Contrasting oxidation states of low-Ti and high-Ti magmas control Ni-Cu sulfide and Fe-Ti oxide mineralization in Emeishan Large Igneous Province

Magmatic Ni-Cu-(PGE) sulfide and Fe-Ti oxide deposits in plume-related large igneous provinces (LIPs) are commonly related to low-Ti and high-Ti series magmas, respectively, but the major factors that control such a relationship of metallogenic types and magma compositions are unclear. Magma fO₂ controls sulfur status and relative timing of Fe-Ti oxide saturation in mafic magmas, which may help clarify this issue. Taking the Emeishan LIP as a case, we calculated the magma fO₂ of the high-Ti and low-Ti picrites based on the olivine-spinel oxygen barometer, and the partitioning of V in olivine. The obtained fO₂ of the high-Ti series magma (FMQ + 1.1 to FMQ + 2.6) is higher than that of the low-Ti series magma (FMQ - 0.5 to FMQ + 0.5). The magma fO₂ of the high-Ti and low-Ti picrites containing Fo > 90 olivine reveals that the mantle source of the high-Ti series is likely more oxidized than that of the low-Ti series. The results using the ‘lambda REE’ approach show that the high-Ti series may have been derived from relatively oxidized mantle with garnet pyroxenite component. The S contents at sulfide saturation (SCSS) of the two series magmas were calculated based on liquid compositions obtained from the alphaMelts modeling, and the results show that the low-Ti series magma could easily attain the sulfide saturation as it has low fO₂ with S being dominantly as S^2-. In contrast, the oxidized high-Ti series magma is difficult to attain the sulfide saturation, but could crystallize Fe-Ti oxides at magma MgO content of ~7.0 wt.%. Thus, contrasting magma fO₂ of low-Ti and high-Ti series in plume-related LIPs may play an important role in producing two different styles of metallogeny.