Genesis Of The Magmatic Ni-Cu Sulfide Deposits In Orogenic Belt

The East Tianshan is situated along the southern margin of the Central Asian Orogenic Belt,and its outcrops of a series of important magmatic Ni-Cu sulfide deposits hosted by mafic-ultramafic intrusions are the focus of recent researches.In this study,we present a systematic study of petrology,mineralogy and geochemistry of the Huangshannan sulfide ore-bearing mafic-ultramafic intrusion of the East Tianshan region,in order to further discuss the origin and nature of its parental magma.The Huangshannan intrusion consists of an ultramafic unit,which is composed of harzburgite,lherzolite and olivine websterite,and a mafic unit,which is composed of(olivine)gabbronorit and diorite.The rocks of the intrusion are characterized by enrichments of large ion lithophile elements,light rare earth elements and strong negative Nb-Ta,Ti anomalies,similar to that of typical arc volcanic.These rocks also have variable isotope compositions[?_Nd(t=282.5Ma)=-1.31-4.22;(⁸⁷Sr/⁸⁶Sr)_i=0.7032-0.7069;(²⁰⁶Pb/²⁰⁴Pb)_i=17.67-18.69],indicating the parental magma was derived from a moderately enriched depleted mantle source and contaminated by 5–20%juvenile arc crust and then by?5%upper crustal materials.According to its highest Fo olivine,the estimated parental magma of the Huangshannan intrusion has 12.11%MgO,11.14%Fe O_Totaland 306 ppm Ni,indicating the picritic magma was generated from partial melting of both the asthenosphere and mantle wedge.The low Ca(?725*10^-6)?100*Mn/Fe(1.18-1.38)and high Ni(1451-2813*10^-6)?Mn/Zn(11.09-23.53)ratios of the Huangshannan olivines indicates the Huangshannan parental magma may be derived from pyroxenite in a modally enriched peridotite mantle source.Therefore,we speculate that the Huangshannan primary magmas were likely derived from a lithospheric pyroxenite mantle source as the result of slab-derived fluid modification during previous subduction.Furthermore,the Huangshannan intrusion may represent dynamic magma conduits through the deep-seated magma chamber to shallower levels.The magma experienced crystallization differentiation and sulfide segregation in the deep-seated magma chamber,but the Ni-Cu sulfide ores result from a second-stage sulfide segregation.