Fluid origin and evolution of the Pusangguo Cu–Pb–Zn skarn deposit in Tibet: Constraints from fluid inclusions and isotope compositions

Abstract

he Pusangguo Cu–Pb–Zn skarn deposit is a representative Co-bearing deposit in the Gangdese metallogenic belt. The mineralization mainly has a close relationship with the biotite granodiorite. Here we identified four paragenetic stages during the ore formation: prograde skarn stage (stage 1) dominated by garnet and less diopside, retrograde stage (stage 2) which is characterized by epidote, chlorite and actinolite, quartz polymetallic sulfide stage (stage 3) featured by chalcopyrite, sphalerite, galena with minor cobaltite and aikinite, and quartz calcite stage (stage 4). In this study, fluid inclusion, H–O, He–Ar, S and Pb isotope compositions were determined to constrain the sources and evolution of the ore-forming fluids, metal deposition mechanisms and metallogenetic model for the Pusangguo deposit. The He–Ar isotopic compositions (3He/4He = 0.39–0.55 Ra, 40Ar/36Ar = 299–366) show that the ore-forming fluids predominantly derived from crust source with minor mantle input. The H–O isotopic analysis results (δ18OH2O = −13.3 to + 4.9 ‰, δD = −168 to −79.5 ‰) suggest that the ore-forming fluids were sourced from a magmatic origin that mixed with some meteoric water. The S–Pb isotopic results suggest that the ore-forming materials were mostly derived from the upper crust with a minor amount of mantle materials and the biotite granodiorite magma contributed most of the lead and possibly other metals. Fluid inclusion petrography and microthermometry results show that the fluid inclusion assemblages developed in the different mineralization stages of Pusangguo deposit changed from type II (vapor-rich two-phase fluid inclusions) + type III (daughter mineral-bearing three-phase fluid inclusions) to type I (liquid-rich two-phase fluid inclusions) + type II + type III + type IV and eventually evolved into type II. Correspondingly, the ore-forming fluids mainly belonged to H2O-NaCl solutions and changed from high temperature (413–532 °C), moderate–high salinity (15.5–53.1 wt% NaCl equiv.) fluid system in stage 1 to a lower-temperature (176–234 °C) and lower-salinity (0.7–4.2 wt% NaCl equiv.) meteoric water during stage 4. Fluid boiling and mixing were likely the vital factors controlling the metal precipitation.