Re–Os systematics of sulfides (chalcopyrite, bornite, pyrite and pyrrhotite) from the Jiama Cu–Mo deposit of Tibet, China

Abstract The Jiama Cu–Mo deposit, one of the largest skarn-porphyry deposits in China currently being mined, is located in the eastern part of the Gangdese porphyry Cu belt, Tibet, China. Chalcopryite and bornite concentrate in the Skarn Ore Zone at Jiama with molybdenite concentration, whereas molybdenite occurs in the Porphyry Ore Zone and Hornfels Ore Zone relatively by itself. Cu and Mo have different distributions in the Porphyry Ore Zone with Cu concentrating above the porphyry and with Mo inside of porphyry intrusion. The direct Re–Os dating on Cu sulfides will be helpful for us to understand the Cu and Mo mineralization at Jiama. This study is the first attempt to use the Re–Os isotope system to directly date chalcopyrite, bornite, pyrite and pyrrhotite at Jiama. The contents of Re, common Os and 187 Os in these four sulfides are 0.145–32.9 ppb, 0.0022–0.0669 ppb and 0.0003–0.0118 ppb, respectively. They have very low ratios of 187 Os to common Os (0.09–0.55), therefore, they are different from low-level highly radiogenic (LLHR) sulfides. The plot of 187 Re/ 188 Os and 187 Os/ 188 Os did not yield an isochron age based on the scatter points. Even if age of 15 Ma based on molybdenite Re–Os age is used to calculate the initial 187 Os/ 188 Os, the ratios show a wide range and low ratios of 187 Re/ 188 Os ( 187 Os/ 188 Os. The geology suggests the Cu and Mo mineralizations at Jiama are both related to porphyry intrusions, and the Cu mineralization should also have occurred in Miocene based on the molybdenite Re–Os age and porphyry zircon U–Pb ages.