Characteristics Of Ore-forming Fluid Of The Zhunuo Porphyry Copper Deposit In The Gangdese Belt, Tibet

The Gangdese porphyry copper belt is one of the most important copper belts in China. Many large and super-large porphyry Cu-Mo(-Au) deposits have been discovered in the eastern part but only one large porphyry Cu deposit at Zhunuo was found in the western part of the belt. The metallogenesis of porphyry deposit in the western part of Gangdese belt is poorly constrained, which is not helpful for studying the difference of ore-forming condition and metallogenesis between western and eastern belt. Therefore, we studied the characteristics of the petrography, microthermometry, and Raman spectrum of fluid inclusions(FIs) of the Zhunuo porphyry copper deposit and compared them with the Qulong deposit in the eastern part of the Gangdese belt. Four types of fluid inclusions(FIs) are distinguished in this study based on petrographic and microthermometric criteria, i.e. rich vapor phase aqueous, rich liquid phase aqueous, rich CO2 and daughter mineral-bearing fluid inclusions. The inclusions from A vein through B vein to D vein have homogenization temperatures ranging from 350°-550°C, 250°-350°C, 250°-300°C and salinities ranging from 5%-55%NaCleqv, 5%-40%NaCleqv, and 2%-10%NaCleqv, respectively, which reveals that the temperature and salinity decreased from early to late stage. At B vein stage, different types of FIs were observed in one field of vision and show a similar temperature and wide-ranged salinity, indicating fluid boiling and decrease of pressure which are benefit for the minerals precipitation. The pressure estimation in Zhunuo deposit shows that the depth is 2.9 km, 2.7 km, and 2.3 km from early stage to the late stage. The δ18O and δD deplete mainly because of water-rock reaction, magmatic degassing progress and meteoric water. The types, temperatures, and salinities of FIs for the Zhunuo deposit are similar to those for the Qulong deposit. However, the ore-forming depth at Zhunuo is relatively lower than that at Qulong, which is helpful for the prospecting in the western part of the Gangdese belt because the erosion degree of the western part is lower than that of the eastern part. In addition, anhydrites were not widely present in the Zhunuo deposit compared with Qulong deposit, indicating that the oxygen fugacity of fluid at Zhunuo is lower than that at Qulong.