Fluid evolution of the Nulasai Cu deposit, Xinjiang, NW China: Evidence from fluid inclusions and O-H-C isotopes

The Nulasai deposit is the earliest Cu mine in Xinjiang, NW China, and has been mined and smelted since the warring state period. Another striking characteristic of the deposit is the high-grade (ave. 1.34 % Cu) with bornite, chalcopyrite and chalcocite as main ore minerals. Through a field and petrographic investigation, three phases of hydrothermal evolution have been recognized at the Nulasai Cu deposit, including the stage I magnetite – quartz ± chalcopyrite ± bornite veins, the stage II calcite – barite ± chalcopyrite ± chalcocite ± pyrite ± bornite veins, and the stage III calcite – barite ± chalcopyrite ± sphalerite ± pyrite ± galena veins. Fluid inclusions of the stage I veins were captured under two-phase condition indicates that existence of both contemporaneous daughter mineral-bearing and liquid-rich fluid inclusions; they have an intermediate-high salinity (11.5 ∼ 38.5 wt% NaCl equiv) and homogenization temperature (266 ∼ 376 °C), with entrapment pressures from 47 to 167 bar (depth of approximately 0.5 to 1.7 km). The coexistence of vapor-rich and liquid-rich fluid inclusions was a defining feature of the stage II fluid inclusions, and they have an intermediate salinity (6.2 ∼ 10.7 wt.%NaCl equiv) and homogenization temperature (211 ∼ 287 °C), with entrapment pressures from 18 to 70 bar (depth of approximately 0.2 to 0.7 km). Fluid inclusions of the stage III veins were captured under two-phase condition, and intermediate-low salinity (3.2 ∼ 9.2 wt.%NaCl equiv) and homogenization temperature (155 ∼ 241 °C), with entrapment pressures of 6 to 32 bar (depth of 0.1 to 0.6 km). According to the stable isotope (O-H-C) data for the three mineralization phases, magmatic water containing organic carbon made up the majority of the early ore-forming fluids, whereas abundant meteoric water has been involved during the late stage of mineralization. Therefore, we suggest that fluid mixture between magmatic water and meteoric water may have led to simultaneous decrease of fluid temperature, salinity and their stable isotope values, and finally facilitates the Cu ore precipitation. This study emphasizes how high-grade Cu deposition in orogenic belts is initiated by fluid dilution.