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

Porphyry Cu deposits are typically associated with oxidized (ΔFMQ +1.0 to +3.0, where ∆FMQ is the deviation of logfO₂ from the fayalite–magnetite–quartz, FMQ, buffer) and volatile-rich magmas, however, the factors controlling their Cu fertility have not been completely constrained. Here we use time-resolved geochemistry of zircon and apatite inclusions within zircon to characterize the time–temperature–composition evolution of a small (0.11 Mt Cu at 0.6 % Cu) porphyry Cu system at Ermi in northeastern China. All zircons from Ermi yield ²⁰⁶Pb/²³⁸U dates of about 95 Ma. From core to rim, the Eu/Eu* values (0.31–0.08) of zircon systematically decrease, Yb/Gd_N (15.4–46.6) and Ce/√(U×Ti) (0.27–0.95) values systematically increase, whereas Th/U (0.46–1.16) and 10000×(Eu/Eu*)/Yb (0.27–2.76) values do not show systematic variability. Such variations are linked to plagioclase-dominated fractionation in the upper crust and oxidation of the magma from ΔFMQ 0.55 to 1.40 as a result of hydroxylation–oxidation reactions during fluid exsolution. Fluid exsolution caused a decrease in Cl concentration (from 0.21% to 0.01%) of the sulfur-poor melt (<0.05 wt. % SO₃ in apatites) with crystallization, as reflected by the chemistry of apatite inclusions in zircon grains. Considering the small size of the Ermi deposit, we propose that even if the evolved porphyry magma were appropriately oxidized, the low volatile (S and Cl) contents of the parent magma would have limited the availability of chalcophile elements for the later exsolved fluids and caused the overall porphyry system to be metal poor. Therefore, the budget (which is ultimately controlled by the magma volume) and behavior of volatiles, as well as the fO₂ evolution of a magma, play important roles in determining the Cu fertility of porphyry systems.