Abstract: Bairendaba Ag-Pb-Zn polymetallic ore deposit, Inner Mongolia, belongs to a mesothermal vein-type deposit controlled by fractures, which are mainly E-W trending compresso-shearing faults with the relative flat-dipping other than steep-dipping parts being the favorable ore-hosting locations. The wall rocks of the ore bodies are mainly Hercynian quartz diorite. It is suggested that the Hercynian mafic rocks are the source for ore-forming materials while Yanshanian felsites might provide the heat driving meteoric water cycling. Based on fluid inclusion study, it has been recognized that the ore-forming hydrothermal fluid has the features like low salinity（5.0％~7.8% NaCl）, low density（0.66-0.95 g/cm³）,intermediate temperature（280-300℃）. The ore-forming pressures are from 76 to 80 MPa, which reflect the mineralising depth ranges from 6.8 to 7.2 km. The sulfur isotope of the ore pyrites varies from -0.99‰ to 0.36‰ with an average value of -0.315‰, which is similar to mantle sulfur value. According to geological study, it is suggested that there exists a post-ore fault that extends along the NW-striked valley in the district, which dissects the ore district into two parts, i.e. north and south sections, and causes the former uplifted with bigger denudation and the latter relatively down. Therefore, the ore bodies are shallower within the north section than in the south section while the latter still has good potentiality for further ore prospecting.

Key words: Inner Mongolia, Bairendaba Ag-Pb-Zn polymetallic ore deposit, ore-forming fluid, ore-controlling conditions