Geochemical and morphological characteristics of gold particles from recent river deposits and the fossil placers of the Witwatersrand

A relationship has been established between morphological features and fineness of gold particles and the distance over which they have been transported in recent alluvial placer deposits, such as the rivers of the Barberton Mountain Land, South Africa and the river Rhine in Germany. It was possible to show that most gold particles from the Witwatersrand conglomerates retained their detrital morphology and by comparing them with particles from recent alluvial gold deposits it was possible to estimate the distance of transport for the Witwatersrand gold, which in most cases ranged from 10 km to 30 km. Gold particles in recent placers show a characteristic increase in fineness with increasing distance of transport because of the leaching of the silver from them. The Witwatersrand gold particles on the other hand, have retained their primary fineness, because leaching of silver in the oxygen-deficient Precambrian atmosphere was not feasible chemically.     

SHRIMP U–Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins

SHRIMP (Sensitive High‐Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable ²⁰⁷Pb/²⁰⁶Pb data are more readily obtained than ²⁰⁶Pb/²³⁸U, which also favours application to the Precambrian. However, it is demonstrated that ²⁰⁶Pb/²³⁸U dating of larger overgrowths (>10 μm) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of ± 7 Ma. In contrast, greenschist‐facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin.     

Composition and genesis of Precambrian metalliferous conglomerates

Precambrian metalliferous conglomerates are the most important source of gold, uranium, and other metals. They concentrate no less than 30% of world gold reserves and provide 30–50% of world gold production. The metalliferous conglomerates are known at various chronological levels of the Early Precambrian: the Neoarchean (Witwatersrand Supergroup, South Africa), the Neoarchean-Paleoproterozoic (Huronian Supergroup, Canada), and the Paleoproterozoic (Tarkwaian Group, West Africa; Roraima Group, the Guiana Shield; Jacobina and Sierra de Carrego groups, the Brazil Shield; Mount Bruce Group, West Australian Shield). They are related to different stages of the tectonic evolution: preorogenic stage (Huronian Supergroup), orogenic stage (Tarkwaian Group), and postorogenic or protoplatformal stage (Witwatersrand). Long-term stabilization of the Earth’s crust and deposition of thick sedimentary sequences were the most favorable conditions for the formation of metalliferous conglomerates.     

新疆且干布拉克超大型蛭石矿床的成因与成矿模式

新疆尉犁县且干布拉克超基性杂岩体位于塔里木北缘隆起带库鲁克塔格前寒武纪地块中西段,兴地塔格断裂带附近。该蛭石矿床是继南非Palabora、美国Libby和俄罗斯Kovdorsk蛭石矿之后的第4个超大型蛭石矿床。根据野外地质观察和室内薄片鉴定结果,结合该区已有的岩石地球化学和矿床学资料,并通过与国外同类型矿床进行详细对比,从含矿杂岩体的岩石组合、矿体发育特征和矿物共生组合等不同层面探讨了该杂岩体的演化过程和矿床成因,并建立了相关成岩成矿模型。结果表明,且干布拉克杂岩体是裂谷背景下,富集岩石圈地幔低程度部分熔融岩浆经岩浆分异和不混熔作用并以橄榄岩-黑云母透辉岩-碳酸岩等岩相依次侵入的产物,蛭石矿床中的金云母是岩浆晚期高温富碱质热液交代超基性岩的产物,云母类矿物在剥蚀到近地表后,在大气降水作用下发生表生风化最终形成蛭石矿床。矿床的形成过程复杂,经历了从地幔交代、超大陆裂解、岩浆分异、热液蚀变到表生风化作用等五大地质过程。这一模式的建立将为进一步寻找和深入研究该类矿床提供理论依据和实践指导。     

Detrital pyrite in Witwatersrand gold reefs: X-ray diffraction evidence and implications for atmospheric evolution

