NEARLY 60 per cent of the world鈥檚 gold comes from rocks formed during the Archean eon, which ended 2.5 billion years ago. Now it seems that seismic activity in bends in ancient fault lines produced the mysteriously high concentrations of gold in some of these deposits.
David Groves of the University of Western Australia in Crawley and his colleagues traced the distribution of gold fields along an ancient fault zone in Western Australia. They found that four of the richest known gold deposits, including the 1800-tonne giant Golden Mile at Kalgoorlie, occur at regular intervals of 30 to 40 kilometres along this fault zone, at bends in the fault. Levels of gold are low along the straight parts of the fault.
According to Groves, a combination of seismic activity and bends in the fault created the deposits. First, a latticework of cracks formed to accommodate the bends. Mineral-rich water seeped into these cracks, and earthquakes helped to push the water towards the bends. 鈥淲hen you get a seismic event, the water is pumped into these areas,鈥� says Groves.
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The earthquakes also released gases from the water, changing its pressure and causing the minerals to precipitate. Extraordinary concentrations of gold, about 10,000 times the background level, are seen in these deposits today (Geology, vol 32, p 545).