Alaskan-type nature and PGE mineralization of the Wuxing mafic–ultramafic complex in eastern part of the Central Asian Orogenic belt

Abstract

The Wuxing mafic–ultramafic complex is located in the eastern part of the Central Asian Orogenic Belt (CAOB). It consists of hornblende-olivine clinopyroxenite, hornblende clinopyroxenite, clinopyroxenite, gabbro and diorite. Hornblende occurs in all the rock types of the complex and is pargasite in composition. Clinopyroxene is diopsidic in composition with high CaO and low TiO2 contents. The flat REE and sub-parallel 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. Enrichment in large ion lithophile elements relative to high field strength elements of the silicate minerals, together with the variable sulfur isotope compositions (δ34S = -2.55 ~ 11.01‰) of the sulfides, indicate the involvement of subducted oceanic materials into the mantle source. These petrological, mineralogical and geochemical features strongly 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 Wuxing complex is a unique Pt-Pd-rich Ni-Cu sulfide deposit in the CAOB. The ore minerals, which are dominated by pyrite, pyrrhotite, pentlandite, chalcopyrite and platinum-group mineral (PGM), mainly occur in clinopyroxenite and hornblende clinopyroxenite. The sulfide ores are predominantly present as net-textured and disseminated textures. Pentlandite and chalcopyrite associated with pyrrhotite commonly occur in the marginal zone of the pyrrhotite grains. The PGMs occur as interstitial grains at the contact between base-metal sulfides and silicate minerals or along the crack of pyrrhotite or clinopyroxene, or as inclusions in pyrrhotite. The occurrences of sulfides and PGMs in the Wuxing complex, together with sulfur isotope compositions and PGE patterns, reveal that the PGE enrichment of the Alaskan-type complexes can be attributed to the high-degree partial melting of the mantle source, sulfur involvement in the source, and PGE transportation of hydrous melts.