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:H_2O-CO_2 inclusions,carbonic fluid inclusions,aqueous fluid inclusions,and daughter mineral-bearing fluid inclusions.Microthermometric studies show that solid CO_2 melting temperatures(T_(m,CO2)) of H_2O-CO_2 inclusions in Ql are from-62.3℃ to-58.5C,clathrate melting temperatures(T_(m,clath)l) are from 0.5 C to 7.5 C,partial homogenization temperatures(T_(h,CO2)) vary from 3.3℃ to 25.9℃(to liquid),and the total homogenization temperatures(T_(h,tot)) vary from 285℃ to 378℃,with the salinities being 4.9%-15.1%NaCl eqv.and the CO_2-phase densities being 0.50-0.86 g/cm~3.H_2O-CO_2 inclusions in Q2 have T_(m,CO_2) from-61.9℃ to-56.9℃,T_(m,clath)from 1.3℃ to 9.5℃,T_(h,CO2) from 3.4℃ to 28.7℃(to liquid),and T_(h,tot) from 242℃ to 388℃,with the salinities being 1.0%-15.5%NaCl eqv.and the CO_2-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 δ~(34)S 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 theδ~(18)O_(H2O) values calculated from δ~(18)O of quartz range from-0.2‰ to 8.3‰(SMOW).The δD-δ~(18)O_(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.

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,co2) of H₂O–CO₂ inclusions in Q1 are from ?62.3°C to ?58.5°C, clathrate melting temperatures (T_m,clath) are from 0.5°C to 7.5°C, partial homogenization temperatures (T_h,co2) vary from 3.3°C to 25.9°C (to liquid), and the total homogenization temperatures (T_h,tot) vary from 285°C to 378°C, with the salinities being 4.9%‐15.1% NaCl eqv. and the CO₂‐phase densities being 0.50–0.86 g/cm³. H₂O–CO₂ inclusions in Q2 have T_m,c02 from ?61.9°C to ?56.9°C, T_m,clath from 1.3°C to 9.5°C, T_h,co2 from 3.4°C to 28.7°C (to liquid), and T_h,tot from 242°C to 388°C, with the salinities being 1.0%‐15.5% NaCl eqv. and the CO₂‐phase densities being 0.48–0.89 g/cm³. The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260–360 MPa and 180–370 MPa, respectively. The δ³⁴S values of pyrite from the volcanic sedimentary period vary from 2.3‰ 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 the δ¹⁸O_H2O values calculated from δ¹⁸O of quartz range from ?0.2‰ to 8.3‰ (SMOW). The δD‐δ¹⁸O_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.