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The Sheahan Diamond Literature Reference Compilation - Technical Articles based on Major Region - Namibia
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]
The Coastal Diamond Deposits of German Southwest Africa. The Probable Origin of the Diamonds and Alleged Cretaceous Gravels Along the Coast. Processes of Concentration.
South African Mining Journal, Vol. 9, PT. 2, SEPT. 16TH. P. 56.
The Story of de Beers. with Some Notes on the Company's Financial, Farming, Railway and Industrial Activities in Africa and Some Introductory Chapters on the River Diggings and Earlt Kimberley.
Ein Vorzeitlicher Traenkplatz Auf Den Diamant feldern Bei Luederitzbucht in Deutsch Suedwest Afrika und Seine Bedeutung Fuer die Geschnichte der Namibwueste.
Geol. Deutsch. Fur Gesell., Vol. 84, P. 569. (abstract.).
Review of 1989 international mineral industry activities.Brief mention Of diamonds in several countries. ie. South Africa, Zaire, Namibia, Angola, Guinea
Mining Engineering, Vol. 42, No. 7, July, pp. 665-675
South Africa, Democratic Republic of Congo, Namibia, Angola, Guinea
Diamond resources on the continental shelf of southern Africa
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) ., Session on Diamonds at The Canadian Institute of Mining, Metallurgy and Petroleum (CIM) Annual Meeting April, Vol. 84, No. 947, March p. 99. Abstract
Paragenetic sequence of pyrrhotite alterations to marcasite, pyrite, magnetite, hematite and goethite in pyroxene and pegmatitic carbonatites and fluorite ores.
11th. Quadrennial Iagod Symposium And Geocongress 2002 Held Windhoek, Abstract p. 26.
Synorogenic melting of mafic lower crust: constraints from geochronology, petrology and Sr Nd, Pb and O isotope geochemistry of quartz diorites, Damara Orogen
Synorogenic melting of mafic lower crust: constraints from geochronology, petrology and Sr Nd Pb O isotope geochemistry of diorites from Damara Origin.
Contributions to Mineralogy and Petrology, Vol. 143, 5, pp.551-66.
The timing of mantle and crustal events in South Namibia, as defined by SHRIMP dating of zircon domains from a garnet peridotite xenolith of the Gideon Kimberlite province.
Journal of African Earth Sciences, Vol. 39, 3-5, pp. 147-157.
The timing of mantle and crustal events in South Namibia as defined by SHRIMP dating of zircon domains from a garnet peridotite xenolith of the Gibeon kimberlite province.
Journal of African Earth Sciences, Vol. 39, 3-5, June pp. 147-157.
Wall, F., Niku-Paavola, V.N., Storey, C., Muller, A.,Jeffries, T.
Xenotime from carbonatite dykes at Lofdal Namibia - an extension of carbonatite REE mineralization, first dating of xenotime overgrowths on zircon.LA-ICP-MS-U-Pb
Frontiers in Mineral Sciences 2007, Joint Meeting of Mineralogical societies Held June 26-28, Cambridge, Abstract Volume p. 89-90.
Wall, F., Niku-Paavola, V.N., Storey, C., Muller, A.,Jeffries, T.
Xenotime from carbonatite dykes at Lofdal Namibia - an extension of carbonatite REE mineralization, first dating of xenotime overgrowths on zircon.LA-ICP-MS-U-Pb
Frontiers in Mineral Sciences 2007, Joint Meeting of Mineralogical societies Held June 26-28, Cambridge, Abstract Volume p. 89-90.
Trumbull, R.B., Reid, D.L., De Beer, C., Van Acken, D., Romer, R.L.
Magmatism and continental breakup at the west margin of southern Africa: a geochemical comparison of dolerite dikes from northwestern Namibia and the Western Cape.
South African Journal of Geology, Vol. 110, 2-3, Sept. pp. 477-502.
Wall, F., Niku-Paavola, V.N., Storey, C., Muller, A., Jeffries, T.
Xenotime - (Y) from carbonatite dykes at Lofdal, Namibia: unusually low LREE:HREE ratio in carbonatite, and the first dating of xenotime overgrowths on zircon.
