The Ventersdorp Contact placer: a gold-pyrite placer of stream and debris-flow origins from the Archaean Witwatersrand Basin of South Africa

Five local varieties of the Ventersdorp Contact placer at the East Driefontein Gold Mine, Carletonville, are distinguished by clast-type assemblage, reflecting discrete provenances. The placer further comprises six lithofacies which are not restricted to specific provenance-varieties. The six lithofacies are: massive, matrix-supported conglomerate (facies Gms): massive, or crudely bedded, clast-supported conglomerate (facies Gm); channel-based conglomerate (facies Gt); single clast layers (facies Glag); horizontally stratified sandstone (facies Sh); and trough cross-stratified sandstone (facies St). Facies Gms represents debris-flow deposits, whereas the other facies are stream deposits. Lithofacies and provenance-varieties are complexly interrelated in local environments, which are themselves complexly related to geomorphic elements of a degradation surface. The maximum thickness of the placer is 7 m. The placer was deposited on a hard, i.e. non-alluvial, bedrock surface, suggesting that it is a pediment mantle. It owes its preservation to a capping of conformably overlying basalts. Economic gold mineralization is present in all the conglomerate facies and all the provenance-varieties. Gold concentrations are high in gravel-bar deposits and channel fills but are not restricted to these environments. A major feature of gold concentration is its variability between provenance-controlled varieties. The gold concentration of a specific primary source is interpreted to be a major control on gold concentration in the final deposit. The origin of the placer as a pediment mantle, and its subsequent preservation by lava-flows, suggests that there was little potential for selective reworking of heavy minerals. Nevertheless, the Ventersdorp Contact placer on the Carletonville Goldfield is one of the richest gold placers in the Witwatersrand Basin.     

Textural and geochemical discrimination between xenotime of different origin in the Archaean Witwatersrand Basin, South Africa

Xenotime (YPO₄) of detrital, diagenetic, and hydrothermal origin within siliciclastic rocks of the Archaean Witwatersrand Basin, South Africa, has been identified on the basis of petrography and in situ ion microprobe (SHRIMP) age data. The chemical composition of xenotime, determined by in situ electron microprobe analysis, can be correlated with its origin. This allows the origin of any xenotime grain to be assessed by a non-destructive microanalytical method prior to ion microprobe geochronology. The main chemical discriminators are MREE-HREE abundance, normalised HREE slope and Eu anomaly, and, in some cases, U and Th contents. Igneous-detrital xenotime (> 2800 Ma) is distinguished from diagenetic (∼2780 Ma) and hydrothermal (< 2780 Ma) xenotime in having lower Eu, Dy, and Gd concentrations and a distinctively lower Gd/Yb ratio. Hydrothermal xenotime has distinctively lower U and Th concentrations when compared to igneous-detrital and diagenetic xenotime. Three separate hydrothermal fluid events and episodes of post-diagenetic xenotime growth are recognised in the geochemical and geochronological data, which correspond in time to the extrusion of the Ventersdorp lavas at ∼2720 Ma, the emplacement of the Bushveld Igneous Complex at ∼2061 Ma, and an event of unknown affinity at ∼2210 Ma. Although geochemical discrimination of the xenotime types from the Witwatersrand Basin, in combination with careful petrography, appears achievable, universal application of these discriminators to xenotime in other sedimentary basins remains untested.     

Silver and mercury in single gold grains from the Witwatersrand and Barberton,South Africa

The contents of silver and mercury in 323 spots on gold grains from seven localities of the Witwatersrand palaeo-placer and Archaean vein deposits from Barberton were measured using an electron microprobe. The objective was to obtain information on the extent of gold alteration during fluvial transport and post-depositional geological processes. The results, however, show that Ag and Hg are distributed homogeneously in the gold grains studied. No indications were found that the gold was transported in solution nor that leaching took place in an oxidizing fluvial environment. This strongly suggests that the Ag and Hg contents in Witwatersrand gold grains represent geochemical ‘fingerprints’ inherited from their eroded primary sources. Combined analysis for Ag and Hg in Witwatersrand gold grains by electron microprobe can therefore be a valuable tool in establishing the types of primary sources for the gold.     

