A Fluid Inclusion Study of Vein-type Copper Mineralization in the East Wulasigou Pb-Zn-Cu Deposit, Altay, Northwestern China

The Wulasigou Cu-Pb-Zn deposit, located 15km northwest of Altay city in Xinjiang, is one of many Cu-Pb-Zn polymetallic deposits in the Devonian Kelan volcanic-sedimentary basin in southern Altaids. Two mineralizing periods can be distinguished: the marine volcanic sedimentary Pb-Zn mineralization period, and the metamorphic hydrothermal Cu mineralization period, which is further divided into an early bedded foliated quartz vein stage (Q1) and a late sulfide-quartz vein stage (Q2) crosscutting the foliation. Four types of fluid inclusions were recognized in the Q1 and Q2 quartz from the east orebodies of the Wulasigou deposit: H2O-CO2 inclusions, carbonic fluid inclusions, aqueous fluid inclusions, and daughter mineral-bearing fluid inclusions. Microthermometric studies show that solid CO2 melting temperatures (T-m,T-CO2) of H2O-CO2 inclusions in Q1 are from -62.3 degrees C to -58.5 degrees C, clathrate melting temperatures (T-m,T-clath) are from 0.5 degrees C to 7.5 degrees C, partial homogenization, temperatures (T-h,T-CO2) vary from 3.3 degrees C to 25.9 degrees C (to liquid), and the total homogenization temperatures (T-h,T-tot) vary from 285 degrees C to 378 degrees C, with the salinities being 4.9%-15.1% NaCl eqv. and the CO2-phase densities being 0.50-0.86 g/cm(3). H2O-CO2 inclusions in Q2 have T-m,T-CO2 from -61.9 degrees C to -56.9 degrees C, T-m,T-clath from 1.3 degrees C to 9.5 degrees C, T-h,T-CO2 from 3.4 degrees C to 28.7 degrees C (to liquid), and T-h,T-tot from 242 degrees C to 388 degrees C, with the salinities being 1.0%-15.5% NaCl eqv. and the CO2-phase densities being 0.4-1.89 g/cm(3). The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260-360 MPa and 180-370 MPa, respectively. The delta S-34 values of pyrite from the volcanic sedimentary period vary from 2.3 parts per thousand to 2.8 parts per thousand (CDT), and those from the sulfide-quartz veins fall in a narrow range of -1.9 parts per thousand to 2.6 parts per thousand (CDT). The OD values of fluid inclusions in Q2 range from -121.0 parts per thousand to -100.8 parts per thousand (SMOW), and the delta O-18(H2O) values calculated from delta O-18 of quartz range from -0.2 parts per thousand to 8.3 parts per thousand (SMOW). The delta D-delta O-18(H2O) data are close to the magmatic and metamorphic fields. The fluid inclusion and stable isotope data documented in this study indicate that the vein-type copper mineralization in the Wulasigou Pb-Zn-Cu deposit took place in an orogenic-metamorphic enviroment.