Volcanic heat speeds up gold deposition

Under the collapsed cone of an extinct volcano on Lihir Island, Papua New Guinea, is about 1,300 tons of gold-one of the largest gold deposits known. Using deep geothermal wells drilled in 1997 to facilitate mining operations, geochemist Stuart F. Simmons of the University of Auckland, New Zealand, and a colleague have taken a rare opportunity to analyze the superheated brine from which the overlying ore presumably formed (Science 2006, 314, 288). From the measured 15-ppb gold concentration in the brine and rates of upward flow of hot water, the researchers conclude the deposit could have formed in as little as 55,000 years, instead of on a much longer time scale. "It is a terrific natural geochemical laboratory," says Randolph A. Koski of the U.S. Geological Survey, Menlo Park, Calif. Geologists studying ore formation usually have access only to tiny inclusions of "ore fluids" trapped in minerals that formed long ago.