Central Rand Group Research Articles

Uranium distribution in the promise reefs of the Mesoarchean Westrand Group, Witwatersrand Supergroup, South Africa

The Mesoarchean Westrand Group (2932 – 2923 Ma) constitutes the lower portion of the Witwatersrand Supergroup. Compared to the overlying Central Rand Group which is well known for its world-class conglomerate-hosted gold-uranium deposits, the economic potential of palaeoplacers in the Westrand Group has not been fully explored. This study aims to assess the uranium potential of the Promise Reefs in the Westrand Group. Field exposures of the Promise Reefs and boreholes drilled down-dip from the surface outcrops were investigated in the Westrand area. Three reef zones were recognized consisting of well-mineralized, matrix-to clast-supported, small pebble conglomerates with associated medium- to coarse-grained quartzite.Radioactive minerals found in the conglomerates comprise leucoxene, brannerite, carbon nodules, monazite, allanite, thorite, and uraninite.Analyses of U and Th from borehole samples reported mean values of 120 ppm for U and 60 ppm for Th. Maximum U and Th concentrations are 630 ppm and 125 ppm, respectively. The highest U/Th ratio was 17.4 with a mean value of 2. The conglomerates from surface outcrops have maximum U and Th concentrations of 111 ppm and 140 ppm, respectively, with a mean U value of 19.5 ppm and a mean Th value of 26.6 ppm, and a mean U/Th ratio of 0.6. Comparison of the data shows that surface outcrops generally report lower U concentrations than their unweathered equivalents.Also, U assays by γ-spectrometry and INAA of the same samples from surface outcrops yielded inconsistent results. As U exploration by field γ-spectrometry relies on the existence of 234U/238U equilibrium and because of the U depletion in the surface samples, further investigations using α-spectrometry and Pb-isotope analyses were employed.The results obtained from α-spectrometry point to complex U redistribution processes in the surface conglomerate with several stages of uranium loss and/or gain. The bulk of the U was removed during an early stage of surface exposure and supergene alteration, probably more than 1.4 Ma ago.The estimation of the original pre-weathering U concentration in the surface conglomerates is based on the measured Pb-isotope ratios 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb and the Pb and Th concentrations. The data were inserted into U-Th-Pb age equations which were reformulated and solved for the U concentrations. To test the method, for a suite of borehole samples the same treatment was applied. The calculated fresh-rock U tenor indicates the occurrence of supergene uranium losses which are of the same magnitude as the U losses caused by experimental leaching of fresh conglomerate samples. The high amount of radiogenic Pb found in the surface samples points to severe uranium losses earlier than 100 Ma ago.The data show that U redistribution in weathered Mesoarchean conglomerates is not governed by systematic processes which obscure the original pre-weathering U concentration. Application of the Pb isotope technique proves to be the only method yielding usable estimates of the fresh-rock uranium tenor in deeply weathered terranes.

Linking Archaean climate change with gold metallogeny

First large-scale concentration of gold to ore grade in Earth’s crust took place at around 2.9 Ga in quartz-pebble conglomerates, well before endogenous deposits, such as porphyry and epithermal systems or orogenic-type deposits, started to play a significant role from ca. 2.75 Ga onwards. The conglomerate-hosted gold placers, typified by those in the Mesoarchaeaen Witwatersrand Basin in South Africa, are thought to have been sourced from the leaching of background levels of gold in the Archaean continental crust, promoted by deep chemical weathering under a reducing acidic atmosphere. Gold dissolved in meteoric waters was trapped by possibly acidophile microbes, fossil remnants of which are preserved as kerogen layers in the 2.9 Ga lower Central Rand Group of the Witwatersrand Supergroup. Mechanical reworking of the delicate microbial mat-bound gold led to rich gold placers from 2.9 Ga onwards. Well-endowed gold placers older than 2.9 Ga are conspicuously missing, although suitable siliciclastic host rocks, even containing relics of former microbes, are known from continental sediment sequences as old as 3.22 Ga. A review of climate-sensitive rock types and geochemical data on the extent of chemical weathering reveals that the period from 2.96–2.91 Ga was cold, with repeated glaciations as evidenced by glaciogenic diamicite, whereas warmer and (or) wetter climates reigned from 2.90 to 2.78 Ga. In concert with the temporal distribution of placer gold accumulation, the conclusion is reached that chemical weathering rate, dictated by climate, was a key determining factor in the exogenous binding of Au into sedimentary deposits.

