Witwatersrand Basin Research Articles

Constraining the Far-Field Stress State near a Deep South African Gold Mine

The deep gold mines in the Witwatersrand Basin of South Africa are some of the deepest underground mines in the world. For over a century, numerous, often deadly, mining induced earthquakes have been observed. In this work, we develop and test a new technique for determining the virgin state of stress near the TauTona gold mine. This work was completed as part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project. The technique we use to constrain the far-field stress state follows an iterative forward modelling approach that combines observations of drilling induced borehole failures in borehole images, boundary element modelling of the mining-induced stress perturbations, and forward modelling of borehole failures based on the results of the boundary element modelling. The final result is a well constrained stress state consistent with all the observed stress indicators. We find that the stress state is a normal faulting regime with principal stress orientations that are slightly deviated from vertical and horizontal (denoted with a *). The maximum principal stress, Sv*, is deviated 0-20o from vertical towards the NNW and has a magnitude gradient of 27 ± 0.3 MPa/km. The intermediate principal stress, SHmax*, is inclined 0-20o from horizontal with an azimuth of 145o to 168o and has a magnitude gradient of 21.5 to 26 MPa/km. The least principal stress, Shmin*, is inclined 0-10o from horizontal with an azimuth of 235o to 258o and has a magnitude gradient of 13 to 15.5 MPa/km. This stress state indicates that the crust is in a state of frictional faulting equilibrium, such that normal faulting is likely to occur on pre-existing fault planes that are optimally oriented to the stress field. The stress concentrations caused by the mining activities can dramatically alter the range of fault orientations upon which fault slip could occur.

Impact cratering — fundamental process in geoscience and planetary science

Impact cratering is a geological process characterized by ultra-fast strain rates, which generates extreme shock pressure and shock temperature conditions on and just below planetary surfaces. Despite initial skepticism, this catastrophic process has now been widely accepted by geoscientists with respect to its importance in terrestrial — indeed, in planetary — evolution. About 170 impact structures have been discovered on Earth so far, and some more structures are considered to be of possible impact origin. One major extinction event, at the Cretaceous-Paleogene boundary, has been firmly linked with catastrophic impact, but whether other important extinction events in Earth history, including the so-called “Mother of All Mass Extinctions” at the Permian-Triassic boundary, were triggered by huge impact catastrophes is still hotly debated and a subject of ongoing research. There is a beneficial side to impact events as well, as some impact structures worldwide have been shown to contain significant (in some cases, world class) ore deposits, including the gold-uranium province of the Witwatersrand basin in South Africa, the enormous Ni and PGE deposits of the Sudbury structure in Canada, as well as important hydrocarbon resources, especially in North America. Impact cratering is not a process of the past, and it is mandatory to improve knowledge of the past-impact record on Earth to better constrain the probability of such events in the future. In addition, further improvement of our understanding of the physico-chemical and geological processes fundamental to the impact cratering process is required for reliable numerical modeling of the process, and also for the correlation of impact magnitude and environmental effects. Over the last few decades, impact cratering has steadily grown into an integrated discipline comprising most disciplines of the geosciences as well as planetary science, which has created positive spin-offs including the study of paleo-environments and paleo-climatology, or the important issue of life in extreme environments. And yet, in many parts of the world, the impact process is not yet part of the geoscience curriculum, and for this reason, it deserves to be actively promoted not only as a geoscientific discipline in its own right, but also as an important life-science discipline.

