Fluid inclusions, isotopes and mineralogy of the Kuruer epithermal Cu-Au deposit, Chinese Western Tianshan: Implications for ore genesis and exploration

The Kuruer Cu-Au deposit is located in the Wusun Range of Yili Block, Chinese Western Tianshan, where numerous epithermal Au, skarn Fe-Cu and porphyry Mo-Cu deposits have been recognized. Petrography, fluid inclusions, S-Pb-H-O isotopes, field emission-scanning electron microscopy (FE-SEM) plus energy dispersive spectrometer (EDS) and electron microprobe analysis (EMPA) have been carried out to constrain the genesis of the Kuruer deposit and its implication for further exploration. The minerlization is hosted by volcanic rocks and occur as veins and veinlets structurally controlled by NW-trending faults. Three ore-forming stages were recognized, including quartz-sulfides-bismuth-gold stage (I), quartz-chalcopyrite stage (II) and calcite-chloritechalcopyrite stage (III). Six types of fluid inclusions were identified in stage I quartz, including monophase vapor inclusions (type 1), liquid-rich biphase inclusions (type 2), vapor-rich biphase inclusions (type 3), H2O-rich CO2-H2O triphase inclusions (type 4), CO2-rich CO2-H2O triphase inclusions (type 5) and solid halite-bearing triphase inclusions (type 6), whereas only type 2 inclusions were recognized in stage II quartz and stage III calcite. Types 2, 3, 4 and 6 inclusions in stage I have homogenization temperature (T-h) ranges of 184-359 degrees C, 209-252 degrees C, 250-319 degrees C and 229-263 degrees C, with salinity ranges of 4.1-8.5 wt% NaCleq, 5.9-6.9 wt% NaCleq, 1.0-6.0 wt% NaCleq and 32.7-33.9 wt% NaCleq, respectively. Type 2 inclusions in stages II and III have Th varying from 144 to 212 degrees C and 114 to 163 degrees C, with salinities of 0.2-6.7 wt% NaCleq and 0.1-3.1 wt% NaCleq, respectively. The in-situ delta S-34 values of sulfides in stage I range from -4.66 to 0.34 parts per thousand. The Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 ratios of stage I chalcopyrite vary from 18.295 to 18.954, 15.568 to 15.673 and 38.109 to 38.817, respectively, which are consistent with those of Early Carboniferous Dahalajunshan Formation volcanic rocks and granitoids from the Wusun Range. The values of delta(OH2O)-O-18-VSMOW calculated from delta(18)Oquartz and delta DH2O-VSMOW of stage I quartz range from 0.0 to 2.4 parts per thousand and -118.6 to-109.3 parts per thousand, respectively. These isotopic compositions indicate that both metals and sulfur were sourced from the hosting volcanic rocks and hidden causative pluton, while the initial ore-forming fluids were magmatic in origin, with increasing involvement of meteoric water with time. Evidence from mineralogy, mineral geochemistry and fluid inclusions indicates that the initial fluid phase separation triggered the precipitation of "invisible" gold within pyrite and chalcopyrite during stage I. Subsequently, the decrease of f(S2) due to the escape of H2S(g) and sulfide precipitation and cooling caused the formation of liquid bismuth from solutions. The bismuth melts have scavenged Au in situ from the residual solutions, resulting in the formation of Bi-Au inclusions within the fractures and intergranular spaces of earlier pyrite. For stages II and III, dilution and cooling due to input of meteoric water facilitated the chalcopyrite precipitation. The Kuruer Cu-Au deposit is classified as a subepithermal mineralization associated with porphyries. Porphyry Cu-Au mineralization could have developed beneath the Kuruer and its vicinity.