The relationship of silver and gold in the basal reef of the Witwatersrand System,South Africa

Ag/Au ratios of individual gold particles and of ore-samples obtained from the Basal Reef conglomerate horizon of the Upper Witwatersrand System in the Orange Free State Goldfield (South Africa) are discussed. The Ag/Au values of individual gold grains were found to be exceptionally constant at about 0.0812, whereas the Ag/Au ratios of the ore-samples have markedly higher values and show a large variation. These values indicate that the detrital gold has been homogenized during transportation and sedimentation and that part of the original silver content has been redistributed. It is likely that the first refining of detrital gold particles occurred in streams which eventually drained into the Witwatersrand basin. Further redistribution of the silver relative to the gold took place in the littoral zone of the basin, where the conglomerates were reworked by wave action and erosion. It is contended that supergenic enrichment of gold might also have taken place close to the original shore-line. These processes are believed to account for an increase in the Ag/Au values of ore-samples towards the northwest in the area investigated. Finally, the gold has been homogenized during the metamorphic period of the Witwatersrand System, when pseudohydrothermal transport and redeposition of certain constituents took place. It is suggested that the main portion of the silver content of the original alloy, which was redistributed during the various processes outlined above, is now present as finely dispersed silver minerals. The hypothesis of an elevation control of the Ag/Au values has been found to be improbable.

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Es werden Ag/Au Verhältnisse von Goldkörnern und von Erzproben aus dem Basal Reef-Konglomerathorizont des Oberen Witwatersrand-Systems im Oranje-Freistaat-Goldfeld mitgeteilt. Die Ag/Au-Werte der Goldkörner sind außerordentlich konstant und bewegen sich um 0,0812, während die Ag/Au-Verhältnisse der Erzproben merklich höher sind und starken Schwankungen unterliegen. Das detritische Gold muß daher während seines Transportes und während seiner Ablagerung homogenisiert worden sein und dabei einen Teil seines ursprünglichen Silbergehaltes verloren haben. Eine erste Raffinierung des Goldes geschah wahrscheinlich während des Flußtransportes der Partikel in den Sedimentationsraum. Weitere Umlagerungen von Silber relativ zu Gold fanden in der Küstenzone des Witwatersrand Beckens statt, im Bereich starker Wellenbewegung und intensiver Aufbereitung des Sedimentmaterials. Es wird zudem angenommen, daß in der Nähe der ehemaligen Küste deszendente Prozesse aktiv waren, welche Gold anreicherten. Diese Silberumlagerungen werden als Grund dafür betrachtet, daß im Untersuchungsgebiet die Ag/Au-Werte von Erzproben in der Haupttransportrichtung gegen Nordwesten hin zunehmen. Schließlich wurde das Gold im Gefolge der Metamorphose der Witwatersrand-Sedimente pseudohydrothermal über kurze Distanzen umgelagert und dabei weitgehend homogenisiert. Es wird angenommen, daß die Silbergehalte, welche bei den verschiedenen Umlagerungsprozessen freigesetzt wurden, heute größtenteils in Form fein verteilter Silbermineralien vorliegen. Eine Abnahme der Ag/Au-Werte mit zunehmender Teufe konnte nicht bestätigt werden.

     

南非维特沃特斯兰德盆地绍斯迪普金矿床地质特征、成矿模式和找矿模型

对南非兰德砾岩型金矿床的研究成果进行总结,概括了绍斯迪普金矿床的区域地质背景、岩石地层划分,较为系统地分析了矿床的地质特征和矿床成因。得出如下结论:1金的主要来源可能与太古宙含金花岗绿岩带有密切关系;2金矿体主要受太古宙文特斯多普砾岩地层和中兰德群中的Turffontein亚群埃尔斯伯格组砾岩地层控制;3金矿床可能经历了早期沉积成岩成矿阶段—后期改造富集多个(Klipriviersberg火山活动作用时期和与2050Ma布什维尔德杂岩体有关的变质作用高峰时期)阶段,为复合成因的层控改造型矿床。研究结果对该类矿床的深入研究和走出去的中国企业有重要的参考价值。     