Pyrite is easily oxidized, and therefore unambiguous evidence of detrital pyrite grains in metasediments is a significant constraint on when an oxygenated atmosphere developed. Compact rounded pyrite in the Witwatersrand gold reefs of South Africa has a detrital habit and is texturally equivalent to and spatially associated with detrital zircon and chromite. X-ray precession photography reveals that petrographically featureless As-poor grains are untwinned single crystals of high diffraction quality. This new evidence from crystallography is consistent with mechanically abraded pyrite from primary lode gold deposits, and excludes an origin by replacement of a pre-existing detrital phase. Further evidence of a detrital origin for the compact rounded pyrite is afforded by isolated grains of arsenian pyrite displaying truncated As-rich growth bands. The geographically extensive Witwatersrand fluvial conglomerates evidently had a matrix of quartz and pyrite sand and pyritic mud in their unconsolidated state and, thus, the late Archean atmosphere of Earth was likely essentially anoxic.     

Microbial hydrocarbon gases in the Witwatersrand Basin, South Africa: Implications for the deep biosphere

In this study, compositions and δ¹³C and δ²H isotopic values of hydrocarbon gases from 5 mines in the Witwatersrand basin, South Africa, support the widespread occurrence of microbially produced methane in millions of years-old fissure waters. The presence of microbial methane is, to a large extent, controlled by the geologic formations in which the gases are found. Samples from the Witwatersand Supergroup have the largest microbial component based on δ¹³C and δ²H signatures and CH₄/C₂+ values. Based on mixing between a microbial CH₄ component and a more ¹³C-enriched and ²H-depleted C₂+-rich end member, conservative estimates of the % contribution of microbial CH₄ to the gas samples range from >90% microbial CH₄ at Beatrix, Masimong, and Merriespruit, to between 5 and 80% microbial CH₄ at Evander, and <18% microbial CH₄ at Kloof. The Witwatersrand basin’s history of thermal alteration of organic-rich ancient sedimentary units suggests a thermogenic origin for this ¹³C-enriched end member. Alternatively, the potential for an abiogenic origin similar to hydrocarbon gases produced by water-rock interaction at other Precambrian Shield mines is discussed. Microbial methane is predominantly found in paleo-meteoric fissure waters with δ¹⁸O and δ²H values that fall on the meteoric waterline, and have temperatures between 30 to 40°C. In contrast, fissure waters with a larger component of nonmicrobial hydrocarbon gases show a trend towards more enriched δ¹⁸O and δ²H values that fall well above the meteoric waterline, and temperatures of 45 to 60°C. The enrichment in ¹⁸O and ²H in these samples, and their high salinity, are similar to the isotopic and compositional characteristics of saline groundwaters and brines produced by water-rock interaction at Precambrian Shield sites elsewhere. The reported 100 Ma ages of fissure waters from the Witwatersrand and Ventersdorp formations suggest that these microbial hydrocarbon gases are the product of in situ methanogenic communities in the deep subsurface of the Witswaterand basin. Small subunit ribosomal RNA genes were amplified using archaeal-specific primer sets from DNA extracts derived from several of these waters. Fissure waters with a high proportion of microbial methane also contained sequences resembling those of known methanogens.     

Evidence in pre-2.2 Ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota

The loss of Fe from some pre-2.2 Ga paleosols has been considered by previous investigators as the best evidence for a reduced atmosphere prior to 2.2 Ga. I have examined the behavior of Fe in both pre- and post-2.2 Ga paleosols from depth profiles of Fe3+/Ti, Fe2+/Ti, and sigma Fe/Ti ratios, and Fe3+/Ti vs. Fe2+/Ti plots. This new approach reveals a previously unrecognized history of paleosols. Essentially all paleosols, regardless of age, retain some characteristics of soils formed under an oxic atmosphere, such as increased Fe3+/Ti ratios from their parental rocks. The minimum oxygen pressure (PO2) for the 3.0-2.2 Ga atmosphere is calculated to be about 1.5% of the present atmospheric level, which is the same as that for the post-1.9 Ga atmosphere. The loss of sigma Fe, common in paleosol sections of all ages, was not due to a reducing atmosphere, but to reductive dissolution of ferric hydroxides formed under an oxic atmosphere. This reductive dissolution of ferric hydroxides occurred either (1) after soil formation by hydrothermal fluids or (2) during and/or after soil formation by organic acids generated from the decay of terrestrial organic matter. Terrestrial biomass on the early continents may have been more extensive than previously recognized.     

