Genesis and fluid evolution of the Yuku porphyry Mo deposit, East Qinling orogen, China

The Yuku is a large porphyry Mo deposit (1.5 Mt at 0.12% Mo) in the Luanchuan ore district of the East Qinling orogen, central China. The economic Mo ore bodies occur as veins, veinlets, and disseminated ore and are developed mainly within the late Mesozoic Yuku porphyritic granite. Molybdenum mineralization is generally associated with potassic and phyllic alteration. Hydrothermal processes in the Yuku deposit are divided into four stages: (I) a quartz–K‐feldspar–biotite ± pyrite stage, (II) a quartz–molybdenite ± pyrite ± K‐feldspar ± sericite stage, (III) a quartz–polymetallic sulphide stage, and (IV) a calcite ± quartz ± fluorite ± pyrite stage. The fluid evolution during these hydrothermal stages was constrained through systematic investigation of fluid inclusions (FIs) and H−O isotopes. Four types of primary or pseudosecondary FIs were recognized in hydrothermal quartz and calcite: two‐phase liquid‐rich inclusions (L‐type), two‐phase vapour‐rich inclusions (V‐type), halite‐bearing (hypersaline) inclusions (H‐type), and three‐phase CO2‐bearing inclusions (C‐type). FIs within Stages I–IV have homogenization temperatures of 375 to >550, 297–400, 198–298, and 149–188°C, with salinities of 0.53–13.18, 0.35–40.23, 5.11–11.81, and 5.71–9.73 wt% NaCl equiv., respectively. In Stage I, coexisting vapour‐rich (V‐type) and liquid‐rich (L‐type) FIs have similar homogenization temperatures (488 to >550°C) and distinct salinities, indicating fluid boiling during the formation of quartz–K‐feldspar–biotite veins at a pressure of 550–700 bar and a lithostatic depth of 2.3–2.8 km. In Stage II, FIs in quartz were also trapped under boiling conditions, as evidenced by coexisting hypersaline H‐type (34.13–40.23 wt% NaCl equiv.) and low‐salinity V‐type (0.35–2.24 wt% NaCl equiv.) inclusions, which formed at temperatures of 309–361°C and hydrostatic depths of 1.0–2.0 km, equivalent to pressures of 100–200 bar. During Stage III, the ore‐forming fluids were cooler (198–298°C) and more dilute (5.11–11.81 wt% NaCl equiv.) due to the involvement of meteoric water, with minimum trapping pressures estimated at