Fluid evolution characteristics of the sulfate-rich iron-oxide-apatite deposit and its significance: A case study of the Luohe deposit in eastern China

• Meteoric water addition led to a large amount of anhydrite and pyrite precipitation in the Luohe. • The S isotopes of Luohe pyrite are controlled by thermodynamic and redox fractionation. • The amount and timing of evaporite addition jointly influence the formation of IOA. Luohe magnetite apatite deposit is the largest iron deposit in the Luzong volcanic basin in eastern China. Its early hydrothermal stage developed a diopside + magnetite + anhydrite assemblage, and developed a large amounts of anhydrite + pyrite in late stage. This provides a good research opportunity for the evolution process of hydrothermal fluid in sulfate rich IOA deposit (Fe-P-SO 4 system). SHRIMP in situ oxygen isotope analysis showed the early fluid δ 18 O is 9.73 ‰ − 10.76 ‰, higher than that of magmatic water, indicating that the ore-forming hydrothermal fluid had equilibrated with Triassic marine carbonate and evaporite strata. Luohe has five alteration/mineralization stages; at the end of stage II δ 18 O fluid decreased (5.04 ‰ average), then increased in stage III (10.66 ‰ average), and decreased again from the end of stage III (−4.23 ‰ – 2.52 ‰). Combining fluid inclusion study results, the high temperature, high salinity, and high δ 18 O fluid deposited diopside, epidote, chlorite, magnetite, and apatite. The low temperature, low salinity, low δ 18 O fluid is interpreted as meteoric water, and corresponds to the formation of anhydrite + pyrite. The addition of meteoric water led to a decrease in fluid temperature and ƩSO 4 2- /ƩH 2 S ratio, resulting in the change from magnetite to pyrite stability. Pyrite and anhydrite sulfur isotopes show that SO 4 2- was not reduced at stage II, and that 1/3 of the sulfur in the stage IV pyrite is derived from the reduction of SO 4 2- . The precipitation of large amounts of anhydrite indicates that a significant portion of SO 4 2- did not participate in the oxidation of Fe 2+ . Combining with the related research on other IOA deposits in the region, we have established an evaporite contamination model of the IOA system, propose that the incorporation of a large amount of evaporite during the magmatic stage or the initial fluid exsolution stage can facilitate the complete oxidation of Fe 2+ , leading to the formation of high-grade IOA-type ore deposits.