Copper-rich fluids arising from sulfide resorption by hydrous arc melts

Abstract

Abstract Increasing global demand for copper (Cu) related to the energy transition requires that we understand the mechanisms by which Cu is enriched in the upper crust via magmatism. Porphyry Cu deposits (PCDs) are associated with arc volcanic systems and form under rare circumstances by precipitation from Cu-rich magmatic fluids. Here we develop models to delineate the magmatic conditions under which the Cu concentration and flux may be maximised in exsolved hydrous magmatic fluids. We show that ubiquitous sulfide saturation is a critical limitation on the Cu and sulfur load of exsolved magmatic fluids, owing to the strong partitioning of Cu into sulfide. Sulfide saturation in arc magmas may usually only be avoided under the most hydrous or oxidised conditions, which the global volcanic rock record suggests is not commonplace. However, thermally mature arc crust is likely to develop deep crustal cumulate zones in which sulfides may accumulate over time. When sulfide-undersaturated water-rich mafic melts percolate through these zones they may resorb sulfides during reactive flow. On volatile saturation, Cu-rich fluids will be generated that are viable precursors to PCDs.