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Much that glitters is really old

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2:44pm, September 23, 2002

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.

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