Magma fO2 and Volatiles in the Sub-Economic Ermi Porphyry Cu System in Northeastern China: Implications for Porphyry Cu Fertility.

This article focuses on the magmatic evolution and factors controlling copper (Cu) fertility in the Ermi porphyry Cu deposit in Northeastern China. Using time-resolved geochemistry of zircon and apatite inclusions, the study reveals that the Ermi magma evolved mainly through plagioclase-dominated fractionation in the upper crust, with episodic mafic magma recharge, and experienced an increase in oxidation state (∆FMQ from +0.6 to +1.4) due to hydroxylation–oxidation reactions during fluid exsolution. Despite this oxidation, the magma’s low volatile content—particularly sulfur (S) and chlorine (Cl)—limited the availability of chalcophile elements to exsolved fluids, resulting in a relatively low Cu endowment (0.11 Mt at 0.6% Cu). The findings emphasize that the budget and behavior of volatiles, alongside magma oxygen fugacity (fO2) evolution, critically influence the Cu fertility of porphyry systems by controlling the efficiency of Cu transfer from melts to hydrothermal fluids.