A positive syn-fold test from the Neoarchaean Klipriviersberg Group of South Africa: Quo vadis Vaalbara?

Abstract The existence of Vaalbara, the combined Neoarchaean to Palaeoproterozoic Kaapvaal-Pilbara supercraton, is questionable during the early Neoarchaean when scrutinised through the lens of recent Australian and South African palaeomagnetic data. Remarkably similar ~2.7 to 2.5 Ga geological successions (with near bed-for-bed correlatability) support a coherent Vaalbara at the end of the Neoarchaean. Here we report palaeomagnetic and rock magnetic results from the Klipriviersberg Group of South Africa, which is the oldest rock sequences used to define Vaalbara originally. A positive syn-fold test illustrated a high-temperature remanence component acquired during the formation of the Witwatersrand syncline. This fold structure predates the Vredefort Impact Structure and its formation is synchronous with the deposition of the Mesoarchaean Central Rand Group and extrusion of the Klipriviersberg Group. The studied rocks of the Klipriviersberg Group are not directly dated, but most are likley younger than 2 780 to 2 789 Ma, based on detrital zircon ages from the lowermost Ventersdorp Supergroup and U-Pb baddeleyite ages for mafic sills that intrude the Witwatersrand Supergroup that are regarded as feeders of the Kliprivierberg Group lavas, but older than the overlying 2 720 to 2 750 Ma Platberg Group. The Klipriviersberg Group pole is at 27.7°S, 32.7°E with an A95 of 11°. A comparison of Meso- to Neoarchaean palaeopoles from the Kaapvaal and Pilbara cratons suggests their shared drift path traversing the polar circle and thus supports the existence of Vaalbara across the 2.78 to 2.70 Ga interval.

Polyphase Archean to Paleoproterozoic deformation along the northwest margin of the Johannesburg Dome, Kaapvaal Craton

Despite extensive gold exploration, the reasons for the dome shape of the Paleoproterozoic Johannesburg Dome (JD) remain elusive. We investigated the structural record of an outlier of Archean to Paleoproterozoic supracrustal rocks (the Zwartkops Hills Outlier, ZHO) overlying the JD to investigate its tectonic evolution. Steep gneissic fabrics in the basement pre-date the intrusion of ∼ 3225 Ma trondhjemite (new U–Pb zircon age). Two Archean deformation events affected West Rand Group rocks and pre-date Platberg Group deposition. To the south of the ZHO, a newly recognized, large-scale, recumbent fold formed during top-to-the-south shearing. In the central part of the ZHO, steeply W-dipping stratification results from E-W shortening. Two more deformation events are Paleoproterozoic in age and affected all units. A shallowly S-SW-dipping cleavage formed in the biotite zone of the greenschist facies is associated with N–S shortening and top-to-the-north shearing. The cleavage transects the large recumbent fold and is observed down to the upper 30 m of the basement. Finally, a weak N–S shortening event is inferred from kink folds and north-dipping axial planar cleavage. These data are consistent with the existing regional models of (i) south- and east-directed fold-and-thrust belts formed during sedimentation of the Archean Central Rand Group, (ii) north-verging schist belt formed during, or after the Transvaal Supergroup deposition, and (iii) impingement of the Bushveld Complex onto the JD during N–S shortening. These findings are integrated with previous regional sedimentological, seismic, and structural studies to suggest that the JD granitoid basement formed a paleo-high during the deposition of the Central Rand and Platberg groups. Once buried in the upper crust, the paleo-high acted as a rigid granitoid promontory that indented weak surrounding cover rocks during Paleoproterozoic N–S contraction events, creating the dome shape of the Johannesburg Dome.

U-Pb-Hf isotopes and shape parameters of zircon from the Mozaan Group (South Africa) with implications for depositional ages, provenance and Witwatersrand – Pongola Supergroup correlations

