Genesis of the Bangbu Orogenic Gold Deposit, Tibet: Evidence from Fluid Inclusion, Stable Isotopes, and Ar‐Ar Geochronology

Abstract

AbstractThe Bangbu gold deposit is a large orogenic gold deposit in Tibet formed during the Alpine‐Himalayan collision. Ore bodies (auriferous quartz veins) are controlled by the E‐W‐trending Qusong‐Cuogu‐Zhemulang brittle‐ductile shear zone. Quartz veins at the deposit can be divided into three types: pre‐metallogenic hook‐like quartz veins, metallogenic auriferous quartz veins, and post‐metallogenic N‐S quartz veins. Four stages of mineralization in the auriferous quartz veins have been identified: (1) Stage S1 quartz+coarse‐grained sulfides, (2) Stage S2 gold+fine‐grained sulfides, (3) Stage S3 quartz+carbonates, and (4) Stage S4 quartz+ greigite. Fluid inclusions indicate the ore‐forming fluid was CO2‐N2‐CH4 rich with homogenization temperatures of 170–261°C, salinities 4.34–7.45 wt% NaCl equivalent. δ18Ofluid (3.98‰–7.18‰) and low δDV‐SMOV (–90‰ to –44‰) for auriferous quartz veins suggest ore‐forming fluids were mainly metamorphic in origin, with some addition of organic matter. Quartz vein pyrite has δ34SV‐CDT values of 1.2‰–3.6‰ (an average of 2.2‰), whereas pyrite from phyllite has δ34SV‐CDT 5.7‰–9.9‰ (an average of 7.4‰). Quartz vein pyrites yield 206Pb/204Pb ratios of 18.662–18.764, 207Pb/204Pb 15.650–15.683, and 208Pb/204Pb 38.901–39.079. These isotopic data indicate Bangbu ore‐forming materials were probably derived from the Langjiexue accretionary wedge. 40Ar/39Ar ages for sericite from auriferous sulfide‐quartz veins yield a plateau age of 49.52 ± 0.52 Ma, an isochron age of 50.3 ± 0.31 Ma, suggesting that auriferous veins were formed during the main collisional period of the Tibet‐Himalayan orogen (∼65–41 Ma).