Witwatersrand Basin Research Articles

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.

Characterisation and modelling of mercury speciation in urban air affected by gold mining - assessment of bioavailability

The growing global concern over the release of mercury to the environment has prompted specific inventories that quantify mercury emissions from various sources. Investigations of atmospheric mercury have been mostly done on gaseous species. Although, to assess human expose to mercury, especially in urban areas, the inhalable dust should be included in a study. The Witwatersrand Basin in South Africa is one of the most important gold mining regions in the world. Mercury (Hg), which occurs in gold-bearing ores, was also used for gold recoveries in previous centuries (19th and early 20th century) and presently in illegal artisanal mining. The consequences of these mining activities were the release of Hg to the environment, mainly due to AMD from tailings dumps which are presently reprocessed. The city of Johannesburg is a multimillion population exposed strongly to industrial pollution. The aim of this study was to determine the magnitude of mercury pollution in this urban area and assess its bioavailability. The gaseous samples were collected by trapping mercury on various gold traps. Dust samples were collected from a ground and on inhalation levels (1–2 m above a ground). They were later separated into different fractions by micro sieving. Bioavailability of mercury in inhalable dust (25 μm) was tested by leaching collected samples with artificial lung fluid (ALF, pH 4.5), Gray’s solution (pH 7.4) and water. The leaching conditions were selected to mimic lungs environment (incubator at 30°C, time 24 hrs, rotation of samples 150 rpm). Total concentrations of mercury in dust fractions were also determined after microwave digestion. The results showed extremely high concentration levels of mercury in air and dust in industrial areas. Especially high levels were found around presently reprocessed old gold tailings dumps, up to 900 000 μgl–1 . The levels dropped significantly in CBD area but still showing elevated concentrations up to 10 μgl−1 . They depend strongly on prevailing wind directions, dry and wet seasons, and day time. Leaching experiments showed that the Gray’s solution extracts more mercury from the dust followed by ALF and much lower by water. This suggests that most of the mercury inhaled with dust will be extracted in the deep interstitial parts of the lungs. In the upper region of lungs extraction will occur to a lesser extent.

The Composition and Depositional Environments of Mesoarchean Iron Formations of the West Rand Group of the Witwatersrand Supergroup, South Africa

This paper documents the sedimentological setting, mineralogy, and geochemistry of several iron formation units interbedded with siliciclastic strata of the Mesoarchean Witwatersrand Supergroup, well known for its world-class conglomerate-hosted Au-U deposits. Four major iron formation beds, with associated magnetic mudstones, are present in two distinctly different lithostratigraphic associations, namely shale- and diamictiteassociated iron formation. The shale association is represented by the Water Tower and Contorted Bed iron formations in the Parktown Formation of the Hospital Hill Subgroup in the lower part of the succession and the diamictite association by the Promise and Silverfield iron formations in the overlying Government Subgroup. The iron formation units have been subjected to lower greenschist facies metamorphism. Oxide (magnetite and limited hematite), carbonate, and silicate facies iron formations are recognized. The iron formations typically overlie major transgressive flooding surfaces in the succession and, in turn, form the base of progradational coarsening-upward increments of sedimentation comprising magnetic mudstone, nonmagnetic shale, and interbedded siltstone-quartzite. The upward transition from iron formation into magnetic mudstone is accompanied by a change in mineralogical composition from hematite-magnetite iron formation at the base in the most distal setting through magnetite-siderite- and siderite-facies iron formation in the transition zone to magnetic mudstone. The siderite with associated ankerite displays highly depleted δ13C values, suggesting crystallization via iron respiration in presence of organic carbon. The iron formations display positive postArchean Australian shale-normalized Eu and Y anomalies with depletion in light rare-earth elements relative to heavy rare-earth elements, indicating precipitation from marine water with a high-temperature hydrothermal component. Integration of sedimentological, petrographic, and geochemical results indicates that the shale-associated iron formation was deposited during the peak of transgression, when reduced iron-rich hydrothermal waters entered the Witwatersrand Basin over a limited vertical extent due to neutral buoyancy, with the top of the plume occurring below the photic zone. It is suggested that chemolithoautotrophic iron-oxidizing bacteria, which would have been able to exploit the difference in chemistry between the iron-enriched plume water and ambient ocean water to fuel metabolic activity in the presence of limited free molecular oxygen, were responsible for precipitation of initial ferric iron oxyhydroxides. The vertical facies associations in the iron formations most likely developed in response to the limited vertical extent of the hydrothermal plume, with (from distal to proximal) hematite preserved where the base of the plume was not in contact with the basin floor, magnetite where the plume water was in contact with bottom sediment, iron-rich carbonates where organic carbon input was high, iron-rich alumosilicates where siliciclastic input became significant in more proximal settings, and iron-poor sediment above the top of the plume. Diamictite-associated iron formations in the Witwatersrand are inferred to have been deposited in a fashion similar to the shale-associated iron formations, with the exception that major transgressions and hydrothermal plume invasion were preceded by glacial ice cover. The climate warming and increased volcanic activity required could have been related to increased tectonic activity inferred for the Witwatersrand Supergroup during deposition of the glacially associated iron formations.

