Fluid evolution of the Qiman Tagh W-Sn ore belt, East Kunlun Orogen, NW China

Abstract

The Qiman Tagh W-Sn ore belt is located in the westernmost sector of the East Kunlun Orogen, NW China. It has been recognized as a unique W-Sn belt that formed in the early Paleozoic and related to closure of the Proto-Tethys. To understand the evolution of ore-forming fluids and its relationship with the tectonic setting of East Kunlun Orogen, we report the results obtained from fluid inclusion and H-O isotopic studies of ores and quartz veins for the Qiman Tagh W-Sn ore belt. Mineralization in Qiman Tagh includes four stages characterized by quartz-cassiterite-wolframite assemblage stage 1, quartz ± scheelite assemblage stage 2, quartz-polymetallic sulfides stage 3, and ore-barren veins stage 4. The former two stages are conducive to mineralization, while the latter two stages are less important. The fluid inclusions are distinguished between CO2-H2O (C-type) and NaCl-H2O (W-type) in composition, containing a trace of CH4, N2, C2H6, SO2, and CO32–. Cassiterite and quartz in stage 1, instead of wolframite, contain a great deal of C-type inclusions. All inclusions in minerals of stage 1 yield homogenization temperatures of 230.1–384.1 °C (peaking at 310–320 °C), with salinities lower than 14.76 wt% NaCl equiv. and bulk densities of 0.63–0.89 g/cm3. The stage 2 minerals contain both two types of inclusions, yielding homogenization temperatures of 183.4–335.9 °C (peaking at 280–290 °C), with salinities lower than 14.53 wt% NaCl equiv. and bulk densities of 0.66–0.97 g/cm3. Fluid inclusions in minerals of stages 3 and 4 are mainly W-type and homogenized at temperatures of 140.6–277.6 °C (peaking 210–220 °C), and 116.9–255.1 °C (peaking 160–170 °C), respectively. The H-O isotopic systematics shows that the fluids were dominated by magmatic water in stages 1 and 2, but by meteoric water in stages 3 and 4. Integrating all the geological and geochemical data, we conclude that the fluids forming the Qiman Tagh W-Sn ore belt evolved from granite-derived, CO2-rich and reducing, to meteoric water-dominated, CO2-poor and oxidizing. Fluid immiscibility, cooling and interaction with rocks are main mechanisms for metallic deposition.