The Mozaan Group represents the youngest unit of the c. 2.9 Ga Pongola Supergroup located along the south-eastern margin of the Kaapvaal Craton. It comprises a ca. 4800 m thick succession of clastic sedimentary rocks intercalated by minor chemical and volcano-sedimentary rocks deposited in shallow marine to fluvial environments, and is stratigraphically correlated with the auriferous Witwatersrand Supergroup. This correlation, however, is speculative, in particular as systematic information about depositional ages and sediment provenances are absent. To address these problems, we present new combined sets of U-Pb ages, Hf isotopes, and shape parameters (width, length, aspect ratios and roundness) of >700 detrital zircon grains from seven samples of the Mozaan type profile in the Hartland area. These data reveal a switch in provenance between the lower and upper Mozaan Group. Zircons in sandstones of the lower Mozaan Group (Sinqeni to Ntombe formations) were supplied from surrounding proto-Kaapvaal Craton, and those in upper Mozaan Group rocks (Delfkom to Ntanyana formations) predominately from a juvenile hinterland, comprising sources as far as the Pietersburg and/or Kimberley blocks, which became amalgamated to the proto-Kaapvaal Craton at 2.97–2.87 Ga. Significant overlap of zircon age spectra, Hf isotope data, and maximum depositional ages (2908 ± 8 Ma to 2866 ± 7 Ma) suggest similar sources for upper Mozaan Group and Central Rand Group sediments of the Witwatersrand Basin. In contrast, sedimentary rocks of the West Rand Group have no counterparts in the Pongola Basin, except for the Orange Grove Formation, which shows good agreement with the Sinqeni Formation. The provenance switch indicated by the age-Hf isotope data is not identified by zircon shape parameters. These rather reflect differences in depositional environment (littoral, fluvial, volcanogenic), related to the duration and energy of sediment transport and reworking, as indicated by specific patterns in grain size vs. roundness diagrams.

Source-to-Sink Analysis of the Gold-Hosting Mesoarchean Main and Mondeor Formations (Central Rand Group) in the Area South of Johannesburg, Witwatersrand Basin, Kaapvaal Province, South Africa

AbstractTo date, all source-to-sink research has focused on the Phanerozoic and Proterozoic stratigraphic record. This study attempts to apply source-to-sink principles to the 2.8–3.0 Ga placer gol...

A geochemical study of the Crown Formation and Bird Member lavas of the Mesoarchaean Witwatersrand Supergroup, South Africa

Abstract The ca. 2.97 to 2.80 Ga Witwatersrand Supergroup, South Africa, represents the oldest intracontinental sedimentary basin of the Kaapvaal craton. Two volcanic units occur in this supergroup: the widespread Crown Formation lavas in the marine shale-dominated West Rand Group and the more geographically restricted Bird Member lavas, intercalated with fluvial to fluvio-deltaic sandstone and conglomerate of the Central Rand Group. These units remain poorly studied as they are rarely exposed and generally deeply weathered when cropping out. We report whole-rock major and trace elements, Hf and Nd-isotope whole-rock analyses of the lavas from core samples drilled in the south of the Witwatersrand basin and underground samples from the Evander Goldfield in the northeast. In the studied areas, both the Crown Formation and Bird Member are composed of two units of lava separated by sandstone. Whereas all the Crown Formation samples show a similar geochemical composition, the upper and lower volcanic units of the Bird Member present clear differences. However, the primitive mantle-normalized incompatible trace element concentrations of all Crown Formation and Bird Member samples show variously enriched patterns and marked negative Nb and Ta anomalies relative to Th and La. Despite the convergent geodynamic setting of the Witwatersrand Supergroup suggested by the literature, the Crown Formation and Bird Member are probably not related to subduction-related magmatism but more to decompression melting. Overall, the combined trace element and Sm-Nd isotopic data indicate melts from slightly to moderately depleted sources that were variably contaminated with crustal material. Greater contamination, followed by differentiation in different magma chambers, can explain the difference between the two signatures of the Bird Member. Finally, despite previous proposals for stratigraphically correlating the Witwatersrand Supergroup to the Mozaan Group of the Pongola Supergroup, their volcanic units are overall geochemically distinct.