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)
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 lithospheric mantle underneath the Gibeon kimberlite field ( Namibia): a mix of old and young components - evidence from Lu-Hf and Sm-Nd isotope systematics.
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).
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.
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.
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-
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.
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
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: 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 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.
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: 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.
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.
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.
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.
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: 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: 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 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.
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
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 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.
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.
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.
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.
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.
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.
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: 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: 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: 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: 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
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.
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.
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.
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: The Neoproterozoic Lofdal alkaline carbonatite complex consists of a swarm of carbonatite dykes and two plugs of calcite carbonatite known as the ‘Main’ and ‘Emanya’ carbonatite intrusions, with associated dykes and plugs of phonolite, syenite, rare gabbro, anorthosite and quartz-feldspar porphyry. In the unaltered Main Intrusion calcite carbonatite the principal rare-earth host is burbankite. As burbankite typically forms in a magmatic environment, close to the carbohydrothermal transition, this has considerable petrogenetic significance. Compositional and textural features of Lofdal calcite carbonatites indicate that burbankite formed syngenetically with the host calcite at the magmatic stage of carbonatite evolution. The early crystallisation of burbankite provides evidence that the carbonatitic magma was enriched in Na, Sr, Ba and light rare earth elements. In common with other carbonatites, the Lofdal burbankite was variably affected by alteration to produce a complex secondary mineral assemblage. Different stages of burbankite alteration are observed, from completely fresh blebs and hexagonal crystals through to complete pseudomorphs, consisting of carbocernaite, ancylite, cordylite, strontianite, celestine, parisite and baryte. Although most research and exploration at Lofdal has focused on xenotime-bearing carbonatite dykes and wall-rock alteration, this complex also contains a more typical calcite carbonatite enriched in light rare earth elements and their alteration products.
Abstract: Community-Based Natural Resource Management (CBNRM) is a form of resource governance that has been widely popularized across southern Africa. CBNRM consists of three main goals or pillars which include economic development, environmental conservation, and community empowerment. It is intended to devolve control of certain natural resources from national government to local communities. The idea being that local residents will manage resources more sustainably and break neopatrimonial webs that have led to mismanagement and protracted underdevelopment. However, with communities there are important politics that often go understudied and there are instances where the same type of issues that going local was intended to circumvent, are re-engendered at the local level. Beyond this, CBNRM cannot only be understood as a form of domestic resource governance that happens in a vacuum and instead there are important politics and power imbalances between local, national, and global actors, that sees the will of some win out over others. As a result of these power asymmetries, I argue that the three goals of CBNRM form a trilemma in which the realization of one goal undermines success in achieving one or both of the others. As a result, CBNRM programs have failed to achieve the success proponents envision. Thus, it becomes integral to understand and account for the politics involved, rather than to analyze CBNRM as an apolitical policy fix for domestic conservation as much of the literature presents it as.
African Journal of Marine Science, Vol. 44, 1, pp. 49-60.
Africa, Namibia
mining
Abstract: Coastal diamond mining in southern Namibia involves constructing seawalls to hold the sea at bay, and seaward accretion of the shoreline by up to 800 m opens what was previously the surf zone for excavation and extraction of bedrock alluvial diamonds. This has created large coastal wetland ponds of up to 380 000 m2 as the sea overtops the seawalls or seeps into the excavated areas. The ages of these ponds span 1-38 years. We investigated physical conditions in the ponds to determine whether they can function as saline wetlands equivalent to blind estuaries. Water temperatures were 6-10 °C higher than in the sea, as expected of shallow enclosed waterbodies. Dissolved oxygen was 82-137%, peaking at midday owing to photosynthesis, and the ponds were never hypoxic. Correlated with oxygen levels, pH values spanned 7.7-8.3, and always exceeded the pH of seawater. Chlorophyll a concentrations matched or exceeded the levels in seawater, reaching 76 µg l?1. The southern and central ponds had salinities close to those of seawater, but the salinity of northern ponds exceeded 80 after ?15 years, thus limiting their capacity to support wetland communities. Apart from this, these ponds are viable habitat that can support flora and fauna typical of saline wetlands, a habitat that is scarce along this arid coastline.
