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.