Age of the Archaean Murchison Belt and mineralisation, South Africa

Abstract The east-northeast-trending Murchison-Thabazimbi Lineament in northern South Africa is one of the world’s most important structures for its control on world-class mineral deposits, Proterozoic sedimentary basins and giant igneous intrusions. The deepest exposed Archaean parts of the lineament are the Murchison Belt. Bounded by granitoids, the belt comprises greenschist to amphibolite facies volcano-sedimentary strata with isoclinal folds and the 7 km thick meta-igneous Rooiwater Complex. The Rooiwater Complex is intruded by a northern regional granitoid dated at 2 929 ± 7 Ma by SHRIMP U-Pb on zircons. Using field relationships, published isotopic age data and new SHRIMP zircon dates we confirm the age Rooiwater Complex at 2 965 Ma, showing it to be contemporaneous with the Archaean volcanic and sedimentary formations, the meta-igneous Complex being the lower sequence in a ~2 980 to 2 960 Ma island arc. Despite being implicated as a source of gold for the world’s largest natural accumulation of gold in the Witwatersrand Basin, the absolute age of Sb-Au mineralisation in the Murchison Belt is poorly constrained. We have utilised SHRIMP U-Pb geochronology to date monazites from a Sb-Au ore sample from the granitoid-hosted Malati Pump orebody and determine ages for two different generations of monazite, both associated with ore minerals. The older age of 2 832 ± 23 Ma is from a minority of grains and is interpreted to date the primary Sb-Au mineralisation, about 120 Ma after belt formation. This age predates, or is possibly synchronous with, sedimentation of the upper-Witwatersrand Central Rand Group. The younger age of 1 968 ± 17 Ma from a majority of monazite grains is unrelated in time to known events and interpreted here as a cryptic hydrothermal reworking of the Sb-Au ores in this deposit.

The Significance of Erosion Channels on Gold Metallogeny in the Witwatersrand Basin (South Africa): Evidence from the Carbon Leader Reef in the Carletonville Gold Field

Abstract Within the eastern portion of the Carletonville gold field, the gold- and uranium-rich Carbon Leader reef of the Central Rand Group (Witwatersrand Supergroup) is truncated by an erosion channel. This channel is asymmetrical and lenticular in shape, measuring 150 to 180 m in width and up to 100 m in depth. High-resolution seismic data show that the erosion channel cuts from the Carbon Leader reef into all older units of the Central Rand Group down to the Roodepoort Formation of the underlying West Rand Group. A total of seven boreholes were drilled into the channel, revealing that it is composed of quartzite at its base (9 m thick), overlain by deformed (lower) and laminated (upper) chloritoid-bearing shale (21 m thick) and quartzite (18 m thick). The Carbon Leader reef is highly enriched in gold (5–40 g/t Au), whereas the gold tenor of the erosion channel fill is in general much lower (<1 g/t Au), although locally grades of as much as 3.8 g/t Au are reached. Detailed seismic, sedimentological, and petrographic analyses revealed that the channel was filled with locally sourced sediments from the Main Formation. A closed-system mass balance further demonstrates that gold in the erosion channel could have been entirely sourced from the Carbon Leader reef. Sediment load played a crucial role in the distribution of gold in the channel, thus supporting a stratigraphically controlled modified placer model for the origin of gold in the Carbon Leader reef.

High-resolution Integrated Geophysical Investigation at the Lancaster Gold Mine, Krugersdorp, South Africa

An integrated geophysical approach using seismics and geoelectrical techniques was employed to investigate the architecture of historic narrow-reef workings and a proposed open-pit mine at Lancaster Gold Mine, near Krugersdorp, South Africa. The mining activities in the area were mainly carried out within the Kimberley Reef Package in the upper Central Rand Group of the Witwatersrand Supergroup, which hosts several gold-bearing conglomerates (locally known as reefs). The reefs are generally thin (≤ 2 m thick) and dip between 28° and 32° south. The low-velocity weathered layer introduces significant static shifts in the reflection seismic data. Moreover, environmental noise from drilling and trucking, and the prominent bedrock-overburden contact that produces various wave conversions (P-S conversion), caused undesirable noise that contaminates the shot gathers as high-amplitude, source-generated and monochromatic noise. The noise was removed from the shot gathers using frequency and velocity filtering techniques. The final depth-migrated sections are characterised by high-resolution images of the subsurface from ~10 to ~150 m depth, which are constrained by the borehole information. The reflection seismic data delineate the interfaces between different rock layers and the stopes. The refraction and resistivity tomograms, on the other hand, provide more detailed images of the top 20–50 m of the subsurface and depict the approximate shallow geometry of fluid migration paths, mined-out areas, and bedrock-overburden boundaries. The integrated results indicate that the study area is characterised by a weathered surface layer with variable low P-wave velocity (400–1200 m/s) and resistivity (150–800 Ωm). The deeper layer reveals an increase in resistivity and velocity, and it’s characterized by discontinuities, weak zones, cavities or water-bearing zones due to the mining activities. The combined borehole and geophysical data provide valuable information regarding the physical characteristics of the subsurface and can be helpful for future risk management decisions, environmental and engineering studies in the area.

