Geochronological, mineralogical and geochemical studies of sulfide mineralization in the Podong mafic-ultramafic intrusion in northern Xinjiang, western China

The Podong mafic-ultramafic intrusion is one of many Ni-Cu sulfide-mineralized intrusions in the Beishan-Tianshan Orogenic Belt in northern Xinjiang, western China. The temporal and lithological variations of the NiCu mineralized intrusions in Beishan are much less constrained than those in Tianshan. This paper provides the first set of geochronological and geochemical data for the Podong magmatic sulfide deposit that is present in the Beishan region. The host intrusion is composed of lherzolite, troctolite, gabbronorite and gabbro. Ni-Cu mineralization occurs as disseminated sulfides in gabbronorite (olivine). The zircon U-Pb age of gabbronorite is 270 2.7 Ma, identical to the zircon U-Pb age of the nearby Poyi magmatic sulfide deposit that is hosted in troctolite and ultramafic rocks. These ages together define a new episode of Ni metallogeny in the Beishan region, which is 10 Ma younger than that in the Tianshan region to the north. The Fo contents of olivine in the Podong mafic-ultramafic intrusion are from 76 to 87 mol%, lower than the values for the nearby Poyi maficultramafic intrusion. The Podong mafic-ultramafic rocks are characterized by eNd(t) values from 0.61 to +1.92, low to elevated initial 87(Sr)/86(Sr) ratios (0.7055-0.7090), and pronounced negative Nb-Ta anomalies. The Sr-Nd isotope data indicate that the parental magma of the Podong intrusion experienced 0-13 wt% crustal contamination, which cannot fully account for the observed negative Nb-Ta anomalies in the rocks. Sulfur isotope compositions of the Podong deposit are similar to those of Pre-Permian sedimentary rocks in the area as well as typical mantle values. The results support the premise that fractional crystallization and assimilation of siliceous crustal materials may have played a key role in triggering sulfide saturation in the Podong magma, although the involvement of external sulfur cannot be completely ruled out. Like other Permian magmatic sulfide deposits in the Beishan-Tianshan Orogenic Belt, the PGE tenors of bulk sulfides in the Podong deposit are extremely low, indicating a severely PGE-depleted parental magma. The Cu/Pd ratios of the Podong magmatic sulfide deposit are significantly higher than the mantle values, consistent with previous sulfide segregation at depth. The results from the Podong magmatic sulfide deposit confirm that widespread Ni metallogeny in a convergent tectonic zone is very protracted and the mineralized lithologies are highly variable, which have significant implications for regional Ni-Cu exploration.