EARTH
The Witwatersrand Gold Field
~2.9 to 2.78 billion years ago
The Witwatersrand basin, in South Africa, is the source of more than a third of all the gold ever mined, and this gold was deposited in river beds between 2.9 and 2.78 billion years ago.
The Witwatersrand basin is a depression filled with sediments, in South Africa, that has produced more than a third of all the gold ever mined. The gold accumulated in the beds of braided rivers, rivers divided into many channels, on a continent of the Archaean, an early eon of the Earth. Microbial mats, thin sheets of microbes, may first have caught the gold along the shore. Researchers have argued for a century over two explanations of the deposit. The first is a detrital placer, in which water washed in grains of gold and concentrated them. The second is a hydrothermal deposit, in which hot fluids brought the gold in later. The explanation now favoured combines the two: water transported the grains, and fluids rearranged them afterwards.
Why it matters
When a continent weathers, that is, when its rock breaks down, under an atmosphere devoid of oxygen, metals are concentrated in ways that no later period can reproduce. The mineral wealth of Johannesburg is a consequence of the composition of the air in the Archaean.
Dating & uncertaintyDisputed
Researchers agree that the gold lies in ancient river gravels. They have argued for a century over how it came there. The gold may have arrived as grains. Hot fluids may have precipitated it later, that is, deposited it from solution. Or microbial mats may first have trapped it, before it was reworked, moved and deposited again. The best-supported reading combines the first two: rivers delivered grains, and fluids rearranged them afterwards.
Earth history as one day
08:40:1015 h 20 min before midnight
if Earth’s 4.54 billion years were compressed into a single day