U-Pb and Hf isotopes of detrital zircons from the Pongola Supergroup: Constraints on deposition ages, provenance and Archean evolution of the Kaapvaal craton

The c.3.0–2.9 Ga Pongola Supergroup located on the south-eastern margin of the Kaapvaal Craton preserves some of the most pristine Archean successions but the timing of volcanic and sedimentary processes, and sediment sources remain relatively poorly known. Combined zircon U-Pb ages and Hf isotope data of sandstones from the Nsuze and Mozaan Groups are presented from key locations of the Northern, Central and Southern Domains of the 270 km long belt. These data indicate important lateral and temporal variations and therefore changes in source areas. The basal Pongola sedimentary rocks (Mantonga Formation) in the Northern Domain were derived from the c.3.4 Ga basement, and also most likely from reworked Moodies Group sedimentary rocks of the Barberton greenstone belt. Zircon detritus in Nsuze and lower Mozaan Groups sandstones shows similar age-Hf isotope spectra, and a remarkable overlap with data from magmatic and (meta)sedimentary rocks of the southeastern Kaapvaal craton. The data point to a proximal source for Nsuze sandstones in the Northern Domain and a different provenance for the Southern Domain sandstones. A possible signature of the extensive 3.10–3.07 Ga Mpuluzi granite is seen only in the Nsuze sandstones and basal Mozaan of the Central Domain. The data indicate for the first time that Cr-rich sandstones of the Ndikwe Thrust Segment of the Southern Domain were deposited at <2.92 Ma, and therefore belong to the uppermost Mozaan Group. Furthermore, they reveal a significant switch in provenance from the lower to upper Mozaan Group from a reworked Archean basement towards a more juvenile hinterland with εHft = +0.9 to +6.3 between 3.07 and 2.92 Ga; similar to the source previously reported for the Central Rand Group of the Witwatersrand Basin. The juvenile zircon detritus in the upper Mozaan Group requires sources as far as the Pietersburg and/or Kimberley Blocks, which became amalgamated to the central block of the Kaapvaal Craton at 2.97–2.87 Ga, but perhaps also a source outside of the present day Kaapvaal Craton, and possibly as far as the Pilbara Craton.

Uranium in South Africa: Exploration and Supply Capacity

The major uranium deposits types are quartz-pebble conglomerate of the Central Rand Group (Witwatersrand Supergroup) and the Dominion Group, sandstone of the Karoo Uranium Province, surficial deposits of the Namaqualand region, as well as coal of the Springbok Flats Basin. Recent exploration activities (both greenfields and near-mine) have increased the resource potentials of these deposits. South Africa's Council for Geoscience has conducted high resolution magnetic and radiometric surveys in the Namaqualand region. Witwatersrand Basin is the only deposit where uranium mining is active at present in South Africa. Total uranium production in 2015 was 448 t. However, with production expected to increase in the Witwatersrand Basin, production is expected to reach 700 t U by the end of 2018. Furthermore, with uranium production envisaged to begin in the Karoo Uranium Province, uranium production may exceed 1000 t U by the year 2020.

