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The Sheahan Diamond Literature Reference Compilation - Technical, Media and Corporate Articles based on Major Region - Other Latin America
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 ŕ 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]
Other Latin America - Technical, Media and Corporate
Ultramafic xenoliths from the Palo-Aike basalts: Implications for the nature and evolution of the subcontinental lithosphere below southern SouthAmerica
Proceedings of the Fourth International Kimberlite Conference, Held Perth, Australia, No. 16, pp. 343-345
Volcanoes of the central Andes.Section of the book -large silicic systemsxeroxed. Not well in black and white as the photographs are in colour in thebook
Mantle xenoliths from the Quaternary Pali-Aike volcanic field of southernmost South America: implications for the accretion of Phanerozoic continentallithosphere
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 395-397
Peru, South America
Petrochemical, mineral chemistry, geothermometry, Basalts
Structural geology and geochronology of subduction complexes along the margin of Gondwanaland: new dat a from the Antarctic Peninsula and southernmostAndes
Geological Society of America (GSA) Bulletin, Vol. 104, No. 11, November pp. 1497-1514
Summary of geological history of Bolivia - precambrian shield, Cordillera Paleozoic belt, Altiplano and western Cordiller volcanic belt, northernpiedmont
A tale of two continents: some tectonic contrasts between the Central Andes and the North America Cordillera as illustrated by their paleomagneticsignatures
A tale of two continents: some tectonic contrast between the central Andes and the North America Cordillera as illustrated by paleomagnetic signatures.
Tectonics, Vol. 13, No.1, Feb. pp. 215-24.
Andes
Tectonics, Geophysics - paleomagnetics - not specific to diamonds
Hauser, N., Matteini, M., Omarini, R.H., Pimentel, M.M.
Constraints on metasomatized mantle under central South America: evidence from Jurassic alkaline lamprophyre dykes from the eastern Cordillera, NM Argentina.
Bulletin of Volcanology, Vol. 79, 2 in press available
South America, Mexico
Diatremes
Abstract: Felsic maar-diatreme volcanoes host major ore deposits but have been largely ignored in the volcanology literature, especially for the diatreme portion of the system. Here, we use two Mexican tuff rings as analogs for the maar ejecta ring, new observations from one diatreme, and the economic geology literature on four other mineralized felsic maar-diatremes to produce an integrated picture of this type of volcano. The ejecta rings are up to 50 m+ thick and extend laterally up to ?1.5 km from the crater edge. In two Mexican examples, the lower part of the ejecta ring is dominated by pyroclastic surge deposits with abundant lithic clasts (up to 80% at Hoya de Estrada). These deposits display low-angle cross-bedding, dune bedforms, undulating beds, channels, bomb sags, and accretionary lapilli and are interpreted as phreatomagmatic. Rhyolitic juvenile clasts at Tepexitl have only 0-25% vesicles in this portion of the ring. The upper parts of the ejecta ring sequences in the Mexican examples have a different character: lithic clasts can be less abundant, the grain size is typically coarser, and the juvenile clasts can be different in character (with some more vesicular fragments). Fragmentation was probably shallower at this stage. The post-eruptive maar crater infill is known at Wau and consists of reworked pyroclastic deposits as well as lacustrine and other sediments. Underneath are bedded upper diatreme deposits, interpreted as pyroclastic surge and fall deposits. The upper diatreme and post-eruptive crater deposits have dips larger than 30° at Wau, with approximately centroclinal attitudes. At still lower structural levels, the diatreme pyroclastic infill is largely unbedded; Montana Tunnels and Kelian are good examples of this. At Cerro de Pasco, the pyroclastic infill seems bedded despite about 500 m of post-eruptive erosion relative to the pre-eruptive surface. The contact between the country rocks and the diatreme is sometimes characterized by country rock breccias (Kelian, Mt. Rawdon). Pyroclastic rocks in the diatreme are typically poorly sorted, and ash-rich. They contain a heterolithic mix of juvenile clasts and lithic clasts from various stratigraphic levels. Megablocks derived from the ejecta ring or the country rocks are often found in the diatremes. Evidence for multiple explosions is in the form of steep crosscutting pyroclastic bodies within some diatremes and fragments of pyroclastic rocks within other pyroclastic facies. Pyroclastic rocks are cut by coherent felsic dikes and plugs which may have been feeders to lava domes at the surface. Allowing for the difference in magma composition, felsic maar-diatreme volcanoes have many similarities with their ultramafic to mafic equivalents. Differences include a common association with felsic domes, inside the crater or just outside (Wau), although the domes within the crater may be destroyed during the eruption (Hoya de Estrada, Tepexitl); the dikes and plugs feeding and invading felsic diatremes seem larger; the processes of phreatomagmatic explosions involving felsic magmas may be different.
