South Africa's Witwatersrand Basin contains the largest region of gold deposits in the world. However, how did all that gold get there?
It turns out that a lot of stinky, sulfur-filled air played a large role in bringing gold to the region.
Christoph Heinrich, professor of Mineral Resources at ETH and the University of Zurich, recently explained how that process created an area that still contains hundreds of tons of gold deposits just lying beneath the Earth's surface.
Scientists have long debated the source of Witwatersrand's gold. One widely held theory is that it came via a river sediment, such as the gold found in California during the gold rush era. That theory holds that a river transported the gold, concentrating its particles into nuggets.
However, there's a problem with that theory. At Witwatersrand, this process would need sediments of at least six miles thick to create the temperature and pressure required for this process. Evidence of such a layer of sediment, however, does not exist there.
Instead, Heinrich proposes that this region saw gold becoming concentrated on the Earth's surface, rather than beneath it. A flowing river still played a part, but the gold dissolved in the water and was then later "collected" into rocks.
For this to work, though, rainwater needs high levels of hydrogen sulphide or sulfur. However, sulfur and gold aren't stable unless the environment is free of oxygen. This means that the processes creating the gold at Witwatersrand probably took place early in the Earth's lifespan, around 3 billion years ago.
"Quite inhospitable environmental conditions must have dominated, which was possible only three billion years ago during the Archean eon," said Heinrich. "It required an oxygen-free atmosphere that was temporarily very rich in hydrogen sulphide -- the smell of rotten eggs."
Volcanic activity at the time also played a part by increasing the sulfur in rainwater in the area. Scientists already have evidence of volcanic eruptions throughout regions of South Africa that align with this time period.
A third factor in the formation of Witwatersrand's gold deposits was a bacteria that grew on the bottom of lakes, assisting in dissolving the gold there.
"It's possible that these primitive organisms actively absorbed the gold," said Heinrich. "But a simple chemical reduction of sulphur-complexed gold in water to elementary metal on an organic material is sufficient for a chemical 'gilding' of the bottom of the shallow lakes."
