Witwatersrand Basin Research Articles

A new interpretation of soft-sediment deformation features (SSDs) in the Parktown Formation (lower West Rand Group); the physical processes of deformation during deposition

Abstract In the Parktown Formation of the lower West Rand Group (Witwatersrand Supergroup), remarkably well-preserved soft-sedimentary deformation features (SSDs) occur in a number of locations in three stratigraphic horizons across the Vredefort Mountainland and in key sites in Johannesburg. The SSDs were the inevitable products of static liquefaction during early deposition in the Witwatersrand Basin. The trigger to static liquefaction is interpreted to be autogenic and related to cyclic fluctuations in pore fluid pressure(s) during capped dewatering. Allogenic triggers are not indicated. We suggest that static liquefaction was an integral physical process to sediment deposition on the Parktown Formation offshore platform, but a process not previously recognised as important to the modification of the West Rand Group stratigraphy across the Witwatersrand Basin.

Mineral Reconnaissance Through Scientific Consensus: First National Prospectivity Maps for PGE–Ni–Cu–Cr and Witwatersrand-type Au Deposits in South Africa

AbstractWe present here the first experimental science (consensus)-based mineral prospectivity mapping (MPM) method and its validation results in the form of national prospectivity maps and datasets for PGE–Ni–Cu–Cr and Witwatersrand-type Au deposits in South Africa. The research objectives were: (1) to develop the method toward applicative uses; (2) to the extent possible, validate the effectiveness of the method; and (3) to provide national MPM products. The MPM method was validated by targeting mega-deposits within the world’s largest and best exploited geological systems and mining districts—the Bushveld Complex and the Witwatersrand Basin. Their incomparable knowledge and mega-deposit status make them the most useful for validating MPM methods, serving as “certified reference targets”. Our MPM method is built using scientific consensus via deep ensemble construction, using workflow experimentation that propagates uncertainty of subjective workflow choices by mimicking the outcome of an ensemble of data scientists. The consensus models are a data-driven equivalent to expert aggregation, increasing confidence in our MPM products. By capturing workflow-induced uncertainty, the study produced MPM products that not only highlight potential exploration targets but also offer a spatial consensus level for each, de-risking downstream exploration. Our MPM results agree qualitatively with exploration and geological knowledge. In particular, our method identified areas of high prospectivity in known exploration regions and geologically and geospatially corresponding to the known extents of both mineral systems. The convergence rate of the ensemble demonstrated a high level of statistical durability of our MPM products, suggesting that they can guide exploration at a national scale until significant new data emerge. Potential new exploration targets for PGE–Ni–Cu–Cr are located northwest of the Bushveld Complex; for Au, promising areas are west of the Witwatersrand Basin. The broader implications of this work for the mineral industry are profound. As exploration becomes more data-driven, the question of trust in MPM products must be addressed; it can be done using the proposed scientific method. Graphical Abstract

Factors responsible for Witwatersrand gold mineralisation

Abstract The exceptional gold mineralisation in quartz pebble conglomerates of the Witwatersrand Basin is attributed to a combination of factors. These factors are linked to the co-evolution of the atmosphere, hydrosphere, lithosphere and biosphere, at a very specific time in Archaean geological history and the evolution of the Kaapvaal Craton. Following craton stabilisation and its subaerial emergence, intense chemical weathering and erosion of large volumes of granitoid-greenstone basement released detrital and dissolved gold. Shallow-marine reworking in a long-lived and slowly subsiding basin subjected to episodic compressional deformation and relative sea-level oscillations led to sedimentary concentration of detrital gold. The interaction between acidic, anoxic, and sulfurous surface runoff and more oxidizing marine waters in a near-coastal oxygen oasis supported microbially mediated diagenetic pyrite formation and incorporation of dissolved gold in the pyrite crystal lattice. Erosion and reworking of diagenetic pyrite gave rise to detrital pyrite that characterise most reefs. Abundance of detrital uraninite in conglomerates, derived from erosion of Mesoarchaean granites, and episodes of hydrocarbon migration through sedimentary strata during deep burial set the scene for further enhancement of gold grades in the reefs. Granular and seam pyro-bitumen formed by radiation-induced polymerisation of hydrocarbons around detrital uraninite. Gold dissolved in migrating hydrothermal fluids was then reduced and precipitated upon interaction with the reef pyro-bitumen during hydrothermal placer modification.