南非维特沃特斯兰德盆地绍斯迪普金矿床地质特征、成矿模式和找矿模型

对南非兰德砾岩型金矿床的研究成果进行总结,概括了绍斯迪普金矿床的区域地质背景、岩石地层划分,较为系统地分析了矿床的地质特征和矿床成因。 得出如下结论：①金的主要来源可能与太古宙含金花岗绿岩带有密切关系;②金矿体主要受太古宙文特斯多普砾岩地层和中兰德群中的Turffontein亚群埃尔斯伯格组砾岩地层控制;③金矿床可能经历了早期沉积成岩成矿阶段—后期改造富集多个（Klipriviersberg火山活动作用时期和与2050Ma布什维尔德杂岩体有关的变质作用高峰时期）阶段,为复合成因的层控改造型矿床。研究结果对该类矿床的深入研究和走出去的中国企业有重要的参考价值。     

SHRIMP U–Pb ages of diagenetic and hydrothermal xenotime from the Archaean Witwatersrand Supergroup of South Africa

SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime ²⁰⁷Pb/²⁰⁶Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event.     

Contaminant leaching from gold mining tailings dams in the Witwatersrand Basin,South Africa: A new geochemical modelling approach

In this study a new geochemical reaction modelling methodology is used to shed light on the geochemical processes within Witwatersrand tailings impoundments and the evaluation of the geochemical impacts for future mining projects is evaluated. Information from international and local studies on tailings sulphidic tailings impoundments is used to develop a conceptual understanding on a typical Witwatersrand gold tailings impoundment. The tailings impoundments consist of 3 distinct geochemical zones: Oxidation Zone (OZ), a Transition Zone (TZ) and a Reduction Zone (RZ). Individual reaction models are developed for each of the 3 zones. The output of one model is used as an input to the next in spatial order. The results of the final model represent the tailings basal seepage characteristics, which indicate the most likely impacts on groundwater resources. The model results agree with existing information on AMD in the Witwatersrand. The results indicate that the tailings basal seepage is likely to be acidic (pH of ∼3.5), containing elevated concentrations of SO₄ and trace metals (Al, Co, Cr, Cu, Mn, Ni, Zn and U). Predicted Fe concentrations are low, due to the low $f_{{\text{O}}_2}$ in the TZ and RZ and the buffering effect of the precipitation of K-jarosite in the OZ and TZ. This study therefore indicates that the methodology employed produces results that can be correlated to existing information and can thus be used as a methodology in the assessments of impacts from sulphidic tailings material for future mining projects.     

Petrogenesis of Late Archaean Flood-Type Basic Lavas from the Klipriviersberg Group, Ventersdorp Supergroup, South Africa

The Klipriviersberg Group is a small continental flood-typetholeiitic suite forming the basal unit of the Ventersdorp Supergroup,an undeformed late-Archaean supracrustal sequence covering 200000km² in the SW part of the Kaapvaal Craton. From the base up,the Klipriviersberg Group consists of the Westonaria, Alberton,Orkney, Jeannette, Loraine, and Edenville formations, with amaximum combined thickness of 1–8 km. Samples were obtainedfrom several borehole cores in the Klerksdorp goldfield closeto the type area of the Klipriviersberg Group. This sample suiteincludes a detailed collection from a 1–36-km core intersection.These samples reveal that the Westonaria Formation is absentfrom the succession and that the Jeannette Formation is representedby a thin sequence of breccias. The remaining units are builtof lavas ranging from siliceous picrite basalts to tholeiiticandesites in composition. All lavas exhibit the effects of greenschistfacies metamorphism but igneous textures are well preservedand reveal that most lavas are aphyric to sparsely porphyritic.A well-developed geochemical stratigraphy exists in the lavasequence and is characterized by an upward trend from evolvedto primitive lavas. The Alberton (mg-number = 0.53–0.43;Zr= 108–137 ppm) and Orkney (mg-number = 0.51–O.42;Zr = 90–110ppm) lavas have a narrow compositional rangeand are more differentiated than the compositionally more variableLoraine/Edenville lavas (mg-number = 0.75 0.47; Zr = 34–97ppm). Unusual inter-element correlations in the Alberton andOrkney lavas suggest that magma mixing processes were importantin their petrogenesis. Compositional variation in the Loraine/Edenvillelavas is consistent with simple fractional crystallization dominatedby orthopyroxene. Collectively, all these lavas are light rareearth element (LREE)-enriched and have negative Nb-Ta, P, andTi anomalies on mid-ocean ridge basalt (MORB)- or primitivemantle-normalized abundance diagrams. In this they resemblemafic lavas from modern subduction-zone environments and manyArchaean and early Proterozoic siliceous high-magnesium basalts.The Klipriviersberg lavas also have initial Nd-isotopic compositionsthat cluster about the chondritic uniform reservoir (CHUR) valueregardless of degree of differentiation. It is unlikely thatcrustal contamination has influenced the composition of theKlipriviersberg lavas to any significant extent, and their geochemicalfeatures have probably been inherited from lithospheric mantlesources.     

