How was this treasure trove of gold formed?

Brett Smith for redOrbit.com – Your Universe Online

A chain of gold strata in the Witwatersrand basin of South Africa comprises around 40 percent of all of the gold that has ever been excavated and how this gold got there has baffled geologists for decades.

Scientists have known that the gold was formed there around 3 billion years ago, and according to a new study in Nature Geoscience, the unique conditions of Earth at that time are responsible for the gold deposition.

Two prevailing theories describe how gold is deposited into relatively high concentrations: the “placer” theory and the “hydrothermal hypothesis”.

According to the placer theory, gold initially from igneous rocks is liberated by erosion and concentrated, as water sweeps away lighter particles of rock made of other minerals. The hydrothermal hypothesis says water heated by magma far below the Earth’s surface dissolves and transports minerals, including gold, as it goes up through stone, sediment and rock. When the water cools down and enters fractures, which form tiny paths, those dissolved minerals can precipitate out to create rich veins or precious metal.

A new theory

The new study combines these two theories to put forth a third possible method for gold deposition, which was fostered by the conditions on Earth 3 billion years ago.

The theory is supported by evidence of large eruptions of flood basalts that took place in the region at the time. These eruptions would have thrown sulfur dioxide and hydrogen sulfide into the air. Both compounds would eventually precipitate onto the ground, with the sulfur dioxide taking the form of sulfuric acid. The volcanic stone eroding underneath the falling rain and flowing water would have held diffuse gold. The acidic, low-oxygen, high-sulfur water it was subjected to during eruptions could have been efficient at dissolving gold, conditions which incidentally are not seen today.

Eventually, that gold-rich water would gather into placid pools and there it would come across beds of microbes, rotting organic matter, or methane. Chemical interactions with that organic and natural carbon would rip the gold atoms from their water-soluble relationship with hydrogen and sulfur, precipitating out the precious metal. Scientists doo see what looks like gold precipitated on microbial beds in these South African deposits.

After the gold precipitated out, small pieces could escape and roll around, rounding off any sharp edges. The outcome would be a formation that looked rather like a placer deposit and a bit like a hydrothermal deposit, explaining why these rocks have baffled geologists for years.

One slight flaw

The study, written by Swiss geologist Christoph Heinrich, does point out a single observation that may be challenging for his theory: some trace element and isotopic information that implies the particles of gold are much older than the sedimentary rocks where they sit. However, there’s some doubt in those measurements that still must be resolved, Heinrich wrote.

If the ETH Zurich scientist is correct, the Witswaterand deposit is only there because of some unique conditions. An oxygen-free atmosphere and acid rain downwind of enormous volcanic eruptions is a situation hostile to most modern life.

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