Bushveld-aged fluid flow, peak metamorphism, and gold mobilization in the Witwatersrand basin, South Africa: Constraints from in situ SHRIMP U-Pb dating of monazite and xenotime

In situ U-Pb dating of monazite and xenotime in gold reefs and unmineralized greenschist facies sedimentary rocks from the Witwatersrand basin, South Africa, reveals two episodes of tectonothermal activity. A major event between 2.06 and 2.03 Ga is recorded in the Wit-watersrand and Transvaal Supergroups in the northwestern and central basin, and broadly coincides with the ca. 2.06 Ga Bushveld event. In the central and southern basin, a previously unrecognized event has been dated between 2.14 and 2.12 Ga. The widespread geographic and stratigraphic occurrence of Bushveld-aged monazite and xenotime, including both auriferous reefs and unmineralized strata, indicates that metamorphism and fluid flow associated with magmatism was pervasive, affecting most of the succession (>10 km thick) in the central and northern parts of the basin. The metamorphic phosphate dates, which are younger away from the complex, indicate a lag of 20–30 m.y. between emplacement and phosphate growth in the central basin (∼100 km south), suggesting that heat related to magmatism was transferred southward at an average rate of 3–5 mm yr −1 . The absence of 2.06–2.03 Ga phosphates in the Welkom goldfield at the southern end of the basin implies that Bushveld-related heating and fluid flow did not affect this part of the basin. The intergrowth of ca. 2.045 Ga monazite with gold in quartz-pebble conglomerate from the West Rand goldfield indicates that fluid flow related to the Bushveld event caused mobilization of gold in the Witwatersrand basin.

Progressive evolution of a late orogenic thrust system, from duplex development to extensional reactivation and disruption: Witwatersrand Basin, South Africa

Abstract This paper examines progressive evolution of fault architectures through late orogenic compression- to post-orogenic extensional deformation in the Witwatersrand Basin, South Africa. The results indicate that rapid extrusion of mafic lavas of the lower Klipriviersberg Group formed a rigid ‘lid’ over the thrust front, changing its mechanical character and thereby driving a change of structural style from fold growth to passive roof duplex. Flexural tightening of folds in the core of the triangle zones at this time may have helped provide the dynamic permeability for distributed ingress of hydrothermal fluids and consequent gold mineralization. Shortly afterwards, the kinematic environment changed to become extensional. However, this study shows sharp lateral partitioning of the duration of kinematic style and structural amplification, such that thrusting and extension coexisted along strike in the upper Klipriviersberg Group. Thus the switch from thrusting to extension was progressive within the region, but locally very rapid. As the local kinematic environment became extensional, the fault system evolved progressively, with the early stages of kinematic changes being dominated by a process of reactivation by architectural scavenging, in which new extensional structures developed by selectively reusing and incorporating geometrical segments of earlier formed thrust and normal faults. Three basic stages can be identified in this evolution: broad extension above underlying detachments, involving reactivation of lateral structures; a period of intensive reactivation and kinematic reworking incorporating frontal structures; and an abandonment stage when the detailed influence of the earlier architecture diminished and the fault system developed larger through-going normal faults. The interaction of the newly developing fault system with the pre-existing architecture constitutes pre-programming of the final geometry, in which individual large faults are composed of a reticulated network of new and inherited segments. The observations are consistent with fault scale being a key control on the fault reactivation involved. This study has involved full integration of a dataset comprising 2D and 3D seismic reflection data, geological mine plans, logging of over 120 km of drill core and underground mapping in deep mine workings that pass 3 km into the seismic volume at 2–3 km depth.

The Deposition of Elemental Gold from Gold(I)-Thiosulfate Complexes Mediated by Sulfate-Reducing Bacterial Conditions

The role of sulfate-reducing bacteria in the precipitation of elemental gold through a geologic stratum was investigated using column experiments. The mixed culture, dominated by sulfate-reducing bacteria, was isolated from the Driefontein consolidated gold mine, Witwatersrand basin, Republic of South Africa. Bacterially mediated gold precipitation from the gold(I)-thiosulfate complex was more efficient than the corresponding abiotic experiments. In the bacterial systems, sulfate-reducing bacteria (i.e., Desulfovibrio sp.) deposited gold inside the cells as spherical nanoparticles. Over time, these nanoparticles of gold were released from the cells and deposited at cell surfaces and in the bulk solutions. Ultimately, these nanoparticles of gold contributed to the formation of micrometer-scale octahedral gold crystals, occasional framboidlike structures (~1.5-μm diam) and millimeter-scale gold foils, typically surrounding the silicate grains. Spherical gold particles (~1-μm diam) were precipitated in the abiotic experiments, and the formation of octahedral gold, framboidlike structures and gold foils were not observed in these experiments. The reduction and enrichment of elemental gold by sulfate-reducing bacteria occurred without the concomitant death of the bacteria, suggesting that in natural systems, this process could go on over geologic time scales, as long as nutrients and gold(I)-thiosulfate were provided to the system.