A Review of the Witwatersrand Basin - The World’s Greatest Goldfield

The world's greatest deposit of gold is hosted by the Archaean Witwatersrand sedimentary basin, situated in the central portion of the Kaapvaal Craton of South Africa. The geological setting of this remarkable clastic sedimentary deposit, which has yielded more than one third of all the gold ever produced on the planet, is discussed. The stratigraphy and structure of the Witwatersrand Supergroup is reviewed together with the sedimentology, mineralogy and geochronology of the more important auriferous conglomerate (reef) horizons. Geophysical methods of exploration are discussed briefly. The Witwatersrand Basin was formed in response to a series of crustal plate movements from the north and west in a foreland basin setting. Deposition commenced in a shallow marine environment with quartzite and shale, including prominent iron rich shale horizons forming the West Rand Group (WRG). The overlying dominantly arenaceous Central Rand Group (CRG) was deposited in a basin which regressed as it filled. Important gold deposits formed on braided fluvial plains which sometimes terminated in a shoreline. Gold reefs at the base of the CRG show evidence of later marine reworking. Higher in the sequence, fluvial deposits become dominant. The focus of this paper is the Central Rand Group where most of the gold mineralisation occurs. Detrital gold (largely within sulphides) and uranium (as uraninite) were originally concentrated along with placer minerals such as chromite and zircon, at the base of fluvial cycles. In addition to the placer mineralisation, prokaryotic bacteria flourished and provided carbon-rich material that was highly effective in the concentration of non-detrital gold. Although most of the lithologies, including reef horizons, have great continuity and can be correlated across much of the Witwatersrand Basin, discrete entry points of reef material, with distributary channels and associated ore shoots, can generally be clearly defined. Three major events reshaped and metamorphosed the Witwatersrand. Lateral crustal plate movements caused severe faulting and folding, especially along the western and northern sides of the Basin. The second event was the catastrophic Vredefort meteorite impact which struck the centre of the basin. The resulting fluid movement which pervaded the Witwatersrand strata caused widespread metasomatic alteration. Localised alteration and remobilisation of the gold mineralisation is thought to have been in partial response to this event. The third major event was the emplacement of the Bushveld Complex, which is the largest layered igneous intrusion of its kind on Earth. The heat generated by this intrusion certainly exceeded that of the Vredefort event, also making it an important agent of mineral alteration and remobilisation, especially along the northern flank. The Great Debate on the origin of the gold has been the subject of heated discussions for decades. The three main points of view are a placer source, a hydrothermal source and a combination of the two called the modified placer theory. Recognition of the problem was realised soon after the discovery of the gold and continues to this day. Evidence has been presented in favour of all arguments, lending more credence for the modified placer process. Although the Witwatersrand is a mature goldfield with declining production, it is estimated that it still contains six times more gold than the world's second largest goldfield. Much of the remaining resource occurs at considerable depths; however, there are still opportunities for extracting some of this resource, as well as generally somewhat lower grade mineralisation, at moderate to shallow depth. The Basin thus remains a major exploration target. Gold mining in the Witwatersrand Basin has been responsible for the creation of the metropolis of Johannesburg and many thriving mining towns and has had a major influence in the shaping of South Africa. It still plays an important role in the country's economy.

Evidence for an Intrabasinal Source and Multiple Concentration Processes in the Formation of the Carbon Leader Reef, Witwatersrand Supergroup, South Africa

Laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) trace element maps of pyrite and gold from the Carbon Leader Reef in the Witwatersrand basin provide new evidence that a significant proportion of the pyrite and gold was intrabasinal, derived from the West Rand Group or equivalent shales stratigraphically below this reef. Rounded detrital pyrite grains in the Carbon Leader Reef vary from compact to porous to sooty, with similar textures and composition to diagenetic pyrites reported from sedimentary rocks in the West Rand Group. Detrital porous and sooty pyrites contain 0.4 to 11.3 ppm solid-solution gold, sulfur isotope δ34S values from -17 to +16%, and high levels of As and Te, plus a wide range of other trace elements (in decreasing order: Ni, Co, Sb, Cr, Cu, U, Pb, Bi, Mo, Zn and Ag). The sooty pyrite is the most Au, Te, and Mo rich and is intergrown with alumino-silicates and organic matter. The detrital pyrite textures, S-isotopes and geochemistry resemble diagenetic pyrite developed under suboxic to anoxic bottom water conditions. The LAICP-MS maps also show that the detrital pyrite grains have euhedral hydrothermal pyrite overgrowths containing micro-inclusions of gold, brannerite, and Ni-As sulfides. The pyrite overgrowths are also enriched in solid-solution gold, tellurium, and other trace elements including, in decreasing order, As, Co, Ni, Cr, Cu, Mn, Pb, Bi, and Ag. Fractured and brecciated pyrite associated with brittle bedding-parallel fracture zones in the Carbon Leader Reef are also enriched in these elements due to alteration of pyrite surrounding the fractures. Laser ICP-MS Pb isotope determinations on the cores of detrital pyrite indicate an age between 2750 and 2950 Ma with hydrothermal overgrowths originating between 2100 and 2020 Ma incorporating highly radiogenic Pb. This study demonstrates that both of the two competing theories for the origin of the Witwatersrand gold reefs are likely to be correct. We suggest that the hydrothermal event was widespread (kilometer scale) and involved basinal fluids that scavenged gold, tellurium, arsenic, and trace elements (Co, Ni, Cr, Cu, Mn, Pb, Bi, Ag) from gold-bearing sedimentary units in the Central Rand Group. © 2013 Society of Economic Geologists, Inc.

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.

