Copper has been in use for 6,000 years and it shows no signs of slowing down. The average home has about a hundred pounds of it and we are going to have more people and homes, not fewer. Volcanoes may be the answer.
A research team led by Professor Jon Blundy of University 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.
Dormant volcanoes, such as Cerro San Antonio on the Bolivia Altiplano, are potential sites for future copper exploration. The exquisite colours of the volcano result from the passage of millions of tonnes of sulphurous and acidic gases, themselves by-products of the ore-forming process. Image credit: Professor Jon Blundy
To resolve this copper-sulfur paradox, BHP Billiton, the world's largest mining company, and the academics 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.
To demonstrate their idea, the researchers simulated the process of copper ore formation in their laboratory using high temperature and pressure apparatus. They were able to replicate many of the features of natural porphyry copper deposits in a capsule measuring just a few millimeters in length.
Blundy said, "This is a remarkable result with far-reaching implications for how we go about searching for new copper deposits."
Finding the link between volcanism and ore formation means that even recently active volcanoes may be targeted as copper mines of the future. Professor Blundy and colleagues speculate that copper deposits are forming beneath many active volcanoes today, including the Soufrière Hills volcano on the tiny Caribbean island of Montserrat that has been erupting since 1995. "Despite its potential for copper deposit formation it is unlikely to be ready for mining for a few centuries yet," said Blundy.
Citation: 'Generation of porphyry copper deposits by gas-brine reaction in volcanic arcs' by J. Blundy, J. Mavrogenes, B. Tattitch, S. Sparks, A. Gilmer, Nature Geoscience