate from volatile-fluxed ambient convecting upper mantle domains beneath relatively thick and cold lithosphere from where previously ‘stranded’ lower mantle and transition zone diamonds can be plucked.
Abstract: During the past two decades significant progress has been made in understanding the origin and evolution of kimberlites, including relationships to other diamondiferous magma types such as lamproites and aillikites. However, the association of kimberlites and carbonatites on continental shields remains poorly understood, and two opposing ideas dominate the debate. While one school of thought argues that primary carbonatite melts transform into hybrid carbonated silicate magmas akin to kimberlites by assimilation of cratonic mantle material, others use geochemical evidence to show that carbonatite magmas can evolve from near-primary kimberlite melts within the cratonic lithosphere. The 1.15 Ga Premier kimberlite pipe on the Kaapvaal craton in South Africa hosts several kimberlite and carbonatite dykes. Reconstructions of magma compositions suggest that up to 20 wt.% CO2 was lost from near-primary kimberlite melts during ascent through the cratonic lithosphere, but the carbonatite dyke compositions cannot be linked to the kimberlite melts via differentiation. Geochemical evidence, including mantle-like ?13C compositions, suggests that the co-occurring kimberlite and carbonatite dykes represent two discrete CO2-rich magma batches derived from a mixed source in the convecting upper mantle. The carbonatites probed a slightly more depleted source component in terms of Sr-Nd-Hf isotopic compositions relative to the peridotitic matrix that was more effectively tapped by the kimberlites (87Sr/86Sri = 0.70257 to 0.70316 for carbonatites vs. 0.70285 to 0.70546 for kimberlites; ?Ndi = +3.0 to +3.9 vs. +2.2 to +2.8; ?Hfi = -2.2 to +0.7 vs. -5.1 to -1.9). Platinum-group element systematics suggest that assimilation of refractory lithospheric mantle material by the carbonatite melts was negligible (<1 vol.%), whereas between 5 - 35 vol.% of digested cratonic peridotite account for the kimberlite compositions, including the low 187Os/188Os signature (?Osi = -12.7 to -4.5). The kimberlite and carbonatite dykes show similarly strong Nd-Hf isotope decoupling (??Hfi = -10.7 to -7.6 vs. -8.8 to -6.1), regardless of the variable lithospheric mantle imprints. This observation suggests a common sublithospheric origin of the negative ??Hf signature, possibly linked to ancient recycled oceanic crust components in the convecting upper mantle to transition zone sources of CO2-rich magmatism. Mesoproterozoic kimberlite and carbonatite magmatism at Premier was coeval with subduction and collision events along the southern Kaapvaal craton margin during the 1,220 -1,090 Ma Namaqua-Natal orogeny associated with Rodinia supercontinent formation. Thermochronology suggests that the entire Kaapvaal craton was affected by this collisional tectonic event, and it appears that the changing lithospheric stress-field created pathways for deep-sourced kimberlite and carbonatite magmas to reach Earth’s surface. We find that collision-induced (e.g., Premier) and continental breakup-related (e.g., Kimberley) kimberlite magmas are compositionally indistinguishable, with the inference that plate tectonic processes aid solely in the creation of magma ascent pathways without a major influence on deep mantle melting beneath cratons. It follows that on-craton kimberlite magmatism in the hinterland of collision zones is not necessarily more likely to entrain large sublithospheric diamonds than kimberlite eruptions linked to continental breakup. This implies that Premier’s world-class endowment with ‘ultradeep’ Type-II diamonds is not causally related to its setting behind an active orogenic front.
Journal of the Geological Society, Vol. 177, pp. 784-798.
Africa, Madagascar
geothermometry
Abstract: Madagascar occupied an important place in the amalgamation of Gondwana and preserves a record of several Neoproterozoic events that are linked to orogenesis of the East African Orogen. In this study, we integrate remote sensing, field data and thermochronology to unravel complex deformation in the Ikalamavony and Itremo domains of central Madagascar. The deformation sequence comprises a gneissic foliation (S1), followed by south- to SW-directed, tight to isoclinal, recumbent folding (D2). These are overprinted by north-trending upright folds that formed during an approximately east-west shortening event (D3). Together these produced type 1 and type 2 fold interference patterns throughout the Itremo and Ikalamavony domains. We show that the Itremo and Ikalamavony domains were deformed together in the same orogenic system, which we interpret as the c. 630 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, and probably formed in response to final closure of the Mozambique Ocean along the Betsimisaraka Suture that amalgamated Madagascar with the Dharwar Craton of India. Apatite U-Pb and novel laser ablation triple quadrupole inductively coupled plasma mass spectrometry (LA-QQQ-ICP-MS) muscovite and biotite Rb-Sr thermochronology indicates that much of central Madagascar cooled through c. 500°C at c. 500 Ma.
Abstract: One of the first appearances of eclogite-facies mineral assemblages in the geological record occurs in the c. 2000 Ma Palaeoproterozoic Usagaran Belt in central Tanzania, where the extended margin of the Tanzanian Craton is interpreted to have been subducted. Mafic rocks are interpreted to have contained the mineral assemblage garnet + omphacite + rutile + quartz ± amphibole. This high-pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene + plagioclase + hornblende + ilmenite ± orthopyroxene. Mineral equilibria forward modelling indicates that the eclogite-facies assemblages reached minimum peak pressure-temperature (P-T) conditions of ~17 kbar and ~700 °C. Inclusions in garnet document a prograde P-T history consistent with burial through upper amphibolite-facies conditions and possible partial melting. Petrological and compositional evidence from garnet suggests that following peak metamorphism, the eclogite-facies rocks were heated while stalled at approximate peak pressures. Temperature estimates derived from Zr concentrations in interpreted texturally retrograde rutile support a near-isothermal post-peak P-T evolution for the eclogite-facies rocks - an evolution that terminates at retrograde P-T conditions of approximately 7.6-8.2 kbar and 680-790 °C. The relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies (the Isimani Suite). The metapelitic gneisses contain garnet + kyanite + biotite + staurolite + hornblende + plagioclase + muscovite + rutile + quartz and preserve minimal evidence of a high-pressure history, conceivably due to post-peak mineralogical recrystallisation. P-T modelling, inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses — similarly to the relict eclogites — experienced burial to minimum peak pressures of approximately 16.5-17 kbar. Compositional zoning patterns in eclogitic garnet suggest the Isimani system was buried, reached peak metamorphic conditions, and was subsequently exhumed within a timeframe of up to 20 Myr. A tectonic regime involving crustal thickening and subduction, followed by extensional exhumation of the entire Isimani Suite is our preferred model for the development of the c. 2000 Ma Usagaran Belt.
Abstract: The transport of carbon into Earth’s mantle is a critical pathway in Earth’s carbon cycle, affecting both the climate and the redox conditions of the surface and mantle. The largest unconstrained variables in this cycle are the depths to which carbon in sediments and altered oceanic crust can be subducted and the relative contributions of these reservoirs to the sequestration of carbon in the deep mantle1. Mineral inclusions in sublithospheric, or ‘superdeep’, diamonds (derived from depths greater than 250 kilometres) can be used to constrain these variables. Here we present oxygen isotope measurements of mineral inclusions within diamonds from Kankan, Guinea that are derived from depths extending from the lithosphere to the lower mantle (greater than 660 kilometres). These data, combined with the carbon and nitrogen isotope contents of the diamonds, indicate that carbonated igneous oceanic crust, not sediment, is the primary carbon-bearing reservoir in slabs subducted to deep-lithospheric and transition-zone depths (less than 660 kilometres). Within this depth regime, sublithospheric inclusions are distinctly enriched in 18O relative to eclogitic lithospheric inclusions derived from crustal protoliths. The increased 18O content of these sublithospheric inclusions results from their crystallization from melts of carbonate-rich subducted oceanic crust. In contrast, lower-mantle mineral inclusions and their host diamonds (deeper than 660 kilometres) have a narrow range of isotopic values that are typical of mantle that has experienced little or no crustal interaction. Because carbon is hosted in metals, rather than in diamond, in the reduced, volatile-poor lower mantle2, carbon must be mobilized and concentrated to form lower-mantle diamonds. Our data support a model in which the hydration of the uppermost lower mantle by subducted oceanic lithosphere destabilizes carbon-bearing metals to form diamond, without disturbing the ambient-mantle stable-isotope signatures. This transition from carbonate slab melting in the transition zone to slab dehydration in the lower mantle supports a lower-mantle barrier for carbon subduction.
Contributions to Mineralogy and Petrology, Vol. 175, 97, 30p. Pdf
Africa, Tanzania
craton
Abstract: U-Pb petrochronology of deep crustal xenoliths and outcrops across northeastern Tanzania track the thermal evolution of the Mozambique Belt and Tanzanian Craton following the Neoproterozoic East African Orogeny (EAO) and subsequent Neogene rifting. At the craton margin, the upper-middle crust record thermal quiescence since the Archean (2.8-2.5 Ga zircon, rutile, and apatite in granite and amphibolite xenoliths). The lower crust of the craton documents thermal pulses associated with Neoarchean ultra-high temperature metamorphism (ca. 2.64 Ga,?>?900 °C zircon), the EAO (600-500 Ma rutile), and fluid influx during rifting (?650 °C (above Pb closure of rutile and apatite) at the time of eruption. Zoned titanite records growth during cooling of the lower crust at 550 Ma, followed by fluid influx during slow cooling and exhumation (0.1-1 °C/Myr after 450 Ma). Permissible lower-crustal temperatures for the craton and orogen suggest variable mantle heat flow through the crust and reflect differences in mantle lithosphere thickness rather than advective heating from rifting.
Msc. Thesis VRIJE University Amsterdam, 166p. Academia available pdf
Africa, Botswana
deposit - Letlhakane
Abstract: The Letlhakane kimberlite pipes are emplaced in the Proterozoic Makondi Fold Belt, Botswana. They belong to a cluster of kimberlite diatremes that also includes the Orapa diamond mine, approximately 40 km to the northwest. In a previous geochemical study on Letlhakane xenoliths it was proposed that the Makondi Fold Belt is underlain by Archaean mantle that belongs to the Zimbabwe Craton. This implies the Letlhakane Kimberlites were sourced from the edge of Zimbabwe cratonic mantle and ascended through Proterozoic Makondi crust, explaining their rather anomalous tectonic setting. In this study we aim to verify the Archaean nature of the lithospheric mantle beneath Letlhakane and assess the origin and extent of metasomatic modifications in comparison to the Kaapvaal and Zimbabwe Cratons. We present an extensive petrological, geochemical and isotopic dataset on a selection of 38 peridotite xenoliths from Letlhakane in order to characterize the mantle that underlies northern Botswana. Whole rock and mineral major and trace elements were measured by electron microprobe (EMP), laser ablation ICPMS and XRF. Sm-Nd and Lu-Hf isotope analyses were performed on garnet, cpx and amphibole separates from 12 selected samples. The sample suite includes spinel harzburgites, spinel lherzolites, a spinel dunite, a wehrlite, a garnet websterite, garnet harzburgites, garnet lherzolites and amphibole bearing garnet lherzolites. The samples are categorized into four groups based on garnet chemistry and modal compositions: I) garnet free samples, II) garnet harzburgites, II) garnet lherzolites and IV) amphibole-bearing garnet lherzolites. Whole rock major elements and olivine magnesium numbers suggest that the SCLM experienced 20 to 60% melt depletion between 5 and 3 GPa. Reconstructed whole rock HREE concentrations and Lu-Yb systematics indicate that up to 20% melting occurred in the absence of garnet, leading to strong fractionation of HREE. The data suggest a shallow decompressional melting regime, consistent with Archaean cratonic genesis models. Preliminary Re-Os data suggest melt depletion occurred at ~2.7 Ga. All samples experienced various amounts of metasomatic re-enrichment expressed by high modal abundances of garnet, opx, clinopyroxene and amphibole. Silica enrichment occurred to a lesser extent than observed in the Kaapvaal, but the Letlhakane samples show significantly more opx-rich assemblages than reported for the Zimbabwe Craton. Sm-Nd and Lu-Hf isotope signatures of garnet harzburgites suggest LREE enrichment occurred around ~2.0 Ga, possibly related to major tectono-magmatic activity associated with accretion of the Makondi Fold Belt to the Zimbabwe Craton. Cpx from the garnet lherzolites show major isotopic interaction and re-equilibration with Group I Kimberlites, which probably crystallized from pre-cursor kimberlitic melts prior to the Letlhakane eruption at ~93 Ma. Trace elements and Nd-Hf istope systematics of the amphibole bearing samples suggest amphibole metasomatism occurred from a LREE, Na, Ka, Ca and Al enriched hydrous melt around 500 to 600 Ma, possibly associated with Pan-African magmatic activity in northern Botswana. The overall lack of negative Nb-Ta anomalies suggests that metasomatic melts were generated in a within-plate geodynamical environment, rather than a subduction related setting.
Abstract: Carbonatites are unusual, carbon-rich magmas thought to form either by the melting of a carbon-rich mantle source or by low-degree partial melting of a carbon-poor (<80 ppm C) mantle followed by protracted differentiation and/or immiscibility. Carbonate-bearing mantle xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatites, have provided key support for a C-rich mantle source. Here, we report unique microscale O and C isotopic analyses of those carbonates, which are present as interstitial grains in the silicate host lava, veins in the xenoliths, and pseudo-inclusions in olivine xenoliths. The ?18O values vary little, from 19‰ to 29‰, whereas ?13C values are more variable, ranging from -23‰ to +0.5‰. We show that such carbonate ?18O values result from the low-temperature precipitation of carbonate in equilibrium with meteoric water, rather than under mantle conditions. In this framework, the observed ?13C values can be reproduced by Rayleigh distillation driven by carbonate precipitation and associated degassing. Together with petrological evidence of a physical connection between the three types of carbonates, our isotopic data support the pedogenic formation of carbonates in the studied xenoliths by soil-water percolation and protracted crystallization along xenolith cracks. Our results refute a mechanism of C enrichment in the form of mantle carbonates in the mantle beneath the Natron Lake magmatic province and instead support carbonatite formation by low-degree partial melting of a C-poor mantle and subsequent protracted differentiation of alkaline magmas.
Geochimica et Cosmochimica Acta, 10.1016/j.gca.2020.10.010 35p. Pdf
Africa, Botswana
deposit - Jwaneng
Abstract: Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (> 100’s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (< 10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (?13C < -21.1‰) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earth’s long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events.
