Cause Of The Baixintan And Lubei Cu-Ni Sulfide Doposits,East Tianshan,Xinjiang

There are a lot of mafic-ultramafic rocks and magmatic sulfide deposits have been found in the east of Kumutage sand ridge in east Tianshan of Xinjiang since the 1970 s,while in the west of which there has been no prospecting breakthrough yet.The Baixintan and Lubei Ni-Cu sulfide-bearing mafic-ultramafic intrusion was discovered in the west part of Kumutage sand ridge in recent years,but the studies of it are relatively weak.The objective of this paper is going to constrain the petrogenesis of Baixintan and Lubei Ni-Cu sulfide-bearing mafic-ultramafic intrusion by analyzing its chronology,major elements,trace elements,S?Sr-Nd and O-Hf isotope data.The variations of the mineral grain size and content in different lithofacies indicate that the Baixintan intrusion was formed by three magmatic stages.The first stage is the hornblende gabbro at the top of the intrusion;The second stage contains the olivine gabbro,plagioclase lherzolite and Pl-bearing olivine websterite,whose mineral contents change gradually.The third stage is the hornblende gobbro atthe bottom of the intrusion,with smaller mineral grain size than the hornblende gabbro.The Baixintan intrusion has the similar trace element partition pattern with the Huangshandong and Huangshanxi intrusions.They are characterized by enrichment in the large ion lithophile elements(Rb?Ba?U?K)and depletion in high field-strength elements(Nb?Ta?Ti).The REE pattern shows a slight enrichment in LREE relative to HREE,These geochemistry characteristics are similar with those of island-arc volcanic rock.The ?Nd(t)(2.84~5.05)and(87Sr/86Sr)(t)(0.704113~0.705682)of the Baixintan rocks ploting between the asthenosphere mantle and lithospheric mantle suggest that the primary magma of the intrusion was the product of interaction between lithosphere and asthenosphere.However,compared with Huangshandong and Huangshanxi intrusions,the Baixintan intrusion has higher incompatible elements contents,lower ?Nd(t)value and La/Ba(0.02~0.15)ratio,higher(87Sr/86Sr)(t)value and La/Nb(2.00~3.33)ratio indicating that there was less involvement of upwelling asthenospheric mantle in the Baixintan intrusion relative to the Huangshandong and Huangshanxi intrusions.The variations of the mineral content in different lithofacies and LA-ICPMS U-Pb zircon dating indicate that the Lubei intrusion was formed by two magmatic stages.the first stage is the plagioclase lherzolite and olivine pyroxenite;The second stage contains the gabbro.The Lubei intrusion has the similar trace element partition pattern with the Huangshandong and Huangshanxi intrusions.Compared with Huangshandong and Huangshanxi intrusions,the Lubei intrusion has higher incompatible elements contents than Huangshandong,the same with Huangshanxi.They are characterized by enrichment in the large ion lithophile elements(Rb?Ba?U?Sr)and depletion in high field-strengthelements(Nb?Ta?Ti).Indicating that the primitive magma may be derived from partial melting of a metasomatized mantle source.The olivine gabbro in Baixintan intrusion has an age of 287±3Ma and the gabbro in Lubei intrusion has an age of 277.8 ± 3.4Ma which is younger than age of the ophiolite(336-503Ma),island arc volcanic rocks(320~322Ma),island arc granite(316~328Ma)and porphyry intrusion(326~334Ma)in this area,but contemporary with the age of A-type granite and bimodal volcanic rocks(284~294Ma).In addition,the chemical composition of clinopyroxene of Baixintan and Lubei intrusion is similar with the mafic-ultramafic cumulate formed in extensional rift environment.These features indicate that Baixintan and Lubei intrusion was formed in an extensional environment after the collision.The characteristics of the isotope and magmatic rock both indicate that the crystallization differentiation of the olivine etc is the factor induces the saturation of the sulfur in Baixintan and Lubei magma.The assimilation and contamination of the crust rock making the sulfides saturated and segregated may exist.The lithospheric thickening and delamination,upwelling of asthenosphere mantle material and partial melting of lithosphere mantel material lead to the formation of the basaltic magma during post-collisional extensional environment in east Tianshan area.These basaltic magma rise along the fault.The crystallization differentiation of olivine and other minerals form the Baixintan and Lubei mafic-ultramafic intrusion.The sulfide was saturated and segregated,the sulfide drips were brought into the flat position by the upwelling magma then sediment and form the orebody,such as Baixintan and Lubei copper-nickel sulfide deposit,because of the contamination of the crust rock.