Source: Science magazine, Nicholas Wigginton (7/16/10)
"Porphyry-type ore deposits are major source of global copper supply."
Major mining operations of porphyry-type ore deposits—emplaced by hot magma that melted and mixed with continental crust—provide a major source of the world's supply of copper. One of the largest copper mines in operation is the 100-year-old open-pit Bingham Canyon Mine in the western United States. By measuring the lead isotope signature of tiny fluid bubbles trapped within minerals adjacent to the copper ore, Pettke et al. suggest that the Bingham Canyon deposits originate from some of the first crust formed on the primitive earth. In this model, metal-rich fluid emerged and concentrated 1.8 billion years ago in the mantle after the ancient crust subducted below what is now North America. The deposits further evolved 37 million years ago when partial melting of the mantle underneath North America occurred during the formation of the Basin and Range province. Because this model does not require metal enrichment to occur contemporaneously with subduction processes (the generally accepted formation mechanism), additional quantities of ore-grade metals may exist in locations not typically considered favorable for metal exploration or mining.