Fluid inclusion and C-O isotopic constrains on the origin and evolution of ore-forming fluids of the Badaguan Cu-Mo deposit, Inner Mongolia

Abstract

The Badaguan porphyry Cu–Mo deposit is located in the Derbugan metallogenic belt. Aqueous (W-type), CO2-rich (WC-type), vapor-rich (V-type), and daughter mineral–bearing inclusions (S-type) were recognized in the hydrothermal quartz. Fluid inclusions in the early stage show high homogenization temperatures (247–374 ℃) and high pressures (66–191 MPa), with salinities of 0.95–12.15 wt% NaCl equiv. The presence of anhydrite in veins and phenocrysts suggests high oxygen fugacity conditions. The veins in the Mo- and Cu mineralization stages were formed by immiscible fluids at temperatures of 244–366 ℃ and 183–342 ℃, and pressures of 39–137 MPa and 30–98 MPa, respectively. They yielded salinities of 1.69–10.52 and 0.35–6.85 wt% NaCl equiv, respectively. Homogenization temperatures of fluid inclusions in late stage range from 194 to 269 ℃ with a salinity of 0.6–5.88 wt% NaCl equiv. It is concluded that the ore-forming fluids in the early stage were magmatic in origin, then gradually diluted and cooled by meteoric water. During this process, fluid immiscibility and CO2 release induced by pressure drop and oxygen fugacity decrease resulted in Mo mineralization, while temperature drop was the main trigger for the precipitation of chalcopyrite. Laser-Raman analyses suggest that the fluid inclusions of the Badaguan porphyry Cu-Mo deposit in Inner Mongolia are rich in CO2. Minor CH4 occurs in the Mo- and Cu-mineralization stage, which may be generated during water–rock interaction. The hybrid carbon isotope compositions of the fluid inclusions, mainly representing CO2, show a very depleted δ13CPDB (−21‰ to −28.3‰), which can be explained by the joint influence of ~ 0.5% subducted oceanic sedimentary contamination in the mantle and carbon isotope fractionation induced by 35–80% CO2 degassing. Petrographic observation, previous studies on the geochemical and isotopic characteristics of ore-causative rocks rule out the possibility of contamination of ascending magma with reducing wall rocks. The large variations in δ18Owater (−6.6‰ to +2.4‰) for quartz and calcite suggest the participation of meteoric water during water–rock interaction.