H–O, S and Pb isotope geochemistry of the Awanda gold deposit in southern Tianshan, Central Asian orogenic belt: Implications for fluid regime and metallogeny

Abstract

The Awanda gold deposit is located within the southern Tianshan Accretionary Complex, a late Paleozoic orogenic belt in the northern margin of Tarim block in NW China. The gold mineralization here occurs in primarily within shear zones and secondary s faults with quartz mica schist as the principal host rock. The sulfide mineral assemblage is dominated by arsenopyrite, and pyrrhotite, with minor pyrite and gudmundite. Alteration includes silicification, sericitization and rare carbonatization. The measured δ18O values for the gold-bearing quartz range from 15.9 to 20.3‰, and the estimated δ18O fluid values range from 7.8 to 12.2‰. The δD values for fluid inclusions from quartz range from −116.3 to −87.6‰. The δ34S values of gold-associated sulfide minerals range from −3.6 to +6.2‰, and the calculated δ34SH2S values of H2S for the hydrothermal fluids range from −4.8 to +5.8‰, with an average of −0.4‰. Lead isotopic compositions of sulfides from gold-bearing ores show 206Pb/204Pb ratios ranging from 17.4625 to 18.5356, 207Pb/204Pb ratios from 15.4943 to 15.6468, and 208Pb/204Pb ratios from 37.8825 to 38.8238, respectively. Collectively, the isotope (H, O, S, Pb) characteristics suggest that the ore-forming fluids were of metamorphic origin derived through dehydration of carbonaceous black shale with the sulfur sourced from crustal components, and the lead from a mixture of host rocks and ore-forming metamorphic fluids. This deposit can be classified as orogenic gold deposit and share many major characteristics with those of Muruntau-type gold deposits. During late Paleozoic, the southern Tianshan Accretionary Complex was intensely deformed into an orogenic belt by folding and thrusting, generating regional thrust faults at different scales. The ore-controlling subsidiary faults also formed subsequently, with the ore-forming fluids derived from dehydration of deeply buried carbonaceous black shale and the ore fluids channeled into shear zones leading to gold precipitation.