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

Abstract

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 <100–150 bar, corresponding to a hydrostatic depth of <1.0 km. In Stage IV, temperatures decreased further to 149–188°C, with lower salinities (5.71–9.73 wt% NaCl equiv.), indicating a post‐ore fluid stage. These data suggest that the mineralizing fluids forming the Yuku Mo deposit changed from early moderate‐ to low‐salinity CO2‐rich fluids in the H2O–NaCl–CO2 system that formed at high temperature and pressure, to late H2O–NaCl low‐salinity fluids that formed at low temperature and pressure. H−O isotopic compositions indicate that the mineralizing fluids had a dominantly magmatic signature but were diluted by meteoric waters. Combining our integrated analysis of the fluid evolution and deposit geology, we propose that fluid boiling and fluid–rock interaction, including intensive potassic alteration, changed the salinity and triggered CO2 escape, leading to a decrease in fO2 and an increase in the acidity of the ore‐forming fluids...

High heat production of the Late Mesozoic Luanchuan granites from East Qinling Orogen, China: Implications for petrogenesis and mineral exploration

“…Generally, apatite U-Pb system has a closure temperature in the range of 500-425 C (Cherniak et al, 1991), whereas apatite fission-track and (U-Th-Sm)/He systems have closure temperatures varying from 120 C to 90 C and 80 C to 55 C (Ketcham et al, 2007;Stockli et al, 2000), respectively. The metallogenic temperatures of Mo-W deposits in Luanchuan region were constrained in the range of 550 C-350 C based on fluid inclusions (Xue et al, 2021;Yang et al, 2013;Yang, Li, & Chen, 2012;Yang, Li, & Yang, 2009;Yang, Wang, et al, 2012;Yang, Zhang, et al, 2009) In this study, the Nannihu pluton has the highest RHP peak value, whilst the Huoshenmiao pluton shows the lowest RHP peak value (Figure 5). This is consistent with the mineralization potential of these two granitic plutons, as evidenced that the Nannihu pluton forms world-class Mo-W deposit (Mo reserves: 664600 tons), whereas the Huoshenmiao pluton only hosts small-medium Mo deposits (Mo reserves: 50000 tons) (Li et al, 2004;Mao et al, 2008;Wang et al, 2019).…”

“…Generally, apatite U–Pb system has a closure temperature in the range of 500–425°C (Cherniak et al, 1991), whereas apatite fission‐track and (U‐Th‐Sm)/He systems have closure temperatures varying from 120°C to 90°C and 80°C to 55°C (Ketcham et al, 2007; Stockli et al, 2000), respectively. The metallogenic temperatures of Mo–W deposits in Luanchuan region were constrained in the range of 550°C–350°C based on fluid inclusions (Xue et al, 2021; Yang et al, 2013; Yang, Li, & Chen, 2012; Yang, Li, & Yang, 2009; Yang, Wang, et al, 2012; Yang, Zhang, et al, 2009), and thus these published age data, including apatite U–Pb, fission‐track and (U‐Th‐Sm)/He dating, could be used to trace the cooling history of ore‐forming fluids and to constrain corresponding timing. The apatite fission‐track and (U‐Th‐Sm)/He ages are all younger (20–80 Myrs) than the corresponding zircon and apatite U–Pb ages (Yang, Jepson, et al, 2022), suggesting secular cooling from 425°C to 55°C for the Huangbeiling pluton.…”