Trace element distribution in uraninite from Mesoarchaean Witwatersrand conglomerates (South Africa) supports placer model and magmatogenic source

The first systematic analyses of the trace and rare earth element (REE) distribution in uraninite from various gold-bearing conglomerates of the Mesoarchaean Central Rand Group in South Africa’s Witwatersrand Basin by in situ laser ablation-inductively coupled plasma-mass spectrometry confirms a placer origin for the uraninite and a magmatogenic provenance thereof. The chemistry of commonly rounded to sub-rounded uraninite is highly variable from grain to grain but generally marked by elevated Th, W, Bi, Mo, Ta, Y, REE contents and unusually high Au concentrations. Especially, the high Th contents and the chondrite-normalised REE patterns are incompatible with post-sedimentary hydrothermal genetic models for the U mineralisation and point to derivation of the detrital uraninite from a high-temperature, magmatogenic, presumably granitic to pegmatitic source. The elevated Au concentrations (of as much as 67 ppm) in this uraninite are unique to the Witwatersrand and hint at a granitic hinterland that was enriched in both U and Au, thus presenting a potential source domain for some of the detrital gold in the Witwatersrand conglomerates. Minute fracture fills of brannerite in close proximity to the larger, rounded uraninite grains are devoid of detectable Bi, Mo, REE and Au and have only very low concentrations of Th, W, Ta and Y. This is explicable by crystallisation from a low-temperature hydrothermal fluid. Thus, Witwatersrand U phases show, analogous to many other ore constituents, such as pyrite and gold, clear evidence of partial, short-range mobilisation of originally detrital particles by post-sedimentary fluids.     

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO₂ (LOI), MgO, Ba, K and Rb, but significant depletion of SiO₂ and, to a lesser extent, Fe₂O₃. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO₂- and H₂S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.     

Fossilized plants in thucholite from Precambrian rocks of the Witwatersrand,South Africa

Examination of columnar carbonaceous material or thucholite from gold-bearing conglomerate revealed internal structures morphologically resembling filamentous, branched and apparently septate cells of obvious biological origin partially encrusted with gold, as well as silicified structures which could be identified as primitive fungi. The columnar structures could be part of a differentiated, apparently symbiotic organism which formed carpet-like colonies of up to several square metres in extent. On the basis of the chemical composition and the apparent ability of the suggested organism to assimilate gold and other inorganic material, the apparent presence of a symbiotic alga and the assumed nature of the Precambrian environment, an organism is proposed that has many morphological similarities with lichens but has otherwise no known living equivalent.     

The gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa)

An investigation of the gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa) has confirmed that the bulk of this metal is concentrated within the bottom parts of the reefs, mostly near the footwall contacts. This, together with other criteria, suggests that the distribution of the metal was governed by sedimentary processes. On the other hand, the shape of the gold grains, as well as the general fabric of the gold-enriched layers, often lack conspicuous sedimentary characteristics. Subsequent metamorphism has obviously produced marked structural modifications and a limited textural rearrangement of the gold and its host-rock, thus obliterating to a certain degree the primary alluvial character of the metallization. The large-scale reconstitution of the gold fraction was effected either by a mechanical reshaping of the malleable and ductile gold particles, or by an intermediate solution process followed by reprecipitation. These processes took place more or less in situ. Accordingly, the original sedimentary distribution pattern was not profoundly affected, whereas the individual allogenic gold grains were transformed into authigenic ones or sometimes even into major gold aggregates. Electron-probe microanalysis established that the gold contains on the average between 9.9 and 12.4% silver, which is also suggestive of an alluvial origin. In spite of some uncertainties arising from the camouflaging effect of pseudohydrothermal reconstitution processes and from the absence of hydraulic equilibrium between gold and several demonstrably detrital heavy minerals, an integration of all available data strongly supports a modified placer theory for the origin of the Witwatersrand gold.

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Zusammenfassung Eine Untersuchung der Goldführung von Konglomeraten des Oberen Witwatersrand-Systems (Oranje-Freistaat-Goldfeld, Südafrika) zeigt, daß die Verteilung des Metalls eindeutig durch sedimentäre Faktoren gesteuert worden ist, obwohl die Form der einzelnen Goldkörnchen sowie ihre Lagebeziehungen zueinander und zu den übrigen Mineralien nicht in jedem Falle mit gleicher Eindeutigkeit für ein sedimentäres Gefüge sprechen. Für diesen scheinbaren Widerspruch ist die Metamorphose verantwortlich, der die Lagerstätte im Laufe ihrer späteren geologischen Geschichte ausgesetzt war und bei der es zu einer teilweisen Umlagerung der Lagerstättensubstanz kam. Davon wurde auch die Goldfraktion betroffen, die dementsprechend heute kaum noch in ihrer ursprünglichen detritischen Form vorliegt, sondern meist pseudohydrothermale Aggregate bildet. Die Umkristallisation des Goldes erfolgte dabei meist in situ, so daß das ursprüngliche sedimentäre Verteilungsschema ausgezeichnet erhalten geblieben ist. — Der Silbergehalt des Goldes konnte mit 9.8–12.4% bestimmt werden, was gleichfalls für eine detritische Herkunft des Metalls spricht. Berücksichtigt man die späteren Umlagerungsvorgänge, dann bereitet der erzmikroskopische Befund einer Deutung der Konglomerate als Seifen keine Schwierigkeiten, sondern spricht mit überwältigender Wahrscheinlichkeit für eine Seifenumlagerungshypothese.

     

Small-scale gold distribution patterns in the Precambrian Kimberley Reef placer: A case-study at marievale GMC., East Rand Goldfield,South Africa

The Kimberley Reef is a Proterozoic gold-uranium-placer which was deposited during the Kimberley stage of the Upper Witwatersrand System, approximately 2.7 × 10⁹ years ago. The investigated site at Marievale GMC. is located towards the fanbase of the East Rand Goldfield. Geological, sedimentological, and geochemical parameters show that the placer formed in a fluviatil depositional environment (ca. 40 cm water depth) with a small-scale meandering stream pattern. Gold anomalies are elongated and frequently arcuate-shaped with sinuosities of ±1.5; their orientation reveals a high degree of variability on the scale of a few metres to tens of metres. They formed at the inside of meandering channel curves next to and alongside elongated conglomerate bodies which correspond to point bars. Concentrations of other heavy minerals formed according to their decreasing specific gravity at the outer, convex side and the downstream part of gold anomalies roughly in the order uraninite — pyrite and chromite — zircon. This sedimentary-controlled gold and heavy mineral distribution pattern was complicated by lateral channel migrations and frequent bed changes of streams, as well as subsequent faulting which had a depletioning effect on gold.     

A petrologist in South Africa: Frederick Henry Hatch and the Witwatersrand Goldfield

The English geologist, Frederick Henry Hatch (1864–1932), today mainly recalled for a textbook on igneous petrology, emigrated to South Africa, where, in the years between 1892 and 1906, he became one of the pioneers of the geology of the Transvaal and the Witwatersrand Goldfield. This article reviews his life and, in particular, his work during this period which led to his becoming a mining geologist with connections to many of the world's goldfields, set against the political background of the Boer War which influenced the course of his work.     