Structural geometry and development of the Witwatersrand Basin, South Africa

Abstract The Witwatersrand Basin is an Archaean basin situated on the Kaapvaal Craton of Southern Africa. The results presented here focus on the structural geometry and development of the basin. Detailed structural sections across the western and northwestern parts of the basin are presented, using seismic data integrated with borehole, mine and outcrop information. The structural development of the basin can be expressed in simple terms, and in a manner that is standard practice within the oil industry. There are several clearly identified stages in basin evolution. The basin was initiated as a rift during Dominion Group times ( c. 3074 Ma), with post-rift thermal subsidence during the early part of West Rand Group times. Thermal subsidence would have been completed by late West Rand Group times ( c. 2900 Ma). Minor volcanic interludes within the West Rand Group sequence may testify to the existence of phases of extension during West Rand Group times. Onset of compression and thrusting outside the thermal basin generated a flexural load and clastic input into the basin as it evolved from thermal sag to foreland basin during late West Rand Group times ( c. 2950 Ma). Foreland basin development culminated in Central Rand Group times, with an increasingly coarse-grained clastic input, and thrust systems that progressively encroached on the basin margins, profoundly influencing structural styles. Thrusting was interrupted during Klipriviersberg Group times (2714 Ma) by the accumulation of basic volcanic rocks. Further thrusting occurred at the end of Klipriviersberg Group times. The basin then returned to an extensional tectonic setting during the Platberg Group rifting (2709 Ma), a major rift event across much of the area. Thermal subsidence related to this rift phase occurred during late Platberg Group times. The overprint of Platberg Group extensional faults breaks up the structural continuity of the Central and West Rand Group sediments within, and adjacent to, the basin, and it is difficult to elucidate the earlier compressional thrust-fold structures. Careful integration of seismic, borehole, mine and outcrop data allows structures from the thrusting event be recognized and more complete sections drawn. These present a larger-scale view of the structure of the basin than has previously been obtained.

About this title

This Special Publication, in memory and celebration of the work of Professor Mike Coward, is about the deformation of the continental lithosphere. The collected papers discuss geometry, structural principles, processes and problems in a wide range of tectonic settings and thereby reflect the breadth of Coward's interests. They encompass the evolution of Precambrian basement gneiss terrains, the geometry and evolution of thrust systems, basement involvement and structural inheritance in basins, syn-orogenic extension, salt tectonics, the implication of structural evolution on hydrocarbon prospectivity and structural controls on mineralization. Examples are drawn from the Lewisian and Moine Thrust Belt of NW Scotland, the Italian Apennines, NW Himalayas, the Cyclades, Oman, Zagros Mountains, Colombian Cordillera, Carpathians, North Sea, offshore Brazil, regional studies of the Irumide Belt (central Africa), Taurus Mountains (Turkey), greater South America, and from the Witwatersrand Basin of South Africa and the Antler Orogeny of SW USA.

Gold-bearing reefs of the Witwatersrand Basin: A model of synsedimentation hydrothermal formation

The current concepts concerning the genesis of the unique ore-bearing reefs of the Witwatersrand Basin and its gold resource potential are considered. The results of microscopic examination of ore from the Black, Ventersdorp Contact, Carbon Leader, and Vaal reefs, as well as of thermobarometric study of quartz, are presented. A model of synsedimentation hydrothermal origin of the reefs in the process of evolution of primary colloidal-disperse systems is substantiated on the basis of these results and the data published by other authors. The formation of these systems is related to the periodic gain of deep ore-bearing gas-saturated fluids. The gold mineralization was formed under conditions of partially closed systems, where various mineral-forming processes developed (metasomatism, crystallization of true solutions and gels, gel metasomatism, dispersion of crystalline phases, segregation of mineral particles, formation of early minerals, etc.). New data on REE specialization of ore-bearing fluids are discussed. The specific features of the gold, carbonic, and uranium mineralization of the intracratonic basin are emphasized.

