Genetic Mineralogy Of Ni-Cu Ore-forming Mafic-ultramafic Rocks In Xiarihamu,East Kunlun

The Xiarihamu deposit is the first discovery super-large magmatic Ni-Cu deposit in the East Kunlun Orogenic Belt(EKOB).In order to establish the mineralogical indicators related to ore-forming mafic-ultramafic rocks,we study on Xiarihamu mafic-ultramafic rocks using genetic mineralogy study.And selected ore-barren Shitoukengde mafic-ultramafic rocks that is located at the same tectonic position as Xiarihamu deposit.We compared and discuss the differences of mineralogy between them.The main results of this study are as follows:(1)The rock assemblage of ore-forming mafic-ultramafic rocks is composed of dunite-harzburgite-Ol-orthopyroxenite-orthopyroxenite-websterite-gabbronorite-gabbro,and characterized by large crystallization of orthopyroxene.The rock assemblage of ore-barren mafic-ultramafic rocks is dunite-harzburgite-lherzolite-wehrlite-Ol-orthopyroxenite-orthopyroxenite-websterite-clinopyroxenite-troctolite-gabbro,characterized by large crystallization of plagioclase and clinopyroxene.(2)The mineral crystallization sequence of ore-forming mafic-ultramafic rocks is(spinel)-olivine-plagioclase-clinopyroxene-pargasite/plagioclase-magnesiohornblende-actinite,and sulfide may form during the crystallization of spinel and olivine.The mineral crystallization sequence of ore-barren mafic-ultramafic rocks is spinel-olivine-plagioclase-pargesite-plagioclase-clinopyroxene,and sulfide formed after the crystallization of olivine.(3)Ore-forming mafic-ultramafic olivine shows a relatively stable range of Fo values and the parent magma Ni contents are relatively high.Olivine Ni content greater than 2000ppm can be used as an ore-forming indicator for mafic-ultramafic rocks.The Mn/Fe and Ni/Co ratios indicate that the ore-forming magma source is the pyroxenite source which is influenced by metasomatism of oceanic crust subduction.Ore-forming olivine is characterized by high Ti and low Ca content,indicating that the source may be metasomatic by Ti-and Fe-rich silicate melt.Ore-barren olivine show a relatively wide range of Fo values and the content of parent magma Ni content is relatively low.Ore-barren olivine Ni contents are below 2000ppm.The Mn/Fe and Ni/Co ratios indicate that the ore-barren magma source is the peridotite source.Ore-barren olivine is characterized by low Ti and high Ca content.(4)The pyroxene component characteristics of ore-forming and ore-barren mafic-ultramafic rocks are similar,which cannot be used as an ore-forming indicator.A large amount of crystallization of orthopyroxene suggest that the ore-forming magma is rich in MgO and poor in CaO,while a large amount of crystallization of clinopyroxene and plagioclase suggest that the ore-barren magma is rich in CaO and poor in MgO.(5)The TiO2 content of amphibole greater than 2.5wt%can be used as an ore-forming indicator of mafic-ultramafic rocks.The ore-forming amphibolite composition zoning indicates that the oxygen fugacity of magma increases first and then decreases,which is favorable for metallogenesis.Assimilation and contamination may significantly change the oxygen fugacity of magma.Magma injection is conducive for sulfide segregation.In the process of regional prospecting,it is necessary to pay attention to the amphibole zoning.