Re-Os dating of molybdenite from the Shijiawan molybdenum deposit in Shaanxi Province and its geological implications

Located in the East Qinling molybdenum belt of Shaanxi Province,the Shijiawan ore deposit is a large porphyry deposit.Mineralization of the deposit consists of quartz-molybdenite veinlets and stockworks which occur in altered granite porphyry and wall rocks.Hydrothermal alteration is composed of potash feldspathization,silicification and sericitization.Direct Re-Os dating of molybdenites collected from different ores in the Shijiawan deposit was carried out to better understand the age of mineralization.The results show that molybdenite samples yield 187Re-187Os model ages ranging from 143.1±2.1 Ma to 145.1±2.2 Ma,with the average of 144.0±1.1 Ma(MSWD=0.91),and give an isochron age of 145.4±2.1 Ma(MSWD=0.83).These ages are close to the zircon U-Pb ages of Shijiawan feldspar-granite porphyry(141.4±0.6 Ma),indicating that mineralization occurred in Late Jurassic-Early Cretaceous period.In combination with the evidence from S isotope,Re content and chemical characteristics of the related pluton,the authors hold that the ore-forming materials were mainly derived from the crust,with the addition of a little mantle material.