Neoarchaean tectonic history of the Witwatersrand Basin and Ventersdorp Supergroup: New constraints from high-resolution 3D seismic reflection data

First-order scale structures in the West Wits Line and West Rand goldfields of the Witwatersrand Basin (South Africa) were mapped using the high-resolution 3D reflection seismic method. Structural models constrain the magnitude of displacement of thrusts and faults, the gross structural architecture and Neoarchaean tectonic evolution of the West Rand and Bank fault zones, which offset the gold-bearing reefs of the basin.The merging of several 3D seismic surveys made clear the gross strato-structural architecture of the goldfields; a macroscopic fold-thrust belt is crosscut by a macroscopic extensional fault array. These are dissected, eroded and overlain by the Transvaal Supergroup above an angular unconformity.The seismic sections confirm that the West Rand Group (ca. 2985–2902Ma) is unconformably overlain by the Central Rand Group (ca. 2902–2849Ma), with tilting of the West Rand Group syn- to post-erosion at ca. 2.9Ga. The seismic sections also confirm that an unconformable relationship exists between the Central Rand Group and the auriferous Ventersdorp Contact Reef (VCR), with an easterly-verging fold-thrust belt being initiated concomitant to deposition of the VCR at approximately 2.72Ga. Fold-thrust formation included development of the (1) newly identified first-order scale Libanon Anticline, (2) Tandeka and Jabulani thrusts which displace the West Rand Group, and (3) parasite folds.The fold-thrust belt is crosscut by a macroscopic extensional fault array (or rift-like system of faults) which incepted towards the end of extrusion of the Ventersdorp lavas, and certainly during deposition of the Platberg Group (2709–2643Ma) when a mantle plume may have heated the lithosphere. The West Rand and Bank fault zones formed at this time and include (1) the West Rand and Bank faults which are scissors faults; (2) second and third-order scale normal faults in the immediate footwall and hanging wall of the faults; (3) drag synclines, and (4) rollover anticlines.

The Hlagothi Complex: The identification of fragments from a Mesoarchaean large igneous province on the Kaapvaal Craton

In this paper, we present geochronological, geochemical and palaeomagnetic results from the Hlagothi Complex and a NW-trending dolerite dyke swarm on the southeastern region of the Kaapvaal Craton in northern KwaZulu-Natal, South Africa. The Hlagothi Complex consists of layered sills of meta-peridotite, pyroxenite and gabbro intruding into the Pongola Supergroup. U–Pb baddeleyite ages on the Hlagothi Complex and a NW-trending dyke of 2866±2Ma and 2874±2Ma, respectively, reveal a ca. 2.87Ga magmatic event on the southeastern Kaapvaal Craton. The geochemical signature of the Hlagothi Complex recognises two discrete groupings, with a magmatic source that is chemically distinct from those of the older rift-related Nsuze and Dominion groups. Additional units on the Kaapvaal Craton can be linked with this new ‘Hlagothi’ event based on spatial and temporal association, and geochemistry: 1) the Thole Complex, 2) parts of the Usushwana Complex, and 3) flood basalts within the Mozaan Group and Central Rand Group. The association between all these units suggests a previously unrecognised large igneous province in the southeastern Kaapvaal Craton. Our palaeomagnetic data identifies a possible primary magnetisation within the least-altered lithologies of the Hlagothi Complex (with a virtual geographic pole at 23.4°N, 53.4°E, dp=8.2° and dm=11.8°). The bulk of samples however, displayed two episodes of remagnetisation. These are likely to be associated with 2.85 to 2.75Ga aged granitoids across the southeastern Kaapvaal Craton, and tectonic activity in the nearby Meso- to Neoproterozoic Namaqua–Natal mobile belt. A short-lived (≤8Ma) mantle plume is proposed to have caused the ca. 2.87Ga magmatism, and also may well have controlled sedimentation within the Pongola–Witwatersrand basin. Volcanism during uplift would have been fed through a series of feeder dykes and sills, of which the Hlagothi Complex and NW-trending dykes are part of.

U–Pb and Hf isotope record of detrital zircons from gold-bearing sediments of the Pietersburg Greenstone Belt (South Africa)—Is there a common provenance with the Witwatersrand Basin?