Origin and timing of the metasomatic silicification of an early archean komatiite sequence,barberton mountain land,South Africa

Stratiform units of pervasively silicified early Archean ultramafic rocks occur near the boundary between the Onverwacht and Fig Tree groups in the Barberton Mountain Land, Republic of South Africa. The origin of these units has been variously ascribed to early Archean subaerial weathering or submarine alteration, to cataclastic metamorphism, and to the alteration of silicic tuffs. An exceptionally well-exposed example of such an alteration sequence along the Skokohla River valley in the central Barberton Mountain Land was studied to determine the nature of the precursor rocks and the timing and nature of the alteration processes.Well-preserved ghosts of spinifex- and culmulate-textured olivines and pyroxenes establish the komatiitic ancestry of the Skokohla rocks. Rock units of several distinctive lithofacies can be correlated over the entire 1.5 km strike of the sequence and were derived from the metasomatic conversion of a series of ultramafic flow units to rocks dominated by quartz, dioctohedral mica, and chlorite. The original ultramafics have been strongly enriched in Si, K, Rb, and Ba, and depleted in Fe, Mg, Ca, Na, Mn, Sr, Zn, and Ni. The elements Al, Ti, P, Cr, Zr, Y, and Nb were immobile.Alteration apparently occurred soon after extrusive activity and before any significant tectonic deformation. It is most probable that alteration resulted from the dynamic, hydrothermal upwelling of fluids through original fracture porosity in the ultramafic flows.     

Banded iron formations and palaeoenvironment: a problem in petrogenesis

The mineralogy, chemistry and stratigraphy of Precambrian banded iron formations have been extensively documented, yet the way such formations are produced remains a subject for debate. Differentiation by varving, microbial precipitation and secondary alteration are all seen as possible mechanisms, but discussion returns to the lack of any modern analogue. Nothing like banded iron forms anywhere in the world today or has done during the entire Phanerozoic. Where do we begin with such enigmatic rocks?     

Paleoclimate changes in the late precambrian: Evidence from the upper precambrian section of the South Urals

The climatic impact on the formation of fine-grained rocks from the Riphean stratotype and Vendian Asha Group on the western slope of the South Urals during the time interval lasting approximately 1200 Ma is considered. It is shown that these rocks are largely represented by “tectonosilicate-dominated” shales. This feature combined with changes in the average K₂O/Al₂O₃ values disavows the hypothesis in (Kennedy et al., 2006), according to which the growth of free oxygen concentration in the Late Riphean and Vendian atmosphere was determined by gradual intensification of the organic carbon extraction from the biosphere by clays. The average values of the hydrolyzate module, chemical index of alteration (CIA), and several lithogeochemical parameters calculated for the Riphean and Vendian clayey rocks provide grounds for the conclusion that intensity of weathering in paleodrainage areas during the accumulation of the Upper Precambrian sedimentary successions was low. The curve reflecting changes of the average CIA values in the Upper Precambrian fine-grained siliciclastic rocks of the South Urals is similar to some extent with the “standard” CIA_correct. curve (GonzalezAlvarez and Kerrich, 2012). It is assumed that changes in microand macrobiotic communities during the Late Precambrian were controlled to a variable extent by climate fluctuations as well. At the same time, these fluctuations most likely left the chemical composition of water in the ocean virtually unchanged, which is evident from analysis of the redox conditions in the ocean and the distribution of primary producers with the average CIA_correct. and CIA values.     

The Reitz ring: a possible circular structure, 350–500 km in diameter,in South Africa

The accurate northwestern rim of the upper Witwatersrand sedimentary basin in South Africa can be regarded as representing 120 degrees of an original circular structure roughly 400 km in diameter. Extrapolation of the perimeter passes, on the east and northeast, near to outcrops of the older, but lithologically similar, Mozaan Series. The center of this circle coincides with a positive Bouguer gravity anomaly, near Reitz (“Ray-tz”), South Africa. If these features are genetically related, the symmetry may reflect an early meteorite impact structure.     

