Distinct magma evolution processes control the formation of porphyry Cu–Au deposits in thin and thick arcs

Abstract

Porphyry Cu–Au deposits are the major global source of copper, an essential metal for the green transition, and a significant source of gold. They occur both in thick continental and thin oceanic arcs at depths between ∼1 and ∼6 km above parental magma chambers situated at ∼5–15 km depth. Although it is believed that the metal precipitation processes for these deposits are the same in thin and thick arcs, it is not clear why porphyry deposits in these two environments display different Cu and Au endowments and different Au/Cu ratios. Using mass balance petrological modeling, I argue that porphyry Cu–Au deposits in thick and thin arcs form by two distinct magmatic evolution precursors. In thick arcs porphyry Cu–Au deposits are tied to the deep crust build-up of large volumes of magmas, volatiles and metals. These large volumes of magmas are H2O-undersaturated and need to rise to shallow level to exsolve fluids and metals. In contrast, porphyry Cu–Au deposits in thin arcs form by little differentiated mantle-derived magmas that rise directly to shallow crustal levels where they exsolve fluids and metals. These two distinct magmatic pathways of porphyry Cu–Au generation are ultimately controlled by the different arc thicknesses in the two environments, even though the processes of metal precipitation in the actual deposits of the shallow crust are similar. The model discussed here may stimulate the development of new tools for the exploration of porphyry deposits in the distinct environments of thick and thin arcs.