Abstract: Colombia is synonymous with fine emerald, and production is believed to date back well over a thousand years. Over the centuries the beautiful verdant gemstone, which emerges from areas that are also a lush green, has been linked to violence and human exploitation. Nevertheless, the desire of the Colombian people to mine for this treasure and strike it rich has endured, with enough dreams coming true to drive their passion. In recent years, industry changes have accelerated, perhaps more profoundly than ever before. While government ownership and regulation, criminal activity, and violence have affected production over the years, the industry’s greatest opportunities may still be ahead. Multinational companies are investing heavily in Colombian emerald mining, which has led to modernization. The government’s position on emerald mining has also improved dramatically in this period. Calls for transparency and traceability have led to branding and a revamping of the industry’s image. The loose system of independent miners (figure 1) is seeing efforts at formalization. These landmark changes are occurring at a time when most of the country’s emerald reserves have yet to be mined. In October 2015, a joint GIA and Colombian team met at the First International Emerald Symposium in Bogotá to interview industry leaders and government officials. Many topics involving industry change were discussed at the symposium. Afterward, the team traveled to Colombia’s major mines and visited dealers and cutters in Bogotá to document the current state of the mine-to-market industry. We were also able to collect rough emerald samples for the GIA laboratory’s country-of-origin reference collection.
Netherlands Journal of Geolsciences, Vol. 95, 4, pp. 491-522.
South America, Suriname
Guiana shield
Abstract: The Proterozoic basement of Suriname consists of a greenstone-tonalite-trondhjemite-granodiorite belt in the northeast of the country, two high-grade belts in the northwest and southwest, respectively, and a large granitoid-felsic volcanic terrain in the central part of the country, punctuated by numerous gabbroic intrusions. The basement is overlain by the subhorizontal Proterozoic Roraima sandstone formation and transected by two Proterozoic and one Jurassic dolerite dyke swarms. Late Proterozoic mylonitisation affected large parts of the basement. Almost 50 new U-Pb and Pb-Pb zircon ages and geochemical data have been obtained in Suriname, and much new data are also available from the neighbouring countries. This has led to a considerable revision of the geological evolution of the basement. The main orogenic event is the Trans-Amazonian Orogeny, resulting from southwards subduction and later collision between the Guiana Shield and the West African Craton. The first phase, between 2.18 and 2.09 Ga, shows ocean floor magmatism, volcanic arc development, sedimentation, metamorphism, anatexis and plutonism in the Marowijne Greenstone Belt and the adjacent older granites and gneisses. The second phase encompasses the evolution of the Bakhuis Granulite Belt and Coeroeni Gneiss Belt through rift-type basin formation, volcanism, sedimentation and, between 2.07 and 2.05 Ga, high-grade metamorphism. The third phase, between 1.99 and 1.95 Ga, is characterised by renewed high-grade metamorphism in the Bakhuis and Coeroeni belts along an anticlockwise cooling path, and ignimbritic volcanism and extensive and varied intrusive magmatism in the western half of the country. An alternative scenario is also discussed, implying an origin of the Coeroeni Gneiss Belt as an active continental margin, recording northwards subduction and finally collision between a magmatic arc in the south and an older northern continent. The Grenvillian collision between Laurentia and Amazonia around 1.2-1.0 Ga caused widespread mylonitisation and mica age resetting in the basement.
Abstract: Recent findings of diamonds in ophiolitic peridotites and chromitites challenge our traditional notion of Earth mantle dynamics. Models attempting to explain these findings involve incorporation of diamonds into chromite near the mantle transition zone. However, the occurrence of metastable diamonds in this context has not been considered. Here, we report for the first time in situ microdiamonds in chromite from ophiolitic chromitite pods hosted in the Tehuitzingo serpentinite (southern Mexico). Here, diamonds occur as fracture-filling inclusions along with quartz, clinochlore, serpentine, and amorphous carbon, thus indicating a secondary origin during the shallow hydration of chromitite. Chromite chemical variations across the diamond-bearing healed fractures indicate formation during the retrograde evolution of chromitite at temperatures between 670 °C and 515 °C. During this stage, diamond precipitated metastably at low pressure from reduced C-O-H fluids that infiltrated from the host peridotite at the onset of serpentinization processes. Diamond was preserved as a result of fracture healing at the same temperature interval in which the chromite alteration began. These mechanisms of diamond formation challenge the idea that the occurrence of diamond in ophiolitic rocks constitutes an unequivocal indicator of ultrahigh-pressure conditions.