A Rights-Based Examination of Residents’ Engagement with Acute Environmental Harm across Four Sites on South Africa’s Witwatersrand Basin

The South African Constitution guarantees everyone’s right to an environment that is not harmful to his/her health or well-being. Nevertheless, the Witwatersrand basin – once a site of environmental beauty - has been ravaged by 160 years of gold mining, which has resulted in severe environmental damage in the form of toxic water and air and dust pollution that has not, to date, been addressed. In this context, we sought to ascertain the extent to which objectively affected residents are sensitized to environmental issues and whether, to what extent and in what circumstances, any sensitization has developed into mobilized action. This article presents the findings of the qualitative social survey we undertook to examine these questions across four residential sites on the Witwatersrand basin.

Application of 3D seismic techniques to evaluate ore resources in the West Wits Line goldfield and portions of the West Rand goldfield, South Africa

As expensive as 3D seismic reflection surveys are, their high cost is justified by improved imaging of certain ore horizons in some of the Witwatersrand basin gold mines. The merged historical 3D seismic reflection data acquired for Kloof and South Deep mines forms an integral part of their Ventersdorp Contact Reef mine planning and development programme. The recent advances in 3D seismic technology have motivated the reprocessing and reinterpretation of the old data sets using the latest algorithms, therefore significantly increasing the signal-to-noise ratio of the data. In particular, the prestack time migration technique has provided better stratigraphic and structural imaging in complex faulted areas, such as the Witwatersrand basin, relative to older poststack migration methods. Interpretation tools such as seismic attributes have been used to identify a number of subtle geologic structures that have direct impact on ore resource evaluation. Other improvements include more accurate mapping of the depths, dip, and strike of the key seismic horizons and auriferous reefs, yielding a better understanding of the interrelationship between fault activity and reef distribution, and the relative chronology of tectonic events. The 3D seismic data, when integrated with underground mapping and borehole data, provide better imaging and modeling of critical major fault systems and zones of reef loss. Many faults resolve as multifault segments that bound unmined blocks leading to the discovery and delineation of resources in faulted areas of the mines.

3D edge detection seismic attributes used to map potential conduits for water and methane in deep gold mines in the Witwatersrand basin, South Africa

Inrushes of ground water and the ignition of flammable gases pose risks to workers in deep South African gold mines. Large volumes of water may be stored in solution cavities in dolomitic rocks that overlie the Black Reef (BLR) Formation, while there are several possible sources for methane, namely, coal seams, kerogen found in some gold ore bodies, or methane introduced by igneous intrusions. Potential conduits that may transport water and methane to underground workings were mapped using 3D reflection seismic data. Edge detection attributes successfully identified many faults, some with displacements as small as 10 m. Faults that displace the Ventersdorp Contact Reef (VCR) and the BLR horizons were of special interest, as known occurrences of fissure water and methane in underground workings show a good correlation with faults that were imaged on the VCR and BLR horizons. Because there are uncertainties in determining the linkage of faults with aquifers and methane sources, it is considered prudent to assume that all structures that displace the VCR and BLR horizons are potential conduits.