The changing geography of traditional medicine: Urban herbalism on the Witwatersrand,South Africa

While medical geographers have generally ignored medical pluralism in developing countries, a small but significant geographical literature on traditional medicine has emerged. Progress for research by geographers on traditional medicine lies through a broader contextualisation of medical pluralism sensitive to the socio-economic and political context of health and disease. In this paper, a brief overview of medical pluralism in South Africa is presented. Issues surrounding the changing geography of traditional medicine are illustrated with reference to urban herbalism on the Witwatersrand.     

Geochemistry of Archean shales from the Witwatersrand Supergroup,South Africa: Source-area weathering and provenance

With a few exceptions, shales from the Archean Witwatersrand Supergroup (~2800 Ma) in South Africa are depleted in Na, Ca, LILE, REE and HFSE compared to Phanerozoic shales. Cr, Co and Ni are enriched in all Witwatersrand shales and Fe and Mg are high in shales from the West Rand Groups (WRG) and lower Central Rand Group (CRG). Shales from the CRG and uppermost WRG are enriched in Na, Al, LILE, REE, HFSE and transition metals relative to shales from the lower WRG. Chondrite-normalized REE patterns for all Witwatersrand shales are enriched in light-REE and exhibit small to moderate negative Eu anomalies. A positive correlation of REE and Al₂O₃ contents in the shales suggests that REE are contained principally in clay minerals.Relative to shales from the CRG, shales from the WRG exhibit depletions of Na, Ca and Sr, a feature probably reflecting intense chemical weathering of their source rocks. CIA indices in Witwatersrand shales are variable (chiefly 70–98), even within the same shale unit. Such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism.Compared to present-day upper continental crust, all but the Orange Grove, Roodepoort, and K8 shales appear to have been derived from continental sources depleted in LILE, REE, and HFSE and enriched in transition metals. Computer mixing models based on six relatively immobile elements (Th, Hf, Yb, La, Sc, Co) and four source rocks indicate that the relative proportions of granite, basalt and komatiite increased with time in sediment source areas at the expense of tonalite. The contributions of basalt and komatiite appear to reach a maximum during deposition of the Booysens shale, and granite during deposition of the K8 shales and possibly during deposition of the Orange Grove shales.     

Morphological and anatomical observations on some precambrian plants from the Witwatersrand,South Africa

The columnar form of carbonaceous layers in some Precambrian Witwatersrand conglomerates after detailed SEM examination has been shown to be a primary plant structure. By oxidation of the organic components at about 500° C a skeleton of length-orientated interconnected filaments is obtained which consist mainly of uranium oxide, lead oxide and gold. Some of the filaments preserved the original microbiological structures in detail.Brief heating of the columns causes a separation into different types of tissue thus making the reconstruction of the original plant possible. A comparison of the inner and outer structure and the probable metabolism with recent plants showed a definite resemblance to certain types of lichens. Following the presently used term of thucholite for the carbonaceous material the name ofThuchomyces lichenoides is suggested for the fossilized plant. A probably parasitic, filamentous fossilized microorganism which occurs in the Basal Reef together with Thuchomyces was namedWitwatersrand conidiophorus. Both fossils are described and their possible environment discussed.

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Zusammenfassung Die säulige Form von kohligen Lagen in den präkambrischen Witwatersrand Konglomeraten wurde durch detaillierte Untersuchungen im REM als primäre pflanzliche Struktur erkannt. Durch Oxydation der organischen Komponenten bei etwa 500° C erhält man ein Skelett aus längsorientierten und einander verknüpften mineralischen Filamenten (meist Uranoxyd, Bleioxyd und Gold), die zum Teil die ursprünglichen mikrobiologischen Formen bewahrt haben. Kurzes Erhitzen der Säulen bewirkt eine Aufspaltung in verschiedene Gewebearten wodurch eine Rekonstruktion der ursprünglichen Pflanze möglich wurde. Vergleiche des inneren und äußeren Aufbaus mit rezenten Pflanzen zeigt deutliche Ähnlichkeiten mit bestimmten Flechtenarten. In Anlehnung an die bisher gebräuchliche Bezeichnung Thucholith für die kohlige Substanz wirdThuchomyces lichenoides als Name für die fossile Pflanze vorgeschlagen. Ein wahrscheinlich parasitärer, fadenförmiger, fossiler Mikroorganismus, der im Basal Reef zusammen mit Thuchomyces vorkommt, wurdeWitwateromyces conidiophorus benannt.Die beiden Fossilien werden beschrieben und ihre mögliche Umwelt diskutiert.

