Characteristics and its geological significance of fluid inclusions in Chehugou porphyry Mo-Cu deposit,Xilamulun molybdenum metallogenic belt

Chehugou Mo-Cu deposit is a typical porphyry Mo-Cu deposit in the Xilamulun molybdenum metallogenic belt on the northern margin of North China Craton ( MNCC) . It locates to the south of the MNCC fault. The orebodies are mainly hosted in granite porphyry. The Mo mineralization is characterized by veinlet-disseminated mineralization as well as breccia type and quartz vein type mineralization. According to crosscutting relationships of different veins and mineral parageneses,ore-forming process can be divided into four stages: ( 1) the pyrite-molybdenite-quartz stage; ( 2) the pyrite -chalcopyrite-quartz ± molybdenite stage; ( 3) the pyrite- quartz stage; and ( 4) the quartz-carbonate ± fluorite stage. Petrographic study of fluid inclusions suggests that six dominant types of fluid inclusions related to metallogensis are present in the deposit: Ⅰ-Type CO2-bearing three-phase ( LCO2 + VCO2 + LH2O) ,Ⅱ-Type daughter-minerals bearing three-phase ( V-L + S) ,Ⅲ-Type gas-rich ( V-L) ,Ⅳ-Type liquid-rich ( L-V) ,Ⅴ-Type pure gas ( V) and Ⅵ-Type pure liquid ( L) phase inclusions. Types of fluid inclusions regulary evolves from the early stage to the late stage. Fluid inclusions in the first and the second stage are characterized with Ⅰ-Type CO2-bearing three-phase ( LCO2 + VCO2 + LH2O) and Ⅱ-Type daughter-minerals bearing three-phase ( V-L + S) inclusion. But in the later the third and the fourth stage which are characterized with Ⅲ-Type gas-rich ( V-L) and Ⅳ-Type liquid-rich ( L-V) inclusion. Homogenization temperatures and salinities of ore-forming fluid also regularly evolves from the early stage to the late stage. The peak value of Homogenization temperatures of different stages are separately 270 ～ 400℃,230 ～ 370℃,160 ～ 290℃,120 ～ 230℃,showing a reducing tendency from the early stage to the late stage; Fluid salinities of the first stage could subdivide into two groups: 3. 39% ～ 14. 25% NaCleqv and 31. 01% ～ 66. 75% NaCleqv. Fluid salinities of the second stage also could be subdivided into two groups: 1. 23% ～ 12. 85% NaCleqv and 31. 14% ～ 64. 33% NaCleqv. Fluid salinities of the third and the fourth stage are separately 1. 05% ～ 21. 47% NaCleqv and 2. 07% ～ 10. 73% NaCleqv,which is showing a reducing tendency. Laser Raman spectroscopic and inclusion groups components studies indicate that components of ore- forming fluid mostly contains H2O,CO2,Cl - ,SO42 - ,Na + and minor F - ,Ca2 + ,Mg2 + . Diagnostic ion ratios indicates that ore- forming fluid is derived from magmatic fluid. Petrographic and microthermometric study indicate that the early first and the second stage high temperature,high salinities,CO2-bearing NaCl-H2O-CO2 system ore-forming fluid occurs boiling and phase separation. With fluid boiling,CO2 escaping and temperature decreasing,abundant metal sulfides precipitate. In the later third and fourth stage,ore-forming fluid system tends to open when atmospheric condensation interfuses into the system; ore-forming fluid evolves to middle-low temperature,middle-low salinitie and CO2-poor NaCl-H2O system. Boiling characterized the early ore-forming stage which leads to the precipitation of molybdenite and chalcopyrite,while in the later third and the fourth ore-forming stage fluid mixing is the leadingminerogenesis mechanism.