Mining seismicity in the Witwatersrand Basin: monitoring, mechanisms and mitigation strategies in perspective

The Kaapvaal Craton in South Africa hosts one of the largest gold placer deposits in the world. Mining in the Witwatersrand Basin here has been the source of about one third to one half of the gold ever produced in the world. Gold was discovered in the Johannesburg area in 1886 and after 120years of continuous operation, mining is currently approaching depths of 4 000m. In spite of the challenges and risks that the industry has had to deal with including rock temperature, ventilation and water, one of the most feared hazards in the basin has been the threat from the ongoing occurrence of seismicity and rockbursts. The problem first manifested itself by way of the occurrence of tremors roughly twenty years after the commencement of mining operations. This paper traces the history of the approach to rockbursts and seismicity during the 120year history of mining in the basin. It portrays a picture of the mining seismicity in terms of monitoring phases, mechanisms and mitigation strategies. The work of research organizations over the years is highlighted with a brief mention of current regulation strategies on the part of the mining inspectorate.

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.

REPLY TO COMMENT BY W.U. REIMOLD ON "PSEUDOTACHYLITE IN THE SOUTH BOUNDARY FAULT AT THE COOKE SHAFT, WITWATERSRAND BASIN, SOUTH AFRICA BY P.W. MAMBANE ET AL., SOUTH AFRICAN JOURNAL OF GEOLOGY 114.2, 109-120

In reply to comments by W.U. Reimold on ”Pseudotachylite in the South Boundary Fault at the Cooke Shaft, Witwatersrand Basin, South Africa by P.W. Mambane et al., S. Afr. J. Geol. 114.2, 109–120. 1. The Introduction statement in Mambane et al. (2011) directs the reader to consider that authors such as Schwarzman et al. (1983), Gibson et al. (1997), Spray (1998), Eiko (2004), Riller et al. (2010) and others, have in the past, considered pseudotachylites as a diagnostic feature of shock tectonics. Mambane et al. make clear in the Introduction that pseudotachylites are formed in a number of settings and under a number of P-T conditions. 2. The perseverant objections to the use by Mambane et al. of the term “pseudotachylite breccia” are not tenable. It is agreed that terminologies used by Shand (1916), Bisschoff (1962), Reimold and Colliston (1994), and Reimold and Gibson (2006), and others, are not well-defined. A better classification scheme is needed to assist in the description of the different types of pseudotachylite, and the meaning of pseudotachylite textures including breccia, “melt breccia”, and “tectonic pseudotachylite”. It seems that Mohr-Westheide and Reimold (2011) and Reimold (2011) are occupied with this task, but that being mostly after review and publication of Mambane et al., which raises the question of how Mambane et al. could consider articles that appeared after the March 2011 acceptation date of their article, or a June 2011 publishing date. For example, Mohr-Westheide and Reimold (2011) was first published on the 17 March 2011, …

COMMENT ON "PSEUDOTACHYLITE IN THE SOUTH BOUNDARY FAULT AT THE COOKE SHAFT, WITWATERSRAND BASIN, SOUTH AFRICA BY P.W. MAMBANE ET AL., SOUTH AFRICAN JOURNAL OF GEOLOGY 114.2, 109-120

Mambane et al. (2011) presented a geological analysis of an occurrence of different fault rock types at the South Boundary Fault on Cooke Shaft, in the West Rand goldfield of the Witwatersrand Basin. They refer to both pseudotachylite and pseudotachylitic breccias (below referred as PTB) being present at that location. They state that based on the conclusion of previous research, namely that “…the Hekpoort Andesite Formation (2241 ± 21 Ma) of the Transvaal Supergroup provides a maximum age for all pseudotachylite in the West Rand goldfield may therefore need revision”, had to be revised, whereas their own findings constrained “the relative age of tectonically–induced pseudotachylite formation…to pre-2.58 Ga…” (their p. 109). Their article does, however, not adequately refer to the previous work on Witwatersrand pseudotachylitic breccias, as borne out of the reference list they provide, and thus I feel obliged to give a plug to previous Witwatersrand pseudotachylitic breccia workers by highlighting some of their findings that seemingly have been overlooked. This may also assist the authors to address their own astonishment about: “It is therefore curious that attempts have been made to correlate pre-2.58 Ga tectonically-induced pseudotachylites from the goldfields with ca. 2.02 Ga impact-induced pseudotachylites in the Vredefort Dome” (p.118), at which instance particular reference to previous work by myself and co-workers is being made. Before I begin to discuss the work by Mambane et al. and their inferences, it is necessary to address a serious problem these authors raise in their Introduction (p.109). They state: “It [ the occurrence of pseudotachylite , the author] is also considered a diagnostic feature of shock tectonics (metamorphism) associated with impact cratering,…”. “Shock tectonics is confusing, as impact-related deformation does not only occur during the shock compression phase, but particularly thereafter – during the modification phase …