14C in Methane and DIC in the Deep Terrestrial Subsurface: Implications for Microbial Methanogenesis

A comparison between the 14C content of the methane and dissolved inorganic carbon (DIC) in deep, terrestrial subsurface systems was used to assess the timing of microbial methanogenesis contributing to gases in fracture water samples from three mines in the Witwatersrand Basin, South Africa. The results demonstrated that the majority of methane was produced over geologic timescales. In four of the samples, the methane contained no significant radiocarbon, indicating that the estimated 90% microbial methane in these samples was produced in the geologic past by indigenous microbial communities. In two samples from different mines, methane Δ14C levels indicated a primarily ancient origin for the microbial methane with the potential for more recent contributions from ongoing indigenous microbial activities constrained to between 0 and 40%, and 0 and 24%, respectively. Microbiological evidence for methanogenic archaea was observed in both of these samples. One sample had a Δ14C CH4 that was higher than the corresponding DIC, indicating an extreme decoupling between these species and raising concerns over the representative quality of this sample. The variations in the Δ14C of DIC and CH4 between and within mines demonstrate the need for a thorough assessment of each sample to obtain an accurate understanding of the role and timing of microbiological gas production in these complex, heterogeneous, terrestrial subsurface systems. The approach detailed here introduces timing as a new and widely applicable signature for the recognition of a major geochemical marker of indigenous life in the deep subsurface.

Structural and Chemical Characterization of a Natural Fracture Surface from 2.8 Kilometers Below Land Surface: Biofilms in the Deep Subsurface

A freshly intersected water-bearing fracture zone from the Mponeng Au mine located in the Witwatersrand Basin, Republic of South Africa was sampled, providing an opportunity to examine the natural, deep subsurface biosphere. The fracture, intersected by an advancing tunnel 2.8 kilometers below land surface, possessed a millimeter thick layer of chlorite group minerals, i.e., chamosite, at the water-mineral interface. Water flowing out from the fracture zone had a temperature of 52°C, pH of 9.16 and Eh of −263 mV. Using scanning electron microscopy, the water-mineral interface was generally found to be clean, i.e., it did not possess any secondary mineral or dominant organic coatings. Irregular patches (10's of μm 2 ) of organic material, however, resembling bacterial exopolysaccharides, occurred in the presence or absence of bacteria. The surface was colonized by highly dispersed individual bacteria or by microcolonies containing up to 5 cells, with an overall cell density of 5 × 104 bacteria cm −2 . This biofilm population, although low, was 2 orders of magnitude greater than the bacteria present within the aqueous phase and provides the first direct observation of the sessile population from the terrestrial deep subsurface. Time of Flight-Secondary Ion Mass spectrometry revealed that the fracture surface was actually coated with a thin, i.e., molecular, organic conditioning film over much of its surface that was separate from the exopolysaccharide layers associated with the mineral water interface and with some of the attached cells.

The Origin and Age of Biogeochemical Trends in Deep Fracture Water of the Witwatersrand Basin, South Africa

Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH 4 and hydrocarbon, occasionally N 2 , originally formed at ∼ 250–300°C and during cooling isotopically exchanged O and H with minerals and accrued H 2 , 4 He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼ 10 Ka to > 1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO 2 and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past ∼100 Myr, a process which inoculates previously sterile environments at depths > 2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 10 7 times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bioavailable chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The 4 He concentrations and theoretical calculations indicate that the H 2 that is sustaining the subsurface microbial communities (e.g. H 2 -utilizing SRB and methanogens) is produced by water radiolysis at a rate of ∼1 nM yr −1 . Microbial CH 4 mixes with abiogenic CH 4 to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from ∼ 100 at < 1 kmbls, to < 0.01 nM yr −1 at > 3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ13C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces.

