The Ore-forming Magmatism Of Xiarihamu Ni-cu Sulfide Deposit In Eastern Kunlun Orogenic Belt, China:Constraints From Petrochemistry And Volatile Geochemistry

The newly-discovered Xiarihamu magmatic Ni-Cu sulfide deposit in the Eastern Kunlun orogenic belt is the largest one ever found in orogenic settings and contains157 million metric tons sulfide ore. The condition of ore-forming magma evolution and processe of sulfur saturation lacked integrated constraints by petrochemistry and valotiles geochemistry. The selected rock samples of this study included peridotite,pyroxenolite and gabbroite in Xiarihamu No.1 intrusion. Through the systematic research of petrochemistry and valotiles geochemistry, we revealed the characteristics of magmatism, process of sulfur saturation and ore-forming dynamic background of Xiarihamu Ni-Cu sulfide deposit. The followings are major conclusions derived from this study:1.The different types of host rock in Xiarihamu No.1 intrusion were byproucts of the same magma evolution. The Harker diagrams of different types of host rock in Xiarihamu Ni-Cu sulfide deposit showed the crystallization differentiation and assimilation of crust. Variation of the host rock composition are mainly controlled by the crystallization of silicate minerals, such as clinopyxene,orthopyroxene, olivine, plagioclase and chromium spine. Trace elements in different types of rock have the similar primitive mantle normalized patterns with enrichment of large ion lithophile elements, such as Rb 、 Sr, and depletion of the high field strength elements such as Nb、Ti、Zr and P. The total content of REE is low and mildly enrich LREE.2.The volatiles of different host rocks released at three heating intervals and rather low H2 O contents with relatively high percentage of H2 indicated a reduced environment of magmatism. The volatiles in pyroxene released at 200-400 ℃ 、 400-900 ℃ and 900-1200 ℃ respectively. The major component is H2O(43vol%), H2 and CO2 with minor constituent of CO, N2, O2 and H2 S. The volatiles from Xiarihamu Ni-Cu deposit are characterized by low content of H2 O and high content of H2 compared to other Ni-Cu sulfide deposits in different tectonic settings which indicated reduced magmatic environment for Xiarihamu deposit. The contents of total volatile from lherzolite, olivine websterite to websterite are 884, 720 and 959mm3/g, which are all dominanted by H2 O.3.The carbon isotopic compositions indicated the mantle derived volatiles mixed with fluids derived from crustal and organic materials. The value ofδ13CCO2 ranges from-23.5‰~-5‰ which were plotted in the δ13C range of crust and mantle. The δ13C values of methane homologues showed the normal carbon isotopic distribution pattern among CH4 to C4H10 in most of samples and partial reversal carbon isotopic distribution pattern in some samples, which are the carbon isotopic features of thermogenic hydrocarbon gases of organic materials in oceanic sedimentary.4.Some key factors having controled the process of sulfide suturation and metallization was determined. The values of RH=log(XH2/XH2O) are beyond-2.82 which indicated reduced magmatic environment that contributes to segregation of sulfide. The total contents of H2 S increased from lherzolite, olivine websterite to websterite which showed S-richment materials was added into magma. Some volatiles from crustal materials can be confirmed by volatiles carbon isotopic compositions which were significant for segregation of sulfide and metallization.5.The composition of parental magma and process of metallization were determined by this study. We calculated the major oxide of parental magma with9.76 wt.% Mg O, 6.54 wt.% Fe O and 59.17 wt.% Si O2. The ratios of trace elements showed that magma source was disturbed by subduction fluids and magma evolved involving minerals crystallization. The assimilation of crust about 10-30 % caused addition of external sulfur into magma.