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Thin layers of gold and pyrite cut through this argillite from the Witwatersrand Basin. Jason Kirk, a graduate student at the University of Arizona in Tucson, and colleagues found that gold in samples similar to this one is older than the surrounding rock. They report that their finding indicates the gold first formed outside the basin and then washed into the basin many millions of years later. Image courtesy of Lori Stiles, University of Arizona.
“Our results support a placer model: the gold is derived from source rocks north and west of the basin and transferred by streams and rivers into the basin and converted into rock,” says Jason Kirk, a graduate student at the University of Arizona in Tucson and lead author of the paper.
Rivers filled the basin with sediments between 2.7 and 2.9 billion years ago. Today the gold lies within thin sedimentary layers, or reefs, that wrap around the edge of the basin and dive to depths of 5,000 meters or more.
If the placer model is correct, prospectors looking for higher ore grades within the basin should concentrate on sediments that accumulated under calm sections of the ancient rivers. Most of the gold would have fallen out in these areas.
Phillips, G.N., and Law, Jonathan D.M., 2000. Witwatersrand goldfields: geology, genesis and exploration. pp439-500 in S.G. Hagemann and P.E. Brown (eds), Gold in 2000. Reviews in Economic Geology, 13. Soc. Econ. Geol.
The Witwatersrand goldfields of South Africa account for more than a third of the world's total gold production since mining started there in 1886, or 48,000 t Au. These goldfields have dominated production throughout the 20th century with a peak of 1000 t produced in 1970, and a steady decline since that time as the Witwatersrand enters its twilight period.
The regional geological framework is well understood as a consequence of intensive study and virtually unrivalled 3D access from drilling and underground opening supplemented by seismic data. Advancements of the last decade have come from the integration of stratigraphy, sedimentology, structural evolution, and a much greater appreciation of thermal and fluid processes.
Gold has been produced from seven major goldfields concentrated around the northern and western margin of the 350 km long Witwatersrand Basin. Each goldfield consists of one major and commonly several minor reef horizons that have been mined semi-continuously for up to 400 square kilometres. Economic mineralization varies from 1 cm to several metres in thickness, and the host rock varies from quartz pebble conglomerate and carbon seam, to polymictic conglomerate and pyritic quartzite. All reefs are either on or within a few metres above unconformity surfaces, and the major reefs are part of a very distinctive reef package of footwall, unconformity, conglomerate, quartz-rich sandstone and/or shale. The actual distribution of gold has been poorly represented in the literature due to grade coding, inappropriate averaging, and omission of the very features that are critical to understanding the controls on the gold. High grades and remarkable lateral continuity have nevertheless facilitated economic exploitation. The mineralogy of the Witwatersrand reefs is dominated by pyrite with lesser pyrrhotite and arsenopyrite, widespread nickel and cobalt sulfarsenides, and low base metal sulfides. A distinctive mineral assemblage of pyrophyllite – chloritoid – muscovite – chlorite – quartz – rutile – pyrite is found in and around the reef package in all goldfields. This assemblage has been used to constrain peak metamorphic temperatures to just above 300oC to the south and east of the Basin, and closer to 400oC in the NW corner west of the Basin near Johannesburg. Two characteristics of the metamorphic event are the near- strata parallel distribution of metamorphic grade, and an extremely high geothermal gradient. The presence of pyrophyllite and chloritoid in conglomerate, quartzite, shale, basalt flows and some dikes has been used to indicate an alteration halo embracing the goldfields and of approximately 300km by 50km (into the Basin) by 3km of sequence. Quartz veins to a few centimetres thickness are common within this alteration halo especially in the reef horizons, but veins of metres thickness are rare. Ubiquitous pyrite throughout the alteration zone indicates elevated levels of sulfur in solution. The strongest chemical associations with gold involve Fe and C; uranium is also associated with many reefs and extensively mined as a co-product.
The origin of the Witwatersrand has been of great interest to economic geologists, and a source of great contention. Historically, there have been placer and hydrothermal theories with much more regional and mine-scale data being used in the debate over the last decade. The 1980s and especially the 1990s has seen a shift from the unmodified to modified placer model, and a more important shift from both placer models to a hydrothermal model. Challenges still facing the placer models include the regional alteration, structural control on gold, the chemical process of gold remobilization and the poor match with modern gold placer processes. However, the outstanding issues remain the demonstration of a viable source area for the gold, and a convincing demonstration that processes during sedimentation concentrated gold at all.