English

Grootvlei Mine is located in the Far East Rand Basin, part of the Witwatersrand Basin, which has been actively mined for over 100 years. In order to maintain access to its reserves, Grootvlei dewaters approximately 75 Ml/day from its workings. The water is treated and discharged to the Blesbokspruit. A portion of the water pumped from underground is believed to originate from surface water. Technical assessments to identify the areas of ingress, quantities of ingress and possible measures to reduce the ingress are the subject of this paper. In addition, progress made with remediation works on one identified ingress area is discussed. The remedial works are being carried out by Grootvlei Mine, with partial funding provided by the Council for Geoscience. The site represents a difficult working environment due to the very wet conditions resulting from historical roadway construction and tailings spills.

Bioaccumulation of gold by sulfate-reducing bacteria cultured in the presence of gold(I)-thiosulfate complex

A sulfate-reducing bacterial (SRB) enrichment, from the Driefontein Consolidated Gold Mine, Witwatersrand Basin, Republic of South Africa, was able to destabilize gold(I)-thiosulfate complex ( Au ( S 2 O 3 ) 2 3 - ) and precipitate elemental gold. The precipitation of gold was observed in the presence of active (live) SRB due to the formation and release of hydrogen sulfide as an end-product of metabolism, and occurred by three possible mechanisms involving iron sulfide, localized reducing conditions, and metabolism. The presence of biogenic iron sulfide caused significant removal of gold from solutions by adsorption and reduction processes on the iron sulfide surfaces. The presence of gold nanoparticles within and immediately surrounding the bacterial cell envelope highlights the presence of localized reducing conditions produced by the bacterial electron transport chain via energy generating reactions within the cell. Specifically, the decrease in redox conditions caused by the release of hydrogen sulfide from the bacterial cells destabilized the Au ( S 2 O 3 ) 2 3 - solutions. The presence of gold as nanoparticles (<10 nm) inside a sub-population of SRB suggests that the reduction of gold was a part of metabolic process. In late stationary phase or death phase, gold nanoparticles that were initially precipitated inside the bacterial cells, were released from the cells and deposited in the bulk solution as addition of gold nanoparticles that already precipitated in the solution. Ultimately, the formation of micrometer-scale sub-octahedral and octahedral gold and spherical aggregates containing octahedral gold was observed.

Palaeoclimates: the first two billion years.

Earth's climate during the Archaean remains highly uncertain, as the relevant geologic evidence is sparse and occasionally contradictory. Oxygen isotopes in cherts suggest that between 3.5 and 3.2 Gyr ago (Ga) the Archaean climate was hot (55-85 degrees C); however, the fact that these cherts have experienced only a modest amount of weathering suggests that the climate was temperate, as today. The presence of diamictites in the Pongola Supergroup and the Witwatersrand Basin of South Africa suggests that by 2.9 Ga the climate was glacial. The Late Archaean was relatively warm; then glaciation (possibly of global extent) reappeared in the Early Palaeoproterozoic, around 2.3-2.4 Ga. Fitting these climatic constraints with a model requires high concentrations of atmospheric CO2 or CH4, or both. Solar luminosity was 20-25% lower than today, so elevated greenhouse gas concentrations were needed just to keep the mean surface temperature above freezing. A rise in O2 at approximately 2.4 Ga, and a concomitant decrease in CH4, provides a natural explanation for the Palaeoproterozoic glaciations. The Mid-Archaean glaciations may have been caused by a drawdown in H2 and CH4 caused by the origin of bacterial sulphate reduction. More work is needed to test this latter hypothesis.

