By Sid Perkins
New isotopic analyses of rock samples from one of the world’s richest gold-mining regions suggest that the flecks of gold in those ores are more than 3 billion years old. That age, scientists say, finally may settle a debate about how the gold and other rare metals ended up in those deposits in the first place.
About 40 percent of the gold mined worldwide in the past 120 years has been taken from South Africa’s Witwatersrand Supergroup of deposits. The mineral particles in those rocks eroded from mountains and settled in river deltas and offshore sediments between 2.89 billion and 2.76 billion years ago, says John Chesley, a geochemist at the University of Arizona in Tucson. Those are the respective ages of the volcanic rocks that lie below and above the gold-bearing sediments and therefore bracket the period when those sediments formed.
However, the ratio of two isotopes of the metal osmium–which often taints gold and other minerals in ore deposits–suggests that the gold flecks in the Witwatersrand Supergroup are actually about 3.03 billion years old. This means that a few million years after forming, gold eroded from existing rocks and was carried to the ancient river deltas, says Chesley.
He and his colleagues, who describe their analyses in the Sept. 13 Science, say the results from these samples cast doubt on a competing theory. It says that hydrothermal fluids carried the precious metals up into the delta sediments after they formed. If that were the case, Chesley explains, the gold would appear to be younger than the surrounding rocks, not older.
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