The Witwatersrand Basin is a geological formation in the Witwatersrand, South Africa. It holds the world's largest known gold reserves and has produced over 1.5 billion ounces (over 40,000 metric tons). The basin straddles the old provinces of Transvaal and the Orange Free State and is of the same period as the Vredefort impact of 2.023 Ga ago, and the Bushveld Igneous Complex.
Nearly half of all the gold ever mined has come from the extensive Witwatersrand Basin that was first found near Johannesburg in 1886. The gold occurs in reefs, or thin bands, that are mined at depths of down to 4,000 m – Mponeng gold mine currently being the world's deepest. Although many of the older mines are now nearly exhausted, the Witwatersrand Basin still produces most of South Africa's gold and much of the total world output. Silver and iridium are recovered as gold-refining byproducts, and the basin also has coal mines, although they are bit players in the overall mining of the Basin.
This sedimented cratonic basin covers an elliptical area with a 300 km long major axis from Delmas in the north-east to Theunissen in the south-west, with a small subsidiary basin at Kinross. Dotted outside the basin are older Archaean granites of between 3 and 3.2 Ga, some of which are exposed while others are covered by the much younger Karroo System. The Witwatersrand System is a sequence of shales, quartzites and conglomerates ranging in age from 2.7 Ga for the Hospital Hill subgroup to 2.4 Ga for the Turffontein subgroup. The Lower Witwatersrand is composed mainly of argillaceous clays and shales with occasional banded ironstone, a tillite and an intercalated lava flow, while the Upper Witwatersrand consists almost entirely of quartzites and conglomerates, with its own volcanic horizon.
The vast majority of the Earth's gold and other heavy metals are locked up in the core. Evidence from tungsten isotope studies indicates that most gold in the crust is derived from gold in the mantle which resulted from a meteorite bombardment some 3.9 billion years ago. The gold bearing meteorite event occurred millions of years after the segregation of the earth's core.
The gold in the Witwatersrand Basin area was deposited in ancient river deltas, having been washed down from surrounding gold-rich greenstone belts to the north and west. Rhenium-osmium isotope studies indicate that the gold in those mineral deposits came from unusual three billion year old mantle-derived intrusions known as komatiite, present in the greenstone belts. The Vredefort Dome impact which lies within the basin and the nearby Bushveld Igneous Complex are both about a billion years younger than the interpreted age of the gold.
Gold mines operating in the Witwatersrand Basin
On the East Rand, Ergo is run by DRDGold Limited, a company involved in the extraction of gold and uranium residue from slimes dams and sand dumps on the Central, East and Far East Rand, and to produce sulphuric acid.
On the West Rand, Mponeng, Savuka and TauTona, formerly known as Western Deep Levels, are managed by AngloGold Ashanti, and Kusasalethu (previously Elandsrand) and Doornkop Mines are managed by Harmony Gold. South Deep, Kloof and Driefontein mines are operated by Goldfields  and Mintails Limited.
|Gold Production on the Witwatersrand
1898 to 1910:134
|1899 (Nov- 1901 Apr)||12||574,043||£2.02||£908|
- Relationships between the Vredefort structure and the Witwatersrand basin within the tectonic framework of the Kaapvaal craton as interpreted from regional gravity and aeromagnetic data
- Mining Weekly
- Battison, Leila; Meteorites delivered gold to Earth, BBC News, Science & Environment, 8 September 2011 
- Kirk, Jason; Joakin Ruiz; John Chesley and Spencer Titley; The Origin of Gold in South Africa, American Scientist, Vol 91, Nov.-Dec 2003, pp. 534–531 
- Yap, Melanie; Leong Man, Dainne (1996). Colour, Confusion and Concessions: The History of the Chinese in South Africa. Hong Kong: Hong Kong University Press. p. 510. ISBN 962-209-423-6.
- Measuring Worth, Relative Value of a UK Pound Amount – average earnings, retrieved on 27 January 2011