Saturday, March 14, 2015

Earth's biggest gold deposit formed by ancient microbes?

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.

However, the Kaapvaal Craton is located in the Limpopo Province in north-east South Africa. How did the gold move several hundred kilometres from there to get to the Witwatersrand basin in Gauteng?
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 volcaneos, rising to form clouds that then fell as acid rain on the mountains.

In a new study published in Nature Geoscience entitled, "Witwatersrand gold deposits formed by volcanic rain, anoxic rivers and Archaean life", Christoph Heinrich, a professor of Economic Geology, argues that the gold didn't just get moved by the rivers – it was first dissolved chemically in volcanic rain. 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 new theory may explain why there's a string of gold beds in the Witwatersrand basin that collectively make up 40 percent of all of the gold that has ever been, or ever will be, dug out of the ground.
Gold is a rare element in the universe that forms only in the hearts of supernovae. The precious metal has been part of Earth since its birth 4.6 billion years ago, and while most of the Earth's gold is locked deep within the planet's core, the rest is largely dispersed throughout rocks at incredibly tiny concentrations of about one part gold per billion.

Occasionally, a physical phenomenon causes the gold to become enriched in certain layers of rock. In the case of the Witwatersrand formation, up to 1 percent of the carbon-rich layers is made up of gold.

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