Abstract: Pyroclastic deposits of the Holocene Igwisi Hills kimberlite volcanoes, Tanzania, preserve unequivocal evidence for rapid, syn-eruptive agglutination. The unusual pyroclasts are composed of ash-sized particles agglutinated to each other by thin necks. The textures suggest the magma was disrupted into droplets during ascent. Collisions between particles occurred within a volcanic plume and on deposition within the conduit to form a weakly agglutinated, porous pyroclastic deposit. Theoretical considerations indicate that agglutination occurred over short timescales. Agglutinated clasts were entrained into weak volcanic plumes and deposited around the craters. Our results support the notion that agglutination can occur during kimberlite eruptions, and that some coherent, dense rocks in ancient kimberlite pipes interpreted as intrusive rocks could instead represent agglutinated pyroclastic rocks. Differentiating between agglutinated pyroclastic rocks and effusive or intrusive rocks in kimberlite pipes is important because of the potential effects that pyroclastic processes might have on diamond concentrations in deposits.
Canadian Journal of Development Studies, Vol. 41, 3, pp. 417-431. pdf
Africa
legal
Abstract: This paper introduces a special section of the Canadian Journal of Development Studies, "The Africa Mining Vision: A Manifesto for More Inclusive Extractive Industry-Led Development?" Conceived by African ministers "in charge of mineral resources" with inputs and guidance from African Union Heads of State, the Africa Mining Vision (AMV) was officially launched in February 2009. The papers presented in this special section reflect critically on progress that has since been made with operationalising the AMV at the country level across Africa; the general shortcomings of the manifesto; and the challenges that must be overcome if the continent is to derive greater economic benefit from its abundant mineral wealth.
Abstract: Newlands and Bobbejaan kimberlites, South Africa, contain suites of highly chromian, garnet-rich peridotites amongst their xenolith population and an investigation of these xenoliths has been targeted because there is an overlap of mineral compositions withthe garnet-chromite-olivine paragenesis found as inclusions in diamonds. A high proportion of garnets and chromites in these rocks plot in the diamond facies fields on Cr2O3-CaO and Cr2O3-MgO wt. % plots respectively. However, it has also been found that many Cr-rich assemblages are clinopyroxene-bearing (lherzolitic) as well as harzburgitic (i.e. forming arange of chromite-garnet peridotite assemblages). Many samples have garnets with inclusions of serpentine ± chromite (+ clinopyroxene in lherzolitic samples) whose arrangement are sometimes indicative of exsolution or annealed exsolution textures. Initial bulk REE patterns were calculated for the pre-exsolution, pre-metasomatism, high-Cr garnets. The patterns found are humped for harzburgitic samples and relatively flat for lherzolitic ones. These appear to be closely linked to the concentration of Ca in garnet indicative of a single-stage formation process for the humped profiles. The profiles were later modified by exsolution of pyroxenes (with a clear link between Ca content of garnet and the garnet/clinopyroxene partition coefficients) and, in a few cases, by metasomatism causing a zonation of REE.Most of the garnets have strongly developed zonation patterns which are a result of diffusion towards the matrix (external zonation) followed by zonation towards inclusions (internal zonation). Cr-Al and Mg-Ca inter-diffuse in both types of zonation; Ti may also be strongly zoned, whereas Fe is not distinctly zoned in any sample. External zonation may be divided into ‘P-T re-equilibration’ and ‘metasomatic’ types, where the former type conforms to down-P-T garnet-spinel transition reaction simulations and the latter does not. All internal zonations conform to down P-T reaction simulations. All the zonation profiles conform well to diffusion controlled reaction models. External P-T re-equilibration is modelled to have occurred on an order of magnitude greater timescale than internal zonation (~5 Ma compared to ~0.5 Ma using DMg= 10-20m2 /s). Metasomatic zonation occurs over the longest diffusion distances and is modelled to have timescales up to 20 Ma. Based upon the geometries of chemical heterogeneity in the minerals analysed, a sequence of events has been proposed for the evolution of the Newlands and Bobbejaan samples: (1) Earliest known mineralogy. Thereis little evidence for the events prior to and during this stage. However, the modal mineralogy is postulated to have been more garnet and olivine-rich than seen in the samples and the crystals more chemically homogeneous. The P and T in samples are modelled to have the highest Ps and Ts of all stages (potentially > 65kb and > 1350°C). (2) Exsolution.This stage represents the initiation of an event which significantly lowered P and T where spinel and pyroxene exsolved from garnet. (3a) External zonation - P-T re-equilibration and metasomatism. These events are contiguous with stage 2 and P-T estimates for this stage indicate further lowering of P and T. This is accompanied by modification of a few samples by the infiltration of metasomatic fluid. (3b) Internal zonation – P-T re-equilibration. This stage represents the final lowering of P and T, yielding final P-T estimates on clinopyroxene inclusion-garnet boundaries of 38-50 kb and 900-1150°C. These P-T estimates place samples plot on a relatively cool continental steady state geotherm. (4) Kimberlite eruption. The initial formation of the garnet-rich rock types with their (calculated) highly chromian composition and particular initial REE compositions appears to be related to a major depletion event with subsequent burial. Differential interaction with a CO2-bearing fluid would generate the range of harzburgitic and lherzolitic compositions found. However it is acknowledged that this is difficult to determine and constrain because of the lack of preservation of evidence prior to Stage 2.The down-P-T event has a similar timescale and associated uplift rate to be related to continent-continent collision according to the time scales for diffusion. The contemporaneous metasomatism leads to postulation that the event may have been related to the continental accretion of the eastern and western parts of the Kaapvaal craton in the late Archaean. In terms of the relation to diamond, it was found that samples with high-Cr harzburgitic garnets tended to yield P-T estimates that were most substantially into the diamond stability field based upon the Cr-Al partitioning between garnet and spinel and also modelling the samples using the computer programme known as‘Perplex’. Clinopyroxene-bearing samples (i.e. those with a bulk rock composition higher inCa) tended to indicate higher temperatures of equilibration. Cr-spinels from the harzburgitic paragenesis have higher Mg and lower Ti than the lherzolitic Cr-spinels, but overlapping Cr compositions at high Cr and may be differentiated on Cr-Mg and Cr-Ti compositional plots.
Canadian Journal of Development Studies, Vol. 41, 3, pp. 432-449. pdf
Africa, Liberia
alluvials
Abstract: Across sub-Saharan Africa, artisanal and small-scale mining (ASM) represents a major source of direct and indirect employment. Yet, despite the livelihood benefits and the growing interest from governments, donors and policy makers to formalise ASM, most artisanal miners still operate informally. Focusing on Liberia, this article critically investigates the question of why formalisation efforts continue to fail and argues that the persistence of informality in the sector needs to first be understood as a rational strategy for those who profit from it. Only then can sustainable mining reforms be linked to broader national and international extractive sector policy frameworks.
Abstract: The structural depression that occupies the Okavango Basin in southern Africa comprises a depo?centre within the intracratonic Kalahari Basin where sediments of the Cenozoic Kalahari Group have accumulated. The Okavango Basin has been formed due to stretching and subsidence at an area of diffused deformation, southwestwards to the main East African Rift System (EARS). Sediments from two full Kalahari Group sequences, located on opposite sides of the Gumare Fault that forms a major fault within the Okavango Basin, were studied to determine their provenance and chronology. Terrestrial Cosmogenic Nuclide (TCN) 26Al/10Be burial dating was used to constrain a chronostratigraphical framework, and Pb, Sr, and Nd isotopic ratios combined with geochemical and sedimentological analyses were applied to track the source areas of the sediments.Results indicate the following sequence of basin filling: (a) Accumulation between ca. 4-3 Ma during which the currently downthrown (southern) block received a mixture of sediments mostly from the Choma?Kalomo, Ghanzi?Chobe, and Damara terranes, and possibly from the Lufilian Belt and/or Karoo basalts during earlier stages of deposition. Simultaneously, the upthrown (northern) block received sediments from more distant Archean sources in the Zimbabwe and/or Kasai cratons, (b) Hiatus in sedimentation occurred at both sites between ca. 3-2 Ma, (c) Sediments on both sides of the Gumare Fault share a similar source (Angolan Shield) with minor distinct contributions to the downthrown block from the Kasai Craton and local sources input to the upthrown block, and (d) Regional distribution of aeolian sand since at least 1 Ma. The change in source areas is attributed to rearrangements of the drainage systems that were probably linked to vertical crustal movements on the margins of the Okavango Basin. The tectonically induced morphodynamics controlled the landscape evolution of the endorheic basin where vast lakes, wetlands and salt pans have developed through time.
Canadian Journal of Development Studies, Vol. 41, 3, pp. 450-466. pdf
Africa, Ivory Coast
history
Abstract: Implementation of the African Mining Vision in Côte d’Ivoire is weak, and AMV domestication is unlikely to happen soon. Focusing on the artisanal and small-scale mining (ASM) sector, a key section of the AMV, we look at the tumultuous recent history of the diamond mining town Tortiya. The subject of a halting and uneven formalisation process, the case is emblematic for the lack of interest shown in ASM at a policy level. This is due to high costs, and low political and economic returns of formalisation. It underscores a broader lack of strategic vision for the mining sector.
White-Gaynor, A.L., Nyblade, A.A., Durrheim, R., Raveloson, R., van der Meijde, M., Fadel, I., Paulssen, H., Kwadiba, M., Ntibinyane, O., Titus, N., Sitali, M.
Geochemistry, Geophysics, Geosystems, 10.1029/GC008925 20p. Pdf
Africa, South Africa
Geophysics, seismic
Abstract: We report new P and S wave velocity models of the upper mantle beneath southern Africa using data recorded on seismic stations spanning the entire subcontinent. Beneath most of the Damara Belt, including the Okavango Rift, our models show lower than average velocities (?0.8% Vp; ?1.2% Vs) with an abrupt increase in velocities along the terrane's southern margin. We attribute the lower than average velocities to thinner lithosphere (~130 km thick) compared to thicker lithosphere (~200 km thick) immediately to the south under the Kalahari Craton. Beneath the Etendeka Flood Basalt Province, higher than average velocities (0.25% Vp; 0.75% Vs) indicate thicker and/or compositionally distinct lithosphere compared to other parts of the Damara Belt. In the Rehoboth Province, higher than average velocities (0.3% Vp; 0.5% Vs) suggest the presence of a microcraton, as do higher than average velocities (1.0% Vp; 1.5% Vs) under the Southern Irumide Belt. Lower than average velocities (?0.4% Vp; ?0.7% Vs) beneath the Bushveld Complex and parts of the Mgondi and Okwa terranes are consistent with previous studies, which attributed them to compositionally modified lithosphere resulting from Precambrian magmatic events. There is little evidence for thermally modified upper mantle beneath any of these terranes which could provide a source of uplift for the Southern African Plateau. In contrast, beneath parts of the Irumide Belt in southern and central Zambia and the Mozambique Belt in central Mozambique, deep?seated low velocity anomalies (?0.7% Vp; ?0.8% Vs) can be attributed to upper mantle extensions of the African superplume structure.
Abstract: Carbonatites are unusual, carbon-rich magmas thought to form either by the melting of a carbon-rich mantle source or by low-degree partial melting of a carbon-poor (<80 ppm C) mantle followed by protracted differentiation and/or immiscibility. Carbonate-bearing mantle xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatites, have provided key support for a C-rich mantle source. Here, we report unique microscale O and C isotopic analyses of those carbonates, which are present as interstitial grains in the silicate host lava, veins in the xenoliths, and pseudo-inclusions in olivine xenoliths. The ?18O values vary little, from 19‰ to 29, whereas ?13C values are more variable, ranging from -23‰ to +0.5‰. We show that such carbonate ?18O values result from the low-temperature precipitation of carbonate in equilibrium with meteoric water, rather than under mantle conditions. In this framework, the observed ?13C values can be reproduced by Rayleigh distillation driven by carbonate precipitation and associated degassing. Together with petrological evidence of a physical connection between the three types of carbonates, our isotopic data support the pedogenic formation of carbonates in the studied xenoliths by soil-water percolation and protracted crystallization along xenolith cracks. Our results refute a mechanism of C enrichment in the form of mantle carbonates in the mantle beneath the Natron Lake magmatic province and instead support carbonatite formation by low-degree partial melting of a C-poor mantle and subsequent protracted differentiation of alkaline magmas.
Canada, Nunavut, Southampton Island, Africa, Kaapvaal
craton
Abstract: Ancient rocks have survived plate tectonic recycling for billions of years, but key questions remain about how and when they were exhumed to the surface. Constraining exhumation histories over long timescales is a challenge because much of the rock record has been lost to erosion. Argon and helium noble gas thermochronology can reconstruct deep-time <350 °C thermal histories by using the distinct temperature sensitivities of minerals such as feldspar, zircon, and apatite, while exploiting grain size and radiation damage effects on diffusion kinetics. Resolution of unique time-temperature paths over long timescales requires multiple chronometers, appropriate kinetic models, and inverse simulation techniques to fully explore and constrain possible solutions. Results suggest that surface histories of ancient continental interiors are far from uninteresting and may merely be misunderstood.
South African Historical Journal, Vol. 72, 3, pp. 431-460.
Africa, Angola
history
Abstract: In the wake of the Anthropocene there is a growing body of literature questioning the colonial and imperialistic underpinnings of geology, mineralogy and mining sciences. When focused on the African continent, these critiques echo and complement previous analyses of the role that anthropology has played as the ‘handmaiden of colonialism’. This article proposes to liken the two debates by focusing on the ethnographic practices promoted by the Angolan diamond mining company Diamang (1917-1988) during the interwar period. It argues that mineral desire, the greed for mineral resources such as diamonds, copper or gold, was the drive behind the introduction of ethnographic collecting and field-working to the Portuguese colony. The implications of this shift in perspective will be explored regarding the ongoing restitution debate. First, the article demonstrates that the history of the objects collected by Diamang disrupts ‘classic’ readings of the history of Portuguese anthropology focused on ‘disciplinary big men’ and their agendas. Second, it shows how the gathering and interpretation of ethnographic and archaeological data were totally integrated into the extractive apparatus of Diamang. The article then concludes by suggesting that the decolonisation of ethnographic collections must consider their entanglements with mining, geology and mineralogy.