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.

Ammonium stability and nitrogen isotope fractionations for [formula omitted]–NH3(aq)–NH3(gas) systems at 20–70 °C and pH of 2–13: Applications to habitability and nitrogen cycling in low-temperature hydrothermal systems

Ammonium/ammonia is an essential nutrient and energy source to support life in oceanic and terrestrial hydrothermal systems. Thus the stability of ammonium is crucial to determine the habitability or ecological structure in hydrothermal environments, but still not well understood. To date, the lack of constraints on nitrogen isotope fractionations between ammonium and ammonia has limited the application of nitrogen isotopes to trace (bio)geochemical processes in such environments. In this study, we carried out laboratory experiments to (1) examine the stability of ammonium in an ammonium sulfate solution under temperature conditions from 20 to 70°C and pH from 2.1 to 12.6 and (2) determine nitrogen isotope fractionation between ammonium and ammonia.Our experimental results show that ammonium is stable under the experimental temperatures when pH is less than 6. In experiments with starting pH greater than 8, significant ammonium was lost as a result of dissociation of ammonium and degassing of ammonia product. Nitrogen concentrations in the fluids decreased by more than 50% in the first two hours, indicating extremely fast effusion rates of ammonia. This implies that ammonium at high pH fluids (e.g., Lost City Hydrothermal Vents, Oman ophiolite hyperalkaline springs) may not be stable. Habitable environments may be more favorable at the leading edge of a pH gradient toward more acidic conditions, where the fluid can efficiently trap any ammonia transferred from a high pH vent. Although modeling shows that high temperature, low pH hydrothermal vents (e.g., Rainbow hydrothermal vent) may have the capability to retain ammonium, their high temperatures may limit habitability. The habitable zone associated with such a hydrothermal vent is likely at the lower front of a temperature gradient. In contrast, modeling of ammonium in deep terrestrial systems, suggests that saline fracture waters in crystalline rocks such as described in the Canadian Shield and in the Witwatersrand Basin, South Africa may also provide habitable environments for life.The nitrogen isotope results of remaining ammonium from the partial dissociation experiments fit well with a batch equilibrium model, indicating equilibrium nitrogen isotope fractionations have been reached between ammonium and its dissociation product aqueous ammonia. Modeling yielded nitrogen isotope fractionations between ammonium and aqueous ammonia were 45.4‰ at 23°C, 37.7‰ at 50°C, and 33.5‰ at 70°C, respectively. A relationship between nitrogen equilibrium isotope fractionation and temperature is determined for the experimental temperature range as:103·lnαNH4+-NH3(aq)=25.94×103T-42.25Integrated with three previous theoretical estimates on nitrogen isotope equilibrium fractionations between ammonium and gaseous ammonia, we achieved three possible temperature-dependent nitrogen isotope equilibrium fractionation between aqueous ammonia and gaseous ammonia:(1)103·lnαNH3(aq)-NH3(gas)=13.73×103T-30.76(2)103·lnαNH3(aq)-NH3(gas)=13.54×103T-30.51(3)103·lnαNH3(aq)-NH3(gas)=12.88×103T-36.00These calibrations provide a new tool to contribute to the study of nitrogen cycling under low temperature subsurface conditions.

