Abstract:The Machangqing Cu- Mo deposit, located in the Sanjiang Tethyan metallogenic belt, is a polymetallic deposit related to the intrusion of alkaline porphyry in the Himalayan period. Previous studies have shown that the Machangqing Cu- Mo polymetallic deposit was formed in a porphyry- skarn metallogenic system. However, due to the lack of systematic mineralogical research, a mineralization process and metallogenic effects of skarn are still unclear, limiting a comprehensive understanding of the mineralization process of this deposit. Herein, this paper considers garnet in skarn mineralization in Machangqing deposit as the research object, and uses H- O isotope, electron probe (EPMA) and LA- ICP- MS in- situ microregion technology to carry out isotope and composition analysis to define the source of ore- forming fluids and invert the evolution process of ore- forming fluids. The garnets can be divided into early euhedral garnets (Grt I) and late anhedral type (Grt II). The Grt I, coexisting with the pyroxene, pyrite and chalcopyrite, belongs to the grossular- andradite garnet solid solution (And49. 37~99. 58 Gro0~49. 79). The Grt II has elevated Fe compared to Grt I, and mainly consists of andradite (And67. 5~99. 85 Gro0~31. 84). Both the types of garnets are enriched in Th, U, LREEs and Nd, depleted in Ba, Sr, Hf and Nb. Based on the chemical compositions of garnet, the Grt I may have formed under mildly acidic, oxidized and low W/R conditions, whereas the Grt II may have formed under acidic, oxidation, and high W/R conditions. In addition, the δDV- SMOW values and δ18Ofluid values of these garnets range from 122. 6‰ to 98. 6‰ and 5. 95‰ to 8. 25‰, respectively, implying that the hydrothermal ore- forming fluid originated from magma- derived fluid. A comprehensive consideration of data indicated that the oxygen fugacity and temperature of the ore- forming fluid presented a decreasing trend during the skarn mineralization process, and the pH of the fluid gradually changes from acid to weak acid neutral. This process may have been the main mechanism to trigger the precipitation of skarn type mineralized copper molybdenum sulfide in the Machangqing deposit.