SAXI-XI Inter Guiana Geological Conferene 2019: Paramaribo, Suriname, 5p. Pdf
South America, Suriname
Guiana shield
Abstract: The ultramafic rocks of the Marowijne Greenstone Belt in Suriname and elsewhere in the Guiana Shield comprise both intrusive dunite-gabbroic bodies and ultramafic lavas and volcaniclastic rocks. They were emplaced in the early stages of the Trans-Amazonian Orogeny (2.26-2.09 Ga), but their petrogenesis and geotectonic significance have still to be elaborated. They present several economically interesting mineralisations, including chromium, nickel, platinum, gold and diamonds. In Suriname diamonds are found since the 19 th century; possible source rocks show similarities with the diamondiferous komatiitic volcaniclastic rocks in Dachine, French Guiana and in Akwatia in the Birimian Greenstone Belt of Ghana. This might point to a regionally extensive diamond belt in the Guiana Shield and its predrift counterpart in the West-African Craton.
Geophysical Research Letters, Vol. 46, 4, pp. 2012-2024.
South America, Colombia, Australia
craton
Abstract: Significant amounts of landmasses are brought together in a hemispheric supercontinent, then breaks up, disperse, and reform in a new supercontinent in every 400-450 Myrs. During the supercontinent cycle, global?scale continental magmatism and orogenic activity increased. The assembly and breakup of Pangaea, the latest supercontinent, are well understood today. However, the evidence becomes more sparse further back in geological history. The geological and paleomagnetic data are insufficient to determine the exact geometries of Rodinia and Columbia supercontinents. Hence, we trace the position of Cathaysia block in the Columbia supercontinent and its relationship with other continental blocks, based on its Paleoproterozoic magmatisms, metamorphisms, and sedimentations, especially ultradepleted mantle?derived rocks. This work has important implications for the mantle heterogeneity in supercontinent reconstruction.
Geochimica et Cosmochimica Acta, in press available, 32p.
Asia, Tibet, Andes
melting
Abstract: The lithospheric mantle beneath Archean cratons is conspicuously refractory and thick compared to younger continental lithosphere (Jordan, 1988, Boyd, 1989; Lee and Chin, 2014), but how such thick lithospheres formed is unclear. Using a large global geochemical database of Archean igneous crustal rocks overlying these thick cratonic roots, we show from Gd/Yb- and MnO/FeOT-SiO2 trends that crustal differentiation required continuous garnet fractionation. Today, these signatures are only found where crust is anomalously thick (60-70?km), as in the Northern and Central Andes and Southern Tibet. The widespread garnet signature in Archean igneous suites suggests that thickening occurred not only in the lithospheric mantle but also in the crust during continent formation in the late Archean. Building thick crust requires tectonic thickening or magmatic inflation rates that can compete against gravitational collapse through lower crustal flow, which would have been enhanced in the Archean when geotherms were hotter and crustal rocks weaker. We propose that Archean crust and mantle lithosphere formed by thickening over mantle downwelling sites with minimum strain rates on the order of 10?13-10?12 s?1, requiring mantle flow rates associated with late Archean crust formation to be 10-100 times faster than today.
