The link between volcanism and the formation of copper ore has been discovered by researchers from the University of Bristol, UK. Their findings could have far-reaching implications for the search for new copper deposits.
With global demand for copper high (the average UK house contains about 200 kg of the metal, mostly in electric cables and transformers) and current reserves relatively limited, finding new reserves is a priority.
The researchers, led by Professor Jon Blundy of Bristol’s School of Earth Sciences, studied giant porphyry copper deposits of the variety that host 75 percent of the world’s copper reserves.
Copper forms in association with volcanoes such as those around the Pacific Ring of Fire but the nature of this association has never been entirely clear. Copper ore is predominantly in the form of copper-iron sulphides so an enduring problem has been how to simultaneously create enrichments in both copper and sulfur. Volcanoes rich in copper tend to be poor in sulfur and vice versa.
To resolve this copper-sulphur paradox, the Bristol team, working in collaboration with BHP Billiton, the world’s largest mining company, drew on observations of modern arc volcanoes, including several in Chile, source of most of the world’s copper, to postulate a two-step process for porphyry copper formation.
They proposed that first, salt-rich fluids, or brines, separate from large magma bodies and become trapped in the crust at a depth of a few kilometers. These brines have the ability to concentrate copper from the magma from which they separate. At a later stage, sulfur-rich gases ascend from deeper in the same volcanic system. When they meet the trapped, copper-rich brines they react explosively to form sulphide ores and hydrogen chloride gas.
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