New constraints on oxygen fugacity of basalts in large igneous provinces using Cr isotopes: Implications for Fe-Ni-PGE mineralization

The genetic linkage between large igneous provinces (LIPs) basalts and large-scale mineralization has been extensively documented. Oxygen fugacity is a critical metallogenic factor controlling magmatic systems. This study presents the first Cr isotope analysis of Permian high-Ti and low-Ti basalts from the Emeishan large igneous province (ELIP), revealing distinct isotopic signatures between the two basalt groups. The δ53Cr values in low-Ti basalts are slightly lighter than that in high-Ti basalts on average. Both basalt types demonstrate significant enrichment in heavy Cr isotopes compared to mid-ocean-ridge basalts (MORB) and ocean-island basalts (OIB). Trace element patterns and Sr-Nd isotopic compositions of ELIP basalts indicate that low-Ti basalts are geochemically similar to an enriched mantle type II mantle source, whereas high-Ti basalts exhibit affinities to OIB-like compositions, providing crucial insights into mantle source characteristics. Comparative analysis of Cr isotopes in MORB and Emeishan basalts reveals that the oxygen fugacity of low-Ti basalts (ΔlogfO2 = FMQ −1.5 ± 1) is slightly lower than that of MORB, whereas high-Ti basalts show higher oxygen fugacity (ΔlogfO2 = FMQ +0.5 ± 1) is higher, indicating a more oxidized state compared to MORB. This elevated oxygen fugacity in Emeishan basalts creates favorable conditions for mineralization, offering new constraints on the redox conditions of the ELIP mantle sources and their implications for magmatic processes and associated mineralization events.