The Magma Source And Evolution Process Of The Cu-Ni Sulfide-bearing Intrusions In The Beishan Block, Xinjiang

The geochemical data of chromites and olivines, platinum group elements andRe-Os isotopic compositions of rocks from Ni-Cu sulphide-bearing mafic-ultramaficintrusions in the Beishan of Xingjiang have been analyzed in this paper. The resultsshow that the olivines are characterized by low Ni and high Mg, and the chromites arecharacterized by high Al and Fe, suggesting high magma temperature>1330℃. Thetotal PGE contents are quite low, with low Pd/Ir,high Cu/Pd and Ti/Pd ratios, andPPGE enrichment relative to IPGE, indicating affected by magmatism, and the sulfurin the primary magmas had reached saturation and immiscible sulfides dropletssegregated from silicate magmas before their emplacement. The relationship betweenNi/Cu and Pd/Ir indicated that the parental magma was high-magnesium basalticmagmas. The high γOsand Os indicate the possible magma source from mantle plumematerial located at core-mantle boundary. The variable（187Os/188Os）i suggestingthat abundant crustal material was mixed with the ascending magma. Thefractionation of olivine and contamination by crustal material probably trigger thesulfide saturation and segregation, and the Ni-Cu-(PGE) mineralization as well. Theintrusions are generated from the magma that had undergone crystallizationdifferentiation of olivine and the segregation of sulfide. The Ni-Cu sulphide-bearingmafic-ultramafic intrusions, widespread in the Beishan area, are the products ofCarboniferous to Permian plume magmatism.