Magma generation and sulfide saturation of Permian mafic-ultramafic intrusions from the western part of the Northern Tianshan in NW China: implications for Ni-Cu mineralization

Abstract

Compared to Permian magmatic Ni-Cu sulfide deposits in the eastern part of the Northern Tianshan, such as Huangshan and Huangshandong, those in the western part (e.g., Baixintan and Lubei) are poorly documented. Zircon ID-TIMS U-Pb age data (279.7 ± 0.2 Ma) of lherzolite and LA-ICPMS U-Pb age data (277.8 ± 3.4 Ma) of a hornblende gabbro suggest the Lubei intrusion was emplaced in the Early Permian. The Baixintan and Lubei mafic-ultramafic intrusive rocks are characterized by enrichments of large ion lithophile elements and Th and U relative to the high field strength elements, as well as depletion of Nb and Ta relative to mid-ocean ridge basalt. The (87Sr/86Sr)t (0.70354–0.70713), εNd(t) (2.80 to + 7.05), and εHf(t) (2.50–18.4) values for the intrusions indicate that they have assimilated Tianshan basement rocks, whereas the Se/S ratios (181 × 10−6 –418 × 10−6 ) and δ34SCDT values (− 0.13‰ to 0.90‰) of the sulfides suggest that crustal sulfur has not been incorporated into the magma. The decrease in Pd/Zr with increasing (Th/Nb)PM indicates that sulfide saturation accompanied crustal contamination. The olivine compositions and whole-rock chalcophile elements of the Baixintan and Lubei intrusions imply that the parental magma of the intrusive rocks in the west contains higher chalcophile elements than those in the east, which suggests that the mafic-ultramafic intrusions in the western part of the Northern Tianshan should be regarded as favorable Ni-Cu sulfide exploration targets.