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

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 <0. Biotite crystals from these intrusive rocks and MMEs also yielded magmatic fO2 values of NNO <0. Based on the hornblende thermobarometer and hygrometer, the parental magma to the Ermi Cu deposit mainly evolved at pressures <3 kbar and contained 4.38−5.94 wt% water. In addition, the water-saturated and oxidized Ermi magma experienced fluid exsolution during differentiation, as evidenced by the lower Cl contents (0.94−0.03 wt%) of apatite. The transition from sulfate-dominated to sulfide-dominated (S2−) fluids occurred when the system was recharged by the reduced mafic melt. We attribute the pyrrhotite-dominated ore assemblage in many porphyry Cu deposits to a decrease in the S6+/S2− ratio of the magmatic-hydrothermal systems, which resulted from the addition of reduced materials (e.g., magmas and organic carbon-rich sedimentary rocks) to the oxidized magmatic systems in the shallow crust. This reduction process could also have led some of the Cu to be lost from the magmatic-hydrothermal systems by early sulfide segregation prior to fluid ascent and ore deposition.