The hydrothermal replacement model invokes transport of gold into the Witwatersrand Supergroup during metamorphism and associated widespread alteration in a reduced, low salinity fluid analogous to other gold-only deposits. Fluids were channeled by structures, unconformity surfaces and bedding, and gold precipitation was dominated by reaction with carbon- and/or iron-bearing rocks. These Fe- and C-rich rocks are concentrated immediately above unconformity surfaces. The model proposes that the source of gold as crustal rocks (probably mafic) beneath the Witwatersrand Supergroup. The hydrothermal model links the tectonic evolution of the basin to mineralizing processes, and can thus be used to target other basins with potential for similar mineralization.
From the hydrothermal model arise a number of factors that contribute to the enormous size of the Witwatersrand goldfields.
The age of Witwatersrand Basin predating a major period of gold introduction.
The retro-arc foreland basin.
Major thrust structures around the basin margin.
Unconformities in the upper Witwatersrand.
Iron-rich nodules on unconformity surfaces.
Carbon distribution.
High geothermal gradient during gold deposition.
Witwatersrand metamorphism and regional alteration.
Limited erosion.
Special source regions for detritus and special sorting processes during sedimentation do not appear to have been critical. The Witwatersrand gold formed by the optimization of a set of processes found in other gold provinces, not by unique processes unrepresented elsewhere.
The full article on the Witwatersrand Goldfields is published in SEG Reviews, vol. 13, 2000, p. 439-500. The paper was originally presented at the ‘Gold in 2000' Short Course held in Reno in November last year. The authors have also compiled a series of slides on CD in Microsoft Powerpoint format with a brief commentary on the key points. Many of these slides formed the basis of the Reno presentation but this more detailed compilation is aimed specifically at 3rd and 4th year economic geology students and could probably be easily incorporated into a lecture series. Copies of the article and CD are available from Jonathan Law (mail to PO Box 147, South Melbourne, VIC, 3205, Australia; email to law_jonathan@hotmail.com) or Neil Phillips (oria@enternet.com.au).
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Kirk, J., Ruiz, J., Chesley, J., Titley, S. and Walshe, J., in press. A detrital model for the origin of gold and sulfides in the Witwatersrand Basin based on Re-Os isotopes. Geochim. Cosmochim. Acta.
The Re-Os systematics of gold and sulfides from the Witwatersrand basin were utilized to determine whether the gold is detrital or was introduced by hydrothermal solutions from outside the basin.Gold from a gravity concentrate from the Western Areas Gold Plant and gold from the Vaal Reef have very high Os concentrations of approximately 73-10,000 ppb and 3-32 ppb Re, resulting in 187Re/188Os ratios of 0.01 to 0.19. The gold has sub-chondritic 187Os/188Os ratios between 0.1056-0.1099 and an average value of 0.1067. Rhenium depletion ages (TRD) range from 3.5 Ga to 2.9 Ga, with a median age of 3.3 Ga.
Pyrite from the Vaal Reef have Os concentrations ranging from 0.26-0.68 ppb, Re-concentrations of 1.7-2.8 ppb and 187Re/188Os ratios of approximately 13-83. The pyrite samples have measured 187Os/188Os ratios of 0.84-4.1 and define an isochron with an age of 2.99 ± 0.11 Ga (MSWD = 0.77).
The Os isotopic data from the direct measurement of gold preclude introduction of gold to the Witwatersrand basin from crustally derived metamorphic or hydrothermal fluids between 2.7-2.0 Ga. The unradiogenic 187Os/188Os ratios, old TRD ages of the Western Areas and Vaal Reef gold samples, as well as the contemporaneously old age of the Vaal Reef pyrite are consistent with detrital deposition of gold during the formation of the Witswatersrand basin. The Os data will allow for minor hydrothermal remobilization and/or overprinting of hydrothermal gold on pre-existing detrital gold grains but does not support the introduction of gold solely by hydrothermal fluids.
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