A practical approach to determine the role of rockbolts in stoping gullies

A research project was carried out on a gold mine in the Vaal River region of South Africa (Witwatersrand Basin) to evaluate the effect of two particular aspects of rockbolt installation - the angle of inclination of the bolt relative to the orientation of the rock surface, and the protruding length of the bolt - on the mine's gully support system. A keyblock analysis approach indicated that these factors do not have an adverse impact on the stability of the strike gullies. This was confirmed by extensive empirical data collected from the case study mine. While safety is the primary concern, cost saving opportunities can be realized through reconsidering historically inherited support standards that are perceived to be correct and appropriate. The research described in this paper sets forth a practical approach, that is easy to implement and repeatable, to evaluate the role of rockbolts in stoping gullies.

The Temporal Distribution of the Host Rocks to Gold, the Archean Witwatersrand Basin, South Africa

The hosts to gold around the Witwatersrand Basin span over 400 my, through 14 km of stratigraphy in a variety of host rocks and in tectonic settings that include periods of rifting, thermal subsidence, foreland basin, flood basalt outpouring, graben development, and further thermal subsidence. A geological model that assumes placer processes to explain this diversity implies a super-long-lived and special source of the detrital gold, transport, and highly effective sorting processes over a time span of 400 my. There is no evidence of a special source and sorting over such a long time period. In the Phanerozoic, this would be equivalent to the special source and sorting processes operating continually over an equivalent period of geological time spanning from the Devonian up until the present day; this is as yet recognised nowhere else on the planet. With regard to the geological model that assumes a placer process, this is untenable because of these scientific shortcomings and its lack of success in exploration. A better use of funds may be to consider alternative approaches and epigenetic models in exploration.

Impact‐Generated Permeability and Hydrothermal Circulation at the Vredefort Impact Structure, South Africa

AbstractThe 2.02 billion year old Vredefort impact structure in South Africa offers a unique opportunity to study large‐scale impact processes on Earth. Vredefort's large size (∼250 km in diameter) and eroded topography provides the opportunity to study the effects of shock physics at depth and post‐formation hydrothermal alteration. In this work, we simulate the formation of the Vredefort structure building upon recent shock physics (iSALE) simulations. We expand those simulations to cover a wider range of input conditions, and compute impact‐driven porosity and permeability. The latter quantities are used to perform fluid mobility simulations (HYDROTHERM). We find that the Vredefort event produced significant impact‐generated porosity (up to 30%) in an annulus from about 50 to 100 km from the center and up to several kilometers in depth. The corresponding estimated permeability (up to 10−12 m2) would have allowed for large scale subsurface fluid flows. Our hydrothermal calculations show that the Vredefort impact event could have generated a complex crustal fluid pattern within the crater rim that lasted for hundreds of thousand years, with localized flow concentration regions, opening a new interpretation for the mobilization and location of the ore deposits in the Witwatersrand basin. The combined approach utilizing impact and hydrothermal simulations constitute a powerful tool to understand geochemical processes at Vredefort, as well as to assess the ability of large impacts to drive crustal chemistry with far‐reaching consequences for the prebiotic evolution of the early Earth.