The melt rocks of the Vredefort impact structure – Vredefort Granophyre and pseudotachylitic breccias: Implications for impact cratering and the evolution of the Witwatersrand Basin

The unique combination of its large size (250–300 km diameter), deep levels of erosion (>7 km), and widespread regional mining activity make the Vredefort impact structure in South Africa an exceptional laboratory for the study of impact-related deformation phenomena in the rocks beneath giant, complex impact craters. Two types of impact-generated melt rock occur in the Vredefort Structure: the Vredefort Granophyre – impact melt rock – and pseudotachylitic breccias. Along the margins of the structure, mining and exploration drilling in the Witwatersrand goldfields has revealed widespread fault-related pseudotachylitic breccias linked to the impact event. There, volumetrically limited melt breccia occurs in close association with cataclasite or mylonitic zones associated with bedding-parallel normal dip-slip faults that formed during inward slumping of the crater walls, and in rare subvertical faults oriented radially to the center of the structure. This association is consistent with formation of pseudotachylites by frictional melting. On the other hand, rocks in the Vredefort Dome – the central uplift of the impact structure – contain ubiquitous melt breccias that range in size from sub-millimeter pods and veinlets to dikes up to tens of meters wide and hundreds of meters long. Like fault-related pseudotachylites in the goldfields and elsewhere in the world, they display a close geochemical relationship to their wallrocks, indicating local derivation. However, although mm/cm- to, rarely, dm-scale offsets are commonly found along their margins, they do not appear to be associated with broader fault zones, are commonly considerably more voluminous than most known fault-related pseudotachylites, and show no consistent relationship between melt volumes and slip magnitude. Recent petrographic observations indicate that at least some of these melt breccias formed by shock melting, with or without frictional melting. Consequently, the non-genetic term “pseudotachylitic breccia” has been adopted for these Vredefort occurrences. These breccias formed during the impact in rocks at temperatures ranging from greenschist to granulite facies, and were subsequently annealed to varying degrees during cooling of the central uplift. In addition to the pseudotachylitic breccias, nine clast-laden impact melt dikes (Vredefort Granophyre), each up to several kilometers long, occur in vertical radial and tangential fractures in the Vredefort Dome. Unlike the pseudotachylitic breccias, they display a remarkably uniform bulk composition and clast populations that are largerly independent of their wallrocks, and they contain geochemical traces of the impactor. They represent intrusive offshoots of the homogenized impact melt body that originally lay within the crater. U–Pb single zircon and Ar–Ar dating indicates that the Vredefort Granophyre and pseudotachylitic breccias, and the Witwatersrand pseudotachylites all formed at 2020±5 Ma – the age of the impact event, making the breccias a convenient time marker in the evolution of the structurally complex Witwatersrand basin with its unique gold deposits.

Geochemically Generated, Energy-Rich Substrates and Indigenous Microorganisms in Deep, Ancient Groundwater

Recent studies have shown that the biosphere extends to depths that exceed 3 km, raising questions regarding the age of the microbes in these deep ecosystems and their sources of energy for metabolism. Abiogenic energy sources that are derived from in situ, purely geochemical sources and thus independent from photosynthesis have been suggested. We sampled saline fracture water emanating from a 3.1-km deep borehole in a Au mine in the Witwatersrand Basin of South Africa and characterized the chemical constituents (including stable isotopes), groundwater age, and indigenous microorganisms. Salinity data and ratios of dissolved noble gases indicate that extremely ancient (2.0 Ga) saline fracture water has mixed with meteoric water to yield an average subsurface residence time of 20–160 Ma, the oldest age of any waters collected to date in the Witwatersrand Basin. H2 isotope data suggest the water originated from a depth of 4 to 5 km. Sulfur isotope fractionation indicates biological sulfate reduction. Calculations of free energies and steady state energy fluxes based on water chemistry data also support sulfate reduction as the dominant terminal electron accepting process. Lipid and flow cytometry data indicate a sparse microbial community (103 cells ml−1), despite the presence of relatively high concentrations of energy-rich compounds (H2, CH4, CO, ethane, propane, butane, and acetate). The H2 can be explained by radiolysis of water. Stable isotopic signatures of the CH4 and short chain hydrocarbons indicate abiogenic synthesis. The persistence of energy-rich compounds suggests that other factors are limiting to microbial metabolism and growth, e.g., availability of an inorganic nutrient, such as Fe or phosphate.

