The role of magma differentiation in optimizing the fluid-assisted extraction of copper to generate large porphyry-type deposits

Porphyry copper deposits (PCDs) are the main source of copper globally, with the metals transported in and deposited from aqueous magmatic fluids. Processes that define the volume of magma and concentration of copper in the magma required to form PCDs, however, are not well understood. Here, we present the results of quantitative modeling of the behavior of Cu and Cl during magma evolution in the upper crust. We show that fractional crystallization is the most important process promoting efficient Cu extraction, and that high concentrations of Cu in the ore-forming hydrothermal fluids can be reached with moderate Cl concentrations. Unusually high concentrations of Cl and Cu in the magma and large magma volumes are not required. Arc magmas of modest volume (<10 3 km 3 ) and modest initial Cu and Cl concentrations can generate large PCDs, if a sufficient mass of magmatic fluid is exsolved at an advanced stage of crystallization.