Big geochemical data through remote sensing for dynamic mineral resource monitoring in tailing storage facilities

Evolution in geoscientific data provides the mineral industry with new opportunities. A direction of geochemical data generation evolution is towards big data to meet the demands of data-driven usage scenarios that rely on data velocity. This direction is more significant where traditional geochemical data are not ideal, which is the case for evaluating unconventional resources, such as tailing storage facilities (TSFs), because they are not static due to sedimentation, compaction and changes associated with hydrospheric and lithospheric processes (e.g., erosion, saltation and mobility of chemical constituents). In this paper, we generate big secondary geochemical data derived from Sentinel-2 satellite-remote sensing data to showcase the benefits of big geochemical data using TSFs from the Witwatersrand Basin (South Africa). Using spatially fused remote sensing and legacy geochemical data on the Dump 20 TSF, we trained a machine learning model to predict in-situ gold grades. Subsequently, we deployed the model to the Lindum TSF, which is 3 km away, over a period of a few years (2015-2019). We were able to visualize and analyze the temporal variation in the spatial distributions of the gold grade of the Lindum TSF. Additionally, we were able to infer extraction sequencing (to the resolution of the data), acid mine drainage formation and seasonal migration. These findings suggest that dynamic mineral resource models and live geochemical monitoring (e.g., of elemental mobility and structural changes) are possible without additional physical sampling.

Where is all the gold?

The Witwatersrand Basin contains by far the most gold ever found, and has hosted mining from its discovery in 1886 to the present. For many years, South Africa was the world's largest producer of gold, nearly all of which came from the Witwatersrand. Since 2000, South Africa has fallen back several positions because of declining gold output. There are many complex and varied reasons for this; however, declining gold resources in the Witwatersrand Basin are not one of them. As far as the author knows, there are no qualified estimates in the literature of gold remaining in the Witwatersrand Basin. This paper collects mining data from several sources and then, using elementary gold accounting and error analysis, attempts to estimate the amount of gold remaining in the Witwatersrand Basin. It includes gold underground in existing and defunct mines, in evaluated ore resources, and dumps. Compiled data from 1887 to 2019 shows that 50 200 t of gold were produced by Witwatersrand mines, while it is estimated that a further 48 100 t remain underground, both within and outside of mine leases, a further 1 600 t on the surface in tailings, and an unknown amount in rock dumps. Nearly all of this gold will remain inaccessible with current mining methods, and major technical developments in mining will be necessary before any of the gold can be categorized as a code-compliant resource or reserve. To win this prize, the mining industry will have to rethink its approach to mining, both in old mining leases and in the unexploited ground. It will also have to find effective means of preventing gold theft and informal mining, which are on the rise.

Machine Learning-Based Delineation of Geodomain Boundaries: A Proof-of-Concept Study Using Data from the Witwatersrand Goldfields

Machine-aided geological interpretation provides an opportunity for rapid and data-driven decision-making. In disciplines such as geostatistics, the integration of machine learning has the potential to improve the reliability of mineral resources and ore reserve estimates. In this study, inspired by existing geostatistical approaches that use radial basis functions to delineate domain boundaries, we reformulate the problem into a machine learning task for automated domain boundary delineation to partition the orebody. We use an actual dataset from an operating mine (Driefontein gold mine, Witwatersrand Basin in South Africa) to showcase our new method. Using various machine learning algorithms, domain boundaries were created. We show that based on a combination of in-discipline requirements and heuristic reasoning, some algorithms/models may be more desirable than others, beyond merely cross-validation performance metrics. In particular, the support vector machine algorithm yielded simple (low boundary complexity) but geologically realistic and feasible domain boundaries. In addition to the empirical results, the support vector machine algorithm is also functionally the most resemblant of current approaches that makes use of radial basis functions. The delineated domains were subsequently used to demonstrate the effectiveness of domain delineation by comparing domain-based estimation versus non-domain-based estimation using an identical automated workflow. Analysis of estimation results indicate that domain-based estimation is more likely to result in better metal reconciliation as compared with non-domained based estimation. Through the adoption of the machine learning framework, we realized several benefits including: uncertainty quantification; domain boundary complexity tuning; automation; dynamic updates of models using new data; and simple integration with existing machine learning-based workflows.

