Geochronology, petrology and Sr-Nd-Hf-S isotope geochemistry of the newly-discovered Qixin magmatic Ni-Cu sulfide prospect, southern Central Asian Orogenic Belt, NW China

Abstract

The Qixin mafic-ultramafic complex is present in the southern margin of the Central Asian Orogenic Belt in northern Xinjiang where many magmatic Ni-Cu sulfide deposits occur. To determine the temporal association of the newly-discovered Ni-Cu prospect associated with the Qixin complex with the known major magmatic Ni-Cu sulfide deposits in the region, as well as the origin of sulfide mineralization in the Qixin complex, we have carried out geochronological, petrological and geochemical studies of this complex. Gabbro, olivine gabbro, norite, olivine gabbronorite, troctolite, websterite and lherzolite are the major rock types of this complex. Two drill cores completed to date show two types of sulfide mineralization associated with the complex: (1) disseminated sulfides that occur near the base of bodies of lherzolite and troctolite; (2) blotchy-massive sulfides in the hornfels country rocks. The disseminated sulfides are composed of pyrrhotite, pentlandite and chalcopyrite, whereas the blotchy-massive sulfides in the country rocks are composed of pentlandite, violarite, pyrrhotite and chalcopyrite. Zircon crystals from the Qixin olivine gabbronorite yield a U-Pb age of 286.5 ± 2.5 Ma, which is among the oldest post-subduction magmatic Ni-Cu sulfide deposits in the region. The TiO2/Al2O3 ratios of Cr-spinel and clinopyroxene from the Qixin complex are similar to those from the older, subduction-related mafic-ultramafic intrusions in the region. This and other arc-like geochemical characteristics such as whole-rock negative Nb-Ta anomalies coupled with low initial 87Sr/86Sr (0.7036–0.7057), positive εNd(t) (+2.3 to +6.0) and high εHf(t) of zircon (+8.3 to +15), indicate that the parental magma of the Qixin complex was derived from a mantle source modified by pre-Permian metasomatic processes, similar to other contemporaneous mafic-ultramafic intrusions in the region. Mixing calculations using Sr-Nd-Hf isotopes indicate that the parental magma for the Qixin mafic-ultramafic rocks experienced no more than 15 wt% contamination with siliceous country rocks. The δ34S values of different sulfide assemblages of the Qixin Ni-Cu prospect are similar and vary from 1.1 to 1.7‰, which are within the range of typical mantle values as well as that of the country rocks, providing no direct evidence for addition of external sulfur. However, the role of external sulfur in sulfide mineralization at Qixin still cannot be ruled out, as forward modelling indicates that fractional crystallization plus crustal contamination involving siliceous country rocks free of S could not have induced sulfide saturation in the Qixin magma. This may also be true for the contemporaneous magmatic Ni-Cu sulfide deposits in the region. Based on the results from this study, we suggest that fractures in the solid country rocks nearby the Qixin complex and the ultramafic bodies of the complex are preferred targets if exploration continues.