www.minsocam.org/ MSA/Centennial/ MSA_Centennial _Symposium.html The next 100 years of mineral science, June 20-21, p. 36. Abstract
Asia, Pakistan, Kashmir, South America, Colombia
sapphire, emerald
Abstract: The geographic origin of gemstones has emerged as one of the major factors affecting their sale on the colored stone market, in large part due to the prestige attributed to certain regions (e.g. sapphires from Kashmir or emeralds from Colombia) but also because of political, environmental and ethical considerations. Identifying the geographic provenance of a colored stone has, therefore, developed into one of the main tasks for gem-testing laboratories, providing a strong motivation to establish accurate scientific methods. The properties and features of individual gemstones reflect the specific geological conditions of their formation and the main challenge of origin determination is to find the link between the two. In addition, access to a complete collection of authentic reference samples and analytical data for all economically relevant mining areas worldwide is key. Different techniques have been developed for determining gemstone provenance, including a range of gemological observations, and spectroscopic, chemical, and isotopic analyses[1]. These have proven useful in distinguishing the origin of gemstones from different geological settings but for many gemstones (including ruby and sapphire) to reliably distinguish between gems from different geographic regions that share a similar geological setting is not always possible. So far, no unique fingerprint exists, and the geographic origin remains a challenge, especially for high-clarity stones, emphasizing the need for a more powerful tool. Here we will give an overview of the current techniques, and outline some of the challenges and limitations of geographical origin determination of colored gemstones. In addition, we present new trace element data and the first radiogenic isotope compositions (Sr and Pb) obtained for ruby and sapphire from several different localities of geologically similar deposits. The acquisition of quantitative data of a range of ultra-trace elements along with the most commonly observed elements in ruby and sapphire (Mg, Fe, Ti, Ca, Ga, V and Cr) makes it possible to explore new elements as potential provenance discriminators. Among the elements consistently above the limits of quantification (Zn, Nb, Ni, and Pb), Ni in particular shows promise as a discriminator for amphibolite-type ruby. Measured 87Sr/86Sr and Pb isotope ratios clearly show distinct ranges for the different localities of amphibolitetype ruby, ranges for marble-related ruby and metamorphic blue sapphires from different geographic regions overlap. These results suggest that radiogenic isotopes potentially offer a powerful means of provenance discrimination for different localities of amphibolite-type ruby, their potential for geographical origin determination among marble-hosted ruby and metamorphic sapphire, however, appears to be limited.
Abstract: Recent findings of diamonds in ophiolitic peridotites and chromitites challenge our traditional notion of Earth mantle dynamics. Models attempting to explain these findings involve incorporation of diamonds into chromite near the mantle transition zone. However, the occurrence of metastable diamonds in this context has not been considered. Here, we report for the first time in situ microdiamonds in chromite from ophiolitic chromitite pods hosted in the Tehuitzingo serpentinite (southern Mexico). Here, diamonds occur as fracture-filling inclusions along with quartz, clinochlore, serpentine, and amorphous carbon, thus indicating a secondary origin during the shallow hydration of chromitite. Chromite chemical variations across the diamond-bearing healed fractures indicate formation during the retrograde evolution of chromitite at temperatures between 670 °C and 515 °C. During this stage, diamond precipitated metastably at low pressure from reduced C-O-H fluids that infiltrated from the host peridotite at the onset of serpentinization processes. Diamond was preserved as a result of fracture healing at the same temperature interval in which the chromite alteration began. These mechanisms of diamond formation challenge the idea that the occurrence of diamond in ophiolitic rocks constitutes an unequivocal indicator of ultrahigh-pressure conditions.
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.
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, Peńas 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.
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.
SAXI-XI Inter Guiana Geological Conference, held Paramaribo, Suriname., 5p. Pdf
South America, Suriname
diamond
Abstract: The ultramafic rocks of the Marowijne Greenstone Belt in Suriname and elsewhere in the Guiana Shield comprise both intrusive dunite-gabbroic bodies and ultramafic lavas and volcaniclastic rocks. They were emplaced in the early stages of the Trans-Amazonian Orogeny (2.26-2.09 Ga), but their petrogenesis and geotectonic significance have still to be elaborated. They present several economically interesting mineralisations, including chromium, nickel, platinum, gold and diamonds. In Suriname diamonds are found since the 19 th century; possible source rocks show similarities with the diamondiferous komatiitic volcaniclastic rocks in Dachine, French Guiana and in Akwatia in the Birimian Greenstone Belt of Ghana. This might point to a regionally extensive diamond belt in the Guiana Shield and its predrift counterpart in the West-African Craton.
Journal of Gemmology, Vol. 37, 2, pp. 180-191. pdf
South America, Suriname
deposit - Paramaka Creek
Abstract: Alluvial diamonds have been found in Suriname since the late 19th century, but to date the details of their origin remain unclear. Here we describe diamonds from Paramaka Creek (Nassau Mountains area) in the Marowijne greenstone belt, Guiana Shield, north-eastern Suriname. Thirteen samples were studied, consisting mainly of euhedral crystals with dominant octahedral and dodecahe-dral habits. They had colourless to brown to slightly greenish body colours, and some showed green or (less commonly) brown irradiation spots. Surface features showed evidence of late-stage resorption that occurred during their transport to the earth’s surface. The studied diamonds were predominantly type IaAB, with nitrogen as both A and B aggregates. In the DiamondView most samples displayed blue and/or green luminescence and concentric growth patterns. Their mineral inclusion assemblages (forsterite and enstatite) indicate a peridotitic (possibly harzburgitic) paragenesis.