Using legacy 3D seismic data and source parameters of mining‐induced earthquakes to mitigate the risk of rockbursting in Kloof Gold Mine, South Africa

AbstractRockbursts pose a significant risk in deep gold mines in the Witwatersrand Basin of South Africa as they may damage the excavation, injure workers, and delay production. We analysed the source mechanisms of 75 large mining‐related seismic events (ML 1.5–2.7) that caused damage to stopes in Kloof Gold Mine and used legacy 3D reflection seismic data to delineate the ore body and geological structures that may be correlated with mining‐related seismic events. The 75 seismic events took place at depths of 1600–4200 m below surface. Most events were located close to stopes mining the Ventersdorp Contact Reef ore body. The S‐to‐P‐wave energy ratio (Es/Ep) from the analysed seismic events ranged from 1.1 to 19.9. The source mechanisms of mining‐induced seismic events aid in understanding the dominant modes of failure. Planes of weakness may be the result of mining‐induced stresses or pre‐existing geological structures such as faults and dykes. The Es/Ep ratio, focal mechanism and moment tensor solutions were correlated to the underground mapped and seismically defined geological structures. Approximately 44% of the events showed strong correlation with the known underground mapped and seismically imaged geological structures (faults and dykes), whereas 56% of the events were related to elements of the mining geometry (dip pillars, abutments and remnants). This information enables mining layouts to be modified to minimize the risk of rockbursting.

Environmental Risk Assessment, Principal Component Analysis, Tracking the Source of Toxic Heavy Metals of Solid Gold Mine Waste Tailings, South Africa

This study collected 21 soil samples from 30 to 120 cm depth, from 9 solid gold mine waste dumps in the Witwatersrand basin, South Africa. The samples were analysed using ICP-OES method to determine the concentrations of heavy metals. Principal component analysis (PCA) was performed to track sources of heavy metals, migration passages, their distributions and spatial variations at the study sites. The results of this study showed that the soil screening values (mg/kg) ranged from 2.20–7,070.00 Pb, 0.20–11.00 As, 0.06–2,630.00 Hg, 1.00–69.00 U, 0.02 − 4.20 Cd, and exceeded the background concentrations of the continental crust. The study results further showed that the average concentrations (mg/kg) of Pb (432.08), U (13.75), Tl (4.41), Ag (1.07), Cd (0.34) were highest at Durban Deep. Arsenic (59.86) and Sn (2.03) were highest at Krugersdorp, Kagiso, New Canada, and Shaft 17. The average values of Hg (625.56 µg/kg) and Au (17.67 µg/kg) were highest at Mogale City, Fleurhof, Krugersdorp, Shaft 17, and Davidsonville. These values reflected moderate to extreme contamination, supported by geoaccumulation index (Igeo), pollution load index (PLI), and contamination factor (Cf i) values. According to PCA, F1 was responsible for heavy metal generations from the local gold processing and waste dumping facilities. The F1 loading for Cd and Pb was 0.845 and 0.835, respectively, whereas F1 and F2 contribution of the variable % were greater than 0.70 for Ag, As, Au, Cd, Pb, Sn, Tl, and U, suggesting that they were generated from the waste dumps, transported by rain water and subsequently deposited in downstream soil. The heavy metal concentrations were traced upstream at the dumps where the highest averages were recorded, whereas the lowest values were measured from downstream samples. Thus, the gold mine tailing dumps are considered the primary sources of the high measured values of the heavy metals in the study sites. The ecological risk values were ranked in the order of As > Cd > Hg > Pb and exceeded the permissible health risk value suggested by the South African and global standards. Excessive Hg concentration with 160 ≤ E r i < 320 was recorded at Krugersdorp, whilst Pb with 320 ≤ E r i was recorded at Durban Deep. Thus, the estimated high E r i values insinuate higher ecological risk of heavy metals that can accumulate in the human body through absorption, inhalation and ingestion of dust, or ingestion of food grown from the contaminated soil.