Characterization of cyanide in a natural stream impacted by gold mining activities in the Witwatersrand Basin, South Africa

Water systems in the Witwatersrand Basin are heavily contaminated with pollutants from gold-mine tailing leachates and spillages. Cyanide speciation was studied in relation to the Natalspruit, a natural stream within a gold mining area of Johannesburg. The part of the studied stream was adjacent to an abandoned gold-mine tailings dump. The results of this study pointed to a variation in surface and groundwater chemistry. The groundwater had low pH and contained elevated concentrations of pollutants including secondary cyanide species, e.g. thiocyanate, cyanate, and ammonium. Metal-cyanide complexes that are weak-acid dissociable (CNWAD) and strong-acid dissociable (CNSAD) cyanides were found to dominate the speciation of cyanide, with iron cyanide complexes being most prevalent in groundwater. Decreases in the concentration of 63% for , 83% for , 69% for , and 47% for were observed from groundwater to surface water largely due to dilution and dissociation (in the case of the first two complexes). Speciation modeling was used to estimate the concentrations of CNWAD and CNSAD based on total cyanide (CNT), total metal, and total anion concentrations. Other cyanide complexes as well as dominant sulfate complexes were also predicted.

The origin of NO3− and N2 in deep subsurface fracture water of South Africa

Deep (>0.8km depth) fracture water with residence time estimates on the order of several Ma from the Witwatersrand Basin, South Africa contains up to 40μM of NO3−, up to 50mM N2 (90 times air saturation at surface) and 1 to ~400μM NH3/NH4+. To determine whether the oxidized N species were introduced by mining activity, by recharge of paleometeoric water, or by subsurface geochemical processes, we undertook N and O isotopic analyses of N species from fracture water, mining water, pore water, fluid inclusion leachate and whole rock cores.The NO2−, NO3− and NH3/NH4+ concentrations of the pore water and fluid inclusion leachate recovered from the low porosity quartzite, shale and metavolcanic units were ~104 times that of the fracture water. The δ15N–NO3− and δ18O–NO3− of the pore water and fluid inclusion leachate, however, overlapped that of the fracture water with the δ15N–NO3− ranging from 2 to 7‰ and the δ18O–NO3− ranging from 20 to 50‰. The δ15N–NO3− of the mining water ranged from 0 to 16‰ and its δ18O–NO3− from 0 to 14‰ making the mining water NO3− isotopically distinct from that of the fracture, pore and fluid inclusion water. The δ15N–N2 of the fracture water and the δ15N–N from the cores ranged from −5 to 10‰ and overlapped the δ15N–NO3−. The δ15N–NH4+ of the fracture water and pore water NH3/NH4+ ranged from −15 to 4‰. Although the NO3− concentrations in the pore water and fluid inclusions were high, mass balance calculations indicate that NO3− accounts for ≤10% of the total rock N, whereas NH3/NH4+ trapped in fluid inclusions or NH4+ present in phyllosilicates account for ≥90% of the total N.Based on these findings, the fluid inclusion NO3− appears to be the source of the pore water and fracture water NO3− rather than paleometeoric recharge or mining contamination. Irradiation experiments indicate that radiolytic oxidation of NH3 to NO3− can explain the fluid inclusion NO3− concentrations and, perhaps, its isotopic composition, but only if the NO3− did not attain isotopic equilibrium with the hydrothermal fluid 2billion years ago. The δ15N–N, δ15N–N2 and δ15N–NH4+ suggest that the reduction of N2 to NH4+ also must have occurred in the Witwatersrand Basin in order to explain the abundance of NH4+ throughout the strata. Although the depleted NO3− concentrations in the fracture water relative to the pore water are consistent with microbial NO3− reduction, further analyses will be required to determine the relative importance of biological processes in the subsurface N cycle and whether a complete subsurface N cycle exists.

