LAUSR

Abstract The Qunjsai copper deposit in West Tianshan, Xinjiang, NW China, with a reserve of 7.5 million metric tons of ore grading 0.57 wt% Cu, is characterized by mineralization occurring as sparsely disseminated chalcopyrite, bornite and chalcocite in a diabase dike. The deposit is different from most magmatic-hydrothermal Cu deposits in that the mineralization is mostly constrained to the dike and is not associated with extensive wall rock alteration. On the other hand, the deposit differs from typical orthomagmatic deposits in that it lacks significant Ni and PGE mineralization. Zircon grains from a diabase sample yield a weighted 206Pb/238U age of 289.9 ± 1.4 Ma, which is broadly coeval with the Re–Os isochron age of 307 ± 10 Ma from six Cu concentrate samples (mixture of bornite+chalcopyrite+chalcocite with an average Cu grade of 29%), suggesting that they are coeval. Petrochemical studies suggest that the diabase dikes in the Qunjsai area were derived from a subduction-related, fluid-metasomatized sub-continental lithospheric mantle, with assimilation of crustal materials in a post-collisional extensional setting. Measured δ34S (−3.8 to 0.8‰ CDT) and lead isotopes of sulfides, as well as initial 187Os/188Os ratios of Cu concentrates (0.42 ± 0.19) are consistent with the notion that the sulfur and metals were derived from a mantle source and were contaminated by crustal components. The flat patterns of mantle-normalized chalcophile element diagrams are comparable with those from typical orthomagmatic Ni–Cu sulfide deposits. Based on these data, we propose that the Qunjsai deposit is of orthomagmatic origin, the depletion of Ni and PGE, being due to source rocks originally rich in Cu and to segregation of a Cu-rich sulfide melt from a Ni-rich sulfide melt. This orthomagmatic mineralization was followed by hydrothermal alteration (mainly chlorite and epidote alteration) by magmatic fluids, which may have introduced additional amounts of Cu and S and reworked the magmatic sulfides.