An investigation into arsenic speciation in a wetland impacted by acid mine drainage

The formation of acid mine drainage (AMD) and release of toxic contaminants, such as arsenic (As), is a serious environmental problem encountered worldwide. In this study, we investigate the crucial role the Klip River wetland system plays in attenuating As arising from gold mining activities within the Witwatersrand Basin in Johannesburg, South Africa. Mining operations in the region commenced over 130 years ago and have been associated with the widespread pollution of water resources by AMD. We investigated As concentrations, bioavailability and speciation in a peat core from the Klip River wetland as well as in samples from the main tributaries and tailing storage facilities (TSFs) in the upper catchment. Total As concentrations in tributary and TSFs samples ranged between 10.1-89.9 mgkg-1 and 77.4-106 mg kg-1, respectively, with concentrations in the wetland varying between 1.91-73.8 mg kg-1. In general, As bioavailability was low in both catchment (19%) and wetland (4%) samples, with elemental associations suggesting the majority is bound in an immobile form to organic matter and sulfide. As(v) was the predominant species detected in all samples (0.0901-16.6 mg kg-1), with As(m), MMA and DMA present in lower concentrations. Strong correlations between As and S suggest that speciation and methylation are dependent on both chemical and microbial activity. The study highlights the vital role that wetlands can play in sequestering As in the environment.

Hydrogeochemical and isotopic signatures elucidate deep subsurface hypersaline brine formation through radiolysis driven water-rock interaction

Geochemical and isotopic fluid signatures from a 2.9–3.2 km deep, 45–55 °C temperature, hypersaline brine from Moab Khotsong gold and uranium mine in the Witwatersrand Basin of South Africa were combined with radiolytic and water–rock isotopic exchange models to delineate brine evolution over geologic time, and to explore brine conditions for habitability. The Moab Khotsong brines were hypersaline (Ca-Na-Cl) with 215–246 g/L TDS, and Cl− concentrations up to 4 mol/L suggesting their position as a hypersaline end-member significantly more saline than any previously sampled Witwatersrand Basin fluids. The brines revealed low DIC (∼0.266–∼1.07 mmol/L) with high (∼8.49–∼23.6 mmol/L) DOC pools, and several reduced gaseous species (up to 46 % by volume H2) despite microoxic conditions (Eh = 135–161 mV). Alpha particle radiolysis of water to H2, H2O2, and O2 along with anhydrous-silicate-to-clay alteration reactions predicted 4 mol/L Cl− brine concentration and deuterium enrichment in the fracture waters over a period > 1.00 Ga, consistent with previously reported 40Ar noble gas-derived residence times of 1.20 Ga for this system. In addition, radiolytic production of 7–26 nmol/(L × yr) H2, 3–11 nmol/(L × yr) O2, and 1–8 nmol/(L × yr) H2O2 was predicted for 1–100μ g/g 238U dosage scenarios, supporting radiolysis as a significant source of H2 and oxidant species to deep brines over time that are available to a low biomass system (102–103 cells/mL). The host rock lithology was predominately Archaean quartzite, with minerals exposed on fracture surfaces that included calcite, pyrite, and chlorite. Signatures of δ18Ocalcite, δ13Ccalcite, Δ33Spyrite,δ34Spyrite and 87Sr/86Sr obtained from secondary ion mass spectrometry (SIMS) microanalyses suggest several discrete fluid events as the basin cooled from peak greenschist conditions to equilibrium with present-day brine temperatures. The brine physiochemistry, geochemistry, and cellular abundances were significantly different from those of a younger, shallower, low salinity dolomitic fluid in the same mine, and both were different from the mine service water. These results indicate the discovery of one of few long-isolated systems that supports subsurface brine formation via extended water–rock interaction, and an example of a subsurface brine system where abiotic geochemistry may support a low biomass microbial community.