Saline waters in basement rocks of the Kaapvaal Craton, South Africa

The chemical composition and geochemical evolution of saline waters found in deep (1.9–3.5 km) gold mines in the Kaapvaal Craton, South Africa, are reported and discussed. Twenty eight samples, representing the most saline waters available were studied. The salinity of the samples ranges between 2.5 and 99 g·TDS·L − 1 with chloride being the dominating anion. Magnesium is practically absent from most samples. The Kaapvaal waters were sampled in the Carletonville region (Driefontein, Kloof, Mponeng and Tau Tona mines) and at Evander. The samples, defined as fracture waters, were retrieved from the Witwatersrand Basin Proterozoic quartzite rocks and from metamorphosed basalts and basaltic andesites. Based on water salinity, Na/Cl–Br/Cl ratios and Ca–Mg–SO 4 relationship, we conclude that the chemical composition of the waters stems from ancient seawater that experienced three major processes by the following order: (1) concentration of seawater by freezing; (2) density-driven infiltration into the underlying crystalline basement, accompanied by water–rock interaction (WRI); and (3) dilution of the brine by freshwater. Because the last glaciation in South Africa occurred in Carboniferous–Permian (Gastaldo et al., 1996) we attribute this age to the Kaapvaal brines, rather than to a much earlier period proposed previously. After their cryogenic concentration in a marginal trough around the Gondwanan ice sheet, the brines infiltrated and migrated into the underlying crystalline rocks through fissures and shear zones, displacing less dense resident freshwater. Therein, the brines were further modified via water–rock interaction, which included chloritization and albitization of Ca-plagioclase. Chloritization was responsible for the almost total removal of Mg and SO 4 from solution and for the significant Ca enrichment observed. Albitization, which affected only a part of the samples, caused a loss of Na and an additional, equivalent gain in Ca. In view of similar findings, limited so far to crystalline shields in the Northern Hemisphere, our interpretation of the South African data may have a much farther-reaching meaning. We argue that similar saline waters should be expected at depth in all rocks, crystalline and sedimentary alike, in regions that were covered by major glaciers, such as eastern South America, large parts of Australia, India and Antarctica during the Carboniferous–Permian.

PSEUDOTACHYLITE IN THE SOUTH BOUNDARY FAULT AT THE COOKE SHAFT, WITWATERSRAND BASIN, SOUTH AFRICA

The steeply-dipping South Boundary Fault in the Cooke Section on the West Rand goldfield hosts a series of crosscutting pseudotachylite fault and injection veins, and pseudotachylitic breccias that formed progressively by abrasive wear in a brittle (elastico-plastic) regime. The crosscutting pseudotachylite veins and breccias are hosted by faulted supracrustal rocks of the Witwatersrand Basin that are exposed by underground workings and production-development borehole cores. Evidence of frictional melting is represented by quenched vein margins, corona texture, turbulent flow and glassy texture. Underground mapping and logging of boreholes has confirmed the existence of at least three generations of pseudotachylite, which were tectonically-induced. The relative age of tectonically-induced pseudotachylite formation is constrained to pre-2.58 Ga, or before deposition of the lower Transvaal Basin sequences, because 3D seismic data show that pseudotachylite-bearing faults do not penetrate the base of the Transvaal Basin. The conclusions made in previous research that the Hekpoort Andesite Formation (2224 ± 21 Ma) of the Transvaal Supergroup provides a maximum age for all pseudotachylite in the West Rand goldfield may therefore need revision.

Peatlands as Filters for Polluted Mine Water?—A Case Study from an Uranium-Contaminated Karst System in South Africa—Part I: Hydrogeological Setting and U Fluxes

Located downstream of goldfields of the Witwatersrand basin, the Gerhard Minnebron (GMB) Eye—as major water source for downstream community of some 300,000 people—may be impacted on by mining-related water pollution especially with uranium (U). Containing up to 5 m-thick deposits of peat that is frequently reported to act as a filter for U and other heavy metals, this paper is the first part of a series that aims to quantify the ability of the GMB peatland to act as buffer against current and future U pollution. In a first step, this paper outlines the geohydrological conditions and discusses how deep–level gold mining impacted on the dolomitic aquifers. Subsequently, the potential influx of U into the wetland is estimated and associated sources and pathways analyzed. Finally, a model is proposed explaining the significant differences in degree and dynamics of U observed within a single groundwater compartment.

The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?