Petrography and classification of NWA 7402: A new sulfide-rich unequilibrated ordinary chondrite

We classify a new chondritic find Northwest Africa (NWA) 7402. This meteorite is highly unequilibrated, and is therefore potentially significant for the study of primitive Solar System materials. Mineralogy, mineral chemistry, and modal abundances of minerals indicate that NWA 7402 is most likely an L chondrite. However, the specimen contains a higher abundance of sulfide than commonly seen in ordinary chondrites. The structural order of organic matter in the matrix and the chromium content of Fe-rich olivine grains indicate a petrologic type of 3.1. NWA 7402 largely escaped thermal metamorphism, and secondary phases formed by aqueous alteration are rare to absent. Minor planar fractures and undulatory extinction of olivine grains suggest that NWA 7402 experienced shock up to stage 2 or 3. Terrestrial weathering is heterogeneous in the specimen; much of the stone's exterior shows substantial Fe oxidation (weathering grade 2), while some parts of the interior remain relatively fresh (weathering grade 1). NWA 7402 has some unusual features that should be investigated further. The sulfide abundance is higher than reported sulfide contents for other L chondrites, and the chromium content of the olivines does not fall on the trend established for unequilibrated ordinary chondrites by Grossman and Brearley (2005).     

Precambrian phosphorites in the Bijawar rocks of Hirapur-Bassia areas,sagar district,madhya pradesh,India

The Precambrian phosphorites of Bijawar Group of rocks show characteristics of a epicontinental sea with restricted and very shallow marine environment of formation along some shoals, which existed during the iron-rich Precambrian times. These phosphorite deposits located in the Hirapur-Bassia areas show extensive leaching of carbonate and phosphate minerals during episodes of weathering. X-ray diffraction studies indicated that carbonate-flourapatite is the major apatitic phase in these phosphorites while crandallite developed on the surface outcrops. There is a general tendency for the depletion of CO₂ in these apatites leading to formation of flourapatite. This CO₂ is an indicator of hidden weathering in the rocks. Major and trace element determinations of phosphorite have been used to indicate various correlation factors responsible for the concentration of elements in these Precambrian leached phosphorites.The paper is a contribution to the aims and objectives of IGCP Project 156The paper is dedicated to Prof. Dr. R. C. Misra, who as a teacher and guide had been a source of inspiration to the senior author for the last two decades     

Mechanisms to explain the loss of heavy minerals from the Upper Palaeozoic tillites of South Africa and Australia and the late Precambrian tillites of Australia

Heavy mineral studies on Pleistocene tills from North America, Upper Palaeozoic tillites of South Africa and Australia, and late Precambrian tillites of South Australia show that the heavy mineral suites of the Pleistocene tills are dominated by amphiboles, the Upper Palaeozoic tillites by garnet, and the late Precambrian tillites by zircon and tourmaline. About half of the garnets in the Upper Palaeozoic tillites show evidence of having been rounded, and retain delicate surface chattermark trails, which indicates that these garnets have not undergone chemical attack since deposition. Although the remainder of the garnets show, by way of etching, that intrastratal solutions were active in the sediments, it is suggested that amphiboles, pyroxenes and epidote, which must have been present in the original Upper Palaeozoic heavy mineral suites, were lost primarily by the action of sorting and mechanical abrasion in beach environments prior to, and during interglacial periods. The absence of garnet and the etching of tourmaline and zircon in the late Precambrian tillites is attributed to the action of alkaline intrastratal solutions over the long time interval during which the tillites were buried in the Adelaide Geosyncline.     