Lithos, doi.org/10.1016/ j.lithos.2020.105918 67p. Pdf
Africa, South Africa
deposit - Roberts Victor
Abstract: Platinum-group elements (PGE) display a chalcophile behaviour and are largely hosted by base metal sulphide (BMS) minerals in the mantle. During partial melting of the mantle, BMS release their metal budget into the magma generated. The fertility of magma sources is a key component of the mineralisation potential of large igneous provinces (LIP) and the origin of orthomagmatic sulphide deposits hosted in cratonic mafic magmatic systems. Fertility of mantle-derived magma is therefore predicated on our understanding of the abundance of metals, such as the PGE, in the asthenospheric and lithospheric mantle. Estimations of the abundance of chalcophile elements in the upper mantle are based on observations from mantle xenoliths and BMS inclusions in diamonds. Whilst previous assessments exist for the BMS composition and chalcophile element budget of peridotitic mantle, relatively few analyses have been published for eclogitic mantle. Here, we present sulphide petrography and an extensive in situ dataset of BMS trace element compositions from Roberts Victor eclogite xenoliths (Kaapvaal Craton, South Africa). The BMS are dominated by pyrite-chalcopyrite-pentlandite (± pyrrhotite) assemblages with S/Se ratios ranging 1200 to 36,840 (with 87% of analyses having S/Se this editing is incorrect. This should read "(with 87% of analyses having S/Se < 10,000)" Please note the 100 ppm) and are characteristically enriched in Os, Ir, Ru and Rh. Nano- and micron-scale Pd-Pt antimonide, telluride and arsenide platinum-group minerals (PGM) are observed spatially associated with BMS. We suggest that the predominance of pyrite in the xenoliths reflects the process of eclogitisation and that the trace element composition of the eclogite BMS was inherited from oceanic crustal protoliths of the eclogites, introduced into the SCLM via ancient subduction during formation of the Colesberg Magnetic Lineament c. 2.9 Ga and the cratonisation of the Kaapvaal Craton. Crucially, we demonstrate that the PGE budget of eclogitic SCLM may be substantially higher than previously reported, akin to peridotitic compositions, with significant implications for the PGE fertility of cratonic mafic magmatism and metallogenesis. We quantitatively assess these implications by modelling the chalcophile geochemistry of an eclogitic melt component in parental magmas of the mafic Rustenburg Layered Suite of the Bushveld Complex.
South African Journal of Geology, Vol. 123, 4, pp. 597-614. pdf
Africa, South Africa
alkaline rocks
Abstract: Numerous Mesoproterozoic alkaline intrusions belonging to the Pilanesberg Alkaline Province are present within the Transvaal sub-basin of the Kaapvaal Craton. The Pilanesberg Complex is the best-known example; it represents one of the world’s largest alkaline complexes, and is associated with a northwest-southeast trending dyke swarm that extends from Botswana to the southwest of Johannesburg. This paper documents the results of a petrological and geochemical study of a thin mafic sill (here referred to as an alkaline igneous body, AIB), which intrudes the ca. 2 200 Ma Silverton Formation close to the southernmost part of the Pilanesberg dyke swarm. The AIB has only been observed in cores from a borehole drilled close to Carletonville. It is hypocrystalline, containing randomly oriented elongated skeletal kaersutite crystals and 6 to 8 mm varioles mainly composed of radially oriented acicular plagioclase. These two textures are related to undercooling, probably linked to the limited thickness (70 cm) of the AIB coupled with a probable shallow emplacement depth. Ar-Ar dating of the kaersutite gives an age of ca. 1 400 Ma, similar to the age of Pilanesberg Complex. However, the AIB is an alkaline basaltic andesite and is thus notably less differentiated than the Pilanesberg Complex and some of its associated dykes, such as the Maanhaarrand dyke, for which we provide whole-rock geochemical data. Literature data indicate that the Pilanesberg dyke swarm also contains mafic hypabyssal rocks suggesting a link between the dyke swarm and the AIB. The AIB is characterized by strongly negative ?Nd and ?Hf, that cannot be related to crustal contamination, as shown by positive Ti and P anomalies, and the absence of negative Nb-Ta anomalies in mantle-normalised trace element diagrams. The AIB magma is interpreted to have been derived from a long-lived enriched, probably lithospheric mantle reservoir. The AIB thus provides important information on the magma source of the Pilanesberg Alkaline Province.
Abstract: Mesozoic diamondiferous lamproite pipes occur along the Kapamba River within the Luangwa Valley of eastern Zambia, which is a ca. 300-200?Ma old Karoo-age precursor branch to the East African Rift System. The Luangwa Rift developed above a reactivated mega-shear zone that cuts through the Proterozoic Irumide Belt between the Congo-Tanzania-Kalahari cratons and thus it provides a rare snapshot of early-stage cratonic rift evolution. The primary mineralogy of the fresh volcanic rocks suggests that they represent a continuum between primitive olivine lamproites and slightly more evolved olivine-leucite lamproites. Mineral compositions and evolutionary trends, such as the strong Al-depletion at Tisingle bondF enrichment in groundmass phlogopite and potassic richterite, resemble those of classic lamproite provinces in circum-cratonic settings (e.g., the Leucite Hills of Wyoming and the West Kimberley field in Australia). However, there are some similarities to orangeites from the Kaapvaal craton (formerly Group-2 kimberlites), type kamafugites from the East African Rift, and ultramafic lamprophyres from a key region of the rifted North Atlantic craton, which implies a complex interplay between source-forming and tectonic processes during Karoo-age lamproite magma formation beneath south-central Africa. The bulk compositions of the Kapamba volcanic rocks fall within the range of ‘cratonic’ low-silica lamproites, but there is overlap with orangeites, in particular with the more evolved leucite- and sanidine-bearing orangeite varieties. Modelling of the process by which most of the original leucite was transformed into analcime suggests that the primitive alkaline magmas at Kapamba contained ~6-9?wt% K2O and had high K2O/Na2O ratios between ~1.6-6.2 at >10?wt% MgO - confirming the ultrapotassic nature of the mantle-derived magmatism beneath the Luangwa Rift. The virtually CO2-free, H2O-F-rich Kapamba lamproites present an extension of the geochemical continuum displayed by the members of the CO2-H2O-rich kamafugite / ultramafic lamprophyre group. Hence, we suggest that the Kapamba lamproites and the type kamafugites, located within separate branches of the East African Rift System, represent melting products of similar K-metasomatized cratonic mantle domains, but their formation occurred under contrasting volatile conditions at different stages during rift development (i.e., incipient versus slightly more advanced rifting). Temperature estimates for peridotite-derived olivine xenocrysts from the Kapamba lamproites suggest that the Luangwa Valley is an aborted cratonic rift that retained a relatively cold (?42?mW/m2) lithospheric mantle root down to ~180-200?km depth during the Mesozoic. Olivine major and trace element compositions support the presence of an Archean mantle root (up to 92.4?mol% forsterite contents) that is progressively metasomatized toward its base (e.g., increasing Tisingle bondCu contents with depth). For south-central Africa, it appears that significant volumes of Archean cratonic mantle domains ‘survived’ beneath strongly deformed and granite-intruded Proterozoic terranes, which suggests that the continental crust is more strongly impacted during collisional or rift tectonics than the ‘stabilizing’ mantle lithosphere.
Abstract: The majority of this paper is a transcription from the video of the AusIMM Webinar at the Western Australian, South West Branch on the 30th July 2020. To view the video go to https://vimeo.com/464013825/1ed4a0c752 . Start listening at about 5 minutes in. The language in this paper is thus vernacular and not geologese. This should make it more easily read and understood by the majority of readers. Africa is a rich source of minerals. The main mining fields in Africa are located on the ring structures and linears from the surface right to the limits of detailed data at 400 km depth. The mechanism and source of the fluid for most mineralisation may have been discovered by this research. This Part 6 section describes the relationship of the metal and diamond mineralisation to the linear and ring structures observed in African seismic tomography at 170-250 km depth.
Lithos, doi.org/10.1016/ jlithos.2020.105880, 26p. Pdf
Africa, South Africa
deposit - Bultfontein
Abstract: The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage. The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/? PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80-120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.
Nature Scientific Reports, doi.org/10.1038/ s41598-020-76800-0 10p. Pdf
Australia, Africa, South Africa
alkaline magmatism
Abstract: Large-scale mantle convective processes are commonly reflected in the emplacement of Large Igneous Provinces (LIPs). These are high-volume, short-duration magmatic events consisting mainly of extensive flood basalts and their associated plumbing systems. One of the most voluminous LIPs in the geological record is the ~?2.06 billion-year-old Bushveld Igneous Complex of South Africa (BIC), one of the most mineralised magmatic complexes on Earth. Surprisingly, the known geographic envelope of magmatism related to the BIC is limited to a series of satellite intrusions in southern Africa and has not been traced further afield. This appears inconsistent with the inferred large size of the BIC event. Here, we present new radiometric ages for alkaline magmatism in the Archean Yilgarn Craton (Western Australia), which overlap the emplacement age of the BIC and indicate a much more extensive geographic footprint of the BIC magmatic event. To assess plume involvement at this distance, we present numerical simulations of mantle plume impingement at the base of the lithosphere, and constrain a relationship between the radial extent of volcanism versus time, excess temperature and plume size. These simulations suggest that the thermal influence of large plume events could extend for thousands of km within a few million years, and produce widespread alkaline magmatism, crustal extension potentially leading to continental break-up, and large ore deposits in distal sectors. Our results imply that superplumes may produce very extensive and diverse magmatic and metallogenic provinces, which may now be preserved in widely-dispersed continental blocks.
Abstract: Very few zircon-bearing, kimberlite-hosted mantle eclogite xenoliths have been identified to date; however, the zircon they contain is crucial for our understanding of subcratonic lithospheric mantle evolution and eclogite genesis. In this study, we constrain the characteristics of zircon from mantle eclogite xenoliths based on existing mineralogical and geochemical data from zircons from different geological settings, and on the inferred origin of mantle eclogites. Given the likely origin and subsequent evolution of mantle eclogites, we infer that the xenoliths can contain zircons with magmatic, metamorphic and xenogenic (i.e. kimberlitic zircon) origins. Magmatic zircon can be inherited from low-pressure mafic oceanic crust precursors, or might form during direct crystallization of eclogites from primary mantle-derived melts at mantle pressures. Metamorphic zircon within mantle eclogites has a number of possible origins, ranging from low-pressure hydrothermal alteration of oceanic crustal protoliths to metasomatism related to kimberlite magmatism. This study outlines a possible approach for the identification of inherited magmatic zircon within subduction-related mantle eclogites as well as xenogenic kimberlitic zircon within all types of mantle eclogites. We demonstrate this approach using zircon grains from kimberlite-hosted eclogite xenoliths from the Kasai Craton, which reveals that most, if not all, of these zircons were most likely incorporated as a result of laboratory-based contamination.
Earth and Planetary Science Letters, doi.org/10.1016/ j.epsl.2020. 116720 19p. Pdf
Africa, South Africa
deposit - Bellsbank
Abstract: The nature of the deep calcium geochemical cycle through time is unresolved, in part due to the dearth of information about the calcium isotope composition of Archean recycled oceanic crust. Remnants of such ancient oceanic crust are preserved in the form of cratonic mantle eclogites, brought to surface as xenoliths in kimberlite magma eruptions. The ? 44 / 40Ca of fresh mantle-derived eclogite xenoliths (i.e., garnet and omphacite mineral separates) from the Bellsbank kimberlite on the Kaapvaal craton in South Africa are presented here in combination with their trace element compositions, garnet Fe3+ contents and ?18O values. The studied Bellsbank eclogite xenoliths have geochemical compositions that indicate oceanic crustal protoliths, with bulk Al2O3 from 15 to 27 wt.%, Eu anomalies from 0.8 to 2.6 and, significantly, garnet ?18O values from +2.7 to +6.2‰. Garnet Fe3+/?Fe contents yield logfO2(?FMQ) values between -4.0 and -1.2 for a depth range of 110-180 km, recording strong redox heterogeneity of the eclogite component within the Archean Kaapvaal mantle lithosphere. Reconstructed bulk eclogite MgO contents correlate negatively with fO2, suggesting that the redox compositions are related to magmatic differentiation during oceanic crust formation, excluding secondary metasomatic overprints. These data may thus emphasize that Archean basaltic oceanic crust had a similarly variable redox composition to modern MORB-type crust. Reconstructed bulk ? 44 / 40Ca values for the Bellsbank eclogites range from +0.28 to +1.56‰. Although some of the xenoliths have ? 44 / 40Ca values that overlap with the average mantle composition and modern MORB (+0.94 ± 0.1 and +0.83 ± 0.05‰), half of our dataset shows excursions to more extreme Ca isotopic compositions. Both higher and lower ? 44 / 40Ca relative to mantle compositions are recorded by the eclogites, with a general negative correlation with ?18O suggestive of seawater-alteration of oceanic crust. The combined low ? 44 / 40Ca (+0.28‰) and ?18O (+3.4‰) measured for one eclogite xenolith may record a subtle imprint by carbonate-rich mantle melts, which are known to contain isotopically light calcium contributed by recycled sediments. In contrast, the high ? 44 / 40Ca of up to +1.56‰ for some eclogite xenoliths, coupled with strong LREE depletion, can be explained by calcium isotope fractionation during partial melting. The protracted history of recycled oceanic crust as probed by cratonic mantle eclogites is recorded by their highly variable ? 44 / 40Ca-?18O-fO2 signatures. Whereas some of this heterogeneity can be linked to processes that operated on the Archean ocean floor such as seawater-alteration of basaltic crust, other sources of compositional variability are introduced by loss and addition of melts during subduction recycling and mantle residence. The observed ? 44 / 40Ca complexity of ancient recycled oceanic crust components at the scale of a single mantle-derived eclogite xenolith suite implies that mantle plume sourced intraplate magmas should reveal similarly strong calcium isotope variations contributed by apparently essential recycled crust components - as observed in the global oceanic island basalt record.
Basin Research, doi.org/10.111 /bre.12527 23p. Pdf
Africa, Namibia
Thermochron, temperature logging
Abstract: Products of onshore passive continental margin erosion are best preserved in offshore sedimentary basins. Therefore, these basins potentially hold a recoverable record of the onshore erosion history. Here, we present apatite fission track (AFT) data for 13 samples from a borehole in the southern Walvis basin, offshore Namibia. All samples show AFT central ages older or similar to their respective stratigraphic ages, while many single grain ages are older, implying none of the samples has been totally annealed post?deposition. Furthermore, large dispersion in single grain ages in some samples suggests multiple age components related to separate source regions. Using Bayesian mixture modelling we classify single grain ages from a given sample to particular age components to create ‘subsamples’ and then jointly invert the entire dataset to obtain a thermal history. For each sample, the post?depositional thermal history is required to be the same for all age components, but each component (‘subsample’) has an independent pre?depositional thermal history. With this approach we can resolve pre? and post?depositional thermal events and identify changes in sediment provenance in response to the syn? and post?rift tectonic evolution of Namibia and southern Africa. Apatite U?Pb and compositional data obtained during the acquisition of LA?ICP?MS FT data are also presented to help track changes in provenance with time. We constrain multiple thermal events linked to the exhumation and burial history of the continental and offshore sectors of the margin over a longer timescale than has been possible using only onshore AFT thermochronological data.
