Genesis of the Arqiale Pb-Zn-Cu Deposit in the Western Tianshan, Xinjiang: Evidence from Fluid Inclusions and Isotopes

The Arqiale Pb–Zn–Cu deposit is located in the southwestern margin of the Wusun Mountain in the Western Tianshan, Xinjiang Province. The orebodies occur in the limestone of Lower Carboniferous Akeshake Formation and are generally consistent with the strata in occurrence. Considering that the orebodies are stratabound and no magmatic rocks are identified in the orefield, whether the deposit is related to magmatism remains controversial. Ore–forming process can be divided into four stages, including garnet-pyroxene stage (I), actinolite–ilvaite stage (II), quartz–calcite–polymetallic sulfide stage (III) and carbonate stage (IV). Two types of inclusions have been identified in the actinolite from stage Ⅱ and quartz, calcite and sphalerite from stage Ⅲ, including the two–phase aqueous inclusions (L–V type) and mono-phase liquid aqueous inclusions (L type). The L–V type inclusions in actinolite have homogenization temperatures and salinities ranging from 278℃ to 425 ℃ and 2.1 wt.% NaCl eqv to 13.0 wt.% NaCl eqv, respectively. By contrast, the L–V type inclusions in stage III hydrothermal minerals have homogenization temperatures and salinities ranging from 162℃ to 342 ℃ and 0.5 wt.% NaCl eqv to 9.0 wt.% NaCl eqv, respectively. Fluid inclusions and C–H–O isotopic compositions indicate that the initial ore-forming fluids were mainly source from magmatic water, with increasing input of meteoric water with time, leading to the decrease of temperatures and salinities, as well as the precipitation of ore-forming materials. The δ³⁴S rations of sulfides in the ores have a wide range (−7.57‰～1.30‰), and the Pb isotopic compositions have the characteristics of crust–mantle mixing. Combined evidence from geology, fluid inclusions and S–Pb–C–H–O isotopes indicate that the Arqiale Pb–Zn–Cu deposit belongs to the distal skarn type deposit, with the ore–forming materials sourcing partially from the magmatic rocks at depth and partially from the strata. The orebodies in the ore field gradually transit from Pb–Zn orebodies at shallow in the south to Cu ± Zn orebodies at depth in the north, implying that the concealed causative intrusions and skarn Cu orebodies in the contact zone may occur in the deep part in the north of the mining area.