Genesis of Cu deposits hosted by volcanic rocks in Yunxian-Jinggu Arc, Western Yunnan: Constraints from fluid inclusions and C-O-S isotopes

• Guanfang, Bazijie, and Minleyichang deposits have similar ore-forming fluid temperature and salinity. • Calcite C-O isotope and sulfide S isotope data indicate the ore-forming materials contribution from sedimentary strata. • The Cu mineralization in the Triassic volcanic rocks of the Yunxian-Jinggu area is closely related to the basin brine. A series of vein-type Cu deposits have been preserved in the Triassic volcanic rocks of the Yunxian-Jinggu area. The Guanfang, Bazijie, and Minleyichang deposits are typical examples of regional Cu mineralization. The deposits are preserved in the intermediate-basic volcanic rocks, and the ore bodies are controlled by NNE-trending structures with similar alteration features and mineral assemblages. The fluid inclusions are recognized in quartz and calcite at the three deposits, dominated by aqueous inclusions (L) and liquid phase-rich vapor–liquid two-phase inclusions (L-V) with similar temperature (270-140℃) and salinity (14.0–20.2 wt% NaCl eq.). The three deposits have a medium–low temperature, moderate salinity, and higher oxidation rate than the volcanic host rocks, confirmed by the fluid inclusion, alteration, and mineral assemblage. The O isotope values for early quartz associated with sulfides in the three deposits are comparable (δ 18 O SMOW = 15.23 to 20.01 ‰). Late quartz in the Guanfang and Bazijie deposits shows a deficit in O isotope values compared to early quartz (δ 18 O SMOW = 9.89 to 13.98 ‰), indicating the incorporation of meteoric water or basin fluid during the late stage of mineralization. The early calcite in the Guanfang and Bazijie deposits have similar C-O isotopic signatures (δ 13 C V-PDB = −10.51 to −4.75 ‰, δ 18 O V-SMOW = 11.29 ‰ to 12.63 ‰), and δ 18 O fluid (3.84 to 5.18 ‰) calculated from calcite is approximately the same as the δ 18 O fluid (2.08 to 3.95 ‰) calculated from early quartz. The early calcite and early quartz in the Guanfang and Bazijie deposits may be connected to the same fluid source based on the similar δ 18 O fluid signatures and salinities. The calcite C-O isotopic compositions of the Minleyichang deposit are 13 C V-PDB = −23.03 to −17.74 ‰ and δ 18 O V-SMOW = 19.72 to 28.32 ‰, which exhibit the characteristics of biogenic carbon and indicate that the material originated from sedimentary layers in the Simao Basin. The sulfide δ 34 S V-CDT ranges from −17.2 to 3.2 ‰ in the three deposits, which is related to the addition of reduced S from the sedimentary strata in the Simao Basin. The fluid characteristics of medium to low temperature, moderate salinity, and the S-C isotope signatures indicate that the ore-forming fluids are the basin brine from the Simao Basin. The Cu mineralization in the Yunxian-Jinggu area is closely related to the thrust-fold structure system formed during the Late Cretaceous to the Cenozoic period. The ore-forming fluids may have come from the brine in the central part of the Simao Basin, which migrated through the thrust fault and assimilated the materials in the regional volcanic rocks and sedimentary rocks, forming Cu deposits in the Triassic volcanic rocks of Yunxian-Jinggu area.