An ocean of sediment, a fortune of gold
The Witwatersrand gold begins nearly three billion years ago on the Kaapvaal Craton, one of Earth's first stable microcontinents. An immense inland sea, often called the "Witwatersrand Sea," covered a vast area. For about 260 million years, starting around 2.97 billion years ago, powerful river systems flowed into this basin from highlands to the north and west. These rivers eroded ancient granite-greenstone mountains, which held gold.
The prevailing scientific explanation for the gold's concentration is the "modified placer theory". The initial process was mechanical. The rivers carried gold particles and other heavy minerals, like pyrite and uraninite, into the basin. As the rivers entered the calmer waters of the sea, they deposited their sediment loads in massive fan deltas with many braided channels. The heavy gold particles, along with quartz pebbles, settled out and were concentrated in layers of conglomerate rock. This process laid down the Witwatersrand Supergroup, a sequence of sedimentary rocks between 5,000 and 7,000 meters thick. Radiometric dating shows the gold itself is older than the sediments it rests in; gold grains have been dated to 3.01 billion years old, while the host conglomerates are between 2.76 and 2.89 billion years old.
Buried, cooked, and altered
After the sediments were deposited, the process continued. Around 2.7 billion years ago, massive volcanic eruptions covered the entire basin with several kilometers of lava. This burial, combined with later tectonic activity, subjected the gold-bearing conglomerates to intense heat and pressure. This is the "modified" part of the placer theory. Hot, mineral-rich fluids (hydrothermal fluids) circulated through the porous conglomerate layers. These fluids dissolved the original detrital gold flakes and reprecipitated them over short distances. This process altered the shape of the gold, sometimes giving it a crystalline appearance, and cemented it firmly within the conglomerate matrix, known locally as "banket".
This multi-stage process of deposition and modification created the world's largest known gold deposit. Since its discovery in 1886, the Witwatersrand Basin has produced over 50,000 tonnes of gold, which accounts for as much as 40% of all the gold ever mined globally. Some of the gold-bearing reefs are now pursued to extreme depths. The Mponeng Gold Mine, the deepest in the world, operates at depths greater than 4 kilometers below the surface, where rock temperatures can reach 60°C.