African Journal of Marine Science, Vol. 44, 1, pp. 61-68.
Africa, Namibia
mining
Abstract: Coastal diamond mining in southern Namibia involves constructing seawalls to hold the sea at bay, and seaward accretion of the shoreline by up to 800 m opens what was previously the surf zone for excavation and extraction of bedrock alluvial diamonds. This has created large coastal wetland ponds of up to 380 000 m2 as the sea overtops the seawalls or seeps into the excavated areas. The ages of these ponds span 1-38 years. We investigated physical conditions in the ponds to determine whether they can function as saline wetlands equivalent to blind estuaries. Water temperatures were 6-10 °C higher than in the sea, as expected of shallow enclosed waterbodies. Dissolved oxygen was 82-137%, peaking at midday owing to photosynthesis, and the ponds were never hypoxic. Correlated with oxygen levels, pH values spanned 7.7-8.3, and always exceeded the pH of seawater. Chlorophyll a concentrations matched or exceeded the levels in seawater, reaching 76 µg l?1. The southern and central ponds had salinities close to those of seawater, but the salinity of northern ponds exceeded 80 after ?15 years, thus limiting their capacity to support wetland communities. Apart from this, these ponds are viable habitat that can support flora and fauna typical of saline wetlands, a habitat that is scarce along this arid coastline.
Abstract: The Epembe Alkaline Carbonatite Complex (EACC) in northwestern Namibia was emplaced along a fault zone into medium- to high-grade Palaeoproterozoic basement rocks of the Epupa Metamorphic Complex (EMC), and extends over a distance of 9 km in a south-easterly direction with a width of 1 km. Nepheline syenite with minor syenite constitute the main lithologies, cross-cut by a calcite?carbonatite dyke. Apatite grains from one syenite, six nepheline syenite and five carbonatite samples were studied using cathodoluminescence (CL) imaging, trace element and Sr-Nd isotope compositions as well as U-Pb geochronology. Syenite-hosted apatite is homogenous in CL and contains the highest concentration of REE (9189-44,100 ppm) with light rare-earth element (LREE) enrichment (LaN/YbN = 4-91) relative to heavy (H) REE consistent with a magmatic origin. Negative Eu anomalies (Eu/Eu* = 0.4-0.9) in syenite apatite are attributed to the formation of apatite in an evolved mantle-derived melt associated with plagioclase fractionation. Nepheline syenite and carbonatite-hosted apatite is also commonly homogeneous in CL, while core-rim zoning and patchy textures are observed occasionally. Both texturally homogeneous and core-rim zoned apatite are enriched in LREE (LaN/YbN = 24-9) relative to HREE, consistent with a magmatic origin. Core-rim zoned apatite is characterized by rim-ward increase in REE concentrations, which can be attributed to mineral fractionation. Patchy apatite is depleted in Na, Y and REE, particularly the LREE (LaN/YbN = 4-19) relative to other nepheline syenite apatite, reflecting interaction with fluids (metasomatism). The strontium isotope composition of metasomatic apatite and magmatic apatite is indistinct suggesting a magmatic origin of the alteration fluids. No Eu anomalies (Eu/Eu* = 1) in chondrite-normalized REE patterns are observed in any apatite hosted by nepheline syenite and carbonatite. An LA-ICPMS U-Pb age of 1216 ± 11 Ma (MSWD = 4.3, 2 SE) for apatite constrains emplacement of the syenite, while magmatic nepheline syenite apatite ages are 1193 ± 14 Ma, 1197 ± 17 Ma and 1194 ± 16 Ma (MSWDs <4.0, 2 SE). The Sr and Nd isotopic composition of apatite in syenite (87Sr/86Sr(i) = 0.7035-0.7048; ?Nd(t) = +2.5 to +3.2), nepheline syenites (87Sr/86Sr(i) = 0.7031-0.7037; ?Nd(t) = +1.5 to +4.4) and carbonatite (87Sr/86Sr(i) = 0.7031-0.7033; ?Nd(t) = 0 to +3.3) overlap, pointing to a common but heterogeneous source, located in the sub-lithospheric mantle.
