Ore genesis and hydrothermal evolution of the Kendenggao’er copper-molybdenum deposit, western Tianshan: Evidence from isotopes (S, Pb, H, O) and fluid inclusions

Abstract The Boluokenu Range is one of the most important metallogenic belts in northwest China, and it contains numerous Fe, Cu, Mo, Au etc deposits. The Kendenggao’er Cu-Mo deposit is located in the eastern segment of the Boluokenu metallogenic belt within the Western Tianshan orogeny. Skarn and Cu-Mo orebodies occur as lamellae or lenses along the contacts between different units of the Upper Carboniferous Dongtujinhe Formation and the Aimujin granitic intrusions. Based on field investigations and petrographic observations, three main metallogenic stages of skarn formation and ore deposition have been distinguished: (1) The pre-ore stage is characterized by the formation of garnet, wollastonite and diopside and high-temperature, hypersaline hydrothermal fluids with oxygen isotopic compositions that are similar to those of magmatic fluids. These fluids were most likely generated by the separation of brine from the silicate melt due to the immiscibility of primary magmatic fluids that occurred at a depth of approximately 1 km with inferred lithostatic pressure conditions of 300 bars. (2) The main-ore stage coincides with the replacement of garnet and diopside by vesuvianite, epidote, sulfides and quartz. The hydrothermal fluids of this stage are represented by V-type (vapor-rich) fluid inclusions that coexist with L-type (liquid-rich) inclusions. The oxygen isotope (δ 18 O H2O ) values of the main-ore stages are in a narrow range of 7.5–9.9‰, which indicates that the mineralization-related fluid is also of magmatic origin. Based on the fluid inclusion and stable isotopic data, it can be concluded that the ore-forming fluid evolved by the boiling of the magmatic fluids that likely occurred at a depth of ∼1 km and hydrostatic pressure of ∼100 bars, and that this was the predominant mechanism for the large-scale precipitation in the Kendenggao’er Cu-Mo deposit. (3) The post-ore stage is characterized by the development of calcite, quartz and chlorite. The fluids during this stage are represented by the L-type inclusions, which mainly show low-salinity and low-homogenization temperatures. These fluids yield the lowest δ 18 O H2O values (−2.8 to 3.7‰). The above data indicate that the post-ore stage fluids originated from the mixing of residual main-ore stage fluids with various proportions of external water (low salinity, low temperature). No boiling occurred at this stage at the low temperatures. The δ 34 S values of the sulfides range from −4.3 to 1.9‰, and the 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb values of the sulfides range from 18.220 to 18.554, 15.608 to 15.812 and 38.080 to 39.055, respectively. The S and Pb isotopic compositions indicate that the ore materials were mainly derived from magmas. Taken together, these geologic, geochemical, and isotopic data confirm that Kendenggao’er is a typical skarn deposit.