Abstract: The Great Xingan Range tin metallogenic belt is the only mineable tinpolymetallic belt in northern China. The newly discovered Weilasituo Sn polymetallic ore deposit in Inner Mongolia, in the southern Great Xingan Range, is a part of the XinganlingMongolian Orogenic Belt in the eastern Central Asian Orogenic Belt. The deposit is a largescale porphyryhydrothermal vein type Snpolymetallic ore deposit, its mineralization is mainly hosted in the top portion of the quartz porphyry and quartz veins above it. Based on the crosscutting relationship of veins and ore structure, we recognized four metallogenic stages during the formation of the western Weilasituo SnZn polymetallic ore deposit, namely the dropletlike SnZn in quartzporphyry (stage Ⅰ), molybdenum (stage Ⅱ), quartzcassiteritewolframite (stage Ⅲ) and quartzpolymetallic sulphides (stage Ⅳ) mineralization. Fluid inclusions can be divided into three types: liquid and vaporrich twophase aqueous inclusions, and daughter mineralbearing threephase inclusion. Microthermometric analyses indicate that the homogenization temperature of dropletlike SnZn mineralization in quartz porphyry varied from 324 to 333 ℃, with salinity of 6.5%7.5% NaCleqv and density around 0.730.74 g/cm3. The fluid inclusions from quartz porphyrystyle mineralization exhibit a homogenization temperature range of 201324 ℃, with salinity and density of 3.4%9.9% NaCleqv and 0.730.92 g/cm3, respectively, consistent with the characteristics of moderatehigh homogenization temperature, low salinity and moderate density fluids in this deposit. The analytical results by laser raman spectroscopy suggest that the composition of the oreforming fluid is mainly H2O in the liquid phase and CH4, CO2 and H2O in the vapor phase. The data obtained by δ18O and δD analyses imply that the oreforming fluids in the quartz vein mineralization stage originated from the mixture of magmatic and meteoric waters, with magmatic water as the main component. Therefore, the principle mechanism for the deposit mineralization was mixing of magmatic and meteoric waters followed by a cooling period during fluid evolution.

Key words: Great Xingan Range tin metallogenic belt, Weilasituo Sn polymetallic ore deposit, fluid inclusion, mechanism of mineralization