IN: Nemeth, K., Carrasco-Nunez, G., Aranda-Gomez, J.J., Smith, I.E.M. eds. Monogenetic volcanism GSL Special Volume, Vol 446, 31p. Pdf * note date
Europe, Germany , United States, Australia, Mexico
maars
Abstract: We report here a growth model for phreatomagmatic maar-diatreme volcanoes with respect to the number of eruptions documented in the tephra beds of maar tephra rings and the upper bedded diatreme facies. We show that the number of tephra beds in large diatremes is larger than that in maar tephra rings. Base surges that lack sufficient momentum to scale high maar crater walls deposit their tephra only inside the crater. Thus the total number of eruptions at large maar-diatreme volcanoes will be larger than the number recorded in maar tephra rings. As many maar-diatreme volcanoes erupt dominantly accidental clasts, an incremental mathematical model was applied to study the growth of diatremes. The model is based only on the ejection of distinct amounts of accidental clasts per unit eruption and the chosen number of eruptions is assumed to be identical. The incremental growth of cone-shaped diatremes follows cube-root functions with respect to diameter and depth and slows down with ongoing eruptions. In nature, small and large maar-diatreme volcanoes are formed and filled syn-eruptively, mostly by tephra, depending on the duration and quantity of magma involved in phreatomagmatic eruptions. In our opinion, this mathematical model is the only current method able to model the growth of diatremes.
Abstract: Diamond is commonly regarded as an indicator of ultra-high pressure conditions in Earth System Science. This canonical view is challenged by recent data and interpretations that suggest metastable growth of diamond in low pressure environments. One such environment is serpentinisation of oceanic lithosphere, which produces highly reduced CH4-bearing fluids after olivine alteration by reaction with infiltrating fluids. Here we report the first ever observed in situ diamond within olivine-hosted, CH4-rich fluid inclusions from low pressure oceanic gabbro and chromitite samples from the Moa-Baracoa ophiolitic massif, eastern Cuba. Diamond is encapsulated in voids below the polished mineral surface forming a typical serpentinisation array, with methane, serpentine and magnetite, providing definitive evidence for its metastable growth upon low temperature and low pressure alteration of oceanic lithosphere and super-reduction of infiltrated fluids. Thermodynamic modelling of the observed solid and fluid assemblage at a reference P-T point appropriate for serpentinisation (350 °C and 100 MPa) is consistent with extreme reduction of the fluid to logfO2 (MPa) = ?45.3 (?logfO2[Iron-Magnetite] = ?6.5). These findings imply that the formation of metastable diamond at low pressure in serpentinised olivine is a widespread process in modern and ancient oceanic lithosphere, questioning a generalised ultra-high pressure origin for ophiolitic diamond.
Abstract: The history of the Chivor emerald mine in Colombia is rife with legend and adventure. The tale traces from early exploitation by indigenous people, to work by the Spanish in the sixteenth and seventeenth centuries, to 200 years of abandonment and jungle overgrowth. The story then picks up with rediscovery near the turn of the twentieth century by the Colombian mining engineer Francisco Restrepo using clues from a historical manuscript. Still the saga continued, with repeated shortages of investment funds driving multiple ownership changes and little progress toward mining the largely inaccessible deposit. The German gem merchant Fritz Klein, in cooperation with Restrepo, pursued limited mining activities with a small number of workers for a few months prior to the outbreak of World War I. After the war, the American company Colombian Emerald Syndicate, Ltd., took ownership, and mining operations resumed under the new leadership. Ownership changed yet again in the 1920s, followed by multiple cycles of expanding and shrinking mining activity, interrupted by completely unproductive periods.
Mineralogy and Petrology, Vol. 115, 19p. Doi.org/101007 /s00710-020- 00723-x pdf
South America, Colombia
REE
Abstract: A multi-methodological study was conducted in order to provide further insight into the structural and compositional complexity of rare earth element (REE) fluorcarbonates, with particular attention to their correct assignment to a mineral species. Polycrystals from La Pita Mine, Municipality de Maripí, Boyacá Department, Colombia, show syntaxic intergrowth of parisite-(Ce) with röntgenite-(Ce) and a phase which is assigned to B3S4 (i.e., bastnäsite-3-synchisite-4; still unnamed) fluorcarbonate. Transmission electron microscope (TEM) images reveal well-ordered stacking patterns of two monoclinic polytypes of parisite-(Ce) as well as heavily disordered layer sequences with varying lattice fringe spacings. The crystal structure refinement from single crystal X-ray diffraction data - impeded by twinning, complex stacking patterns, sequential and compositional faults - indicates that the dominant parisite-(Ce) polytype M1 has space group Cc. Parisite-(Ce), the B3S4 phase and röntgenite-(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite-(Ce), the B3S4 phase and röntgenite-(Ce) reveal different intensity ratios of the three symmetric CO3 stretching bands at around 1100 cm?1. We propose to non-destructively differentiate parisite-(Ce) and röntgenite-(Ce) by their 1092 cm?1 / 1081 cm?1 ?1(CO3) band height ratio.