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Résumé L'étude détaillée, au microscope électronique à balayage, de la forme colonnaire de couches charbonneuses de certains conglomérats Précambriens du Witwatersrand, a permis d'y reconnaître une structure végétale primaire. Par l'oxydation des composants organiques a environ 500°, on obtient un squelette de filaments reliés entre eux et orientés dans le sens de la longueur, principalement à base d'oxyde d'uranium, d'oxyde de plomb, et d'or. Quelques-uns de ces filaments ont conservé leurs structures microbiologiques originelles dans tous leurs détails.Un bref réchauffement des colonnes cause la séparation des différentes sortes de tissus à partir desquels il est possible de reconstituer la plante originelle. Une comparaison de la structure interne et externe et du métabolisme probable avec des plantes récentes indique une ressemblance indéniable avec certaines sortes de lichens. Compte tenu de la désignation de thucholite pour la substance charbonneuse, le nom deThuchomyces lichenoides est suggéré pour cette plante fossile.Un autre micro-organisme filamenteux, probablement parasitaire, découvert dans le Basal Reef, avec Thuchomyces, a reçu le nom deWitwateromyces conidiophorus. Les deux fossiles sont décrits et leur environnement probable, discuté.

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. 600° C — . . , —, . , . . «Thucholit» «Thuchomyces lichenoides», , , — «Witwateremyces conidiophorus». .

     

New data on hydrocarbons in auriferous conglomerates of the Witwatersrand ore region, Republic of South Africa

Genesis of deposits in the world’s largest Witwatersrand auriferous region (South Africa) remains a debatable issue over several decades. Based on geological and mineralogical-geochemical investigations, we previously proposed a sedimentary-exhalative hypothesis for its origin. In the present communication, we have attempted to develop and scrutinize this concept based on the study of organic matter in ore and nonore conglomerates, quartz in conglomerate pebbles, and fluid inclusions in quartz from pebbles and cement of ore and nonore conglomerates.     

Geochemistry of late Archaean metagreywackes from the Western Dharwar Craton,South India: Implications for provenance and nature of the Late Archaean crust

Late Archaean metagreywackes of the Ranibennur Formation, Dharwar Supergroup, in the Dharwar–Shimoga schist belt of the Western Dharwar Craton (WDC) are texturally and mineralogically immature of the quartz-intermediate type. The SiO₂ content in them ranges from 60.58 to 65.26 wt.%. Chemical Index of weathering (CIW) values varies between 50 and 65. 4 indicating a low degree of chemical alteration of the provenance rocks. A high degree of correlation between K₂O and Al₂O₃ (r = ? 0.73) and low Rb/Sr ratios also suggest a low degree of alteration of provenance rocks. Abundances of transition group elements (Cr = 118–221; N = 89–154; V = 89–192 and Sc = 11–16 ppm) as well Zr (132–191 ppm) suggest a mixed mafic–felsic provenance for the metagreywackes. Low HREE and Y content, and low Tb/Yb ratios (0.23–0.41) suggest the presence of tonalite as an important component in the provenance areas. Values of Eu/Eu?(0.78) and Th/Sc (0.55) suggest that the granodioritic upper crust had evolved prior to serving as the provenance. Mixing calculations suggest 50–55 vol.% tonalite, 20–25 vol.% granite, 18–20 vol.% basalt and ~ 5 vol.% komatiite composition for the provenance. Geochemical characteristics of the Ranibennur metagreywackes suggest that sedimentary basin formed in the vicinity of a magmatic arc in a continental island arc setting, and the detritus were shed from the arc rock.