Origin of the Dachang gold deposit,NW China: constraints from H,O, S,and Pb isotope data

The Dachang gold deposit is located in the Late Triassic Songpan-Ganzi Fold Belt, NE Tibetan Plateau. Gold ore is concentrated as veins along secondary faults and fracture zones in the Bayan Har Group metaturbidites. No exposed felsic plutons are present in the vicinity of the deposit. The auriferous veins contain <15% sulphide minerals, mainly arsenopyrite, pyrite, and stibnite. Gold is commonly enclosed within arsenopyrite and pyrite. Typical alteration around the ore bodies includes silicification, sericitization, and weak carbonatization.

Gold-bearing quartz samples have δ¹⁸O values of 16.9–21.2‰ (V-SMOW) from which δ¹⁸O_H2O values of 6.2–9.6‰ can be calculated from the fluid inclusion temperatures (or 10.0 to 12.7‰ if we used the average arsenopyrite geothermometer temperature of 301°C). The δD values of fluid inclusions in quartz range from –90‰ to –72‰. δ³⁴S values of gold-bearing sulphides mainly range from –5.9‰ to –2.8‰ (V-CDT). Pyrite and arsenopyrite in ores have ²⁰⁶Pb/²⁰⁴Pb ratios of 18.2888 to 18.4702, ²⁰⁷Pb/²⁰⁴Pb ratios of 15.5763 to 15.6712, and ²⁰⁸Pb/²⁰⁴Pb ratios of 38.2298 to 38.8212. These isotopic compositions indicate that the ore-forming fluids were of metamorphic origin, and the S and Pb may have been derived from the host metaturbidites of the Bayan Har Group. The Dachang Au deposit has geological and geochemical features similar to orogenic gold deposits. We propose that the ores formed when the Songpan-Ganzi Fold Belt was intensely deformed by Late Triassic folding and thrusting. Large-scale thrusting resulted in regional allochthons of different scales, followed by secondary faults or fracture zones that controlled the ore bodies.