A combined fluid inclusion and S–H–O–He–Ar isotope study of the Devonian Ashele VMS-type copper–zinc deposit in the Altay orogenic belt, northwest China

Abstract

The Altay orogenic belt of Kazakhstan and China incorporates one of the world-class volcanogenic massive sulfide (VMS) polymetallic metallogenic belt. More than 12 large and superlarge VMS deposits have been found in this belt. The Ashele Basin is located at the western end of the Chinese Altay bordering Kazakhstan and hosts the large Ashele copper–zinc deposit, which is a typical VMS deposit and the largest one in this basin. The principal orebodies are stratabound and located between basalt and tuff units in the Ashele Formation. The mineralization has a two-layered structure that consists of an upper stratiform, concordant, massive Cu–Zn (or barite) orebody, and a lower veinlet–disseminated and stockwork Cu (or Cu–Pb–Zn–Ag) orebody associated with silicic alteration. Several exhalative rocks, such as barite, hematitic jasper, pyrite and silicified units also occur.Here we investigate the ore-forming fluids of the exhalative–sedimentary units including jaspilte, stratiform barite and Cu–Zn orebodies which carry fluid inclusions that show homogenization temperatures of 100–410 °C, with two peaks around 230 and 150 °C, and low salinity (clustering between 2 and 8 wt% NaCl equiv), corresponding to NaCl–H2O fluids. The ore-forming fluids of the vein orebody and silicified zones are characterized by low to medium temperature (peak homogenization temperatures between 120 and 280 °C), low to medium salinity (0.7–12.3 wt% NaCl equiv), and H2O–CO2 (±CH4/N2)–NaCl fluids. The δ34S values of barite associated with the stratiform barite orebody range from 20.4‰ to 21.4‰, indicating that the sulfur was derived from the seawater. The sulfur isotope values of sulfide (−3.7‰ to 7.7‰ with a mean of 3.1‰) are similar to those of VMS deposits from other parts of the world. The host volcanic rocks are inferred to be the major source from which the mineralizing fluid leached the ore-forming elements. The 3He/4He ratios of fluid inclusions in pyrite are in the range of 0.136–0.260 (R/Ra), broadly similar to the helium ratios of the crust. The 40Ar/36Ar ratios range from 394 to 9515, and are higher than those of atmospheric argon. The 40Ar/4He ratios of the ore fluids range from 0.002 to 0.064 with a mean of 0.034. The helium and argon isotope compositions of fluid inclusions suggest that the ore fluids of the Ashele deposit were mainly derived from the crust and were mixed with a minor amount of mantle component. The δ18OSMOW values of quartz from the silicified zones and vein orebody range from 8.3‰ to 11.1‰, with corresponding δ18Ofluid values of −4.45‰ to 2.24‰, whereas the δD values of fluid inclusions are between −140‰ and −90‰. The δ18OSMOW values of barite from the stratiform barite orebody range from 7.7‰ to 9.1‰, with corresponding δ18Ofluid values of –3.03‰ to 1.79‰, with δD values of fluid inclusions showing a range of –162‰ and –135‰. The combined isotopic data (H–O and He–Ar) suggest that the ore-forming fluids of the Ashele deposit were mainly derived from deep circulation of seawater and mixed with magmatic fluids. The decrease in temperature and pressure, water–rock exchange, and changes in the composition of ore fluids played important roles in the ore-forming processes of the Ashele Cu–Zn deposit.