EARTH

The Greatest Gold Field

~2.9 to 2.78 billion years ago

Archean · Mesoarchean

One South African basin has produced more than a third of all the gold ever mined. The gold gathered in the beds of braided rivers on an Archean continent, perhaps caught first by microbial mats along the shore; a century of argument has settled into a compromise, grains washed in by water and rearranged later by hot fluids.

Why it matters

A continent weathering under an atmosphere without oxygen concentrates metals in ways no later world could repeat. The wealth of Johannesburg is a consequence of Archean air.

Dating & uncertaintyDisputed

That the gold sits in ancient river gravels is not in doubt. Whether it was washed in as grains, precipitated later by hot fluids, or first trapped by microbial mats and then reworked, has been argued for a century. The best-supported reading combines the first two: grains carried in by rivers, rearranged afterwards by fluids.

Sources

1. Frimmel (2019), Regional Geology Reviews · The Witwatersrand Basin and its gold deposits
2. Frimmel & Hennigh (2020), Geology of the World's Major Gold Deposits and Provinces · Geologic evidence of syngenetic gold in the Witwatersrand goldfields
3. Tucker et al. (2016), Episodes · A review of the Witwatersrand Basin: the world's greatest goldfield

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