Nature And PGE Mineralization Of The Wuxing Mafic-ultramafic Complex In Eastern Part Of The Central Asian Orogenic Belt

A large number of Triassic mafic-ultramafic complexes are exposed in the eastern part of the Central Asian Orogenic Belt?CAOB?.They host several important Ni-Cu sulfide deposits with low platinum-group element?PGE?contents.The Wuxing mafic-ultramafic complex in Heilongjiang province is located within Xingkai massif in the eastern part of the CAOB and close to the Dunhua-Mishan fault.It is a unique Pt-Pd-rich Ni-Cu sulfide deposit in the CAOB.In this study,we provide petrology,mineralogy and geochemistry of the Wuxing complex to redefine the nature of this complex,and to constrain its parental magma,tectonic environment and tectonic environment and controlling factors of PGE mineralization.The Wuxing complex consists of hornblende-olivine clinopyroxenite,hornblende clinopyroxenite,clinopyroxenite,gabbro and diorite.Its rock-forming minerals are clinopyroxene,hornblende and plagioclase with lack of orthopyroxene.Hornblende occurs in all rock types of the complex and is pargasite in composition.Clinopyroxene is diopsidic in composition with low TiO₂ contents.Plagioclase is mainly labradotite and albite.The flat REE and similar trace element patterns of both clinopyroxene and hornblende suggest that these rocks were formed by fractional crystallization from common magma with no significant crustal contamination.The low abundances of trace elements of the silicate minerals with enrichment in large ion lithophile elements relative to high field strength elements show the characteristic of arc magmas.The sulfur isotopic values of the sulfides from the Wuxing complex are mostly in range of 6?9‰,higher than the normal mantle value,revealing the addition of subducted materials to the mantle source.These petrological,mineralogical and geochemical features suggest the Alaskan-type nature of the Wuxing complex.The hydrous parental magma of the Wuxing complex was derived from partial melting of the mantle wedge,which had been metasomatized by subducted melts/fluids.The sulfide ores of the Wuxing complex are predominantly present as net-textured,disseminated and sulfide droplet textures.The ore minerals,which are dominated by pyrite,pyrrhotite,pentlandite,chalcopyrite and platinum-group minerals,mainly occur in clinopyroxenite and hornblende clinopyroxenite.Pentlandite and chalcopyrite associated with pyrrhotite commonly occur in the marginal zone of the pyrrhotite grains or along its crack.The PGMs mainly include sperrylite and native platinum.They are closely associated with sulfides in clinopyroxenite and hornblende clinopyroxenite and occur as interstitial grains at the contact between base-metal sulfides and silicate minerals or as inclusions in pyrrhotite,or along the crack of pyrrhotite or clinopyroxene.The occurrence of sulfides and PGMs in the Wuxing complex,together with sulfur isotope compositions and PGE patterns,reveal that the PGE enrichment and differentiation of the Wuxing complex with an Alaskan-type nature can be attributed to the PGE enrichment in mantle source,high-degree partial melting,sulfur involvement in the source,and PGE transportation in hydrous melts.