The Constraints Of Crustal Contamination In The Shitoukengde Ni-Co Ore-Forming Magmatism In The East Kunlun Orogenic Belt,China:Evidence From C-He-Ne-Ar Isotopes

The discovery of Xiarihamu ultra-large Ni-Co sulfide deposit(410Ma)makes the East Kunlun orogenic belt a beneficial dynamic settings for ultra-large nickel-cobalt mineralization in the global orogenic belt settings.The Shitoukengde mafic-ultramafic intrusion in the central East Kunlun orogenic belt had been found Ni-Co sulfide deposit formed at different ages(334Ma).Petrogeochemistry of the Shitoukengde mafic-ultramafic intrusion shows a significant crustal contaminated contribution in the magmatic mineralization,but the type,source and mechanism of the contaminated crustal material need the systematic constraints of C-He-Ar isotopes.In present study the carbon and noble gas isotopes compositions have analyzed for olivine,pyroxene and plagioclase from different types of rocks in different positions of shitoukengde No.I intrusion,the types and sources of the volatiles in the Shitoukengde mafic-ultramafic intrusionwere determined,and the types,sources,proportions and the mechanism of the crustal contaminated materials in ore-forming magma were discussed,which provided evidences for understanding nickel-cobalt sulfide mineralization in the orogenic belt settings.The following main conclusions are obtained:1.The carbon isotopic compositions of carbon dioxide and methane in the carbonaceous fluids of magmatic minerals in the Shitoukengde intrusion varies widely,the?¹³C_CO2values ranged from-22.7 to+6.0‰(v.PDB),the?¹³C_CH4values ranged from-52.0 to-12.3‰.The pyroxene and olivine have heavier?¹³C_CO2and?¹³C_CH4,and the?¹³C_CO2value of pyroxene is heavier than that of olivine.The?¹³C of methane homologue mainly shows positive distribution patterns,while some samples show reverse distribution patterns.2.The noble gas isotopic compositions of magmatic minerals in the Shitoukengde intrusion show the characteristics of obvious crustal and atmospheric components.The ³He/⁴He values range from 0.17 to 0.64 Ra,⁴⁰Ar/³⁶Ar values rang from 362.3 to 946.3.The ²¹Ne/²²Ne and ²⁰Ne/²²Ne are plotted mainly within the continental crust and radioactive orgins.There is a negative correlation between³He/⁴He and ⁴He contents,indicating that radioactive helium exists,and the ⁴⁰Ar/³⁶Ar ratio and the ⁴⁰Ar contents show a weak negative correlation.3.The isotopic compositions of carbon and noble gases in the Shitoukengde intrusion reveal a few lithospheric mantle component added a large number of crustal and atmospheric components.The He-Ar isotope model estimates that the proportions of lithosphere mantle,crust and atmosphere components are 5.8%,26.7%and 67.5%,respectively.4.Carbon isotopic compositions of methane and carbon dioxide in the Shitoukengde intrusion reveal that the contaminated crustal components are mainly thermogenic organic matter and a small amount of crustal components.?¹³C_CO2and³He/⁴He indicate that the contamination sedimentary organic matter may be mainly altered oceanic crust materials and sedimentary components,the altered oceanic crust also contains a large amount of atmospheric saturated fluid.5.The ore-forming magma in the Shitoukengde originated from the lithospheric mantle,and the ratio of ³He/⁴He from olivine,pyroxene to plagioclase gradually increased,indicating that crust-derived materials were contaminated in before the magma chamber,and there are altered oceanic crust materials in the crust contamination components.It is speculated that the subduction oceanic crust recycles and brings a large amount of sedimentary organic matter and atmospheric components to the magma source area,which promotes the partial melting degree and the reduction of oxygen fugacity of ore-forming magma,and is beneficial to nickel-cobalt sulfide mineralization.