Overberg Geoscientists Group
2900 Mya
Witwatersrand Basin formation
Gold has been mined in the Witwatersrand Basin since 1886. It is the largest Gold Field ever found in the world.
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The Kaapvaal craton, the centre of oldest continent on Earth had stabilized by around 3,100 million years ago. There was no further activity that produced ocean floor volcanic rock, and the continent was stable. There was some subsidence, and marine deposits started to be laid down, these were the products of the erosion of mountains that had formed to the north and west. These were initially sands which formed the West Rand Group sediments. They became the hard, resistant quartzites that form the ridge running trough Gauteng and known as the Witwatersrand. The basin continued to subside and large amounts of sediments, muds and sands, were brought into it by the rivers mainly from the north and west.
As the sediments started to fill the Witwatersrand basin, the sea retreated and the rim became the leaving a wide almost flat coastal plain over which rivers from the north formed wide braided river deltas, into some of which rich deposits of gold were deposited.
The resulting 2500 m thick layer of rock is termed the Central Rand Group.
Gold was first found in the southern portion of the Witwatersrand ridge on the farm Langlaagte in 1886, 5 km west of Johannesburg. It was found in a conglomerate which represented one of the braided rivers across the plain. It continued down dip and could also be traced east and west, thus it was a large area of pebbles at the time of deposition. Gold and other heavy minerals were brought down by the rivers and deposited in the pebbly conglomerates.
The main gold mining areas are demarcated on this image. More than 50,000 tns of gold has been produced since that initial discovery, that is more than 30% of the gold ever mined on Earth.
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Some of the gold bearing beds consist of carbon strands with the gold embedded in it. The origin of the carbon is not known.
There is also controversy over how the gold was introduced into the beds. At one end, there is the sedimentary origin as outlined here. The other end of the spectrum the hydrothermal origin of the gold is posited. This states that the gold was introduced in hot mineral waters and crystalized out in the conglomerates. Indeed, some of the gold fragments are so fine and filigree that it is unlikely they could have been washed down by rivers. The real answer may lie somewhere in the middle, with gold having been deposited by the rivers and them hot waters may have then remobilized some, or even introduced more.
References.
McCarthy, T.; Rubridge, B. (2005). The Story of Earth and Life. Cape Town, S.A.: Struik Publishers.
Norman, N.; Whitfield, G. (2006) Geological Journeys. pp. 38–49, 60–61. Cape Town: Struik Publishers.
Beukes, N. & Nelson, J., 1995. Sea-level fluctuation and basin subsidence controls on the setting of auriferous palaeoplacers in the Archaean Witwatersrand Supergroup: a genetic and sequence stratigraphic approach
Dankert, B. & Hein, K., 2010. Evaluating the structural character and tectonic history of the Witwatersrand Basin. Precambrian Research,
Handley, J., 2004. Historic overview of the Witwatersrand Goldfields. Howick: Handley.
Robb, L.J., and Meyer, F., 1995. The Witwatersrand Basin, South Africa: Geological framework and mineralization processes. Ore Geol. Rev. 10(2)
Viljoen,MJ and Reimold, UW. 1999 An Introduction to South Africa’s Geological and Mining Heritage. Mintek, Johannesburg.
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D J Mourant Aug 2023