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Ancient Microbes Formed Earth’s Biggest Hoard of Gold

There have been some big breakthroughs in the world of science lately regarding how gold was formed in the early earth atmosphere.

We think mankind does most the work when it comes to extracting gold from the Earth, but it could be that Nature is the true workhorse. The gold miners during the California Gold Rush mined came originally from igneous rocks, where it was sparsely distributed among other minerals. Erosion over time freed the gold and concentrated it as water carried away lighter particles.

That is not the only way for gold to become concentrated enough to be of value. Another way entails the movement of water, heated by magma deep below the surface, dissolving and carrying minerals – like the gold – as it rises through rock. As it cools and traverses fractures, which act as little highways, the dissolved minerals “precipitate out to form rich veins.”

South Africa’s Witwatersrand Basin is among the world’s best gold deposit.  The Vaal Reef deposit, for example, has produced three thousand tons of gold or $100 billion at today’s prices. Geologists still debate, to be sure, about how gold got there – riverine placer or hydrothermal precipitation are the two options outlined above.

Gold is discovered in layers within materials which once acted as ancient sediment laid down by rivers, however these rivers flowed roughly three billion years ago. The deposits have become metamorphic rock in the intervening period.

There are clues that suggest the gold had precipitated, which could have taken place hundreds of millions of years later as hydrothermal fluids flowed through the deposits. So you have a case for hydrothermal deposition, too.

A new paper published in Nature Geoscience, written by ETH Zürich geologist Christoph Heinrich,  submits a third possibility that could explain everything. Heinrich’s explanation centers on that three billion years ago held a much different earth environment than today’s. This period was “after the origin of microbial life but before the oxygenation of the atmosphere.”

As Ars Technica writes,

The hypothesis is built on the idea that voluminous eruptions of flood basalts that occurred in the area at the time would also have ejected a lot of volcanic gases—namely sulfur dioxide and hydrogen sulfide. Both those compounds would get into the river by riding in raindrops, with the sulfur dioxide forming sulfuric acid. The gold in these deposits is accompanied by a lot of fool’s gold (pyrite), which is composed of iron and sulfur, but very little of other iron-containing compounds. That would make sense if the water was loaded with sulfur, which grabbed the iron and then precipitated.

The volcanic rock that was weathering and eroding beneath the falling rain and flowing river water then would have contained diffuse gold. The acidic, low-oxygen, high-sulfur water it was exposed to during eruptions would have been good at dissolving gold—conditions you wouldn’t see today.

Downstream in quiet pools, that gold-laden water (at a concentration of maybe a part per billion) would encounter mats of living microbes, dead organic matter, or methane. Chemical reactions with that organic carbon would steal the gold atoms from their water-soluble partnership with hydrogen and sulfur, precipitating out the metal. And in fact, we do see what looks like gold precipitated onto microbial mats in these deposits.

After it precipitated, bits could break free and roll around, smoothing off rough edges. The end result would be something that looked a bit like a placer deposit and a bit like hydrothermal precipitation, which would explain why these rocks have been such a puzzle for geologists.

The volcanic eruptions weren’t constant, so the gold transport would have occurred in pulses. That massive Vaal Reef deposit, for example, could have accumulated over a million years, assuming a decade-long eruption every ten thousand years.

Heinrich does point out one observation that is problematic for his hypothesis: some trace element and isotopic data that suggests the particles of gold are older than the sedimentary rock it calls home. That shouldn’t be the case if the gold was precipitating in the way he’s proposing. However, there’s some uncertainty in those measurements that still needs to be worked out.

If the idea is right, this 3 billion year old bonanza only exists because of some special circumstances. An oxygen-free atmosphere and caustic rain downwind of massive volcanic eruptions sounds pretty unfriendly to life like us, but unfazed microbial life snatched and hoarded the gold liberated by that rain. All we did was dig up their buried treasure.

Regardless, it is interesting to read about the biological, chemical and physical originations of gold, which has been used as many.

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