Geology and geochemistry of the Bulong quartz–barite vein-type gold deposit in the Xinjiang Uygur Autonomous Region, China

Abstract

The Bulong gold deposit, located in Akqi County, Southwest Tianshan, China, occurs in Upper Devonian fine-grained clastic rocks. Gold mineralization is controlled by a series of gently tilted fractured zones. Four ore-bearing quartz–barite veins have been recognized in the Bulong ore district. These range in length from 15 to 660 m and are up to 1 m thick. On the basis of field evidence and petrographic analysis, four stages of vein emplacement and hydrothermal mineralization can be distinguished: (1) an early quartz stage, characterized by the occurrence of massive quartz veins; (2) a barite vein stage; (3) a barite–quartz stage; this represents the main stage of gold mineralization in the Bulong deposit and is characterized by the formation of laminated quartz veins, barite–quartz veins and locally calcite–barite–quartz veins in quartz–barite veins; and (4) a late-stage ankerite–calcite veinlet stage. The ore in these veins occurs as native gold (± pyrite) and is predominantly hosted in quartz and barite. Two types of fluid inclusions, namely H 2O–NaCl and H 2O–CO 2 (± CH 4/N 2)–NaCl types, have been recognized in quartz, barite and calcite. Homogenization temperatures of fluid inclusions vary from 159 to 390 °C, with T h during the main stage of gold mineralization ranging from 200 to 340 °C. Salinity ranges from 2.4 to 46.2 wt.% NaCl equiv. Ore fluid densities range from 0.73 to 1.13 g/cm 3. The δ 34S values of pyrite associated with gold mineralization range from 14.6‰ to 19.2‰ and those of barite from 35.0‰ to 39.6‰, indicating that the sulfur was probably derived from the sedimentary host rocks. The 3He / 4He ratios of fluid inclusions in pyrite are 0.24–0.82 ( R / R a), approximating helium ratios of the crust. The 40Ar / 36Ar ratios vary from 338 to 471, slightly higher than those indicative of atmospheric argon. The 40Ar / 4He ratios of the ore fluids range from 0.02 to 0.41 with a mean of 0.15. Helium and argon isotope compositions of fluid inclusions suggest that the ore fluids of the Bulong gold deposit were mainly derived from the crust. The δ 13C PDB values of fluid inclusions in vein quartz from the Bulong gold deposit define a narrow range of − 4.6‰ to − 1.4‰. The δ 18O SMOW values of vein quartz range from 17.2‰ to 21.1‰, with corresponding δ 18O fluid values of 6.7‰ to 14.7‰, and δD values of fluid inclusions of between − 70‰ and − 55‰. The combined isotopic data imply that the ore-forming fluids of the Bulong gold deposit were mainly derived from basinal fluids, with some minor contributions from magmatic fluids and meteoric water, and the carbon in the ore fluids was mainly derived from marine carbonate rocks. The temperature and pressure decrease, local ore-forming fluid boiling, water–rock exchange, changes in composition of ore fluids played important roles in ore-forming processes of the Bulong gold deposit.