Chronological and gechemical charcateristics of the Baixintan Ni-Cu deposit in Eastern Tianshan Mountains, Xinjiang, and their implications for Ni-Cu mineralization

Abstract：The medium-sized Baixintan Ni-Cu deposit is located at northern Dacaotan fracture, Eastern Tianshan Mountains. The intrusion is emplaced in middle Ordovician Qiaganbulake Group, and the direct country rocks are dacite and tuff. The Baixintan deposit mainly consists of gabbro, olivine pyroxenite and wehrlite, with wehrlite being the main ore-bearing rock type. Orebodies are mostly in bedded or lenticular form. From the center outwards the basicity shows a decreasing trend. Zircon U-Pb La-ICP-MS age of plagioclase-bearing wehrlite is (277.9±2.6) Ma, suggesting Early Permian. With No. 24 exploration line as the dividing line, the Cu/Ni ratio of western orebody is higer than that of the eastern orebody. The values of MgO and FeOT show positive correlation, and there is a negative relationship between MgO and SiO2, CaO and TiO2. The m/f ratio is between 2.43 and 3.9, belonging to the iron series ultramafic rocks. Chondrite-normalized REE patterns show LREE enrichment and negative or no Eu anomaly. The rocks are enriched in LILE and depleted in HFSE (Nb, Ta). The fractionation crystallization/cumulation was dominated by olivine and pyroxenes in the process of magmatic evolution. The geochemical features of analyzed rocks indicate that the parental magma of the rocks experienced weak contamination by middle-lower crust components during its upward migration. The magma source was metasomatized by subduction fluid. The chronological and geochemical characteristics, together with regional evolution history of Earsten Tianshan area, indicate that the Baixintan Cu-Ni deposit might be the product of post-collision extension and mantle plume activities. There is still huge potential for Cu-Ni mineralization on the weatern side of the Huangshandong-Jingerquan-Tulargen Cu-Ni ore belt. The Dacaotan fracture may be another main structure which controls the mafic intrusion and the Ni-Cu deposit.