The effect of thiosulfate-oxidizing bacteria on the stability of the gold-thiosulfate complex

An Acidithiobacillus thiooxidans spp., isolated from the Driefontein Consolidated Gold Mine, Witwatersrand Basin, Republic of South Africa was able to precipitate gold from Au(S 2O 3) 2 3− in the presence of up to 0.26 mM gold. In chemical control experiments and with the presence of dead bacteria, gold was not precipitated under similar experimental conditions and duration. During growth, the pH of the culture medium decreased from pH 5.4 to 1.9, while the Eh increased from 0.3 to between 0.5 to 0.6 V within a period of 75 days. In the active (live) bacterial culture systems, acid production enhanced thiosulfate disproportionation, after which the elemental sulfur and any other intermediate sulfur species were oxidized completely to sulfate. The gold, Au(S 2O 3) 2 3−, was stable in the bacterial systems until sulfur oxidation was complete; then the bacteria precipitated gold from Au(S 2O 3) 2 3−. The bacterial systems (0.02–0.26 mM gold) precipitated 87 to 100% of the gold under diurnal light exposure, while only 11 to 69% of the gold was precipitated in the dark. The presence of gold (≥0.08 mM) reduced bacterial growth, disrupted cell division causing cell elongation, and was ultimately toxic to this bacterium, killing the cultures. The gold was precipitated inside the bacterial cells as fine-grained colloids ranging between 5 and 10 nm in diameter and in the bulk fluid phase as crystalline micrometer-scale gold. Observations using transmission electron microscopy revealed that the gold was deposited throughout the cell; however, it was concentrated in the cell envelope, especially along the cytoplasmic membrane, suggesting that gold precipitation was likely enhanced via electron transport processes associated with energy generation. Seven months after population growth had stopped, the gold had formed coiled or wire gold, irregular and rounded structures with an approximate size ranging from 0.5 to 5 μm, and crystalline octahedral gold.

Radiolytic H2 in continental crust: Nuclear power for deep subsurface microbial communities

H2 is probably the most important substrate for terrestrial subsurface lithoautotrophic microbial communities. Abiotic H2 generation is an essential component of subsurface ecosystems truly independent of surface photosynthesis. Here we report that H2 concentrations in fracture water collected from deep siliclastic and volcanic rock units in the Witwatersrand Basin, South Africa, ranged up to two molar, a value far greater than observed in shallow aquifers or marine sediments. The high H2 concentrations are consistent with that predicted by radiolytic dissociation of H2O during radioactive decay of U, Th, and K in the host rock and the observed He concentrations. None of the other known H2‐generating mechanisms can account for such high H2 abundance either because of the positive free energy imposed by the high H2 concentration or pH or because of the absence of required mineral phases. The radiolytic H2 is consumed by methanogens and abiotic hydrocarbon synthesis. Our calculations indicate that radiolytic H2 production is a ubiquitous and virtually limitless source of energy for deep crustal chemolithoautotrophic ecosystems.

