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 187Os in these four sulfides are 0.145–32.9ppb, 0.0022–0.0669ppb and 0.0003–0.0118ppb, respectively. They have very low ratios of 187Os to common Os (0.09–0.55), therefore, they are different from low-level highly radiogenic (LLHR) sulfides. The plot of 187Re/188Os and 187Os/188Os did not yield an isochron age based on the scatter points. Even if age of 15Ma based on molybdenite Re–Os age is used to calculate the initial 187Os/188Os, the ratios show a wide range and low ratios of 187Re/188Os (<5000) is not suitable to calculate the model ages. The Re–Os results of these four types of sulfides indicate that Re–Os isotope system at Jiama has been affected by the disturbance. The black shale in the Cretaceous Linbuzong Formation is one possible source for Re–Os as we can see from the wide range of initial 187Os/188Os. 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.