Abstract: Madagascar hosts several Paleoproterozoic sedimentary sequences that are key to unravelling the geodynamic evolution of past supercontinents on Earth. New detrital zircon U-Pb and Hf data, and a substantial new database of ?15,000 analyses are used here to compare and contrast sedimentary sequences in Madagascar, Africa and India. The Itremo Group in central Madagascar, the Sahantaha Group in northern Madagascar, the Maha Group in eastern Madagascar, and the Ambatolampy Group in central Madagascar have indistinguishable age and isotopic characteristics. These samples have maximum depositional ages > 1700 Ma, with major zircon age peaks at c. 2500 Ma, c. 2000 Ma and c. 1850 Ma. We name this the Greater Itremo Basin, which covered a vast area of Madagascar in the late Paleoproterozoic. These samples are also compared with those from the Tanzania and the Congo cratons of Africa, and the Dharwar Craton and Southern Granulite Terrane of India. We show that the Greater Itremo Basin and sedimentary sequences in the Tanzania Craton of Africa are correlatives. These also tentatively correlate with sedimentary protoliths in the Southern Granulite Terrane of India, which together formed a major intra?Nuna/Columbia sedimentary basin that we name the Itremo?Muva?Pandyan Basin. A new Paleoproterozoic plate tectonic configuration is proposed where central Madagascar is contiguous with the Tanzania Craton to the west and the Southern Granulite Terrane to the east. This model strongly supports an ancient Proterozoic origin for central Madagascar and a position adjacent to the Tanzania Craton of East Africa.
Abstract: Tectonically the considered area of junction of four lithospheric plates (Nubian, Arabian, Aegean- Anatolian and Sinai) belongs to the Eastern Mediterranean, with its Cyprus-Levantine marine and Anatolian-Nubian-Arabian continental framing. The anomalousness of the region is manifested in the tectono-structural features of the mantle, lithosphere, hydrosphere and specifics of atmospheric, biospheric processes, and Hominid evolution.
Abstract: The Ubendian Belt between the Archean Tanzania Craton and the Bangweulu Block, represents a Paleoproterozoic orogeny of these two constituents of the Congo Craton assembled at ~1.8?Ga, forming the Central African Shield, during the Columbia Supercontinent cycle and consolidated during the Gondwana assembly. Metagranitoids from the Southern and Northern Ufipa Terranes (Western Ubendian Corridor) and those of the Bangweulu Block are compositionally similar and are contemporaneous. The protolith of the Ufipa Terrane is originated from the collided crustal rocks of the Bangweulu Block. New LA-ICPMS zircon U-Pb age of metagranitoids and granoporphyries confirmed magmatic events from 1.89 to 1.85?Ga. The metagranitoids of the Western Ubendian Corridor and that of the Bangweulu Block cannot be distinguished by their trace element characteristics and ages. Geochemically, they belong to high-K calc-alkaline to tholeiite series. The 1.89-1.85?Ga metagranitoids and granoporphyries are characterized by evolved nature, which are common for slab-failure derived magmas. Such geochemical features and the presence of ~2.0?Ga eclogites suggest an Orosirian oceanic subduction and subsequent slab break-off. Melt derived from the mafic upper portion of torn slab led to the partial melting of crust which formed high-K and calc-alkaline, I- and S-type magmatism in the Bangweulu Block and the Ufipa Terrane. Zircons from two metagranites from the Northern Ufipa Terrane show Neoproterozoic (Ediacaran) overprints at ~570?Ma, suggesting the Bangweulu Block collided with the continental margin of the Tanzania Craton. However, we found non-annealed Orosirian apatites in metagranitoids from the Southern Ufipa Terrane and the Kate-Ufipa Complex, implying that areal heterogeneity of the Pan-African tectonothermal overprint in the Ufipa Terrane. All evidences suggest that the Bangweulu Block and the Ubendian Belt participated in the amalgamation of the Central African Shield as separated continents surrounded by oceanic crusts during the Paleoproterozoic Eburnean and the Neoproterozoic Pan-African orogenies.
Geochimica et Cosmochimica Acta, Vol. 293, pp. 1-17. pdf
Africa, Botswana
deposit - Jwaneng
Abstract: Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8?Ga) and one Neoproterozoic (0.85?Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (>100?s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (<10?km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (?13C?
Abstract: Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich "fibrous" and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.
Journal of African Earth Sciences, Vol. 174, 104059, 17p. Pdf
Africa, Madagascar
melilitites
Abstract: The olivine melilitites from the southern part of the 6.8 Ma-old Takarindiona volcanic field (Eastern Madagascar) are olivine ± chromite -phyric lavas, with zoned titanaugite, perovskite, melilite, nepheline, monticellite, Ba-Ti-mica and Fe-Ti oxides as microphenocrysts and groundmass phases. The rocks are very primitive, rich in incompatible trace elements (e.g., Ba = 1049 ± 153 ppm, Sr = 1050 ± 167 ppm, Nb = 98 ± 13 ppm; La/Ybn = 41 ± 5; La/Nb = 0.88 ± 0.05), and have restricted ranges of initial 87Sr/86Sr (0.70391-0.70410) and 143Nd/144Nd (0.51272-0.51282). The rocks follow a differentiation trend controlled by ab. 20% removal/addition of phenocryst olivine ± chromite. The olivine melilititic magmas are the product of small degrees of partial melting (1-3%) of a peridotitic source, enriched in highly incompatible trace elements by CO2-, F-, and H2O-rich melts, located within the garnet stability field (3-3.5 GPa and ~100 km depth) of sub-continental lithospheric mantle, where carbonates (dolomite) and possibly phlogopite were stable phases. Mantle xenoliths within the volcanics are mostly spinel harzburgites having mineral modes and chemical compositions suggesting variable degrees of "basalt" melt extraction. Based on textural and chemical evidence, and quantitative thermobarometric estimates, the xenoliths were incorporated at a pressure of ~1.1 GPa (~35-40 km depth), far shallower than the source of the melilititic magmas, and along a predictably cool geotherm beneath Archean continental lithosphere. Highly resorbed orthopyroxene xenocrysts mantled by augite indicate that the melilitites may have also entrained lower crustal materials or underplated subalkaline rocks. The mantle sources of the lavas and mantle xenoliths of the Takarindiona district indicate stratification of the lithospheric mantle, and help constraining the lithospheric features and the magmatic history of the Eastern Madagascar craton.
Vancouver Kimberlite Cluster, Feb. 24, recorded update https://www.youtube.com/ channel/UCcZvayDnqDD azIHAh1Otreg gets you into the VKC
Africa, South Africa
deposit - Koffiefontein
Abstract: Diamonds and their mineral inclusions preserve mantle processes over space and time. Forming over a protracted period, diamonds also provide snapshots of early craton formation and mantle evolution over much of Earth’s history. The lithosphere beneath Koffiefontein is extremely depleted and is characterised by high-Mg# olivine and low-Ca garnet. In addition to garnet LREE enrichment, Koffiefontein experienced a unique K-Nb-Ta-rich metasomatism event that resulted in new minerals. The lack of clinopyroxene and co-existing garnet-orthopyroxene assemblages lead to the use of the electron microprobe for trace element analysis of Al in olivine. Geothermobarometry indicates that upper mantle diamond formation conditions are 1100-1300 °C and 4-7 GPa. Koffiefontein diamonds have a main ?13C mode for both peridotitic and eclogitic diamonds similar to mantle carbon. Relationships of ?15N-[N] and ?13C-?15N indicate that nitrogen was derived from subducted sources and suggests that formation of not only eclogitic but also peridotitic diamonds involved fluids derived from altered oceanic crust. Lower mantle diamonds with coexisting ferropericlase and former bridgmanite indicate formation at or below 660 km. The high bulk Mg# of this assemblage is consistent with the diamond substrate originating from the depleted lithospheric mantle portion of an oceanic slab. Diamond formation at Koffiefontein dominantly takes place in depleted peridotite at both lithospheric and lower mantle depths. The ?13C-?15N systematics suggest the same subducted source for both peridotitic and eclogitic diamonds. Subduction has played an important role in the formation and evolution of the Kaapvaal Craton and subsequent diamond formation.
Journal of African Earth Sciences, Vol. 177, 104154, 16p. Pdf
Africa, Angola
deposit - Bailundo
Abstract: Pyrochlore is a common accessory in carbonatite rocks and its composition can provide useful insights on petrogenetic and post-magmatic metal-enrichment processes, especially those which favour its occurrence and concentration. Comprehensive compositional and textural characterization of a large set of pyrochlores from the Bailundo Carbonatite Complex (SW Angola) and associated surface rocks was the basis to (i) evaluate the main effects of metasomatism and weathering as causes of metal leaching or concentration; and (ii) assess pyrochlore compositions as potential petrogenetic or metallogenetic tools, with particular emphasis on pyrochlore enrichment in economic components, such as Ta, REE, U, Th and Pb, during weathering processes. Unweathered fluor- and hydroxyl-calciopyrochlores from deep-seated carbonatitic rocks (provided by a 600 m-deep drill-core) often present high Ta/Nb ratios, as well as high U and Th contents, and comparatively low overall REE concentrations. Metasomatic effects are not easy to assess, given the extreme compositional variability of these pyrochlores. On the other hand, some systematic trends can be established in pyrochlores from weathered surface rocks: these pyrochlores usually show strong depletion in most A-site cations (e.g., Na, Ca, U), and clear enrichment in Nb and in large-ion metals (e.g., Ba, Sr, Pb) usually absent in unweathered pyrochlores. REE seem to be relatively immobile and to become concentrated during weathering. Along with some REE phosphates and oxides, pyrochlore is often present in several domains of the weathering profile, occurring in the outcropping weathered carbonatite as well as in the regolith immediately overlying the intrusion. Thus, both the Bailundo carbonatite intrusion and its weathering products, concentrated inside the ridge formed by differential erosion of the fenitic aureole, constitute good exploration targets for Nb (±Ta ± REE). However, future exploration work should also include a 3-D understanding of the chemical and geological processes at work in both geological environments.
Comminocacoes Geologicas ( Researchgate), July, 7p. Pdf
Africa, Angola
deposit - Bailundo
Abstract: Pyrochlore group minerals are common accessory phases in many rock types of the Bailundo Carbonatite Complex. These minerals record compositional and textural features that provide useful information regarding their genesis and accumulation, monitoring magmatic, metasomatic and weathering events. In drill core samples, primary compositions (significant Ta and U contents, and relatively low Nb and F values) are found in relict cores of strongly metasomatized pyrochlore grains; irregular patches in pyrochlore rims, typically enriched in F, Na and Nb, reflect fluid alteration fronts. At shallower levels, preserved pyrochlores show well-defined concentric zoning and substantially higher values of F and Nb. In the weathering profile, alteration processes include replacement of F, Na and Ca by Ba, Sr, Pb and H2O. These data suggest the possibility of Nb concentration in late-magmatic fluids as fluoride complexes, and its subsequent mobilization and crystallization in the form of pyrochlore at shallower levels of the Bailundo Carbonatite Complex.
Abstract: Crowningshieldite is the natural analog of the synthetic compound ?-NiS. It has a NiAs-type structure and is the high-temperature polymorph relative to millerite (?-NiS), with an inversion temperature of 379 °C. Crowningshieldite is hexagonal, space group P63/mmc, with a = 3.44(1) Å, c = 5.36(1) Å, V = 55.0(2) Å3, and Z = 2. It has an empirical formula (Ni0.90Fe0.10)S and dcalc = 5.47(1) g/cm3. The five strongest lines in the powder X-ray diffraction data are [dmeas in angstroms (I) (hkl)]: 1.992 (100) (102), 1.718 (55) (110), 2.978 (53) (100), 2.608 (35) (101), and 1.304 (17) (202). Crowningshieldite was found as part of a multiphase inclusion in a gem-quality, colorless, type IIa (containing less than ~5 ppm N) diamond from the Letseng mine, Lesotho. The inclusion contains crowningshieldite along with magnetite-magnesioferrite, hematite, and graphite. A fracture was observed that extended from the inclusion to the diamond exterior, meaning that fluids, possibly kimberlite-related, could have penetrated into this fracture and altered the inclusion. Originally, the inclusion might have been a more reduced, metallic Fe-Ni-C-S mixture made up of cohenite, Fe-Ni alloy, and pyrrhotite, akin to the other fracture-free, pristine inclusions within the same diamond. Such metallic Fe-Ni-C-S primary inclusions are a notable recurring feature of similar type IIa diamonds from Letseng and elsewhere that have been shown to originate from the sublithospheric mantle. The discovery of crowningshieldite confirms that the ?-NiS polymorph occurs in nature. In this case, the reason for its preservation is unclear, but the relatively iron-rich composition [Fe/(Fe+Ni) = 0.1] or the confining pressure of the diamond host are potential factors impeding its transformation to millerite. The new mineral name honors G. Robert Crowningshield (1919-2006) (IMA2018-072).
Abstract: A growing number of people are entering the artisanal and small?scale mining (ASM) sector worldwide. In Madagascar, millions of individuals depend on this informal activity. Through a case study in the Alaotra?Mangoro region of Madagascar, our research aimed to understand the "bottom?up" dynamics and ripple effects of the sector, by looking at the realities for rural communities where inhabitants are both directly and indirectly affected by ASM. We were interested in community members' and miners' perceptions of the socio?economic and environmental impacts of ASM, and in identifying the factors attracting people living in one of the country's agricultural hubs to this activity. Our results show a wide diversity of push and pull factors leading people to enter the sector. Although many positive impacts of ASM exist for miners and communities within the vicinity of mines, most miner participants considered themselves worse off since starting to mine, highlighting the high risk and low probability of return of ASM. ASM's potential for local and national development will remain squandered if its negative impacts continue to go unmanaged. Accounting for local contexts and the ripple effects of ASM will be crucial in achieving safety and security for miners, and to tap into the benefits it may offer communities while minimising environmental damage.
