Abstract <p>The paper presents mineralogical characterization of antimony ore occurrences of the Iyul’skoe group within the boundaries of the Tyrgetui-Zhipkosh area (Eastern Transbaikalia), which are localized in terrigenous rocks of Late Permian and Early Triassic age. Low-temperature antimony mineralization is associated with tectonic breccias, which spatially tend towards zones of crushing and argillization of the host rocks. The main ore mineral of breccias is stibnite, which in near-surface conditions (in the zone of hypergenesis) is replaced by “antimony ochres” represented by minerals of the romeite group; the cement of breccias consists of quartz and rare carbonates of calcite–dolomite series. According to the results of microprobe analysis of ore minerals, primary ore mineralization in breccias with quartz and carbonate–quartz cement includes four different mineral associations: bertierite–stibnite, arsenopyrite–marcasite, arsenopyrite–pyrite, and pyrite–tetrahedrite. Analyses of sulfur isotopes in stibnite indicate the participation of organic source in the formation of ore mineralization, and a significant range of obtained values indicates the participation of two or more sources of sulfur in the ore formation process.</p>