Magmatic-hydrothermal fingerprints in lode gold deposits: A case study of the Liushaogou deposit in the West Qinling Orogen, China

Abstract

The genesis of gold deposits in the northern belt of the Western Qinling Orogen, hosting numerous world-class gold deposits, remains a contentious issue. The Liushaogou deposit in this region is a quartz vein-style gold deposit, which is strictly controlled by brittle faults, and along with weak silicification and sericitization. Five hydrothermal mineralization stages were identified, including barren quartz (stage I), pyrite-quartz (stage II), quartz-pyrite (stage III), quartz-polymetallic sulfides (stage IV), and quartz-carbonate (stage V). Fluid inclusions within stages I–IV quartz and stage V calcite were studied. Stage I fluids are typified by high temperatures (up to ∼400 °C) and medium to high salinity (8–40 wt% NaCl equiv.), with δ¹⁸O_fluid values of 6.21–8.31 ‰, and δD values of −96.1 to −91.6 ‰, aligning with the characteristics of magmatic fluids. Subsequent stages (II-IV) fluids retain magmatic isotopic signatures, while stage V fluids record a shift toward meteoric water (δ¹⁸O_fluid: −5.44 to −4.84 ‰; δD: −80.1 ‰ to −74.9 ‰), reflecting late-stage meteoric influx. Physicochemical conditions of the mineralization fluids were estimated to be near-neutral, relatively reduced, and H₂S-rich, favoring the transportation of gold as hydrosulfide complexes. Fluid immiscibility and boiling are the primary mechanisms for gold precipitation. Sulfur isotopes (δ³⁴S_H2S: 4.9–7.9 ‰) in Liushaogou pyrites closely overlap Triassic magmatic rocks and regional magmatic-hydrothermal deposits, contrasting sharply with heterogeneous δ³⁴S values (−28 ‰ to + 47 ‰) of Paleozoic sedimentary/metamorphic rocks. Homogenous Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb: 18.225–18.596; ²⁰⁷Pb/²⁰⁴Pb: 15.623–15.722; ²⁰⁸Pb/²⁰⁴Pb: 38.368–39.115), which are comparable to those observed in Triassic magmatic rocks, further trace metals to Triassic magmas. The evidence presented here, in conjunction with the spatiotemporal links to Triassic intrusions and coeval Au-Mo-Cu mineralization, collectively point to the magmatic-hydrothermal origin of the Liushaogou deposit. This study emphasizes Triassic magmatism as a significant ore-forming process in the northern belt of the Western Qinling Orogen, with implications for targeting concealed intrusions and associated fault systems in regional exploration.