Abstract: Thirty-nine garnet harzburgites from Kimberley in the Kaapvaal Craton (South Africa) were studied to constrain the origin, age and evolution of sub-cratonic lithospheric mantle (SCLM). In order to avoid chemical overprinting by recent metasomatism, only garnet harzburgites that appeared clinopyroxene-free to the naked eye were sampled. The majority of garnets were, however, in equilibrium with clinopyroxene (24 of 39). Whole rock and mineral major-trace element geochemistry and garnet Sr-Nd-Hf isotope data are presented. Equilibration pressures range from 3.8-6.1?GPa, indicating the harzburgites were derived from a large portion of the SCLM (~115-185?km). High olivine Mg# (~93.4, n?=?39) and low whole rock heavy rare earth elements (HREE) contents are consistent with large degrees of partial melting (>45%) and garnet exhaustion leaving a dunitic residue with olivine ?90%, orthopyroxene ?10% and HREE <0.01 times chondrite. Mineral modes, whole rock Al2O3 (0.5-3.2?wt%) and SiO2 (43.1-49.1?wt%), however, indicate heterogeneous re-introduction of garnet (?13%) and orthopyroxene (?50%). Harzburgites with high garnet and relatively low orthopyroxene modes (mostly ~7-13% and?~?9-30%; n?=?6) are characterised by mildly sinusoidal garnet REE patterns (Tbsingle bondDy minimum and high HREE) and Archaean depleted Hf TDM ages (2.7-3.3?Ga; ?Hfe: +190 to +709). In contrast, harzburgites with high orthopyroxene and relatively low garnet and modes (~1.5-7.5% and?~?25-50%; n?=?19) are characterised by highly sinuous REE patterns (Hosingle bondYb minimum and low HREE) and Proterozoic enriched Hf TDM ages (0.7-1.6?Ga; ?Hfe: ?16 to +6). It is inferred that Archaean G10 garnet re-introduction caused a significant increase in HREE, making melt depletion models based on HREE inaccurate. Orthopyroxene addition, a few hundred million years later, most likely at ~2.7?Ga and associated with Ventersdorp magmatic activity, caused partial consumption of garnet and olivine, and changed garnet compositions leading to: 1) Cr/Al ratio increase; 2) HREE decrease; 3) more sinusoidal REE patterns; and 4) un-radiogenic 176Hf/177Hf. Garnets define a Lusingle bondHf isochron age of 2702?±?64?Ma (?Hfi?=?+44, n?=?31), which is interpreted as a consequence of partial isotopic equilibrium within the SCLM and mixing of the garnet- and orthopyroxene-rich metasomatic components. The low LILE contents and absence of Nbsingle bondTa anomalies are consistent with modal metasomatism caused by intra-plate magmatism. In addition, the REE signatures of metasomatic agents in equilibrium with the garnets suggest that carbonatitic melts and SiO2-rich hydrous melts were responsible for re-introduction of garnet and orthopyroxene, respectively. Srsingle bondNd isotope systematics were disrupted associated with kimberlite magmatism (Nd isochron: 217?±?58?Ma, ?Ndi?=?+4; n?=?34), consistent with recent G10 garnet transformation into G9 garnets (Ca?+?Fe-enriched). This event may have caused garnet addition (up to 1%), suggesting that garnet was formed or destroyed in at least 4 different events: i) initial extensive polybaric melting, ii) asthenospheric melts re-introducing the bulk of the garnet, iii) orthopyroxene addition and garnet loss, all in the Archaean, and iv) minor garnet addition possibly related to recent kimberlite magmatism prior to eruption.
Abstract: A review of the compositional features of Tunisia, Algeria, and Morocco phosphorites is proposed in order to assess and compare the paleoenvironmental conditions that promoted the deposit formation as well as provide information about their economic perspective in light of growing worldwide demand. Since these deposits share a very similar chemical and mineralogical composition, the attention was focused on the geochemistry of rare earth elements (REEs) and mostly on ?REEs, Ce and Eu anomalies, and (La/Yb) and (La/Gd) normalized ratios. The REEs distributions reveal several differences between deposits from different locations, suggesting mostly that the Tunisian and Algerian phosphorites probably were part of the same depositional system. There, sub-reducing to sub-oxic conditions and a major REEs adsorption by early diagenesis were recorded. Conversely, in the Moroccan basins, sub-oxic to oxic environments and a minor diagenetic alteration occurred, which was likely due to a different seawater supply. Moreover, the drastic environmental changes associated to the Paleocene-Eocene Thermal Maximum event probably influenced the composition of Northern African phosphorites that accumulated the highest REEs amounts during that span of time. Based on the REEs concentrations, and considering the outlook coefficient of REE composition (Koutl) and the percentage of critical elements in ?REEs (REEdef), the studied deposits can be considered as promising to highly promising REE ores and could represent a profitable alternative source for critical REEs.
Abstract: The tectonic-geodynamic characteristics of the North African-Arabian region are complicated by interaction of numerous factors. To study this interaction, we primarily used the satellite gravity data (retracked to the Earth's surface), recognized as a powerful tool for tectonic-geodynamic zonation. The applied polynomial averaging of gravity data indicated the presence of a giant, deep quasi-ring structure in the Eastern Mediterranean, the center of which is located under the Island of Cyprus. Simultaneously, the geometrical center of the revealed structure coincides with the Earth's critical latitude of 35?. A quantitative analysis of the obtained gravitational anomaly made it possible to estimate the depth of the upper edge of the anomalous body as 1650?1700 km. The GPS vector map coinciding with the gravitational trend indicates counterclockwise rotation of this structure. Review of paleomagnetic data on the projection of the discovered structure into the earth's surface also confirms its counterclockwise rotation. The analysis of the geoid anomalies map and seismic tomography data commonly approve presence of this deep anomaly. The structural and geodynamic characteristics of the region and paleobiogeographic data are consistent with the proposed physical-geological model. Comprehensive analysis of petrological, mineralogical, and tectonic data suggests a relationship between the discovered deep structure and near-surface processes. The revealed geological deep structure sheds light on specific anomalous effects in the upper crustal layer, including the high-intensity Cyprus gravity anomaly, counterclockwise rotation of the Mesozoic terrane belt, configuration of the Sinai plate, and the asymmetry of sedimentary basins along the continental faults.
Appendix to previous Lithos article in March 2021, 11p. Pdf
Africa, Botswana
deposit - Damtshaa, Orapa
Abstract: Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich “fibrous” and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.
Abstract: A major debris flow, the Trig Point Hill flow, originating from Kerimasi volcano (Tanzania) contains numerous blocks of extrusive/pyroclastic carbonatites similar to those exposed at the rim of the currently inactive crater. The blocks of calcite carbonatite consist of: (1) large clasts of corroded and altered coarse grained calcite; (2) primary prismatic inclusion bearing phenocrystal calcite; and (3) a matrix consisting primarily of fine-grained prismatic calcite. The large clasts are inclusion free and exhibit a ‘corduroy-like’ texture resulting from solution along cleavage planes. The resulting voids are filled by brown Fe-Mn hydroxides/oxides and secondary calcite. The prismatic or lath-shaped phenocrystal calcite is not altered and contains melt inclusions consisting principally of primary Na-Ca carbonates which contain earlier-formed crystals of monticellite, periclase, apatite, Mn-Mg-magnetite, Mn-Fe-sphalerite and Nb-perovskite. Individual Na-Ca carbonate inclusions are of uniform composition, and the overall range of all inclusions analysed (wt.%) is from 28.7 to 35.9 CaO; 16.7-23.6 Na2O; 0.5-2.8 K2O, with minor SO3 (1.1-2.2) and SrO (0.34-1.0). The Na-Ca carbonate compositions are similar to that of shortite, although this phase is not present. The Na-Ca carbonates are considered to be primary deuteric phases and not secondary minerals formed after nyerereite. Monticellite shows limited compositional variation and contains 2-4 wt.% MnO and 12 wt.% FeO and is Mn-poor relative to monticellite in Oldoinyo Lengai natrocarbonatite. Periclase is Fe-bearing with up to 13 wt.% FeO. Spinels are Cr-free, Mn-poor and belong to the magnetite-magnesioferrite series in contrast to Mn-rich spinels of the magnetite-jacobsite series occurring in Oldoinyo Lengai natrocarbonatite. The matrix in which the ‘corduroy’ clasts and phenocrystal calcite are set consists of closely packed small prisms of calcite lacking melt inclusions, with interstitial fine-grained apatite, baryte, strontianite and minor fluorite. Pore spaces are filled with secondary Mn-Fe hydroxides/oxides, anhydrite and gypsum. The hypothesis that flow-aligned calcite in volcanic calciocarbonatites from Kerimasi, Tinderet, Homa and Catanda is altered nyerereite is discussed and it is considered that these calcite are either primary phases or altered melilite. The nyerereite alteration hypothesis is discussed with respect to the volumetric and compositional aspects of pseudomorphism by dissolution-precipitation replacement mechanisms. This study concludes that none of the volcanic calciocarbonatites containing flow-aligned calcite phenocrysts are altered natrocarbonatite.
Abstract: Samples from three petrographically distinct, intrusive kimberlite bodies and associated kimberlite dykes from the eastern lobe of the Du Toitspan kimberlite pipe, Kimberley, South Africa, have been analysed for their bulk rock major and trace element compositions and their olivine and phlogopite compositions. The two dominant intrusive bodies (D13, D14) are distinguished by the one (D13) being phlogopite-rich and best classified as a macrocrystic hypabyssal phlogopite kimberlite, and the other (D14) being phlogopite-poor and best classified as a macrocrystic hypabyssal monticellite kimberlite. The minor D17 intrusive body is classified as a macrocrystic transitional hypabyssal serpentinized phlogopite kimberlite. The associated kimberlite dykes range texturally from aphanitic to macrocrystic and are classified as calcite kimberlites. The major kimberlite intrusions and their associated dykes show no evidence of crustal contamination and are characterised by broadly overlapping geochemistry except for distinctly higher K2O (> 2?wt%) and Al2O3 (>3?wt%) and flattening HREE patterns (Gd/YbN?=?6.5-7.0) in the D13 - phlogopite kimberlite compared to the D14 - monticellite kimberlite and the calcite kimberlite dykes (Gd/YbN?=?9.6-12.1). These distinguishing geochemical features of the D13 - phlogopite kimberlite are comparable to typical Group II kimberlites in southern Africa. However, their diagnostic incompatible trace element ratios (for example, Th/Nb, La/Nb, Ce/Pb, and Ba/Nb) are instead comparable to other kimberlite intrusions analysed in this study and to southern African Group I kimberlites in general. Semi-quantitative modelling suggests that these kimberlite intrusions could have derived by low (<1%) degrees of partial melting of a source region that is enriched in LREE (Lan?=?~6.1; Ybn?~?1.47) comparable to metasomatised peridotites from the underlying lithospheric mantle. The composition of the D13 phlogopite kimberlite is consistent with a partial melt of a modally metasomatised source containing a higher proportion of residual clinopyroxene relative to garnet (compared to that giving rise to the D14 monticellite kimberlite and calcite kimberlite dykes), as well as accessory amounts of phlogopite, i.e. a garnet phlogopite peridotite (GPP). The absence of K-anomalies on primitive mantle normalized diagrams for the D13 phlogopite kimberlite requires that phlogopite was not a residual phase during partial melting and was exhausted shortly before or at the moment of melt segregation. The higher Gd/Yb ratios and lower K2O in the D14 monticellite kimberlite and calcite kimberlite dykes can be explained by partial melting of a cryptically metasomatized, phlogopite - free, garnet peridotite (GP) source, containing a higher proportion of garnet relative to clinopyroxene. The low absolute K and strong negative K-anomaly on primitive mantle normalized diagrams for the D14 monticellite kimberlite were inherited from a source region that previously experienced cryptic metasomatism by a differentiated fluid already carrying a negative K-anomaly.
Abstract: The Quaternary carbonatite-nephelinite Kerimasi volcano is located within the Gregory rift in northern Tanzania. It is composed of nephelinitic and carbonatitic pyroclastic rocks, tuffs, tuff breccias and pyroclastic breccias, which contain blocks of different plutonic (predominantly ijolite) and volcanic (predominantly nephelinite) rocks including carbonatites. The plutonic and volcanic carbonatites both contain calcite as the major mineral with variable amounts of magnetite or magnesioferrite, apatite and forsterite. Carbonatites also contain accessory baddeleyite, kerimasite, pyrochlore and calzirtite. Zr and Nb minerals are rarely observed in rock samples, though they are abundant in eluvial deposits of carbonatite tuff/pyroclastic breccias in the Loluni and Kisete craters. Pyrochlore, ideally (CaNa)Nb 2 O 6 F, occurs as octahedral and cubo-octahedral crystals up to 300 ?m in size. Compositionally, pyrochlore from Loluni and Kisete differs. The former is enriched in U (up to 19.4 wt.% UO 2 ), light rare earth elements (up to 8.3 wt.% LREE 2 O 3 ) and Zr (up to 14.4 wt.% ZrO 2 ), and the latter contains elevated Ti (up to 7.3 wt.% TiO 2 ). All the crystals investigated were crystalline, including those with high U content ( a = 10.4152(1) Å for Loluni and a = 10.3763(1) Å for Kisete crystals). They have little or no subsolidus alteration nor low-temperature cation exchange ( A -site vacancy up to 1.5% of the site), and are suitable for single-crystal X-ray diffraction analysis ( R 1 = 0.0206 and 0.0290; for all independent reflections for Loluni and Kisete crystals, respectively). Observed variations in the pyrochlore composition, particularly Zr content, from the Loluni and Kisete craters suggest crystallisation from compositionally different carbonatitic melts. The majority of pyrochlore crystals studied exhibit exceptionally well-preserved oscillatory- and sometimes sector-type zoning. The preferential incorporation of smaller and higher charged elements into more geometrically constrained sites on the growing surfaces explains the formation of the sector zoning. The oscillatory zoning can be rationalised by considering convectional instabilities of carbonatite magmas during their emplacement.
