The Magmatic Sulfide Deposit In The Huangshan Mafic-ultramafic Rocks Belt, Eastern Tianshan, And Its Mineralization

The Central Asian Orogenic Belt (CAOB) has been the focus of domestic and foreign geologists for the mineralization, and mafic-ultramafic complex are known to occur across CAOB. The Huangshan mafic-ultramafic rocks belt is located in the eastern part of North Tianshan, along the Kangguer-Huangshan shear zone and Kushui fault, from Kumutag sand ridge to Sidingheishan intrusion, distribution nearly EW and about length270km, width20-35km, area5600km2, has now become the world-class magmatic copper-nickel sulfide deposit metallogenic zone. In this rocks belt, exist a series of magmatic Ni-Cu sulfide deposits and mafic-ultramafic intrusions, such as Tudun, huangshandong, Huangshan, Huangshannan, Xiangshan, Hulu, Tulaergen magmatic sulfide deposits, and Erhongwa, Chuanzhu, Mati, Xianshuiquan, Sidingheishan mafic-ultramafic intrusions. The paper selected the Huangshandong, Huangshan, and Hulu deposits for detailed study, focusing on petrology, mineral deposit and geochemistry, through the depth study by the magma source and characteristic of primary magma, the magma evolution process, the material exchange between with the magma and surrounding rocks, the saturation and segregation mechanism of sulfide, and discuss the mineralization and depth process of magmatic sulfide deposits in East Tianshan region. The following are the main results:(1) Researched on many mafic-ultramafic intrusions of the Huangshan rocks belt in East Tianshan, can reflect that most of these intrusions are multiple intrusions and clear facies. The economic mineralized intrusions are composed by mafic-ultramafic rocks; olivine, pyroxene, amphibole, plagioclase and biotite are the main petrogenic minerals; gabbro norite, norite, pyroxenite and peridotite are the mainly economic mineralized rocks; the rocks of the economic mineralized intrusions are highly altered, especially Huangshandong, Huangshan and Hulu intrusion, hardly see the primary mineral in partly.(2) The zircon LA-ICP-MS U-Pb dating for gabbros diorite of Hulu intrusion yielded274.5±3.9Ma, combined with the existing zircon age of study area, which can limited the formation age of the economic mineralized intrusions which endowed with Cu-Ni deposit well, and also limits the formation age of these deposits; these datas suggest that the intrusions were intruded at early Permian, are the result of a post-collisional extensional setting.(3) The magmatic Cu-Ni sulfide deposits in East Tianshan, the total concentration of platinum-group element (PGE) in ores and rocks is very low. The concentration of PGE in rocks between0.24×10-9and8.03×10-9, and26.62×10-9～1352.27×10-9in ores(on the basic of100%sulfide recalculated). Rock and ore samples have similar mantle-normalized PGE patterns, the diversity between IPGE and PPGE is lower. The sulfide pre-segregation of initial magma in deep crust could take away the most of PGE, which may be the mainly reason for depleted PGE of the parental magma in Huangshan region.(4) The εNd(t)(t=274Ma) value varies from4.1to10.5,7.22in average;(87Sr/86Sr)i value from0.7022to0.7081,0.7040in average. In the diagrams of δNd(t) verses (87Sr/86Sr)i, most of samples fall in the second quadrant, within the MORB and OIB area, which reflect that the magma source of these intrusions is depleted mantle. The metasomatism of the overlying mantle because of the subducted slab dehydration and partial melting, which can result some samples have enriched isotopic characteristics of mantle, indicating that the magma source of the mafic-ultramafic intrusions in Huangshan rocks belt composed of wedge matasomasised by subducting slab and aesethenosphere components, are the result of the lithospheric delamination at the root and the upwelling of asthenosphere components.(5) The varying degrees of hydrothermal alteration of rocks were widespread occurrence in the mineralized process, such as serpentinization, uralitization, chloritization, saussurtization. The evolution of each intrusion with the same series, the primary magma is tholeiitic magma with high-Mg content by partial melting of mantle moderately; but the economic intrusions composed of a higher degree of magma evolution than the uneconomic intrusions.(6) The Fo molecules of olivine verses Ni content, Cu/Zr ratio, the mutual relations of PGE characteristics between the incompatible elements of rocks and ores in East Tianshan, the economic intrusions were occurred sulfide pre-segregation of initial magma in deep crust, such as Huangshandong, Huangshan, Tulaergen, Hulu, Xiangshanzhong; but the sulfide pre-segregation of initial magma was not occurred or not obvious in the uneconomic intrusions.(7) The platinum-group elements geochemical character and petrochemical data show that crustal contamination and the fractionation of olivine and pyroxene in the magma revolution process of the magmatic Cu-Ni sulfide deposits related to post-collisional mafic-ultramafic complex in East Tianshan region may be the main factors leading to S-saturation and sulfide segregation in deep crust; the crust sulfur only in the local section or add individual deposits, has some impact of but did not play a leading role for sulfide segregation and mineralization.