Ore genesis of skarn mineralization in continental collision orogens: A case study from the Pusangguo Co-bearing Cu-Pb-Zn deposit in Tibet

The Gangdese belt in southern Tibet, is renowned for hosting numerous collision-related porphyry Cu-Mo deposits. The recent discovery of Pusangguo Co-bearing Cu-Pb-Zn skarn deposits is the first reported Co-bearing deposit in Gangdese belt as well as in Tibet. Researches on the geological and metallogenic characteristics of Pusanguo indicate that this Co-bearing Cu-Pb-Zn deposit was formed in a collisional setting. We document mineralogy, fluid inclusion studies, and H-O-S-Pb isotope systematics of the skarns and ores of the Pusangguo deposit. Intense endoskarn alteration, dominated by green and pink epidote and quartz, is widely disseminated throughout the host granodiorite as well as in veinlets. Exoskarn alteration is characterized by the presence of prograde grandite (andradite(32-100) and grossular(0-68)) with minor pyroxene (diopside(0-)(46), hedenbergite(38-74), and johannsenite(1-)(62)). Retrograde epidotes from both the endoskarn and exoskarn show a narrow range of Fe3+/(Fe3+ + Al) ratios (0.25-0.34). Ore minerals include chalcopyrite, sphalerite and galena, with lesser pyrite, pyrrhotite, bornite, Co minerals and falkmanite. Four paragenetic stages of skarn formation and ore deposition are recognized in the Pusangguo Cu-Pb-Zn deposit: prograde (stage I), retrograde (stage II), early sulfide (stage III), and late sulfide (stage IV). Fluid inclusions in all stages are dominated by two-phase liquid-rich (type L) inclusions. During the major phase of mineralization (stage III), fluid inclusions formed at low temperatures (180-240 degrees C) and with low salinities (0.5-8.6 wt% NaCl equiv.). This is in contrast to other skarn deposits within the Gangdese porphyry copper belt (e.g., the Zhibula, Jiama, and Bangpu deposits) that formed from highly saline fluids at high temperatures. Hydrogen-Oxygen isotopic values obtained from garnet, epidote, quartz and calcite are delta D-H2O = -179 parts per thousand to - 213 parts per thousand and delta O-18(H2O) = -9.1 parts per thousand to 9.3 parts per thousand, whereas S-Pb isotopic values for sphalerite, chalcopyrite, pyrite and galena are delta S-34 = -1.0 parts per thousand to 1.2 parts per thousand, estimated delta S-34(fluid) = - 0.05 parts per thousand. Such data are indicative of ore-forming fluids and metals that originated from a magmatic-hydrothermal source related to granodiorite emplacement during the Miocene post-continental collision. Extensive mixing between the magmatic-hydrothermal and meteoric fluids led to the co-precipitation of Cu, Pb and Zn minerals from low-salinity fluids at relatively low temperatures. The Pusangguo Co-bearing Cu-Pb-Zn skarn deposit shows consistent features with other (porphyry-related) skarn deposits in the Gangdese belt, except for relatively low temperature and salinity. The research is of great significance for understanding cobalt as an associated mineral in the Gangdese sulfide skarn deposit in Tibet.