Journal of Asian Earth Sciences, Vol. 214, 104773, 9p. Pdf
Africa, Sudan
carbonatites
Abstract: Carbonatite from the Arabian-Nubian Shield of Sudan occurs as dykes in the Nuba Mountains. It is composed of calcite with some feldspars, quartz and fluorite. CaO is the major constituent in this carbonatite and accordingly, it is classified as calico-carbonatite. The studied carbonatite shows exceptionally high concentrations of SrO (4.4 to 5.9 wt%). Ba, Pb and Y occur in relatively higher concentrations compared to other trace elements. Concentration of rare earth elements (?REEs) is relatively low (average 1550 ppm) compared to many primary igneous carbonatites. The chondrite-normalized REE patterns display higher light rare earth elements (LREEs) compared to heavy rare earth elements (HREEs) with slight negative Ce/Ce* and Eu/Eu* anomalies. The ?18OV-SMOW values range between 7.48 and 10.05‰, while ?13CV-PDB values vary from ?6.24 to ?7.38‰, which is close to the primary carbonatites values. Occurrence of carbonatite as dykes with cumulate and triple junction textures, plot of the carbonatite in the true carbonatite fields of the Ba-Sr and Ba + Sr-REE + Y diagrams, igneous-derived ?13CV-PDB and ?18OV-SMOW values and high (La/Yb)N ratios indicate its primary igneous origin. The strong positive correlation between REEs and Sr suggests the occurrence of these elements as secondary strontianite, which was confirmed by SEM and EDX analyses. This might indicate that the enrichment of REEs and Sr in the studied carbonatite is not from the primary magma and most probably took place during a sub-solidus metasomatic process after the carbonatite emplacement.
Abstract: The paper brings together the language of diamond numbers and the underlying principles for calculation of diamond liberation, followed by estimation of process efficiency at circuit and complete plant levels. In this way it provides a reference point, albeit a mixture of the theoretical and empirical, to assess the effectiveness of diamond plant accounting systems in the field. Having established today's baseline, the wider aim is ongoing education, peer technical debate, and progression to a more exact science.
Abstract: Investing in a mining venture can be risky and stakeholders need transparent, unbiased reports to understand the Mineral Resources and Mineral Reserves a mining company holds. Readability and textual choice can be used consciously to manipulate perceptions, or it can be done unconsciously. This exploratory study investigates the readability and textual choice of supplementary Mineral Resources and Mineral Reserves Reports of companies listed on the Johannesburg Stock Exchange. The results indicate that narrative manipulation occurs in these reports through word choices that make the reports difficult to read, as well as specific narrative selections. This reduces the informational value of the reports. The results of the study will be useful to various stakeholders, such as mining company management, investors, investment specialists, financial analysts, and even employees and the general community, who all use these reports to make important decisions. It is also useful for the preparers of the Mineral Resources and Mineral Reserves Reports, Competent Persons, and other technical specialists to be aware of readability and that certain textual choices can affect the interpretation of these reports. It is recommended that bodies such as the JSE and the SAMREC and SAMVAL Code committees consider adding a plain language requirement to regulations, guidelines, and codes to ensure transparent, unbiased, and objective reports.
Journal of Geochemical Exploration, Vol. 224, 106757, 13p. Pdf
Africa, South Africa
deposit - Palabora
Abstract: A detailed characterization of alkaline tailing ponds and waste rock dumps from Phalaborwa Igneous Complex (PIC) South Africa, has been accomplished. The study goes beyond the environmental characterization of mining wastes, offering the first insight towards the recycling of the wastes as alkaline reagent to neutralize acid industrial wastewater. To achieve these aims, tailings and waste rocks were characterized using a combination of conventional, novel and modified Acid Rock Drainage (ARD) prediction methodologies, as well as South African leachate tests, sequential extractions and pseudo-total digestions. The scarcity of Fe-sulphide minerals and the abundance of alkaline minerals indicated that PIC wastes are not ARD producers. The highest neutralization potential was found in the carbonatite rocks and East tailing samples (range between 289 and 801 kg CaCO3 eq/t). According to the National Environmental Management Waste Act (59/2008) of South Africa, tailing ponds and waste rock dumps from PIC classify as non-hazardous (Type 3 waste). The sequential extractions showed that the different fractions from most of the samples would mostly release sulphate and non-toxic elements, such as Ca, Mg, Na and K, which might be a concern if leached in high concentration. In addition, relatively high concentrations of radionuclides, such as U and Th (average of 6.7 and 36.3 mg/kg, respectively) are present in the non-labile fraction of PIC wastes, while the leachable concentrations were always below 0.006 mg/L. Among PIC wastes, East tailing would be the best option as alkaline reagent to neutralize acid wastewater because of its high neutralization potential and non-harmful leachate composition. In general, this study exposes the shortcomings in mine waste characterization, particularly for alkaline mine wastes, and introduces the assessment of potential revalorization as a novel practice in mine waste characterization that, if extended as a regular practice, would facilitate a circular economy approach to the mining industry with its consequent economic and environmental benefits.
South African Journal of Geology, Vol. 124, 1, pp. 279-301. pdf
Africa, Zimbabwe
craton
Abstract: A.M. Macgregor (1888-1961) is remembered for his enormous contribution to geology. His maps changed the course of geological thinking in southern Africa. Following in his footsteps we examine aspects of our current understanding of the geological evolution of the Zimbabwe Craton and, using new SHRIMP U-Pb ages of zircons from felsic volcanic and plutonic rocks from northern Zimbabwe and unpublished data related to the seminal paper by Wilson et al. (1995), a synthesis is proposed for the formation of the Neoarchaean greenstones. The data suggest marked differences (lithostratigraphy, geochemistry and isotope data, mineral endowment and deformational history), between Eastern and Western Successions, which indicate fundamentally different geodynamic environments of formation. The Eastern Succession within the southcentral part of the craton, largely unchanged in terms of stratigraphy, is reminiscent of a rift-type setting with the Manjeri Formation sediments and overlying ca. 2 745 Ma Reliance Formation komatiite magmatism being important time markers. In contrast, the Western Succession is reminiscent of a convergent margin subduction-accretion system with bimodal mafic-felsic volcanism and accompanying sedimentation constrained to between 2 715 and 2 683 Ma. At ca. 2 670 Ma, a tectonic switch likely marks the onset of deposition of Shamvaian felsic volcanism and sedimentation. The Shamvaian resembles pull-apart basin successions and is dominated by deposition of a coarse clastic sedimentary succession, with deposition likely constrained to between 2 672 and 2 647 Ma. The late tectonic emplacement of small, juvenile multiphase stocks, ranging in composition from gabbroic to granodioritic was associated with gold ± molybdenum mineralisation. Their emplacement at 2 647 Ma provides an upper age limit to the timespan of Shamvaian deposition. Amongst the youngest granites are the extensive, largely tabular late- to post-tectonic ca. 2 620 to 2 600 Ma Chilimanzi Suite granites. These granites are characterised by evolved isotopic systems and have been related to crustal relaxation and anatexis following deformation events. After their emplacement, the Zimbabwe Craton cooled and stabilised, with further deformation partitioned into lower-grade, strike-slip shear zones, and at ca. 2 575 Ma the craton was cut by the Great Dyke, its satellite dykes and related fractures.
Abstract: The West-African craton is defined by a combination of Archean and Palaeoproterozoic rocks that stabilised at ~2 Ga towards the end of the Paleoproterozoic Eburnean Orogeny, and therefore may reflect the transition from Archean to modern tectonic processes. Exploring its present lithospheric architecture aids further understanding of not only the craton’s stability through its history but also its formation. We investigate the lithospheric structure of the craton through analysing and modelling magnetotelluric (MT) data from a 500-km-long east-west profile in northern Ghana and southern Burkina Faso crossing part of the Baoulé-Mossi Domain and reaching the Volta Basin in the south-eastern part of the craton. Although the MT stations are along a 2D profile, due to the complexity of the structures characterising the area, 3D resistivity modelling of the data is performed to obtain insights on the thermal signature and composition of the subcontinental lithosphere beneath the area. The thermal structure and water content estimates from different resistivity models highlight a strong dependence on the starting model in the 3D inversions, but still enable us to put constraints on the deep structure of the craton. The present?day thermal lithosphere?asthenosphere boundary (LAB) depth is estimated to be at least 250 km beneath the Baoulé-Mossi domain. The area likely transitions from a cold and thick lithosphere with relatively low water content into thinner, more fertile lithosphere below the Volta Basin. Although the inferred amount of water could be explained by Paleoproterozoic subduction processes involved in the formation of the Baoulé-Mossi domain, later enrichment of the lithosphere cannot be excluded.
Abstract: Projects with long histories must be documented in current disclosures with transparency and materiality, using historical data and historical estimates. Historical data may be of great value if it is from a reliable source, and the raw data can be validated and/or duplicated. Historical estimates can and should be reported, but with qualification of the ever-changing economic parameters of ‘Reasonable Prospects for Eventual Economic Extraction’ (RPEEE). The SAMREC Code requires current sampling results and diamond valuations, without which RPEEE cannot be assessed; consequently, historical estimates cannot ever be declared as current Diamond Resources or Reserves. The SAMREC Code defines historical estimates and provides guidance on the use of historical data. Examples from real projects and reports in the public domain are reviewed in this paper. Opinions on use and misuse are those of the writer; judgment on good or bad practice is not the intention and is left to the opinion of the reader. Comparison, with both the JORC Code (Australasia) and CIM Definition Standards and National Instrument 43-101 (Canada), is provided. The SAMREC Code appears to be more closely aligned with the Canadian standards.
Abstract: Alluvial diamond and other gemstone deposits have, typically, been exploited by small artisanal operations with little or no geological control. Over the last decade, however, alluvial deposits have become more interesting to larger (often listed), mid-tier companies wishing to benefit from the higher incomes generated by high-quality stones. The difficulties associated with evaluation and valuation of such alluvial diamond/gemstone deposits are widely known but, regrettably, often not widely understood - leading to several misconceptions over what can and cannot be expected from such deposits. Fortunately, there is a reasonably well-established body of knowledge on alluvial diamonds that has resulted in accepted industry-standard practices of how to evaluate these deposits. The 2016 version of the SAMREC Code includes several sections specific to the requirements of secondary diamond and gemstone deposits, both alluvial and marine. Consequently, it is possible to define Diamond/Gemstone Resources in accordance with the major international Committee for Mineral Reserves International Reporting Standards (CRIRSCO) type codes. This paper outlines some of the requirements and some of the pitfalls that need to be appreciated while estimating Diamond/Gemstone Resources and/or Reserves on such deposits.
Abstract: The Orapa kimberlite field of Botswana is one of the world’s major diamond producing regions. Within this field, there are several small kimberlite pipes that have not been completely explored in terms of their lateral extent, depth, and diamond potential. Two such pipes, BK54 and BK55, were found during a ground gravity and magnetic survey, and subsequent drilling confirmed the presence of kimberlite material. To determine the physical properties of these pipes, their lateral extent, depth, and thickness were estimated using a gravity and magnetic analysis and 2.5D and 3D modeling. Tilt derivatives of the magnetic data indicated that BK54 has a northwest-trending elliptical shape and BK55 has a roughly circular shape. Residual gravity anomaly maps indicate that BK54 does not have a density anomaly whereas BK55 is associated with a gravity maximum. The 3D gravity and magnetic inversion modeling constrained by magnetic susceptibility measurements indicates that BK54 is smaller in volume than BK55 and that neither pipe is thicker than 125 m. The difference in shape and the lack of a gravity anomaly over BK54 implies a different formation mechanism for each kimberlite pipe. Although several mechanisms are suggested, BK54 may have formed by a more explosive eruption producing more tuffistic material in the crater and diatreme facies. The gravity and magnetic analysis also found that the kimberlite pipes, while small, are larger in extent than was determined by drilling and warrant additional drilling to determine their economic potential.
Journal of African Earth Sciences, Vol. 179, 104204, 17p. Pdf
Africa
EMAG2
Abstract: Data from the Earth Gravitational Model (EGM2008) and the Earth Magnetic Anomaly Grid (EMAG2) were used to develop a continental scale crustal thickness model for Africa, and to estimate the depth to the bottom of the magnetic layer (DBML) and the geothermal gradient and heat flow. The results are: (1) the estimated DBML from the magnetic data varies from ~23.0 to ~37.2 km. The shallowest DBML values are located in the northern, eastern, and western parts of the continent, whereas the deepest values are observed in the central and southern regions. (2) The estimated crustal thickness based on gravity data varies from ~29.9 km in the northern and western parts of Africa to ~48.0 km in its southern regions, with an average thickness of 35.1 km for the whole continent. (3) The estimated heat flow varies between high values of 46-59 mW/m2, observed in the northern, eastern, and western regions to low values of ~< 41 mW/m2, observed in the central and southern parts of the continent. (4) The geothermal gradient values vary between 14.5 and 23.6 °C/km (5) The East African rift zone is underlain by shallow DBML characterized by high heat flow values that vary between 42 and 59 mW/m2 (6) The heat flow anomalies in Egypt and Libya may be associated with the zone of the Pelusium megashear system, and it shows heat flow values that vary between 36.3 and 59.0 mW/m2. The current study has taken advantage of the availability of the EGM2008 and EMAG2 datasets to map crustal thickness variations and DBML beneath the continental landmass of Africa.
Abstract: The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpretation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 - 0.74) and high-Cr (Cr# = 0.79 - 0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS2) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but “exotic” zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and diaspore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantle-derived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage.
Journal of African Earth Sciences, Vol. 179, 104206, 21p. pdf
Africa, Cameroon, Central African Republic
geophysics
Abstract: The Bangui Magnetic Anomaly (BMA) is one of the largest magnetic anomalies in the world whose origin is still not known. This research investigated the crustal thickness, Curie depths and thermal structures in the Central African sub-regions - Cameroon, Central African Republic and adjacent countries - which are largely characterized by the Bangui Magnetic Anomaly. To achieve a better understanding and clearer idea of the location of the possible sources of the BMA, analyses of geothermal structures were conducted. Two potential methods were used: gravity to evaluate the crustal thickness and magnetics for geothermal analysis. Spectral analysis of gravity data shows that crustal thickness range between 14 and 55 km. The highest depths were found in Central African Republic. The lower values of crustal thickness were obtained in South-Chad basin with a minimum of roughly 14 km. Geothermal analysis is carried out using the Curie point depth, thermal gradient and heat-flow evaluations. The results show that the BMA is related to a thick crust of roughly 40 km. Depth to the bottom of possible sources does not exceed the lower crust. The mean Curie point depth estimated is 38 km with an error of ±2 km. Geothermal results also show the difference in the thermal behaviour between the crust in the Pan African and Precambrian domain. The mobile zone which constitutes the Pan African domain is associated with a thin crust of high heat-flow values of 65 mW/m2. However, the Precambrian domain beneath the BMA is associated with a thick crust with lower heat-flow values (roughly 45 mW/m2). The difference between crustal thickness and Curie point depths shows that all the sources of the BMA are crustal. The present results are in favour of a geological origin for the Bangui Magnetic anomaly.