Lower Cretaceous paleosols and paleoclimate in Sichuan Basin,China

Abundant Lower Cretaceous (Berriasian–Hauterivian) paleosols have been recognized in the Sichuan Basin, along with the preserved pedogenetic features, e.g., soil horizons, soil structure, root traces and pedogenic nodules. Chemical, geochemical and mineralogical analyses were used to examine the paleosols. These paleosols were classified as Entisols, Inceptisols, Aridisols and Alfisols in terms of the modern soil taxonomic system. Early Cretaceous paleoprecipitation and paleotemperature in the Sichuan Basin were estimated from the degree of chemical weathering for non-calcareous paleosols, and from the depth to the calcic horizon and stable oxygen isotopic composition of pedogenic carbonates in calcareous paleosols, respectively. A temperate semi-arid climate generally prevailed in the Sichuan Basin as a part of the South China Block (SCB) and was controlled by subtropical high-pressure and a rain-shadow effect because the humid air masses from the Paleo-Pacific were impeded by the highlands of the South China Block. Further, several intervals of sub-humid paleoclimate occurred due to strengthened monsoonal circulation in the Early Cretaceous. Using the paleosol barometer, the paleoatmospheric CO₂ levels of the Early Cretaceous are estimated to range from ∼120 to ∼520 ppmv, with a mean of 305 ppmv. Regional temperature is generally coupled with atmospheric CO₂ concentration and is roughly consistent with the sea level fluctuation.     

Trace element crystal chemistry of mantle eclogites

Rare earth element (REE) concentrations have been measured using instrumental neutron activation analysis on clean separates of primary minerals from 11 eclogite samples from the Bobbejaan and Roberts Victor kimberlites, South Africa. Samples were selected to reflect minimal secondary alteration and represent a broad range of eclogite compositions from coesiteand corundum-grospydites through magnesian bimineralic eclogites. Correlations between REE concentrations and major-element compositions suggest that garnet and clinopyroxene crystal chemistry are the dominant control on REE distribution and that these approach solidstate equilibrium distributions. Reconstructed wholerock REE concentration variation with whole-rock major-element compositions are consistent with an origin by high-pressure igneous fractionation followed by reequilibration to lower temperatures at pressures in excess of three GPa.     

Geochemistry of sericite deposits at the base of the paleoproterozoic aravalli supergroup, Rajasthan, India: Evidence for metamorphosed and metasomatised precambrian paleosol

Fine grained sericite deposits occur at the interface between Archean Mewar Gneiss Complex and the Proterozoic Aravalli Supergroup independent of shearing. They show a gradational contact with the basement granites and gneisses and a sharp contact with the overlying quartz pebble conglomeratic quartzites. Rip-up clasts of these sericite schists are found in the overlying conglomerates. The sericite schists are rich in sericite towards the top and contain chlorite towards the base. The sericite in these schists was formed by metasomatic alteration of kyanite and not from the feldspars of the basement granitoids and gneisses. Uni-directional variations of SiO₂ and Al₂O₃, high Al₂O₃ content (>30%), positive correlation between Al₂O₃ and TiO₂, Ti/Al and Ti/Zr ratios, high pre-metasomatic chemical indices of alteration (> 90), and enrichment of heavy rare earth elements relative to the parent granites and gneisses—all these chemical characteristics combined with field evidence suggest that the sericite schists are formed from a paleosol protolith, which developed on Archean basement between 2.5 and ~2.1 Ga in the Precambrian of Rajasthan. The superimposed metasomatic alteration restricts the use of Fe²⁺/Ti and Fe³⁺/Ti ratios of these paleosols for interpretation of PO₂ conditions in the atmosphere.     

Iron in Precambrian rocks: implications for the global oxygen budget of the ancient Earth

Banded iron formations (BIF) are prominent in sediments older than 2 Ga. However, little is known about the absolute abundance of BIF in Archean and Early Proterozoic sediments, and the source of the Fe is still somewhat uncertain. Also unknown is the role that Fe may have played in the maintenance of low oxygen pressures in the Archean and Early Proterozoic atmosphere. An analysis of the chemical composition of Precambrian rocks provides some insight into the role of Fe in Precambrian geochemical cycles. The Fe content of igneous rocks is well correlated with their Ti content. Plots of Fe vs. Ti in Precambrian sandstones and graywackes fall very close to the igneous rock trend. Plots of Fe vs. Ti in Precambrian shales also follow this trend but show a definite scatter toward an excess of Fe. Phanerozoic shales and sandstones lie essentially on the igneous rock trend and show surprisingly little scatter. Mn/Ti relations show a stronger indication of Precambrian Mn loss, perhaps due to weathering under a less oxidizing early atmosphere. These data show that Fe was neither substantially added to nor significantly redistributed in Archean and early Proterozoic sediments. Enough hydrothermal Fe was added to these sediments to increase the average Fe content of shales by at most a factor of 2. This enrichment would probably not have greatly affected the near-surface redox cycle or atmospheric oxygen levels. Continued redistribution of Fe and mixing with weathered igneous rocks during the recycling of Precambrian sediments account for the excellent correlation of Fe with Ti in Phanerozoic shales and for the similarity between their Fe/Ti ratio and that of igneous rocks.     

