PORPHYRY COPPER DEPOSIT FORMATION BY GAS-BRINE REACTION IN VOLCANIC ARCS

Porphyry copper deposits (PCDs) are characterised by a close spatial and temporal association with hypabyssal intrusions of silicic magmas in volcanic arcs. PCD formation requires elevated Cl and H2O to concentrate Cu in magmatic hypersaline liquids (or 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, of the variety favourable to Cu enrichment, 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 dense 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) vapours.

We test this hypothesis with a novel set of “porphyry in a capsule” experiments designed to simulate low-pressure (1-2 kbar) interaction of mafic magma-derived, S-rich gases with brine-saturated, Cu-bearing, but S-free, granite. Experiments were run at 720-850 °C in cold-seal apparatus with andesite, loaded with H₂O and S, situated below dacite that contains sufficient H₂O, Cl and Cu to form coexisting brine and low-salinity vapour, but lacks any S. At run conditions both compositions are substantially degassed and crystallized. S-rich gas from the andesite ascends to react with Cu-rich brine in the dacite. Our experiments result in direct precipitation of Cu-sulphides, in vugs and veins 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 sericitic alteration haloes around PCDs