Evaluation of Radionuclide Levels in Drinking Water from Communities near Active and Abandoned Gold Mines and Tailings in the West Rand Region, Gauteng, South Africa

The history of gold mining in the Witwatersrand Basin has led to exponential growth in the economy, residential development, and the abundance of radionuclides in the environment, including the water system. This study aimed to evaluate the radionuclide levels in drinking water (municipal water and groundwater) and the health risks associated with the ingestion of the water in the communities of the West Rand region of Gauteng Province. The activity concentrations of uranium, radium, and thorium radioisotopes were established through alpha spectrometry and the activities were subsequently used to assess the health impacts. The results indicated that the groundwaters contain elevated activities of most radionuclides owing to prolonged periods of water–rock interactions. Similarly, the highest annual effective doses were recorded in groundwaters with a range of 0.0237–0.3106 mSv/yr, with most samples exceeding the WHO- and EU-prescribed limits of 0.1 mSv/yr. Cancer morbidity and mortality risks were higher in females than in males due to the higher life expectancy of females. Nonetheless, all morbidity and mortality risks were well below the USEPA radiological risk limit of 0.001. Based on the findings of this study, continuous monitoring is paramount to ensure that the activities remain below recommended regulatory limits.

In Situ Determination of Radioactivity Levels and Radiological Doses in and around the Gold Mine Tailing Dams, Gauteng Province, South Africa

The mining and processing of naturally occurring radioactive materials (NORMs) could result in elevated levels of natural radionuclides in the environment. The gold mining in the goldfields of the Witwatersrand Basin of South Africa has resulted in numerous tailing dams that have high concentrations of NORM bearing residue. The aim of this study was to evaluate the radioactivity levels in tailing dams, soils and rocks, and the consequential radiological exposure to the public in the gold mining areas of Gauteng Province, South Africa. The activity concentrations of 238U, 232Th, and 40K were assessed using a gamma ray spectrometer (RS-230), and the activity concentrations ranges in the mine tailings were 209.95–2578.68 Bq/kg, 19.49–108.00 Bq/kg, and 31.30–626.00 Bq/kg, respectively. The radionuclides show significant spatial variability in soils, with high activities recorded in soils located in close proximity to tailings although regionally, the soil radioactivity levels mainly depend on the chemistry of the underlying rocks. The estimated annual effective doses were higher than the recommended regulatory limit of 0.25 mSv/y in particular tailing dams and soil impacted by tailings. Therefore, to ensure the protection of people and the environment, further monitoring and regulatory control measures targeting these areas are required.

A geochemical and mineral chemical assessment of sediment provenance and post-depositional alteration of auriferous conglomerates in the Singhbhum Craton

