Geology, petrology and geochemistry of the Baishiquan Ni-Cu-bearing mafic-ultramafic intrusions in Xinjiang, NW China: Implications for tectonics and genesis of ores

The Baishiquan mafic-ultramafic intrusions associated with magmatic Ni-Cu-(PGE) sulfide deposit are located in the northern margin of the Central Tianshan Block in northern Xinjiang, NW China. The intrusions consist of olivine pyroxenite, pyroxene peridotite, troctolite, hornblendite, gabbro and diorite. The peridotite and pyroxenite are the main host rock for the Cu-Ni ores. The crystallization sequence of the intrusions is generally as follows: olivine - plagioclase - orthopyroxene - clinopyroxene - amphibole - biotite. The chemical compositions of the rocks have SiO2 (38-51%), MgO (7-32%) and Al2O3(4.2-18%), and relatively low TiO2(0.32-0.96%) and K2O+Na2O (0.06-3.4%). In general, they are characterized by enrichment in large ion lithophile elements (LILE, e.g., Rb, Sr, K, U, Pb and Th) and light rare earth elements (LREE), depletion in high field strength elements (HFSE, e.g., Nb, Ta, Ti and P) relative to primitive mantle and MORB. The absolute PGE abundances are low. Sr-Nd-Pb isotope data suggest a mixture of multi-components of mantle sources, including depleted asthenospheric mantle end-member with MORB-like isotopic signature and EMIlend-member related to subducted oceanic crust. These geochemical characteristics indicate that the parental magma was high-magnesium tholeiitic. Sulfur had reached saturation and immiscible sulfides droplets segregated from silicate magmas before their emplacement. Contamination by a small amount of subductedrelated components combined with crystal fractionation resulted in the Ni-Cu-(PGE) mineralization. (c) 2007 Elsevier Ltd. All rights reserved.