Patterns of sedimentation in the Precambrian

The principle of uniformitarianism may be applied to Precambrian basin evolution and to the sedimentary record as a whole. The major difference in the Precambrian Eon lay in variability of rates and intensities of processes controlling weathering, erosion, transport, deposition, lithification, and diagenesis. This paper examines Precambrian sedimentation patterns within the larger framework of Earth evolution. Pre-rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis, resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension, all reworked by marine current systems and localised turbidites. From c. 4 to 3.2 Ga, greenstone belts provided the predominant settings for the thin passive margin carbonates, BIF, stromatolitic evaporites, pelites and quartzites, and lesser synorogenic turbidites, conglomerates, and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings. Common palaeoenvironments were high gradient alluvial fans, low sinuosity braided rivers, and relatively shallow marine settings, subject to wave and tidal action, and turbidity currents. Although continental crustal growth continued largely through greenstone belts until c. 2.7 Ga, the Witwatersrand basin (c. 3.0–2.7 Ga; Kaapvaal craton, South Africa) reflects initial stabilisation of the oldest craton, with an epeiric sea accumulating largely fluvial detritus subject to tidal (inland) and storm-wave (craton-marginal) reworking within a retroarc foreland basin setting. Neoarchaean–Palaeoproterozoic sedimentation is discussed within a framework of two global “superevents”, at c. 2.7 Ga and 2.2–1.8 Ga, each encompassing major changes in Earth's evolution related to the supercontinent cycle, mantle superplumes, peaks in crustal growth rates, and significant biochemical changes within the atmosphere–hydrosphere system. Concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed, and deposition of iron-formation peaked globally at c. 2.5 Ga. The first global glaciation at c. 2.4–2.2 Ga provides little support for the “Snowball Earth” theory, but does suggest a negative feedback loop model whereby intraglacial CO 2-related warming and synglacial decreases in weathering alternated up to three times. Models for palaeo-atmospheric and -oceanic evolution are mutually exclusive, but by c.1.8 Ga, the existence of large landmasses and free oxygen in Earth's atmosphere enabled erg development and red bed sedimentation globally; the full spectrum of Phanerozoic–Modern sedimentary environments was thus present on Earth. A third postulated “superevent” at c. 0.8–0.6 Ga essentially recreated conditions experienced at c. 2.2–1.8 Ga, with, additionally, at least three global refrigeration events.

Kaapvaal craton: Changing first- and second-order controls on sea level from c. 3.0 Ga to 2.0 Ga

Well preserved supracrustal basin-fills including the oldest large depository (Witwatersrand) known makes the Kaapvaal craton an ideal example for studying controls on first- and second-order stratigraphic cyclicity. The duration of the three basins studied (Witwatersrand, Ventersdorp and Transvaal), from c. 3.0 to c. 2.0 Ga is almost twice that of the entire Phanerozoic period, from which the paradigms of classical sequence stratigraphy have been derived. Amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through Earth history. This does not appear to be borne out by the Precambrian record studied here; instead, a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first- and second-orders is preferred. First-order cycles within the c. 3.0–2.0 Ga period on Kaapvaal are synonymous with the entire evolution of a supracrustal basin-fill defined by a specific basin-type; second-order cycles define basin-fill packages of unique character, which reflect large scale shifts in the accommodation–sedimentation balance. Early to Middle Archaean evolution of the Kaapvaal craton encompassed amalgamation of a mosaic of subdomains, to produce an early south–southeastern nucleus by c. 3.1 Ga. Accretion of composite terranes from both north and west followed from c. 3.0–2.7 Ga; this was contemporaneous with evolution of the c. 3.0–2.8 Ga Witwatersrand basin. The high-grade Limpopo terrane was amalgamated with the Kaapvaal craton between 2.95 and 2.69 Ga, although a younger age for this orogensis is also possible. The Witwatersrand first-order depositional cycle was controlled largely by subduction-related tectonic loading and had a duration of c. 190 My. The c. 5 My first-order cycle of the c. 2.7 Ga Ventersdorp basin was essentially controlled by thermal uplift-induced extensional subsidence, and that of the c. 650 My Transvaal basin by extensional and subsequent thermal subsidence. It is noteworthy in the three basins examined that first-order cycles controlled by plate tectonics (Witwatersrand and Transvaal basins) were about two orders of magnitude longer relative to the Ventersdorp “plume tectonic” cycle; second-order cycles also varied greatly, from c. 100 My to c. 1 My. Within the Witwatersrand and Transvaal basins, subsidence allowed extensive seaways to develop, within which classical sequence stratigraphic systems tracts can be recognized, reflecting different stages of shoreline shifts. Within the short-lived Ventersdorp continental basin system, sedimentological criteria can be used to discriminate high and low accommodation systems tracts. The Transvaal first-order cycle was terminated at c. 2.0 Ga by the Bushveld Complex plume event.