The Mesoarchean Dhosrapahar Group and the Phuljhari Formation in the Singhbhum Craton host so far subeconomic gold occurrences in metaconglomerate beds, similar to the world-famous conglomerate-hosted gold deposits of the Mesoarchean Witwatersrand Basin in South Africa. In the studied metaconglomerates from India, gold occurs as invisible inclusions in pyrite and as native gold grains associated with detrital chromite, quartz, zircon and metamorphic chlorite. Unlike most Archean paleoplacer deposits, the inferred detrital gold here appears unrelated to detrital pyrite, thereby highlighting the decoupling of the sources for the various placer components. To this end, we attempt to delineate the possible source rocks and assess the extent of post-depositional alteration to better understand the controls on gold accumulation in these conglomerates. Elevated Ni, Co and As, to a lesser extent V, Cu, and Se contents in Au-bearing inclusion-rich detrital pyrite are suggested to be derived from Paleoarchean banded cherts in the Gorumahisani Greenstone Belt. Relatively immobile trace element ratios, as recorded by the conglomerate matrix, reflect a predominantly felsic source with minor sediment input from mafic/ultramafic sources. Fractionated chondrite-normalized light rare earth and flat heavy rare earth element patterns, along with a distinct negative Eu-anomaly, obtained for the samples from the Phuljhari Formation point to a differentiated felsic source, most likely the c. 3.05 Ga K-rich Mayurbhanj Granite. In contrast, depleted heavy rare earth element patterns and a weak negative Eu-anomaly in the conglomerates from the Dhosrapahar Group indicate derivation from a felsic source that had experienced garnet fractionation, the obvious candidate being the underlying c. 3.37 Ga Rairangpur tonalite-trondhjemite-granodiorite (TTG) suite. Detrital rutile is rich in Cr and V, and Zr-in-rutile geothermometry yielded crystallization temperatures between ~800 and 1000 °C. The most likely source of this rutile are 3047±17 Ma gabbro-anorthosite bodies in the basement. Post-depositional modification of the conglomerates is dominantly manifested as greenschist-facies metamorphic overprint, and is reflected also by detrital chromite texture and chemistry, and the occurrence of secondary rutile, euhedral pyrite, and secondary gold with metamorphic chlorite. Our provenance interpretations do not indicate any likely point-source of detrital gold, suggestive of the entire Archean hinterland, irrespective of composition, as a potential source, as already indicated for the Mesoarchean auriferous conglomerates of the Witwatersrand Basin. This contention is further substantiated by the evidence presented here for moderate-to deep chemical weathering of the Archean hinterland of the Singhbhum Craton.

Granitoid gneisses of the Morokweng impact structure: Implications for Neoarchaean evolution of the western Kaapvaal craton

One of the most enigmatic aspects of the geology of the Kaapvaal craton of southern Africa concerns the age and nature of the rocks involved in the formation of its western parts, which are almost completely obscured beneath extensive Neoarchaean to Cenozoic supracrustal sequences. A 369-m-deep drillhole near the centre of the ~70 km wide Morokweng impact structure intersects a suite of Neoarchaean, calc-alkaline, granitic-granodioritic, trondhjemitic and monzonitic gneisses. The gneisses are LREE-enriched, but most display primitive mantle-like to slightly depleted HREE concentrations suggestive of a mantle source with garnet retention in the restite. The monzonitic gneiss is distinctly more enriched in trace and REE, and particularly LREE, which is attributed to differentiation processes in parent magmas to the granitoids. Microbeam zircon U Pb geochronology indicates an emplacement age of 2922 ± 5 Ma for the oldest granite, with the remaining granitic, granodioritic and monzonitic rocks being emplaced coevally at 2906 ± 6 Ma. Rare xenocrystic cores preserve an inherited >3.3 Ga crustal component. These crystallization ages coincide with the proposed culmination of westward-directed subduction beneath a significant continental fragment – the Kimberley Block - that led to its collision with the proto-Kaapvaal craton (Witwatersrand Block) between 2.93 and 2.88 Ga, and support a magmatic arc setting. Based on the age data, the granitoid gneisses intersected in the M4 drillhole predate ca. 2.88 Ga unfoliated granitoids found in outcrops and other drillcores in the vicinity that have been interpreted as post-orogenic intrusions but which nonetheless show similar geochemical characteristics to the M4 core granitoids. The distinct age of the M4 granitoid gneisses relative to other granitoid rocks in the Morokweng region may reflect the greater exhumation that occurred in the central parts of the 146 Ma Morokweng impact structure relative to its margins. • Granitoid gneisses exposed by the 146 Ma Morokweng impact belong to a 2.90–2.92 Ga suite of calc-alkaline TTG gneisses • Monzonitic gneiss is strongly enriched in REE, suggesting derivation from a residual magma after fractional crystallization • The granitoid gneisses are interpreted as mid-crustal intrusions formed in a magmatic arc setting • Their intrusion marks the climax of collision and amalgamation of the two major crustal blocks constituting the Kaapvaal craton • Collision affected the tectonic style and sedimentation in the Witwatersrand basin, assisting formation of its gold deposits

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.

