Ore-forming fluid geochemistry of Yangchang quartz vein molybdenum deposit in Xilamulun molybdenum metallogenic belt, Inner Mongolia
内蒙古西拉沐伦成矿带羊场石英脉型钼矿床成矿流体地球化学特征研究

The Yangchang Mo deposit is located in the Xilamulun molybdenum metallogenic belt on northern margin of North China Craton,Inner Mongolia.The mineralization is occurred within the NW-to NNW-trending faults and fractures hosted by the Yanshanian biotite monzogranite.The ore-forming hydrothermal process can be subdivided into four stages:quartz vein stage(Ⅰ) ,quartz-pyrite stage(Ⅱ-1) ,quartz-pyrite-molybdenite-chalcopyrite-galena-sphalerite stage(Ⅱ-2) and carbonate stage(Ⅲ) .Three types of fluid inclusions are observed in quartz crystals,i.e.liquid-rich(VH2O<50%) inclusion,gas-rich(VH2O=50%～90%) inclusion and vapor inclusions.Homogenization temperatures of Ⅰ,Ⅱ-1,Ⅱ-2 stages are 173～280℃,180～467℃,151～366℃,respectively.From Ⅰ stage to Ⅱ-1 stage the temperatures become higher,suggesting the magmatic water was introduced into the mineralizing system.Salinities of Ⅰ,Ⅱ-1,Ⅱ-2 stages are in the range of 4.03%～10.61% NaCleqv,2.07%～10.36% NaCleqv,2.41%～9.98% NaCleqv,respectively.The composition of the hydrothermal fluids at different stages are mainly H2O(>94.39mol%) ,with minor CO2,N2,CH4,C2H6,Ar,H2S,and Na+,HS-,Cl-ions.These suggest the Yangchang Mo deposit was formed in reduction condition and the ore-forming fluids were of the NaCl-H2O±CO2 system.Hydrogen and oxygen isotopes of fluid inclusions in various mineralization stages are-119.66‰～-98.79‰ and-0.08‰～1.90‰ respectively,indicating that the ore-forming fluids were the mixture of magmatic water and meteoric water.It is suggested that the mixing of different fluids with distinct natures might be responsible for the precipitation of molybdenite.