In-situ U–Pb, Hf and Re–Os isotopic analyses of the Xiangshan Ni–Cu–Co deposit in Eastern Tianshan (Xinjiang), Central Asia Orogenic Belt: Constraints on the timing and genesis of the mineralization

The timing and genesis of the major Ni–Cu–Co sulfide deposit in the Xiangshan intrusion have been studied based on newly obtained in-situ U–Pb, Hf and Re–Os isotopic analyses. The SIMS U–Pb zircon ages of the gabbro hosting the Ni–Cu–Co sulfide deposit indicate that the Xiangshan intrusion was emplaced at 279.6 ± 1.1 Ma (95% confidence level, MSWD = 1.30, n = 15). On the basis of combined geological and geochronological evidence, we suggest that the Xiangshan and other adjacent Ni–Cu deposits were formed in the same period. Sulphides have low common Os concentrations and high Re/Os ratios, similar to sulphide ores from the Duluth, Sally Malay and Voisey Bay complexes. The Re–Os isotopic data from the disseminated and massive ores from the Xiangshan intrusion do not form a single isochron, as they have different initial Os ratios. The Hf and Os isotopic data suggest that the Xiangshan intrusion and associated Ni–Cu–Co mineralization were derived from crustally contaminated mantle melts. The geochemical data show a tholeiitic affinity and a strong suprasubduction zone signature with negative Nb, Sr, and Ti anomalies similar to N-MORB and E-MORB. We suggest that the mafic–ultramafic rocks and associated Ni–Cu mineralization of the Eastern Tianshan orogen formed in an Alaska-type subduction zone-arc setting. Some diagnostic features of ridge–trench interaction are present in the Chinese East Tianshan orogen (e.g. granites, adakites, high-Mg andesites, near-trench magmatism, Alaskan-type mafic–ultramafic complexes, high-temperature metamorphic belts that prograde rapidly from low-grade belts, and orogenic gold deposits). The above distinctive rock groups are probably related to the same thermal event, ridge subduction, as in the Cenozoic orogen of Alaska. We suggest that ridge subduction is the most plausible mechanism to provide the necessary heat. Ridge subduction provides an important promising model for understanding many aspects of the evolution of the Chinese Eastern Tianshan orogen, and by inference, throughout much of Central Asia.