Fluid inclusions and H-O isotopic compositions of quartz-vein ores in the Baiganhu-Jialesai W-Sn mineralization belts,Qimantage,NW China

There are three types of fluid inclusions in the quartz of quartz-vein ores,Baiganhu-Jialesai W-Sn mineralization belts,Qimantage: Liquid-rich two-phase inclusions,CO2 and CH4-rich three-phase inclusions and water inclusions,of which the first two types are primary inclusions and are closely related with W-Sn mineralization.The peak homogenized temperatures and salinity of liquid-rich two-phase inclusions in quartz,with liquid component of H2O and gas component of CO2 and CH4,range from 220℃ to 260℃ and from 0.88% to 20.82% NaCleqv respectively,the total density of fliuds range from 0.72 to 1.06g·cm-3.The peak homogenized temperatures and salinity of CO2 and CH4-rich three-phase inclusions in quartz range from 260℃ to 280℃ and from 6.63% to 15.212% NaCleqv respectively,the total density of fliud range from 0.60 to 0.91g·cm-3.The gas components of CO2 and CH4-rich three-phase inclusions are mainly CO2,CH4 and less H2S,N2,H2,while the liquid components are mainly H2O and less CO2,CH4,H2S.The value of δ18OH2O and δD of quartz-vein ores range from 4.02‰ to 6.32‰ and -75.5‰ to -42.8‰ respectively,both of which indicate that the fluid is significantly magmatic water.In all,the initial fluid of quartz-vein ores is from magmatic hydrothermal fluid,which belongs to NaCl-H2O-CO2 system with middle-high temperature (220~280℃),middle salinity (10%~14% NaCleqv) and lower density.The W-Sn mineralization is closely related with the Caledonian magmatism,and the uneven fluids are captured when the quartz-vein ores formed.The significant factors of forming of quartz-vein ores are immiscibility of ore-forming fluids,and changes of temperature,pressure and value of pH,Eh,fO2 and fS2.