We briefly examine the possible antiquity of the supercontinental cycle while noting the likely unreliability of palaeomagnetic data >ca.1.8 Ga, assuming a gradual change from a magmatically dominated Hadean Earth to a plate tectonically dominated Neoarchaean system. A brief review of one of Earth's oldest cratons, Kaapvaal, where accent is placed on the lithostratigraphic and geodynamic-chronological history of its cover rocks from ca. 3.1 to 2.05 Ga, forms the factual basis for this article. The ca. 3.1–2.8 Ga Witwatersrand–Pongola (Supergroups) complex retroarc flexural foreland basin developed while growth and stabilization of the craton were still underway. Accretion of relatively small composite granite-gneiss-greenstone terranes (island arc complexes) from both north and west does not support the formation of a Neoarchaean supercontinent, but may well have been related to a mantle plume which enhanced primary gold sources in the accreted terranes and possibly controlled the timing and rate of craton growth through plate convergent processes. Subsequent deformation of the Witwatersrand Basin fill with concomitant loss of ≤1.5 km of stratigraphy must have been due to far-field tectonic effects, but no known mobile belt or even greenstone belts can be related to this contractional event. At ca. 2714–2709 Ma, a large mantle plume impinged beneath the thinned crust underlying the Witwatersrand Basin forming thick, locally komatiitic flood basalts at the base of the Ventersdorp Supergroup, with subsequent thermal doming leading to graben basins within which medial bimodal volcanics and immature sediments accumulated. Finally (possibly at ca. 2.66–2.68 Ga), thermal subsidence enabled the deposition of uppermost Ventersdorp sheet-like lavas and sediments, with minor komatiites still present. Ongoing plume-related influences are thus inferred, and an analogous cause is ascribed to a ca. 2.66–2.68 Ga dike swarm to the north of the Ventersdorp, where associated rifting allowed formation of discrete ‘protobasinal’ depositories of the Transvaal (ca. 2.6–2.05 Ga Supergroup, preserved in three basins). Thin fluvial sheet sandstones (Black Reef Formation, undated) above these lowermost rift fills show an association with localized compressive deformation along the palaeo-Rand anticline, north of Johannesburg, but again with no evidence of any major terrane amalgamations with the Kaapvaal. From ca. 2642 to 2432 Ma, the craton was drowned with a long-lived epeiric marine carbonate-banded iron formation platform covering much of it and preserved in all three Transvaal Basins (TB). During this general period, at ca. 2691–2610 Ma, the Kaapvaal Craton collided with a small exotic terrane [the Central Zone (CZ), Limpopo Belt] in the north. Although farfield tectonic effects are likely implicit in TB geodynamics, again there is no case to be made for supercontinent formation. Following an 80–200 million years (?) hiatus, with localized deformation and removal of large thicknesses of chemically precipitated sediments along the palaeo-Rand anticline, the uppermost Pretoria Group of the Transvaal Supergroup was deposited. This reflects two episodes of rifting associated with volcanism, and subsequent thermal subsidence within a sag basin setting; an association of the second such event with flood basalts supports a plume affinity. At ca. 2050 Ma the Bushveld Complex intruded the northern Kaapvaal Craton and reflects a major plume, following which Kaapvaal–CZ collided with the Zimbabwe Craton, when for the first time, strong evidence exists for a small supercontinent assembly, at ca. 2.0 Ga. We postulate that the long-lived evidence in favour of active mantle (cf. plume) influences with subordinate and localized tectonic shortening, implicit within the review of ca. 3.1–2.05 Ga geological history of the Kaapvaal Craton, might reflect the influence of earlier Precambrian mantle-dominated thermal systems, at least for this craton.

