Geocheminar fall-2014

Formation of porphyry copper deposits (PCDs), that host 75% of the world's economic copper reserves, requires elevated Cl and H2O to concentrate Cu in magmatic brines, and elevated S to precipitate Cu-rich sulphides. These twin requirements are hard to reconcile with experimental and petrological evidence that voluminous Cl-rich, hydrous silicic magmas lack sufficient S to precipitate directly the requisite quantities of sulphides. These features are, however, consistent with observations of active volcanic arcs whereby PCDs can be viewed as roots of dome volcanoes above shallow reservoirs where silicic magmas accumulate over long time spans. During protracted periods of dormancy metal-enriched brines accumulate in and above the silicic reservoir through slow, low-pressure degassing. Meanwhile cogenetic volatile-rich mafic magmas and their exsolved, sulphur and CO2-rich fluids accumulate in deeper reservoirs. Periodic destabilisation of these reservoirs leads to short-lived bursts of volcanism liberating sulphurous gases, which react with the shallow-stored brines to form copper-rich sulphides and acidic (HCl) gas. We test this hypothesis with a novel set of experiments designed to simulate low-pressure interaction of mafic magma-derived, S-rich gases with brine-saturated, Cu-bearing, but S-free, granite. Our experiments result in direct precipitation of Cu-sulphides within the dacite, at magmatic temperatures, supporting previous suggestions of gas-brine interaction as an ore-forming process. The simultaneous production of HCl during sulphide precipitation drives alteration reactions in granites and their wall-rocks that replicate associations of sulphides and alteration haloes around PCDs.