Abstract: Geophysical interpretation of potential field data plays an important role in the integration of geological data. Estimation of density and magnetic susceptibility variations within the upper crust helps evaluating the continuity of geological structures in the field. In the present study we use gravity and magnetic data in NW Amazonian Craton in Colombia. Total horizontal gradient of the reduction to magnetic pole were used to delineate magnetic lineaments and domains showing four zones, each with its own features. Multiscale edge detection (worming) of the data help delineate upper crustal structures that we interpret as tectonic boundaries that correlate with the four zones identified. 3D density and magnetic susceptibility inversion showed high density and/or high magnetic susceptibility sources correlated with these crustal structures. Zone (1) is located south of the Guaviare River, with predominant NW-SE and NE-SW magnetic lineaments; zone (2), located from south of the Guaviare River to the north, present nearly E-W magnetic lineaments and a deep E-W edge interpreted as a possible shear zone parallel to Guaviare, Orinoco and Ventuari rivers; zone (3) from south of the Vichada River to the north, with NE-SW and NW-SE lineaments; N-S zone (4) cuts the zones (2) and (3), characterized by high density/magnetic susceptibility source bounded by N-S deep edges. A more complete tectonic evolution interpretation requires further work, but we speculate that the zone (4) could indicate an aborted rift/collision suture and that the zone (2) is indicative of a younger deformation event. Shear direction at (2) is not clear: geological maps show NEE-SWW right-lateral faulting, but geophysical anomalies suggest left-lateral displacement, highlighted by left dislocation of the Orinoco River. We also speculate that a N-S edge located at the SE of the area can be related with the Atabapo Belt and the limit of Ventuari-Tapajós and Rionegro geochronological provinces.
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: A magnetotelluric survey comprising 18 broadband stations disposed along a 450 km-long profile was carried out at the transition between the Chaco-Paraná (CPB) and the Paraná (PB) intracratonic basins in northeastern Argentina. Three-dimensional inversions of the responses show that the CPB and southern PB lithospheres are resistive (~103 ? m) down to 120 km, but with distinct crustal and upper mantle electrical properties. Also, Bouguer gravity and density anomalies are positive at CPB, whereas they are negative at PB. We associate the CPB lithosphere with the Paleoproterozoic Rio Tebicuary craton and the southern PB lithosphere with an ancient and buried piece of craton, the Southern Paraná craton. Geochemical data of mantle xenoliths from the Cenozoic alkaline/carbonatitic province within the Rio Tebicuary craton suggest a subcontinental lithospheric mantle affected by metasomatic processes, which explains its lower resistivity (reaching values as low as 300 ? m) and higher density (#Mg = 0.87). In contrast, the Southern Paraná craton is more resistive (>103 ? m) and less dense, suggesting a de-hydrated, depleted, and thicker craton. These cratons are separated by a crustal conductor (15 to 20 km depth; 1-10 ? m) that we interpret as a southward continuation of a linear anomaly (Paraná Axial Anomaly) defined in former induction studies within the PB in Brazil. Hence, we redefined the trace of this conductive lineament: instead of bending towards the Torres Syncline, it continues inside the CPB. We propose the lineament to be an Early Neoproterozoic suture zone that controlled the location of maximum subsidence in the intracratonic basins during the Paleozoic. In the Early Cretaceous, the Paraná Axial Anomaly was the site of maximum extrusion and deposition of Serra Geral basalts. This anomaly separates compositionally distinct cratonic lithospheres along its path. Melting of this heterogeneous and enriched mantle created the Paraná igneous province.
https://www.linkedin.com/posts/dev-shetty-64402719_enormous-and-exceptional-fura-gems-to-auction-activity-6914432545065463808-_wHq?utm_source=linkedin_share&utm_medium=ios_app, Mar. 30, 1p.
