The Emplaced Machanism Of Ore-Forming Mafic Magma In The Xiarihamu Ni-Cu Sulfide Deposit,China:Constraints From Noble Gas And Carbon Isotopes

The Xiarihamu Ni-Cu sulfide deposit in the Eastern Kunlun orogenic belt is the largest one formed in orogenic belt settings.There was obvious crustal contamination in the process of sulfide saturation melting and mineralization.However,the source of the contaminated crust component had not been determined.The directions of ore-forming magma intrusion require isotopic composition constraints.This paper analysed the noble gas isotope and carbon isotopic compositions of magmatic minerals in the Xiarihamu ultramafic intrusion,and discusses the source of contaminated components and contaminated mechanism and magma intrusive direction of ultramafic intrusion in the Xiarihamu Ni-Cu sulfide deposit,and obtains the following main understandings:?1?The noble gas isotopic compositions of the Xiarihamu ultramafic intrusion were characterized by the mixing of the lithospheric mantle,crust and atmospheric components:³He/⁴He ratios ranged from 0.03 to 0.39 Ra in olivine and pyroxene minerals;⁴⁰Ar/³⁶Ar ratios ranged from 291.96 to 316.91.The ³He/⁴He ratios and the⁴⁰Ar/³⁶Ar ratios were located on the mixing curve of the crust and atmospheric saturated fluid end-members.²⁰Ne/²²Ne ranged from 9.13 to 10.16;the ratios of²¹Ne/²²Ne ranged from 0.023 to 0.037.The ²⁰Ne/²²Ne and ²¹Ne/²²Ne radios were plotted around the continental crust and nuclear genesis.?2?The carbon isotopic compositions of volatiles in the magmatic minerals of the Xiarihamu ultramafic intrusion was characterized by the mixing of thermogenic,crust and mantle origin:CO₂ and CH₄ carbon isotope composition ranged from-24.808^-0.2‰and-48.9^-15.2‰.The?¹³C_CO2O2 and?¹³C_CH4H4 values of CO₂ and CH₄volatiles released at the 300°C temperature plotted into the ranges of mantle and adjacent areas of thermogenic and methane oxidation origins.The?¹³C_CO2O2 and?¹³C_CH4values at the 500°C and 700°C temperature plotted into the ranges of mantle and adjacent areas of thermogenic,crust and mantle origins.?3?The source of the contaminated components of the ore-forming magma of the Xiarihamu ultramafic intrusion were determined:The ³He/⁴He ratios after deducted by radiogenic ⁴He*were close to the lithospheric mantle ratios,indicated that the ore-forming magma was derived from the lithospheric mantle source.The lower⁴⁰Ar/³⁶Ar ratios were similar to the atmospheric values,indicated the presence of atmospheric saturated fluids.The?¹³C values of methane homologues showed the normal carbon isotopic distribution pattern in most of samples and partial reversal carbon isotopic distribution pattern in some samples,which were the carbon isotopic features of thermogenic hydrocarbon gases of organic materials in oceanic sedimentary.He-Ne-Ar and C isotopes indicated that the addition of crust components from magmatic fluid.?4?The contaminated mechanism of different source of the ore-forming magma was determined:The carbon isotope partitioning pattern of methane homologues indicated that subduction sediments and the country rock fluid components had been added during magma uplift and emplacement.The recycled oceanic sediment materials and saturated air fluid might be brought and added into the magmatic sources by the subducted plate.The ³He/⁴He and ⁴⁰Ar/³⁶Ar values had covariant relation with the ratios of Th/Nb and La/Yb,indicated that the crustal material contaminated during magma evolution.?5?The ore-forming mechanism of the Xiarihamu magma was determined:the ratios of ³He/⁴He decreased from the west to the east and the ratios of ⁴⁰Ar/³⁶Ar increased.The spatial variation of He and Ar isotopic compositions showed that ⁴He and ⁴⁰Ar increased from west to east and crustal materials were added progressively.The magma intrusion direction was from west to east.Combined with the spatial variation of the carbon isotope,fluid composition and spatial variation characteristics of metal elements suggested that the addition of crustal fluids into the magma during the intruding process results in the sulfur saturation and segregation of sulfide in silicate melt.