Geology, geochemistry and genesis of Tabei: A newly identified intermediate‐sulphidation epithermal Pb–Zn deposit adjacent to low‐sulphidation Au deposit in the Tulasu Basin, Chinese Western Tianshan

The Tabei Pb-Zn deposit is located adjacent to the well-known Axi low-sulphidation epithermal Au deposit in the Tulasu Basin, Chinese Western Tianshan. The homogenization temperatures and salinities of H2O-rich biphase inclusions in ore-stage sphalerite and calcite from Tabei range from 100 to 170 degrees C and 0.4 to 6.2 wt% NaCleq, respectively. The delta C-13(V-PDB) and delta O-18(V-SMOW) values of calcite range from 0.9 parts per thousand to 1.5 parts per thousand and 5.6 parts per thousand to 6.7 parts per thousand, respectively. The delta O-18(H2O) and delta D-H2O values of hydrothermal fluids vary from -7.0 parts per thousand to -5.9 parts per thousand and -120.8 parts per thousand to -111.1 parts per thousand, respectively. The in-situ delta S-34(V-CDT) values of sulphides range from 7.5 parts per thousand to 8.0 parts per thousand (average 7.7 parts per thousand). The in-situ Pb isotopic compositions of galena and (Sr-87/Sr-86)(0) ratios of sphalerite resemble those of Au-bearing pyrite and quartz of Axi deposit, respectively, as well as the ore-hosting volcanic rocks. These isotopic compositions indicate that the ore metals were derived from the ore-hosting volcanic rocks, with the ore-forming fluids consisting predominantly of circulating meteoric water. The absence of coexisting two-phase vapour and liquid fluid inclusions and the presence of abundant banded coarse-grained quartz crystals with dog tooth and comb textures at Tabei, suggest that the sulphides precipitated slowly due to conductive cooling. The high sulphide abundance, sulphide assemblage (pyrite + Fe-poor sphalerite + galena + chalcopyrite), ore textures and isotopic compositions suggest that Tabei represents an intermediate-sulphidation epithermal deposit. The Axi-Tabei Au-Pb-Zn epithermal system constitutes a hybrid consisting of Au veins at shallow level and Pb-Zn veins at depth, suggesting that the base metals remain highly prospective beneath the identified Au orebodies at Axi.