Re–Os dating of pyrite from the Tuolugou stratabound Co(Au) deposit, eastern Kunlun Orogenic Belt, northwestern China

The Tuolugou cobalt–(gold) deposit, in the northeastern margin of the Qinghai–Tibetan plateau and central part of the eastern Kunlun orogenic belt, is the most recently discovered, large, cobalt–(gold) discovery in Qinghai Province, NW China. Exploration to date has identified a cobalt resource of greater than 20,000 t Co, with significant potential for expansion. Mineralization is largely confined to an E-W-striking volcano-sedimentary sequence consisting of four lithological units that underwent extensive greenschist-facies metamorphism and deformation. All orebodies occur in stratiform or lenticular bodies within quartz–albitite and sericite–quartz schists, and are concordant with lithological contacts. The ores contain various amounts of pyrite, the dominant ore mineral, and display massive (> 80% pyrite), banded (30 to 50% pyrite), and disseminated (< 30% pyrite) textures. The Tuolugou deposit shares striking similarities with other stratabound Cu–Co and Co–Cu–Au deposits in the world (e.g., Blackbird district, U.S.A.; Werner Lake deposit, Canada; Modum district, Norway).Re–Os dating shows that pyrite from different ore types has very low Re and Os concentrations in the range of 0.122–8.988 ppb and 0.003–0.054 ppb, respectively. Isotope data on seven pyrite samples yield an isochron age of 429 ± 29 Ma (2σ) with an initial 187Os/188Os ratio of 0 ± 2 (mean square weighted deviates = 7.9) on the 187Re/188Os-187Os/188Os plot, which are close to the age of the host strata, suggesting that primary cobalt mineralization in the Tuolugou deposit may have formed from syngenetic–exhalative processes. In addition, our results not only provide new evidence for plate rifting and mineralization taking place during the Early Silurian in the eastern Kunlun orogenic belt, but also suggest that the age of the regionally widespread Nachitai Group is early Paleozoic.