A geologist has come up with a new theory about the Earth's last known gold reserve, which could give mankind a new method for finding gold elsewhere.

Gold was discovered in South Africa's Witwatersrand Basin in 1886, and, since then, around half of all gold mined throughout the world has originated from that source.

Geologists know that the gold reached the Earth's surface by coming up with lava that formed the Kaapvaal Craton mountain range, which is one of the only remaining rock crusts on Earth that formed millions of years ago.

However, the Kaapvaal Craton is located in the Limpopo Province in north-east South Africa, so how did the gold move several hundred kilometres from there to get to the Witwatersrand basin in Gauteng?

How gold formed in Witwatersrand Basin

The most common theory is that, over billions of years, the gold-bearing veins in the mountain range eroded down to the rivers, where the sediment was transported down to the shallow lakes and waterfalls in the basin.

However, Christoph Heinrich, a professor of Economic Geology at the Swiss Federal Institute of Technology, completely disagrees.

In a new study published in Nature Geoscience entitled, "Witwatersrand gold deposits formed by volcanic rain, anoxic rivers and Archaean life", Heinrich argues that the gold didn't just get moved by the rivers – it was first dissolved chemically in volcanic rain.

The dissolved chemical compound of gold and acidic rain was then washed to the basin, where mats of microbes growing in the shallow pools precipitated the gold out of the water.

"We don't know if the gold precipitated out during life or after they died, but basic chemistry tells us that organic life reduces gold chemically from the ionic to the elemental form," Heinrich told New Scientist.

Gold possible due to sulphuric rain

Three billion years ago, the atmosphere contained almost no oxygen and the air consisted primarily of gases containing sulphur. Gases like hydrogen sulphide were pumped out of volcanos, rising to form clouds that then fell as acid rain on the mountains.

Heinrich's theory is that the gold would have formed soluble complexes with the sulphur, which were then absorbed by water, before finally being separated by primitive microbes to form the gold.

The theory works - provided it all happened three billion years ago, because half a billion years later, algae and cyanobacteria began producing oxygen during the Great Oxidation Event (GOE).

"Oxygen would have 'killed' the sulphur compounds that carried the gold," said Heinrich. "The billion-dollar question is whether the same process created other gold deposits."

Heinrich believes that apart from searching for gold in gravel-rich areas like the Gold Country of California (where the California gold rush happened), people should look in areas rich in carbon.

"I would look for carbon because if I'm right, carbon is an essential part of it [gold]. So I would maybe look for carbon-rich shale in the same lake-type environments but without the gravel," he said.