Abstract: Abstract: Located in Mujicun Village of Laiyuan County, Hebei Province, the Mujicun copper ( molybdenum ) deposit has been proved to be a large-size copper (molybdenum) - gold and silver polymetallic deposit in northern Taihang Mountains, geotectonically lying in the secondary faulted basin in the upper block of the detachment zone on the western side of Laiyuan dumbbell-shaped complex rock body, along the northern section of Fuping mantle branch structure. In order to study the metallogenic ages of the Mujicun copper ( molybdenum) deposit, the authors mainly analyzed the Re-Os isotopes of 5 pieces of molybdenite samples from different drill holes at different depths on the basis of the metallogenic geological background, and obtained three important metallogenic ages, i.e., (143.2±2.2) Ma~(144.9±2.3) Ma (model age), (144±1) Ma (weighted mean model age) and (143.8±1.2) Ma (isochron age). The dating result suggests that the ore deposit was formed in the Late Jurassic of Yanshanian period, which is consistent with polymetallic metallogenic event in northern Taihang Mountains. In addition, according to the characteristics of fluid inclusions and the content of Re in molybdenite, this paper also indicates that the inclusions are composed of 4 types, i.e., gas-liquid inclusions, rich gas inclusions, CO₂-bearing inclusions and gas-liquid-solid multiphase inclusions, in which H₂O is the main composition for gas-liquid inclusions and rich gas inclusions. The ore-forming process took place under the condition of medium temperature and middle-low salinity, and the ore-forming materials were mainly derived from the depth of the earth.