The Contribution of Evaporite Layers in the Formation of the Subvolcanic Type Fe Deposit in the Emeishan Large Igneous Province, Southwestern China: Insights from the S and O Isotopic Characteristics of the Kuangshanliangzi Deposit

The Emeishan Large Igneous Province (ELIP) is one of the largest igneous provinces, containing some of the world’s richest mineral resources. It mainly comprises magmatic Fe-Ti-V deposits and Cu-Ni sulfide deposits, with minor subvolcanic-type Fe deposits related to mafic–ultramafic rocks. The evaporite layer is involved in the metallogenic system, yet its contribution has not been examined in detail. In this study, an integrated geological study, single-mineral S and O isotopic analysis, and in situ S isotope analysis were carried out on pyrite and magnetite from the Kuangshanliangzi (KSLZ) subvolcanic-type Fe deposit to examine the role of evaporite layers in Fe mineralization. The O isotopic values of magnetite and the S isotopic values of pyrite were abnormally high in the KSLZ deposit. This indicates that the ore-forming system of the KSLZ deposit is contaminated by 18O- and 34S-enriched evaporite layers, inferred from the Dengying Formation, which significantly increase the oxygen fugacity, sulfur fugacity, and water content of the metallogenic system via the basic–ultrabasic magma-upwelling process, thus promoting the formation of Fe ores. When the SO42− (from evaporite layers) oxidizes Fe2+ to Fe3+, the SO42− is reduced to S2−, and the ore-forming system can be changed from unsaturated sulfide to supersaturated sulfide, which also benefits the Cu-Ni sulfide deposit formation.