Origin of World's Largest Gold Deposit Found?

Charles Choi
United Press International Science News
September 23, 2002

The radioactive decay of metal specks inside South African gold nuggets may have helped an international team of scientists determine the origin of the world's largest gold deposit. The discovery, described in a recent issue of the journal Science, not only sheds light on Earth's early geology, but promises to help future gold prospecting as well.

"If we can find another one of these giant deposits, that's at least a half-trillion dollars in today's prices, if not twice that," said research leader Jason Kirk, a geochemist at the University of Arizona in Tucson.

Nearly 40 percent of all gold mined during recorded history has come in the past 120 years from "the Rand"—the Witwatersrand Basin in South Africa. Scientists estimate roughly one-third of the world's gold resources still lie unmined in the nine million acres of this ancient lake or sea bed, whose name means "ridge of white waters" in Afrikaans.

The origin of these rich deposits has proven controversial. Two theories prevailed, Kirk explained. The placer model says the gold is older than surrounding rock, having washed into the basin from rivers and streams from surrounding mountains and highlands, much as the gold deposits in California that triggered the gold rush there did. The hydrothermal model, on the other hand, says the hot spring fluids deposited the gold inside the rocks.

To resolve the controversy, Kirk and colleagues in Australia and Britain decided to determine the age of the gold itself. If the gold is older than the rocks in which it is found, then the rocks must have built up around the gold, bolstering the placer model. If the gold is younger than the rocks, that means it must have seeped in with fluids, supporting the hydrothermal model. "Gold is never pure. It always has something in it," Kirk said.

Two elements sometimes found inside gold, rhenium and osmium, help serve as a radioactive clock. Rhenium naturally decays into osmium over very long spans of time—it takes about 42.3 billion years for half of a sample of rhenium to transmute, or some 10 times the age of Earth. By dissolving gold grains in acid and measuring the ratio of rhenium to osmium inside the sample, scientists can determine the gold's age.

It turns out gold from three places in the Rand is three billion years old, "a quarter of a billion years older than its surrounding rock," Kirk said, thus supporting the placer model.

This "should put to rest a debate that has been going on for a century, as for the first time the age of the gold itself is nailed down and not just indirectly implied," said geochemist Hartwig Frimmel, of the University of Cape Town in South Africa.

Also, the unique rhenium-osmium ratio the investigators found means the gold comes from the Earth's mantle, not its crust. The gold in the Rand, therefore, may originate in volcanic pebbly rocks known as komatiites, as opposed to granite from the crust, Kirk explained. Such information should help direct gold prospecting.

"(This) gives clear direction on future exploration initiatives in search for Witwatersrand-type gold deposits elsewhere," Frimmel said.

Kirk's team plans to look at other gold deposits around the world to get a broader picture of how they form. It still remains an open question as to why the Rand's deposit is so extraordinarily huge. Kirk theorizes the Rand gold's origins in the mantle may have helped it survive intact, while gold from the crust gets diluted easily over time.

Continued on Next Page >>




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