Witwatersrand Basin

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.

Witwatersrand (meaning 'white water ridge' in Afrikaans) is often called simply "the Rand," and is located in the Gauteng province (formerly a part of Transvaal) of South Africa.

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.