Geochemistry

Prospecting for Copper

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Science  26 Apr 2002:
Vol. 296, Issue 5568, pp. 621
DOI: 10.1126/science.296.5568.621a

Porphyry ore deposits consist of valuable ore minerals, usually copper or gold, disseminated in a matrix of both small and large crystals of other minerals, usually quartz or feldspar. These hydrothermal deposits represent the last remnants of magmatic fluid to crystallize from a larger igneous intrusion. The concentration of metals such as copper is thought to be enhanced by the circulation of groundwater that transports these metals from the host rock.

Harris and Golding measured the hydrogen and oxygen isotopic concentrations of fluid inclusions and of quartz and sericite grains in a porphyry copper and gold deposit from the Goonumbla volcanic complex in Australia. The isotopic signatures indicate that the deposits formed from high-temperature, saline magmatic fluids and that no groundwater circulation was involved. Thus, as has been shown at some other porphyry deposits, the gold and copper metals are coming solely from the magma, and their formation temperature is higher than expected. This revised model for porphyry ore formation may help economic geologists recognize new deposits.—LR

Geology30, 335 (2002).

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