Study of oxygen fugacity during magma evolution and ore genesis in the Hongge mafic–ultramafic intrusion,the Panxi region,SW China

Economic concentrations of Fe–Ti oxides occurring as massive layers in the middle and upper parts of the Hongge intrusion are different from other layered intrusions(Panzhihua and Baima) in the Emeishan large igneous province, SW China. This paper reports on the new mineral compositions of magnetite and ilmenite for selected cumulate rocks and clinopyroxene and plagioclase for basalts. We use these data to estimate the oxidation state of parental magmas and during ore formation to constrain the factors leading to the abundant accumulation of Fe–Ti oxides involved with the Hongge layered intrusion. The results show that the oxygen fugacities of parental magma are in the range of FMQ-1.56 to FMQ+0.14, and the oxygen fugacities during the ore formation of the Fe–Ti oxides located in the lower olivine clinopyroxenite zone(LOZ) and the middle clinopyroxenite zone(MCZ) of the Hongge intrusion are in the range of FMQ-1.29 to FMQ-0.2 and FMQ-0.49 to FMQ+0.82, respectively.The MELTS model demonstrates that, as the oxygen fugacity increases from the FMQ-1 to FMQ+1, the proportion of crystallization magnetite increases from 11 % to 16 % and the crystallization temperature of the Fe–Ti oxides advances from 1134 to 1164 °C. The moderate oxygen fugacities for the Hongge MCZ indicate that the oxygen fugacity was not the only factor affecting the crystallization of Fe–Ti oxides. We speculated that theinitial anhydrous magma that arrived at the Hongge shallow magma chamber became hydrous by attracting the H_2O of the strata. In combination with increasing oxygen fugacities from the LOZ(FMQ-1.29 to FMQ-0.2) to the MCZ(FMQ-0.49 to FMQ+0.82), these two factors probably account for the large-scale Fe–Ti oxide ore layers in the MCZ of the Hongge intrusion.