Abstract: Chemical events involving deep carbon- and water-rich fluids impact the continental lithosphere over its history. Diamonds are a by-product of such episodic fluid infiltrations, and entrapment of these fluids as microinclusions in lithospheric diamonds provide unique opportunities to investigate their nature. However, until now, direct constraints on the timing of such events have not been available. Here we report three alteration events in the southwest Kaapvaal lithosphere using U-Th-He geochronology of fluid-bearing diamonds, and constrain the upper limit of He diffusivity (to D???1.8?×?10?19 cm2 s?1), thus providing a means to directly place both upper and lower age limits on these alteration episodes. The youngest, during the Cretaceous, involved highly saline fluids, indicating a relationship with late-Mesozoic kimberlite eruptions. Remnants of two preceding events, by a Paleozoic silicic fluid and a Proterozoic carbonatitic fluid, are also encapsulated in Kaapvaal diamonds and are likely coeval with major surface tectonic events (e.g. the Damara and Namaqua-Natal orogenies).
South African Journal of Geology, doi:10.25131/sajg.124.0028 10p. Pdf
Africa, South Africa
deposit - Salpeterkop
Abstract: The Salpeterkop volcano forms part of what has been referred to as the Upper Cretaceous Sutherland Suite of alkaline rocks, an igneous province composed of olivine melilitites, carbonatites, trachytes and ultramafic lamprophyres. Salpeterkop is a remnant of the summit tuff ring structure that surrounds a crater which is almost 1 km in diameter and is filled with epiclastic strata. Five pieces of silicified wood were collected from the crater filled sediments, sectioned and identified as a new species of Cupressinoxylon, C. widdringtonioides. This is the first example of the fossil genus in South Africa. Only one member of the Cupressaceae s.l. occurs in southern Africa today. From the wide and indistinct growth rings in the fossil wood it can be deduced that the local climate was warm and humid with little or no seasonality, in support of global records of a warm Late Cretaceous. The preservation of the crater further signifies the low level of erosion the region has experienced since its emplacement.
Abstract: Thermal events and metasomatic processes have influenced the Kaapvaal craton in South Africa. High-density fluids (HDFs) trapped as microinclusions in diamond are main metasomatic agents which provide an insight to these processes in the Earth's mantle. Here we present data for 15 fibrous, HDF-bearing diamonds from the Voorspoed mine, South Africa, reflecting multiple diamond-forming events in a cooling lithosphere. Analyzed by FTIR and EPMA, the Voorspoed fibrous diamonds reveal three populations that differ in their nitrogen aggregation and HDF composition. A silicic?carbonatitic group containing 11-30% B-centers, a saline group containing 5-16% B-centers, and a single high-Mg carbonatitic diamond with 0% B-centers. The distinct nitrogen aggregation of the fibrous diamond groups in Voorspoed and the lack of clear major element evolutionary trends for each HDF type or intermediate compositions between the different types suggest different time-temperature formation histories. Thermobarometry of mineral inclusions in non-fibrous monocrystalline Voorspoed diamonds (Viljoen et al., 2018) indicates that the Voorspoed lithosphere cooled by 100-200 °C since their host diamonds crystallized at high initial temperatures. High temperatures in Voorspoed lithosphere can be correlated with the eruption of the Ventersdorp flood basalts at the central Kaapvaal (ca. 2.7 Ga) or the Bushveld complex (ca. 2.06 Ga), and cooling rates of the lithosphere provide a time frame for a cooling process that originated ~2-3 Ga. Combining these data with the nitrogen aggregation systematics of fibrous and monocrystalline Voorspoed diamonds, we suggest that most Voorspoed diamonds formed during 4 metasomatic events: the oldest one recorded took place between 2 and 3 Gyr as a result of a major thermal perturbation, whereas the following three occurred between 200 and 600 Myr, 30-90 and < 30 Myr before kimberlite eruption in a cooling lithosphere. An even older (or deeper) event is hinted by a few diamonds where all nitrogen is in B-centers. The sequence of events implied by Voorspoed HDF compositional and nitrogen aggregation differences show affinities with other occurrences in South Africa (e.g. Kimberley, Finsch and Koffiefontein) and may reflect thermal and lithological variation between the central and southwest Kaapvaal lithosphere.
Le Bras, L.Y., Bolhar, R., Bam, L., Guy, B.M., Bybee, G.M., Nex, P.A.M.
Three dimensional tectural investigation of sulfide mineralisation from the Loolekop carbonatite-phoscorite polyphase intrusion in the Phalaborwa Igneous Complex ( South Africa), with implications for ore-forming processes.
Mineralogical Magazine, 19p. Pdf doi:10.1180/mgm.2021.32
Abstract: Controversies surround the origin of kimberlite megacrysts, including whether and how they are genetically related to their host kimberlites, the relationship between the Cr-poor and Cr-rich suites and the dominant processes responsible for elemental and isotopic variations of megacrysts from a given kimberlite. We present new in-situ major and trace element and Sr isotopic results for clinoyroxene and garnet megacrysts from four southern African kimberlites: Colossus and Orapa (Group 1 kimberlites on the Zimbabwe craton), and Kalkput and Bellsbank (Group 2 kimberlites on the western Kaapvaal craton), that include both Cr-poor and Cr-rich megacryst varieties. Cr-poor megacrysts are present at Colossus, Orapa and Kalkput and the data exhibit tight, well-defined trends on major element diagrams as well as incompatible and rare earth element abundances similar to those previously reported for Cr-poor megacrysts. Cr-rich megacrysts, which are also present at Orapa and are the only variety present at Bellsbank, generally have higher Mg# values, lack well-defined major element trends and show stronger incompatible element enrichments as well as more radiogenic 87Sr/86Sri ratios than Cr-poor megacrysts from the same kimberlite group. Thermobarometry indicates that the Cr-poor megacrysts equilibrated at temperatures of ?1200 to 1450 °C and pressures of 4.5 to 7.5 GPa. Cr-rich megacrysts, in contrast, extend to temperatures and pressures as low as 700 °C and 3 GPa, respectively. This indicates that, in the studied suites, Cr-poor megacrysts equilibrated at high temperatures in the lower lithosphere (>135 km), whereas Cr-rich megacrysts typically equilibrated at lower temperatures and pressures. Within the Cr-poor megacrysts from Group 1 and Group 2 kimberlites, there is a clear correspondence between kimberlite group, diagnostic incompatible element ratios (e.g., Nb/La) and Sr isotope ratios that parallel the differences noted between whole-rock Group 1 and Group 2 kimberlites. In the case of Cr-poor megacrysts, similar calculated melt compositions in equilibrium with garnet and clinopyroxene from the same kimberlite were obtained using recent high-pressure mineral?carbonated melt partition coefficients. This suggests formation in conditions close to trace element equilibrium, and is consistent with crystallization from primitive melts with kimberlite-like trace element compositions. In the case of Cr-rich megacrysts, differences in the compositions of melts in equilibrium with clinopyroxene and garnet tend to be larger, and melts in equilibrium with Cr-rich clinopyroxene tend to show significantly greater incompatible element enrichments than those of estimated near-primary kimberlite melts. This could be due to the different behaviour of clinopyroxene and garnet during metasomatic melt-rock interaction, but the apparent disequilibrium between clinopyroxene and garnet could also be due to some of the Cr-rich megacrysts actually being peridotitic xenocrysts. We propose a model for the origin of southern African megacrysts in which carbonated protokimberlite melts formed stockwork-like bodies of variable size in the deep lithosphere (>130 km), which fed networks of melt-filled veins extending into the surrounding and overlying mantle. Crystallization of larger melt bodies resulted in megacryst assemblages dominated by Cr-poor megacrysts, and the incompatible element and isotopic characteristics of these dominantly reflect those of the protokimberlite melt. In contrast, crystallization of smaller melt bodies and their vein networks resulted in megacryst assemblages dominated by Cr-rich megacrysts, which formed as a result of extensive assimilation and metasomatic melt-rock interaction between protokimberlite and peridotite wallrock at low melt/rock ratios, particularly in the middle to shallow lithosphere where pre-existing potassic metasomatic heterogeneities are prevalent. The Cr-rich nature and enrichments in incompatible elements and radiogenic Sr in the Cr-rich megacrysts reflect extensive interaction of their parental magmas with this metasomatized peridotite.
AGU Research Letter, 10.1029/2021GL092570 11p. Pdf
Africa, South Africa
geophysics - magnetotellurics
Abstract: Measuring the composition of the Earth’s mantle is important for understanding mantle processes like plate tectonics, but is surprisingly difficult. Our most accurate information comes from mantle rocks, called xenoliths, that have been brought to the surface during volcanic eruptions. However, these rocks only come from a handful of places. We tend to expect that the rest of the mantle has the same composition as the xenoliths but this might be incorrect. We tested whether xenolith compositions really are representative of the broader mantle by comparing them with compositions interpreted from electrical conductivity models of the mantle. We carried out this comparison in the Kimberley region, South Africa, because it has excellent xenolith and electrical conductivity data. Our results show that xenolith compositions do seem to be broadly representative but there are two important differences: Hydrous minerals found in some xenoliths may not be spatially extensive depending on temperature, and the water contents of some other minerals are different from the broader region. This means that the compositions of xenoliths are at least partly controlled by local processes. Electrical conductivity data may be more useful for measuring some aspects of the composition of the broader mantle, especially its water content.
Journal of Gems & Precious Metals, Vol. 1, 1 pp. 1-11. pdf
Africa, South Africa
deposit - Loxtondal
Abstract: At the end of 2014, around the so called Loxtondal Orangeitic (now called Kaapval type lamproites) cluster, in Boshof district, two circular a nomalies (~540 to ~1100 meters in diameter) were identified by Landsat Satellite Images and interpreted as being of "kimberlites" pipes; probable anomaly which were referred to as Olie 1 and Olie 2. Subsequently, 100 kg of soil samples (horizon A/B) were taken for each of these anomalies. From them there was a high concentration of indicator minerals (IM): olivine, garnets (violets, reds and oranges), chromites, ilmenites, rutile, frosting tourmaline, zircon and among them some crystals of micro and macro diamonds. The high concentration of IM on Olie 2 led to focus the work on it . IM of Olie 2 was burned in HFl and by caustic fusion what contributed about 86 macro (<1 mm) and micro diamonds. The previus works contributed to raising the interest of some diamond geology groups that took new samples that provided electron microprobe analysis of hundreds of chromites and hundreds of garnets: chromites; picro-cromites, and G-9-G-10 garnets. The calculated pressure of the formation of chromites and garnets of Oli e 2 released in the information of seismic Vs-1D and tomography (Model TX2011). It would allow more adequately to reproduce these two minerals generated in the facies of diamonds and separate them from those generated in facies of graphite. A task that would allow a better approach to the diamond potential of this anomaly studied. It was found that in Olie 2 chromites of diamond facies and garnets (G 9 and G 10) are very representative. For this time the study of the lithospheric cratonic mantle (Archon), through of the commented seismic Vs-1D and tomography (Model TX2011) on the Loxtondal cluster (Olie-2)/Kimberley-area setting allowed to estimate the surface heat flow as being approximately 37.5 mW/m2 = 280 km depth of cratonic root (or LAB). Environment in which the highest reference diamond grade is the Kimberley pipe with 200 cpht; and so, for this reason, a similar diamonds-grade could be expected on the Olie-2/potential associated pipes-area.
Abstract: The central region of the Kaapvaal craton is relatively understudied in terms of its lithospheric mantle architecture, but is commonly believed to be significantly impacted by post-Archean magmatism such as the ca. 2056 Ma Bushveld large igneous event. We investigate a collection of 17 eclogite xenoliths from the Cretaceous Palmietfontein kimberlite at the Western Limb of the Bushveld Complex for their mineralogical compositions (major and trace elements, plus Fe3+ contents), as well as stable oxygen and radiogenic Pb isotopic compositions to gain further insights into the nature and evolution of the central Kaapvaal cratonic mantle lithosphere. New U/Pb age determinations on mantle-derived zircon yield a magma emplacement age of ca. 75 Ma for the Palmietfontein Group-1 kimberlite, which means that the entrained eclogite xenoliths may record a protracted metasomatic history from the Proterozoic through to most of the Phanerozoic eon. Garnet ?18O values of up to 6.9‰ and positive Eu anomalies for the bulk rocks suggest seawater-altered oceanic crustal protoliths for the Palmietfontein eclogite xenolith suite, which is typical for the eclogitic components of the Kaapvaal root and other cratonic mantle sections worldwide. However, several features of the Palmietfontein eclogites are commonly not observed in other mantle-derived eclogite xenolith suites. Firstly, the samples studied yield relatively low equilibration pressures and temperatures between 2.7 and 4.5 GPa and 740-1064 °C, indicating a relatively shallow residence between 90 and 150 km depths. Secondly, euhedral coarse amphibole is present in several eclogite nodules where it is in equilibrium with ‘touching’ garnet, supporting eclogite residence within the amphibole stability field at uppermost lithospheric mantle conditions. Thirdly, primary omphacitic clinopyroxene is often overgrown by diopside, and is significantly enriched in incompatible trace elements. The clinopyroxene is also characterized by elevated 206Pb/204Pb of 17.28-19.20 and 207Pb/204 Pb of 15.51-16.27, and these Pb isotopic compositions overlap with those of Mesozoic Group-2 kimberlites from the Kaapvaal craton. Our results show that eclogites reside at ~85 km depth beneath the central Kaapvaal craton as part of a layer that corresponds to an approximately 50 km thick seismically-detected mid-lithospheric discontinuity. Mid-lithospheric discontinuities have been interpreted as metasomatic fronts formed by focussed crystallization of hydrous mineral phases from enriched volatile-bearing melts, and as such the strongly overprinted amphibole-bearing eclogite xenoliths from Palmietfontein may represent a physical expression of such seismically anomalous metasomatic layer at mid-lithospheric depth. Our Pb isotope data suggest that the focussed metasomatism can be attributed to volatile-rich melts reminiscent of potassic Group-2 kimberlites, which have been invoked in MARID-style metasomatic overprinting of the lower lithospheric mantle beneath the western Kaapvaal craton. However, the relatively low fO2 recorded by the Palmietfontein eclogites (minimum FMQ-4.5) suggests that the metasomatism at mid-lithospheric depth was less protracted compared to the more intensive and oxidizing metasomatism typically observed near the base of cratonic mantle roots. While it is possible that Proterozoic magmatic events were responsible for the focussed mid-lithospheric metasomatism of the Kaapvaal mantle, on the basis of the Pb isotope constraints the Palmietfontein eclogites were most likely overprinted during ca. 120 Ma Group-2 kimberlite magmatism.
