Geological, fluid inclusion, and C-H-O-S-Pb isotope constraints on the genesis of the Eastern gold deposit in the Songshunangou gold district, North Qilian orogenic belt, China

The Eastern deposit in the Songshunangou district is the first discovered intermediate-sulfidation (IS) epithermal gold deposit in the North Qilian orogenic belt. Gold mineralization is associated with sericite-chlorite and phyllic alteration styles. Petrographic observations of veinlets and their crosscutting relationships define three distinct hydrothermal stages (I to III): The stage I is represented by pyrite and carbonates (calcite and manganocalcite), while the stage II is reflected by the mineral assemblage galena, Fe-poor sphalerite, chalcopyrite, pyrite, native gold, and tennantite, with trace tetrahedrite and electrum. Gangue minerals are quartz and carbonates (manganocalcite, calcite, ankerite, and dolomite). The stage III is defined by carbonates (manganocalcite, calcite, ankerite, and dolomite), quartz, pyrite, and minor native gold. Homogenization temperatures and salinities obtained from fluid inclusions decrease from stage II (112–191°C; 2.74–12.96 wt% NaCl eqv.) to stage III (103–144°C; 1.40–10.49 wt% NaCl eqv.), indicating that the ore-forming fluids of the Eastern deposit were low-temperature, medium- to low-salinity fluids of the NaCl-H₂O system. The isotopic signatures of S (δ³⁴S = +1.31‰ to +9.93‰) and Pb (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb with 18.391–18.701‰, 15.623–15.635‰ and 38.186–38.414‰, respectively) point to a mantle-derived magmatic source for the ore-forming metals, with minor contributions from the upper crust. Carbon (−2.47‰ to −0.18‰), oxygen (−5.30‰ to −2.70‰), and hydrogen (−83.6‰ to −58.7‰) isotope data indicate that both magmatic fluids and meteoric water have been involved during ore precipitation. Our data suggest that the mineralization at the Eastern gold deposit in the Songshunangou gold district was formed by hydrothermal activity related to Late Ordovician magmatism. The ore-forming components, derived from a mantle magma source, were transported upward by the fluid. During migration, these fluids mixed with meteoric water and cooled, causing the precipitation of large amounts of gold.