Andrew Watson reports that the gold fields of the Witwatersrand Basin in South Africa are, by a landslide, the largest single gold producing district in the world. It has produced some 2 billion ounces over a century of mining and at an average grade of 15 g/t Au with a current head grade of 6-10 g/t Au and they still have estimated reserves of some 1,161 billion ounces (36,000 tonnes). Production does not come easy though. Shafts go down some 3,900 meters below the surface, where temperatures reach 60° Celcius (140° F) and deadly rockfalls are common. Still, they support many of the largest gold companies in the world including Harmony, Sibanye, Goldfields, and AngloGold-Ashanti.
Layers of sedimentary rock in the Witwatersrand show that is was once a shallow sea. This sea was formed approximately 3 billion years ago after the collision between the Zimbabwe and Kaapvaal cratons. Ancient river systems feeding into the basin eroded the surrounding highlands and over a period of about 260 million years, deposited layers of quartz pebble conglomerates, sandstones and shales. The conglomerates are rich in pyrite (iron sulfide) pebbles and uraninite (uranium oxide) pointing to an anoxic atmosphere. Basin infilling resulted in more than 11 kilometers of sedimentary rocks and some volcanic rocks. The basin was then overlain by the “Ventersdorp and Transvaal supergroups” (see map below) which buried the gold-bearing sequence under 23 kilometres of sediments and volcanic rocks. Finally, the area was preserved when a massive meteorite hit and formed the Vredefort dome. This pushed up a core area but pulled the surrounding crust down and protected the basin from erosion.
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