Neon identifies two billion year old fluid component in Kaapvaal Craton

The deep gold mines of the Witwatersrand Basin, South Africa have gained recent attention not only because of investigations of the deep fracture water and associated CH 4- and H 2-rich gases found there, but because of recent reports of deep microbial communities persisting to depths of almost 3 km — an exotic outpost of the Earth's deep biosphere. While shallower fluids in the basin (to approximately 1 km) were found to contain abundant populations of methanogens and sulphate-reducing bacteria, the deepest, oldest, most saline fracture waters in the basin hosted hitherto unrecognised low biomass and low biodiversity chemoautotrophic ecosystems independent from the photosphere. Shallow and deep fluids also show distinct differences in gas and fluid geochemistry. Paleometeoric waters are dominated by hydrocarbon gases with compositional and isotopic characteristics consistent with production by methanogens utilising the CO 2 reduction pathway. In contrast the deepest, most saline fracture waters contain gases that are dominated by high concentrations of H 2 gas, and CH 4 and higher hydrocarbon gases with isotopic signatures attributed to abiogenic processes of water–rock reaction. The high salinities (up to hundreds of g/L), highly altered δ 18O and δ 2H signatures, and both 36Cl and measurements of co-occurring nucleogenic noble gases for these fracture waters are consistent with extensive water–rock interaction over geologically long time scales in these high rock/water ratio environments. While the ultimate origin of these fluids has been attributed alternately to saline waters that penetrated the crystalline basement, formation water, or hydrothermal fluids in some cases, their δ 18O and δ 2H isotopic signatures have typically been so profoundly overprinted by the effects of long-term water–rock interaction that, for the most saline end-members, little evidence of their primary composition remains. The key objective of the present study is to further investigate the origin of these fluids by integrating for the first time detailed neon isotope analyses on the dissolved gases. Helium isotopic analysis confirmed that there is no significant mantle-derived component associated with these fluids and gases. Neon isotope results show distinct differences in neon composition that correspond to the different fluid geochemical end-members previously identified. Typical crustal neon signatures (type A) are identified in the paleometeoric waters populated with abundant methanogens. In contrast, the deep more saline fracture waters contain an enriched nucleogenic neon signature unlike any previously reported in crustal fluids. These samples show the highest 21Ne/ 22Ne ratios (0.160 ± 0.003) ever reported in groundwater. Fluid inclusions in these rocks yield even higher 21Ne/ 22Ne ratios between 0.219 and 0.515, consistent with an extrapolated 21Ne/ 22Ne value of 3.3 ± 0.2 at 20Ne/ 22Ne = 0. We show that this enriched nucleogenic neon end-member represents a fluid component that was produced in the fluorine-depleted Archaean formations and trapped in fluid inclusions ≥ 2 Ga ago. The observation of enriched nucleogenic neon signatures in deep fracture water implies the release of this billion year old neon component from the fluid inclusions and its accumulation in exceptionally isolated fracture water systems. The observed association of this Archean neon signature with H 2-hydrocarbon-rich geogases of proposed abiogenic origin dissolved in the same deep groundwater suggests that the fracture systems have also allowed for the accumulation of various products of water–rock reactions throughout geologic times. One of these fracture systems contained the deepest characterised microbial ecosystems on earth — chemolithotrophs eking out an existence at maintenance levels independent from sunlight. Consequently, the enriched nucleogenic neon isotope signature may indicate regions in the Archaean crust where investigations of the deep biosphere might be focused.

Provenance of groundwater in the crystalline aquifer of Johannesburg area, South Africa

Recharge dynamics, possible flow path and human impact within the crystalline aquifer of the Johannesburg area have been described based on the hydrogeochemical characteristics and environmental isotopes. The dominant circulation zone has also been identified in the study. Understanding of the groundwater dynamics is very important for strategic management of the resource as the country is dependent on groundwater for its different activities. It has also been observed that mixing of fresh and polluted groundwater takes place at shallow zones within the weathered crystalline rocks and dissolution cavities in dolomites where the rocks are characterized by the lithological and structural complexities. From the compositional features of the groundwater in the Johannesburg area, it follows that the solute composition of the water basically results from two basic contributions: The reaction of meteoric water with weathered zone in the near surface area and direct infiltration through dolomite dissolution cavities. The results further indicate that the main constituent movement mechanism within the crystalline aquifer is through a diffusive process, which could be due to few fractures with depth and possible sealing by calcite precipitation. Therefore, the provenance of groundwater circulation is limited to the depth of 40 m. The natural water quality is found to be highly impacted by acid mine drainage that is derived from gold mining centers distributed in the upper Witwatersrand basin. Key words: Crystalline aquifer, diffusive circulation, dissolution cavities, Johannesburg, water supply.