Fluid evolution in the Beidabate porphyry Cu-Mo deposit, Xinjiang, Northwest China: evidence from fluid inclusions and H-O-C-S isotopes

Abstract The Beidabate deposit is a porphyry Cu-Mo deposit located in the West Tianshan Orogenic Belt, Xinjiang Uygur Autonomous Region. The fluid evolution path and Cu-Mo mineralization mechanism remain obscured by multiple hydrothermal pulses. Systematic fluid inclusion and H-O-C-S isotope analyses were performed on five identifiable mineralization stages. Variations in fluid composition, temperature, and pressure record the fluid evolution path: (1) During the formation of barren quartz veins (stage I), fluid inclusions were trapped under single- to two-phase transitional conditions, as evidenced by the presence of intermediate-density inclusions as well as coexisting daughter mineral-bearing inclusions and vapor-rich inclusions (boiling FIAs). Fluid phase separation occurred at temperatures of 387–412°C and pressures of 180–300 bar. (2) Fluid inclusions in K-feldspar-quartz veins (stage II) and molybdenite-quartz veins (stage III) were trapped under two-phase conditions. Boiling FIAs were also recognized, with homogenization temperatures of 306–368°C and entrapment pressures of 80–200 bar. Fluid boiling is the main precipitation mechanism of molybdenite. (3) Fluid inclusions in pyrite-chalcopyrite-quartz veins (stage IV) and fluorite-chalcopyrite veins (stage V) are predominantly liquid-rich and CO2-bearing inclusions, which represent extraneous fluids. These inclusions homogenized at temperatures of 192° to 285°C, and minimum entrapment pressures of 40–50 bar. Fluid mixing is the main precipitation mechanism of chalcopyrite. H-O-C-S isotope data (δ18O = 3.2 to 15.2‰, δD = -103.6 to -75.1‰, δ13C= -24.2 to -9.7‰ and δ34S= 4.6 to 7.0‰) indicate that the ore-forming fluids have a predominantly magmatic signature with subsequent dilution by late meteoric water and intense water-rock interaction. We established a genetic model of the Beidabate porphyry Cu-Mo deposit with the aim of providing insight into prospecting at a regional scale.