The role of evaporite layers in the ore-forming processes of iron oxide-apatite and skarn Fe deposits: Examples from the middle-lower Yangtze River metallogenic Belt, East China

Iron oxide – apatite (IOA) and skarn Fe deposits are important sources of Fe in China. During the Mesozoic, a large number of large IOA and skarn Fe deposits formed in the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in a short period. These deposits genetically show a close spatial relationship to Triassic evaporite layers. In this study, the S isotope values of IOA deposits and Fe skarn deposits in the MLYRMB were collected and summarized. Most of the sulfides from Fe deposits have higher S isotope values (5–10‰), and the sulfates have S isotope values (17–21‰) that are similar to those of the evaporite layers (28–30‰), indicating the involvement of the evaporite layers. The effects of the evaporite layers on the mechanism of Fe ore formation are discussed. Evaporite layers are the most important oxidation barrier in the formation of Fe ores, as they can oxidize Fe2+ to Fe3+ in magmatic melts and magmatic-hydrothermal fluids, promoting Fe accumulation. Evaporite layers can also provide Na, Ca, K, Cl, and other agents of mineralization that facilitate the formation and transformation of Fe-bearing fluids and cause the deposits to display large-scale Na and Cl alterations. When sulfate from evaporite layers oxidizes Fe2+ to Fe3+, S6+ is reduced to S2−, becoming a major sulfur source for the formation of either sulfide ores or pyrite deposits after Fe mineralization. Sulfate (gypsum) from evaporite layers could be activated and transferred to the upper or side part of the ore system, forming anhydrite/gypsum ores (veins). Therefore, pyrite, anhydrite, and Fe ores often appear in single deposit.