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 15 km 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 PbZn 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:HO-COinclusions,carbonic fluid inclusions,aqueous fluid inclusions,and daughter mineral-bearing fluid inclusions.Microthermometric studies show that solid COmelting temperatures(T) of HO-COinclusions in Ql are from-62.3℃ to-58.5C,clathrate melting temperatures(Tl) are from 0.5 C to 7.5 C,partial homogenization temperatures(T) vary from 3.3℃ to 25.9℃(to liquid),and the total homogenization temperatures(T) vary from 285℃ to 378℃,with the salinities being 4.9%-15.1%NaCl eqv.and the CO-phase densities being 0.50-0.86 g/cm~3.HO-COinclusions in Q2 have Tfrom-61.9℃ to-56.9℃,Tfrom 1.3℃ to 9.5℃,Tfrom 3.4℃ to 28.7℃(to liquid),and Tfrom 242℃ to 388℃,with the salinities being 1.0%-15.5%NaCl eqv.and the CO-phase densities being 0.48-0.89 g/cm~3.The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260-360 MPa and180-370 MPa,respectively.The δS values of pyrite from the volcanic sedimentary period vary from2.3‰ to 2.8‰(CDT),and those from the sulfide-quartz veins fall in a narrow range of-1.9‰ to 2.6‰(CDT).The δD values of fluid inclusions in Q2 range from-121.0‰ to-100.8‰(SMOW),and theδOvalues calculated from δO of quartz range from-0.2‰ to 8.3‰(SMOW).The δD-δOdata 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.