Reduction of the oxidized felsic magma in the Ermi porphyry Cu deposit, Northeast China: Implications for the formation of reduced ore assemblages in porphyry systems

Abstract The reduction of oxidized felsic magmas at upper-crustal levels plays an important role in the formation of some porphyry Cu deposits with reduced ore assemblages dominated by pyrrhotite and chalcopyrite. However, the timing of this reduction process relative to volatile exsolution, which is critical to understanding the genesis of this variety of porphyry system, is unclear. The Ermi porphyry Cu system in Northeast China preserves robust petrological evidence that the onset of pyrrhotite and chalcopyrite precipitation was triggered by the reduction of the early oxidized magmatic-hydrothermal system through reduced mafic melt replenishment in the upper crust. This mafic melt is preserved as mafic microgranular enclaves (MMEs) that host fine-grained hornblende, which crystallized at oxygen fugacity (fO2) of NNO −0.78 ± 0.22 (where NNO is the nickel–nickel oxide buffer). The fO2 values estimated from hornblende crystals in the quartz diorite and granodiorite hosts to MMEs show that this replenishment event significantly decreased the fO2 of the magmatic system from NNO +1.0 to NNO