Results of combined U–Pb and Hf isotope analyses on detrital zircons from coarse grained clastic sediments of the Pietersburg Greenstone Belt provide evidence for four major magmatic activities in the hinterland, at ca. 3.29, 3.26, 3.21, and 2.96Ga, during which new crust was formed from depleted mantle sources. The latter interpretation is supported by mostly superchondritic ɛHft of −4.3 to +4.2 at 2.96Ga, +0.1 to +5.3 at 3.21Ga, +0.2 to +3.2 at 3.26Ga, and 0.0 to +6.2 at 3.29Ga. The observed ɛHft variations, and the finding of abundant magmatic but also metamorphic zircon overgrowths with ages at ca. 2.96Ga furthermore suggests that the youngest magmatic event took place in a continental arc setting, whereby juvenile magmas were mixed with older reworked crust. A similar setting is also suggested for the three older magmatic events. Based on our datasets deposition and metamorphism of the Pietersburg sediments is bracketed between 2.88 and 2.69Ga, in agreement with the deposition age of the gold-bearing Central Rand Group of the Witwatersrand Basin. The occurrence of 2.98–2.88Ga zircon populations with mostly superchondritic but variable ɛHft in the Pietersburg and Central Rand Group sediments indicate that a vast amount of detritus for both areas was supplied from proximal, relative juvenile magmatic arc sources. However, the general absence of 3.15–3.00Ga zircons in the Pietersburg sediments, but their abundant occurrence in the Witwatersrand Basin indicates that detritus supply for both basins came either from completely different sources, or that 3.15–3.00Ga basement was additionally sampled during southward transport.

New constraints on the auriferous Witwatersrand sediment provenance from combined detrital zircon U–Pb and Lu–Hf isotope data for the Eldorado Reef (Central Rand Group, South Africa)

Combined U–Pb and Lu–Hf isotope analyses of detrital zircon grains from the auriferous Eldorado Reef conglomerate, upper Central Rand Group, reveal new insights into the provenance of the sediments and thus, by implication, possibly also into that of the gold. Most of the detrital zircon grains, which are of magmatic origin, yielded Mesoarchaean ages clustering around 2.94 and 3.06 Ga. A subordinate zircon population gave ages with maxima at 3.28 and 3.44 Ga. The Mesoarchaean zircon grains mostly show super-chondritic ɛHf t of up to +5.2, whereas the Palaeoarchaean zircon grains have nearly chondritic composition with ɛHf t between −1.3 and +2.0. The new dataset of the Mesoarchaean zircon populations provides the first unambiguous evidence of the formation of juvenile crust not only at 3.06 but also at 2.94 Ga. As the analysed zircon grains are from the ruditic fraction, they must be derived from a comparatively proximal source in close vicinity to the Central Rand Basin. Based on currently available data, this source was most likely a magmatic arc that existed at the northern edge of the Witwatersrand Block at 3.06 Ga. An additional source might be the 2.94 Ga magmatic rocks of the Kraaipan Greenstone Belt that occurs to the west of the Witwatersrand Block. The minor fraction of Palaeoarchaean zircon grains in the Eldorado Reef perhaps stem from sources that are isotopically similar to the Barberton Greenstone Belt and the Limpopo Belt but were more proximal to the Central Rand Basin.

O- AND H-ISOTOPE STUDY OF THE CARBON LEADER REEF AT THE TAU TONA AND SAVUKA MINES (WESTERN DEEP LEVELS), SOUTH AFRICA: IMPLICATIONS FOR THE ORIGIN AND EVOLUTION OF WITWATERSRAND BASIN FLUIDS

The Carbon Leader Reef is a ~1 m thick conglomeratic unit with a thin bituminous base, and is one of the major gold-bearing conglomerate horizons in the Central Rand Group of the Witwatersrand Basin. It consists of alternating conglomerate and quartzite layers and was metamorphosed under greenschist facies conditions. Bulk rock δ18O and δD values of the Carbon Leader Reef range from 7.2‰ to 10.8‰ (mean = 10.6‰) and −27 to −65 (mean = −41‰), respectively. The narrow range in δ18O values, together with the lack of correlation between the δ18O value and the modal % matrix minerals, suggests that the original detrital minerals and the authigenic matrix minerals have similar oxygen isotope composition. Calculated δD values of the fluid, assuming that it was in isotope equilibrium with the bulk rock at the estimated peak metamorphic temperature of 350°C, range from −1‰ to −40‰. This suggests that the ultimate origin of the fluid was a mixture of meteoric and metamorphic water. These data are not consistent with the ingress of large quantities of externally derived fluid post burial metamorphism. Gold-rich sections of the Carbon Leader Reef do not have significantly different δ18O values than the adjacent gold-poor section, which suggests that gold mineralization is not related to interaction with significant amounts of externally derived hydrothermal fluids.