Christoph Heinrich (adapted from Nature Geoscience)
The miners who answered the call of the California Gold Rush worked pretty hard in hopes of making it rich. But from a geological perspective, they were pretty lazy. Nature had already done most of the work. Much of the gold they found was floating around in “placer” deposits — river sediment with particles of solid gold. The gold originally came from igneous rocks, where it was terribly sparsely distributed among other minerals. Erosion freed and concentrated the gold, while water carried away lighter particles of other minerals.
That’s one possible mechanism by which gold could become so abundant that it’s economically viable to search for it in a given volume of rock or sediment. Another involves the movement of water heated by magma deep below the surface. The heated water dissolves and transports minerals, including gold, as it rises through rocks. As it cools and moves through fractures, which form tiny highways, those dissolved minerals can precipitate and form rich veins.
Some of the world’s best gold deposits are found in South Africa’s Witwatersrand Basin. The Vaal Reef deposit, for example, is gigantic – three thousand tons of gold has been coughed up, which at today’s prices would be worth more than $100 billion. However, geologists are still debating how the gold got there: river placer or hydrothermal fallout are the two options mentioned above. The gold is found in layers in what was once sediment laid down by rivers, but these rivers flowed rough three billion years past. The deposits have become metamorphic rock in the intervening time.
Sounds like a placer deposit, right? Well, there is also evidence to suggest that the gold precipitated, which could have happened hundreds of millions of years later as hydrothermal fluids flowed through the deposits. So you also have a case for hydrothermal deposition.
In a new article published in Natural Geosciences, geologist Christoph Heinrich of ETH Zurich outlines a third scenario that seems to explain almost all of the observations. This alternative explanation is based on the fact that three billion years ago there was a very different environment – after the emergence of microbial life but before the oxygenation of the atmosphere.
The hypothesis is based on the idea that massive flood basalt eruptions that occurred in the area at the time would have also emitted a lot of volcanic gases, namely sulfur dioxide and hydrogen sulfide. Both compounds would enter the river by driving in raindrops, 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 ferrous compounds. That would make sense if the water was loaded with sulfur, which grabbed the iron and then knocked it down.
The volcanic rock that was weathering and eroding under the falling rain and flowing river water would then have contained diffused gold. The acidic, low-oxygen, and sulfur-rich water it was exposed to during eruptions would have been good at dissolving gold — conditions you wouldn’t see today.
Downstream in still pools, that gold-laden water (with a concentration of perhaps 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, causing the metal to precipitate. And in fact, in these deposits we see what appears to be gold deposited on microbial mats.
After it settled, bits could come off and roll around, smoothing out rough edges. The end result would be something a little like a placer deposit and a little like hydrothermal fallout, which would explain why these rocks have been such a puzzle to geologists.
The volcanic eruptions were not constant, so the gold transport would have happened in pulses. For example, that huge Vaal Reef deposit could have accumulated over a million years, assuming a ten-year eruption every ten thousand years.
Heinrich does point out one observation that is problematic for his hypothesis: some trace elements and isotopic data suggesting that the gold particles predate the sedimentary rock it calls home. That shouldn’t be the case if gold precipitated as he suggests. However, there is some uncertainty in those measurements that has yet to be worked out.
If the idea is correct, this 3-billion-year-old bonanza only exists because of some special circumstances. An oxygen-free atmosphere and acid rain downwind from massive volcanic eruptions sounds pretty unfriendly to life like us, but unfazed microbial life grabbed and hoarded the gold released by that rain. All we did was dig up their buried treasure.
Natural Geosciences2015. DOI: 10.1038/NGEO2344 (About DOIs).