Sedimentological,geochemical and paleontological insights applied to continental omission surfaces: A new approach for reconstructing an eocene foreland basin in NW Argentina

An Eocene foreland basin linked to the Andean uplift in northwestern Argentina has recently been proposed. The basin is divided and partially eroded due to subsequent Neogene orogenic phases, so that a simple reconstruction is insufficient to describe complex field relationships. This presents a new challenge in understanding the initial phases of Central Andean evolution. We propose a multidisciplinary approach in key locations and/or at key geological features as a way to reconstruct the Paleogene basin. In this contribution, we report on sedimentological and geochemical evidence of a conspicuous weathering surface in the continental Eocene Lumbrera Formation and provide an age estimate based on vertebrate mammalian biostratigraphy and an absolute U/Pb zircon age of 39.9 Ma. Weathering surfaces become evident when diagnostic features like paleosols, karsts, and trace fossils are distinctive but, in our case, these characteristics only emerge through detailed sedimentological and geochemical surveys. The Lumbrera paleosurface is represented by a hardened level (20–30 cm thick) characterized by moderately developed reddish paleosols. Moreover, major and trace element profiles show inflections at the top and/or base of the weathered horizon delineating it. A modified form of the chemical index of alteration shows that chemical leaching was moderate and not extensive. In addition, mammalian fossil records substantially differ below and above the weathered paleosol-bearing surface. We conclude that this horizon represents a Middle Eocene omission surface and represents a key level marking a major basin change in northwest Argentina, adding a new constraints for Eocene foreland reconstruction.     

Iron-formation and glaciogenic rocks of the Rapitan Group,Northwest Territories,Canada

In northwestern Canada, iron-formation occurs as part of the Rapitan Group, a dominantly sedimentary succession of probable Late Precambrian age. The Rapitan Group contains abundant evidence of glaciogenic deposition. It includes massive mixtites which contain numerous faceted and striated clasts. Finely bedded and laminated sedimentary rocks of the Lower Rapitan contain many large isolated (ice-rafted?) intra- and extra-basinal clasts. The Lower and Middle Rapitan are interpreted as products of a glacial marine regime. The iron-formation is interbedded with thin mixtite beds and contains large exotic clasts which are probably indicative of the existence of floating ice at the time of deposition of at least part of the iron-formation. If the apparently low paleolatitudes are confirmed, then glacial marine interpretation of the Rapitan, and the probably correlative Toby Conglomerate of southern British Columbia, support the postulate of a very extensive Late Precambrian ice sheet in North America.Similar iron-formations of similar age are present in South America (Jacadigo Series), in South-West Africa (Damara Supergroup) and in South Australia (Yudnamutana Sub-Group). All of these iron-formations are associated with glaciogenic rocks. In addition to the iron-formations, dolostones, limestones and evaporites (?) are intimately associated with Late Precambrian mixtites, considered by many to be glaciogenic.Huronian (Early Proterozoic) and correlative sequences of North America, and rocks of similar age in South Africa also contain closely juxtaposed undoubted glaciogenic rocks, iron-formations, dolostones and aluminous quartzites. The dolostones and aluminous sedimentary rocks have been interpreted as having formed under warm climatic conditions, but might also be explained by invoking higher P_CO2 levels in the Early Proterozoic atmosphere. By analogy with the Huronian succession, preservation of “warm climate” indicators in mixtite-bearing Late Precambrian sequences does not preclude a glacial origin for the mixtites.