东昆仑夏日哈木镍铜硫化物矿床成矿岩浆条件：流体挥发份化学组成与碳同位素组成制约

The volatile conditions of ore-forming magma for the Xiarihamu Ni-Cu sulfide deposit in East Kunlun orogenic belt,western China:Constraints from chemical and carbon isotopic compositions of volatiles

夏日哈木镍铜硫化物矿床是我国东昆仑造山带新发现的全球岛弧环境最大的岩浆镍铜硫化物矿床。早泥盆世镁铁-超镁铁质成矿岩体中不同类型岩石的辉石中流体挥发份化学组成和碳同位素组成表明,流体挥发份总量较低,平均含量为888mm^3.STP/g(STP:标准温度和大气压)。流体挥发份组成以H_2O为主(48mol%),其次为H_2(26%)、CO_2(15%)和CO(5%)。H_2S含量从二辉橄榄岩(10mm^3/g)、橄榄辉石岩(18mm^3/g)到二辉辉石岩(28mm^3/g)逐渐增加,表明岩浆就位结晶演化过程中含硫组分的加入。CO_2的δ^(13)C值(-23.5‰^-5.0‰)位于地壳和地幔值范围之间。CH_4同系物碳同位素组成随碳数具有总体正序、局部反序的分布特征,显示大洋沉积有机质热裂解成因特征。夏日哈木成矿镁铁-超镁铁质岩体流体组分中较低的H2_O含量和较高的H_2含量表明,岩浆演化处于还原性挥发份条件,地幔岩浆源区存在俯冲洋壳沉积有机质,岩浆房围岩的混染和富S流体的加入可能是夏日哈木岩浆镍铜硫化物矿床硫饱和的主要机制。

The newly-discovered Xiarihamu magmatic Ni-Cu sulfide deposit in the East Kunlun orogenic belt, China is the largest one ever found in arc settings. In this study, the chemical and carbon isotopic compositions of volatiles in pyroxene crystals of different types of rocks from the Early Devonian Xiarihamu mafic-ultramafic intrusion were measured with a stepwise heating mass spectrometer. The results showed the total volatiles released from mineral separates in Xiarihamu deposit have relatively low contents with an average of 888mm3.STP/g （STP-standard temperature and pressure）. The volatiles were dominated by H2O （48mol%）, with minor H2 （26%）, CO2 （15%） and CO （5%）. The H2S contents gradually increased from lherzolite （10mm3/g）, olivine websterite （18mm3/g） to websterite （28mm3/g）, indicating that some sulfur-bearing components has been added into the magma during its evolution. The δ13C values of CO2 ranging from -23.5‰ to -5.0‰ were plotted in the area between crust and mantle. The δ13C values of CH4, C2H6, C3H8 and C4H10 etc. methane homologues showed the normal distribution pattern of carbon isotopic composition among CH4 to C4H10 in most of the samples, and partial reversal distribution pattern in some samples, which were attributed to the thermogenic hydrocarbon gases by oceanic organic materials. The rather low H2O and high H2 contents in Xiarihamu mafic-ultramafic rocks indicated that the magma was formed in a reduced volatile condition, and the oceanic sedimentary rock could be subducted into the mantle. Country rock contamination and the addition of S-rich fluid to the magma chamber may be the main mechanism for sulfide saturation in Xiarihamu magmatic Ni-Cu sulfide deposit.