Study of fluid inclusions in quartz veinlets in the Xuejiping porphyry copper deposit,Northwest Yunnan,China.

The Xuejiping porphyry copper deposit is located in the Zhongdian arc of the southern Yidun island arc in Sanjiang district.The porphyries related to copper mineralization consist of Indosinian quartz-dioritic porphyrite and quartz-monzonitic porphyry. There are obvious alteration zonations for mineralized porphyry rocks.From the center to the outer parts of the porphyry body,rocks are experienced from strong silicification,quartz-sericite alteration,clay-quartz-sericite alteration,and propylitic alteration respectively. Copper ore bodies mainly hosted in strong silicification zone and quartz-sericite alteration zone in the center of the porphyry body.The copper mineralization occurred mainly in type of stockwork veining,with little in type of veinlet-dissemination.A series petrographic, microthermometric and laser Raman spectroscopic studies on fluid inclusions (FI) in various kinds of ore rocks from the Xuejiping deposit have been carried out in this paper.Three types of fluid inclusions have been observed.They are aqueous FI,CO_2-bearing FI and daughter mineral-bearing FI respectively.Daughter minerals mainly include salt,calcite,hematite,with minor CaCl_2 hydrate and opaque sulfide.The homogenization temperatures (T_h),salinities,and densities of daughter mineral-bearing FI are mainly in ranges of 230～420℃,33.48%～75.40% NaClequiv.,and 1.01～1.09g/cm~3 respectively.Laser Raman spectroscopy of inclusions indicate that the principal fluid component is H_2O and the gas components are H_2O and CO_2.The coexistence of some aqueous FI and CO_2-rich FI,with similar T_h,and the existence of carbonic inclusions indicate the unmixing of ore-forming fluid generated by "secondary boiling" of primary magmatic fluid.It is believed that the unmixing of CO_2 and aqueous phases,the decrease of pressure and temperature and the increase of pH values of ore-foriming fluid are main factors leading to the sulfide deposition in the Xuejiping deposit.The fluid of low homogenization temperature and low salinity in inclusions of late stage quartz veins could be formed by the mixing of meteoric and evolved magmatic fluids,with insignificance to the mineralization.