Tappe, S., Massuyeau, M. , Smart, K.A., Woodland, A.B., Gussone, N., Milne, S., Stracke, A.
Sheared peridotite and megacryst formation beneath the Kaapvaal Craton: a snapshot of tectonomagmetic processes across the lithosphere-asthenosphere transition.
Abstract: Enrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1-2 km from the Songwe Hill carbonatite, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focusing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid-rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3100 µg g?1, HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing fluids, reducing fluid - country-rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralization within the source carbonatite body at depth.
Eclogitic geotherms of the Rio de la Plata craton archon-core. Estancia Trementina and Puentesino, DPTO. Of Concepcion - Parauay. Compared of two large diamond deposits Argyle ( lamproitic ) and Orapa ( kimberlitic).
Historia Natural, Vol. 11, 2, pp. 5-16. pdf
South America, Paraguay, Australia, Africa, Botswana
Abstract: The objective of our study is to establish an assessment of four volcanic hazards in a country threatened by the eruption of the OlDoinyo Lengai volcano. The last major eruption dates back to 2007-2008 but stronger activity in 2019 has revived the memory of volcanic threats to the Maasai and Bantu communities and human activities (agro-pastoral and tourism). The methods chosen have had to be adapted to the scarce and incomplete data. The volcanic hazards and their probability of occurrence were analysed on the basis of data available in the scientific literature and were supplemented by two field missions combining geography and hydro-geomorphology. Our study enabled us to map the hazards of ash fall, lava flows, lahars and avalanches of debris. Each hazard was spatialised by being ascribed an intensity. They are sometimes synchronous with the eruption sometimes they occur several months or years after a volcanic eruption. The results are the first step towards developing a volcanic risk management strategy, especially for the pastoral communities living around Lengai and for the growing tourist activities in this area.
Geological Society of London Special Publications, doi:https://dori.org/10.1144/SP513-2021-84 30p. Pdf proof
Africa, South Africa
lamproite
Abstract: Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a ca. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic for the bulk rock compositions and is associated with strongly depleted Th-U-Nb-Ta contents at high Cs-Rb-Ba-K concentrations. The resultant high LILE/HFSE ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones. The Bushveld-intersecting orangeite dykes have strongly enriched Sr-Nd-Hf isotopic compositions (initial 87Sr/86Sr = 0.70701-0.70741; ?Nd = ?10.6 to ?5.8; ?Hf = ?14.4 to ?2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti-Nb-Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2-1.1 Ga Namaqua-Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma. Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H2O-CO2-F in the petrogeneses of these potentially diamondiferous alkaline rocks.
Abstract: Plate reconstruction studies show that the Neotethys Ocean was closing due to the convergence of Africa and Eurasia toward the end of the Cretaceous. The period around 75 Ma reflects the onset of continental collision between the two plates as convergence continued to be taken up mostly by subduction of the Neotethys slab beneath Eurasia. The Owen transform plate boundary in the northeast accommodated the fast northward motion of the Indian plate relative to the African plate. The rest of the plate was surrounded by mid-ocean ridges. Africa was experiencing continent-wide rifting related to northeast-southwest extension. We aim to quantify the forces and paleostresses that may have driven this continental extension. We use the latest plate kinematic reconstructions in a grid search to estimate horizontal gravitational stresses (HGSs), plate boundary forces, and the plate's interaction with the asthenosphere. The contribution of dynamic topography to HGSs is based on recent mantle convection studies. We model intraplate stresses and compare them with the strain observations. The fit to observations favors models where dynamic topography amplitudes are smaller than 300 m. The results also indicate that the net pull transmitted from slab to the surface African plate was low. To put this into context, we notice that available tectonic reconstructions show fragmented subduction zones and various colliding micro-continents along the northern margin of the African plate around this time. We therefore interpret a low net pull as resulting from either a small average slab length or from the micro-continents' resistance to subduction.
South African Journal of Geology, Vol. 124, pp. 421-442.
Africa, South Africa
magmatism
Abstract: At least four spatially overlapping Large Igneous Provinces, each of which generated ~1 x 106 km3 or more of basaltic magmas over short time intervals (<5 m.y.), were emplaced onto and into the Kaapvaal Craton between 2.7 and 0.18 Ga: Ventersdorp (2 720 Ma, ~0.7 x 106 km3), Bushveld (2 056 Ma, ~1.5 x 106 km3), Umkondo (1 105 Ma, ~2 x 106 km3) and Karoo (182 Ma, ~3 x 106 km3). Each of these has been suggested to have been derived from melting of sub-continental lithospheric mantle (SCLM) sources, but this is precluded because: (1) each widespread heating event sufficient to generate 1 to 2 x 106 km3 of basalt from the Kaapvaal SCLM (volume = 122 to 152 x 106 km3) would increase residual Mg# by 0.5 to 2 units, depending on degree of melting, and source and melt composition, causing significant depletion in already-depleted mantle, (2) repeated refertilization of the Kaapvaal SCLM would necessarily increase its bulk density, compromising its long-term buoyancy and stability, and (3) raising SCLM temperatures to the peridotite solidus would also have repeatedly destroyed lithospheric diamonds by heating and oxidation, which clearly did not happen. It is far more likely, therefore, that the Kaapvaal LIPs were generated from sub-lithospheric sources, and that their diverse geochemical and isotopic signatures represent variable assimilation of continental crustal components. Combined Sr and Nd isotopic data (n = 641) for the vast volumetric majority of Karoo low-Ti tholeiitic magmatic products can be successfully modelled as an AFC mixing array between a plume-derived parental basalt, with <10% of a granitic component derived from 1.1 Ga Namaqua-Natal crust. Archaean crustal materials are far too evolved (?Nd ~ -35) to represent viable contaminants. However, a very minor volume of geographically-restricted (and over-analysed) Karoo magmas, including picrites, nephelinites, meimechites and other unusual rocks may represent low-degree melting products of small, ancient, enriched domains in the Kaapvaal SCLM, generated locally during the ascent of large-volume, plume-derived melts. The SCLM-derived rocks comprise the well-known high-Ti (>2 to 3 wt.% TiO2) magma group, have ?Nd, 182 values between +10.5 and -20.9, and are characteristically enriched in Sr (up to 1 500 ppm), suggesting a possible connection to kimberlite, lamproite and carbonatite magmatism. These arguments may apply to continental LIPs in general, although at present, there are insufficient combined Sr + Nd isotopic data with which to robustly assess the genesis of other southern African LIPs, including Ventersdorp (n = 0), Bushveld (n = 55) and Umkondo (n = 18).
South African Journal of Geology, Vol. 124, pp. 519-536.
Africa, South Africa
geodynamics
Abstract: The delicate interplay of various Earth’s systems processes in the Critical Zone is vital in ensuring an equilibrium across the different spheres of life. The upper crust forms a thin veneer on the Earth’s surface that is defined by an interconnected network of brittle structures. These brittle structures enable various Earth System processes. Increased anthropogenic interactions within the very upper crust have seemingly resulted in a growing number of negative natural effects, including induced seismicity, mine water drainage and land degradation. Brittle structures across South Africa are investigated. These structures include various fractures and dykes of different ages and geodynamic evolutions. The orientation of these structures is compared to the underlying tectonic domains and their bounding suture zones. The orientations corroborate an apparent link between the formation of the brittle structures and the tectonic evolution of the southern African crust. Reactivation and the creation of new structures are also apparent. These are linked to the variability of the surrounding stress field and are shown to have promoted magmatism, e.g., Large Igneous Provinces, and the movement of hydrothermal fluids. These fluids were commonly responsible for the formation of important mineral deposits. The preferred structural orientations and their relationship to underlying tectonic zones are also linked to fractured groundwater aquifers. Subsurface groundwater displays a link to structural orientations. This comparison is extended to surficial water movement. Surface water movement also highlights an apparent link to brittle structures. The apparent correlation between these Earth’s systems processes and the interconnectivity developed by brittle structures are clear. This highlights the importance of high-resolution geological and structural mapping and linking this to further development of the Earth’s Critical Zone.
Abstract: The Small and Junior diamond mining industry, which is dominated by alluvial diamond miners, and a few remaining small kimberlite operations, produced a high proportion of diamonds in South Africa in the late 1950s and early 1960s, prior to the discovery and development of major kimberlite mines such as Finsch and Venetia, in the 1970’s and 80’s. Subsequent to these discoveries the Small and Junior sector remained an active and important participant in the local diamond industry, particularly in respect of the highly sought after top-quality gemstone diamonds produced from the extensive alluvial deposits of South Africa. Since 2004 the sector has shown a strong decline. This report highlights the challenges faced by the Small and Junior diamond miners and makes recommendations for the revival of this sector.
The impact of the mineral and petroleum resources development act, ACT 28 of 2002 ( MPRDA) on th sustainability of the alluvial diamond mining sector in the northern Cape Province.
Abstract: In the early 1990s several rebel groups turned to natural resource extraction to pay for war. A key form of this is rebel diamond production, commonly referred to as conflict diamonds, which is widely perceived as being highly beneficial to insurgent organisations. Yet in the Angolan Civil War (1992-2002), the use of conflict diamonds by the National Union for the Total Independence of Angola (UNITA) resulted in a decisive insurgent defeat. How can this outcome be explained? Offering a nuanced understanding of how conflict diamonds affect civil war, this article shows that although diamonds generated considerable revenue for UNITA, they were not an effective method for them to take on the Angolan government. This was for two reasons: internally, the rebels greatly struggled to convert their diamond proceeds into sufficient goods and services; and externally, it left the group highly vulnerable to international countermeasures in the form of United Nations Security Council sanctions. Natural resource extraction may therefore not be as useful to rebel groups as is frequently believed.
Earth Science Reviews , Vol. 219, 103616 231p. Pdf
Africa, Namibia
Craton - Congo
Abstract: Otavi Group is a 1.5-3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran?Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S?N crustal stretching (770-655?Ma), followed by post-rift thermal subsidence (655-600?Ma). Rifting under southern Swakop Group continued until 650-635?Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S?N plate divergence. Rift-related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746?Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770-717?Ma; (2) glacial early Cryogenian/Sturtian, 717-661?Ma; (3) non-glacial middle Cryogenian, 661-646?±?5?Ma; (4) glacial late Cryogenian/Marinoan, 646?±?5-635?Ma; and (5) non-glacial early Ediacaran, 635-600?±?5?Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585?measured sections (totaling >190?km of strata) and?>?8764 pairs of ?13C/?18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5-9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3-8.8?m Myr?1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ?1.6?km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9?m regional average thickness, n?=?140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ?90?m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ?0.6?km of paleobathymetric relief. Late Tonian shallow-neritic ?13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770-760?Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737-717?Ma) in Swakop Group. Devede Fm is isotopically heavy, +4-8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746?Ma volcanics and shows two negative excursions bridged by heavy ?13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic ?13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0-3.5‰. Early Ediacaran neritic ?13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (??+?11‰) with extreme point-to-point variability (?10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform ?13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9?±?1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60-0.58?Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ?1.85?km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
Abstract: Copper-sulfides within carbonatites and phoscorites of the Phalaborwa Igneous Complex, South Africa, have been investigated since the middle of the 20th Century. However, aspects of ore formation have remained unclear. This study examines the mechanisms involved in Cu-sulfide mineralisation by micro-focus X-ray computed tomography as applied to sulfide-rich drill core samples. Several texturally distinct assemblages of magmatic sulfides can be identified, including: (1) <500 ?m rounded bornite and chalcopyrite grains disseminated within the gangue; (2) elongated mm-scale assemblages of chalcopyrite and bornite; and (3) mm-to-cm thick chalcopyrite cumulates. Chalcopyrite veins were also observed, as well as late-stage valleriite, documenting late-stage fluid circulation within the pipe, and alteration of magmatic and hydrothermal sulfides along fractures within the gangue, respectively. The results of micro-focus X-ray computed tomography indicate that magmatic sulfides are sub-vertically aligned. Spatial variability of the sulfide assemblages suggests that textural changes within sulfide layers reflect fluctuating magma flow rate during emplacement of carbonatite-phoscorite magmas, through coalescence or breakup of sulfide liquid droplets during ascent. Modal sulfide abundances, especially for disseminated assemblages, differ from one carbonatite-phoscorite layer to another, suggesting a strong control of the mechanical sorting in the formation of Cu-sulfide textures within the Loolekop carbonatite. The alternation of carbonatite and phoscorite within the intrusion suggest that the Loolekop Pipe was emplaced through a series of successive magma pulses, which differentiated into carbonatite and phoscorite by melt immiscibility/progressive fractional crystallisation and pressure drop. Three-dimensional textural analysis represents an effective tool for the characterisation of magma flow and is useful for the understanding of magmatic processes controlling sulfide liquid-bearing phoscorite-carbonatite magmas.
Abstract: Abstract A major debris flow, the Trig Point Hill flow, originating from Kerimasi volcano (Tanzania) contains numerous blocks of extrusive/pyroclastic carbonatites similar to those exposed at the rim of the currently inactive crater. The blocks of calcite carbonatite consist of: (1) large clasts of corroded and altered coarse grained calcite; (2) primary prismatic inclusion bearing phenocrystal calcite; and (3) a matrix consisting primarily of fine-grained prismatic calcite. The large clasts are inclusion free and exhibit a ‘corduroy-like’ texture resulting from solution along cleavage planes. The resulting voids are filled by brown Fe-Mn hydroxides/oxides and secondary calcite. The prismatic or lath-shaped phenocrystal calcite is not altered and contains melt inclusions consisting principally of primary Na-Ca carbonates which contain earlier-formed crystals of monticellite, periclase, apatite, Mn-Mg-magnetite, Mn-Fe-sphalerite and Nb-perovskite. Individual Na-Ca carbonate inclusions are of uniform composition, and the overall range of all inclusions analysed (wt.%) is from 28.7 to 35.9 CaO; 16.7-23.6 Na2O; 0.5-2.8 K2O, with minor SO3 (1.1-2.2) and SrO (0.34-1.0). The Na-Ca carbonate compositions are similar to that of shortite, although this phase is not present. The Na-Ca carbonates are considered to be primary deuteric phases and not secondary minerals formed after nyerereite. Monticellite shows limited compositional vari