Ecological risk of trace metals in soil from gold mining region in South Africa

The Witwatersrand basin in South Africa is one of the well-known gold mining regions worldwide. Plausibly, it is one of the sites in South Africa with the highest environmental impact emanating from trace metal contamination. This study assessed the concentration of Fe, S, Cu, Mn, Cr, Zn, Ni, Co, P, Mo using inductively coupled plasma optical emission spectrometry on 21 soil samples that were collected from 9 independently located historical gold mine tailings. The results revealed high mean concentration (mg/kg) for Fe (36,433.39) > S (5,071.83) > Cu (1,717,28) > Mn (612.81) > Cr (74.52) > Zn (68.67) > Ni (40.44) > Co (9.63) > P (3.49) > Mo > (2.74) in the samples investigated. Estimated degree of contamination by using contamination indices calculations showed that the sites surveyed are on average low to moderately contaminated with Co, Cr, Cu, Mn, Ni, S, and Zn. The trace metals that are most likely to cause ecological risk incorporate Mn and Cu, especially at Durban Deep. While the health risk assessment revealed a low ecological risk (Eri) of Cr, Ni, and Zn, the Eri estimates showed that the residents in the study area are prone to health risk mainly from the covetous effects of Cu (320 ≤ Eri). The residents of the study area are low income earners and the possibility that they can relocate is nonexistent. Thus, mitigation strategies are required to avoid eminent health risks and restraining housing developments near the tailing dumps.

The role of porosity in H2/He production ratios in fracture fluids from the Witwatersrand Basin, South Africa

Abiotic H2 produced in the Precambrian lithospheric crust is a key substrate at the base of the metabolic chain of chemosynthetic and photosynthesis-independent microbial communities, significant to our understanding of life on early Earth and other planets. H2 cycling processes are also relevant to recent hydrogen exploration efforts and engineered subsurface environments such as radioactive waste disposal sites. In the lithospheric crust, H2 is produced through water-rock reactions (serpentinisation) and radiolysis; the latter directly linked to He through radioelement decay (U, Th). The Witwatersrand Basin in South Africa is an ideal place to study the radiolytic production pathway in particular, because of the low abundance of ultramafic and mafic minerals and therefore low potential for serpentinisation reactions. Gas samples and gas flow rate data (n = 12) were collected from the surface of exploration boreholes tapping the Witwatersrand and Ventersdorp Supergroups. The samples were predominantly composed of CH4 (65–99%), N2 (3–27%), He (0.1–15%), and trace amounts of C2+ hydrocarbons. Notably, H2 in these samples was below detection limit, despite the presence of He - providing a critical indicator of processes removing H2 from the system. Using a Bayesian modelling approach, we test the hypothesis that the observed fluids are generated in-situ, driven by radioelement decay and subsequent microbial methanogenesis, and controlled by porosity of the host rock. The observed data is consistent with this hypothesis, and can be accounted for by a variation in porosity between 0.3 and 2.2% (typical values to Precambrian basement) across the different sampling sites. These He-rich hydrocarbon gases observed at the surface originate from a hydrogeological system that is porosity-constrained and isolated from externally-sourced fluids. Radioelement decay is the primary process driving the generation of H2 and therefore energy production in this subsurface system, utilised by hydrogenotrophic methanogens at the base of the deep carbon cycle. Microbial utilisation is the key mechanism for H2 consumptions and, conversely, preservation, suggesting that conditions favourable to commercial H2 discoveries are likely constrained to hypersaline environments where microbial activity is inhibited. The model results under the proposed hypothesis (consistent N2/H2 ratio between different boreholes) raises the possibility that N2, which often co-occurs with He-rich deep fluids, is also produced through radiolysis, and future work is needed to fully evaluate this hypothesis.