Abstract: The Jiguanshan Mo deposit is a very typical porphyry deposit hosted in the Xilamulun molybdenum metallogenic belt. The deposit occurs within the volcanic-intrusive complex. Re-Os isotope dating from five molybdenite samples gives an isochron age of 151.1±1.3Ma, which indicates the deposit formed in the late Jurassic. The Mo mineralization is characterized by a variety of veinlets and disseminated styles. The hydrothermal ore-forming processes can be divided into three stages: stage I-ore-in poor stage, it is characterized by ivory-white quartz veins and minor disseminated pyrites; stage II-the primary Mo mineralization stage, it can be subdivided into two substages: quartz-polymetallic sulfide and fluorite-quartz-polymetallic sulfide substages; stage III-barren stage, it consists of quartzs and carbonates, and it overprints the minerals of the former two stages. Petrographic observation suggests that six major types of fluid inclusion related to mineralization are present in this deposit: gas-rich, liquid-rich, daughter-mineral bearing poly-phase, CO2-bearing three-phase, pure CO2 phase and liquid phase inclusions. Fluid inclusions in stage I are mainly gas-rich ones with small amount of CO2 phase ones, while stage II are multiform inclusions including gas-rich, liquid-rich and daughter-mineral bearing poly-phase inclusions. However, none but liquid-rich fluid inclusions can be observed in stage III. Microthermometric and laser Raman spectroscopic studies indicate that homogenization temperatures of fluid inclusions in quartz-veins from stage I are above 480℃, with the high salinities of above 66.75% NaCleqv. Liquid components are dominated by H2O and vapor bubbles mainly composed of CO2 and H2O. Daughter minerals of the poly-phase fluid inclusions include halite, chalcopyrite and hematite representing an oxidizing condition, which indicates the ore-forming fluid is metal-rich magmatic fluid. The stage II fluid inclusions with the temperature range of 320~480℃ in early phase and 180~320℃ in late phase, respectively, and fluid salinities ranging from 4.65% to 56.76% NaCleqv, reflect Mo mainly mineralized phases. In the stage II, daughter minerals in fluid inclusions, such as halite, calcite, chalcopyrite and minor hematite, also have been recognized. Vapor bubbles are mainly composed of H2O and minor CO2, while liquids generally contain H2O. The daughter mineral-bearing inclusions coexist with the gas-and liquid-rich fluid inclusions, and fluid inclusions with contrasting salinities are homogenized to divergent phases at similar homogenization temperatures. The above evidence strongly reveal that fluid-boiling occurred in the stage II. The homogenization temperatures of stage III fluid inclusions fall into 100~180℃, with corresponding salinities no more than 10.86% NaCleqv. The stage I fluids which are characterized by high temperature, high salinity, high oxygen-fugacity, CO2-rich and ore-bearing elements are magmatic fluids. The boiling of fluid occured in the stage II, leading to CO2-release, oxygen-fugacity decrease and rapid precipitation of ore-forming materials. During the stage III, the fluids, with input of meteoric water, gradually evolved to those with characteristic of low temperature, low salinity, lack of daughter mineral and poor in CO2. The boiling is the major mechanism of the ore-deposition.