The article discusses the features of the composition, age, and genesis of gold–antimony mineralization of the East Sayan based on the example of the largest Tumannoe ore occurrence. It is shown that objects of this type are rare in the considered region. In the course of studies, it was found that stibnite, pyrite, and arsenopyrite predominate in ores; Bi minerals (tetradymite, cobaltite, native Bi), antimonides and sulfosalts (zinkenite, chalcostibite, aurostibite, tetrahedrite, and andorite), and three generations of native gold are also present. The mineralization contains gold–bismuth and gold–antimony ore assemblages, which are the evolutionary products of a single ore-forming system, during which a sequential decrease in the PTX parameters of ore deposition occurred, where temperatures decreased from more than 380 to 180°С. Sulfur activity decreased during ore deposition, which led to the deposition of alternating ore assemblages from simple sulfides to sulfosalts, in the direction of increasing deposited components, and to the formation of different generations of native gold with a gradual increase in fineness from early to late assemblages. The results of mineralogical–genetic and isotope–geochemical studies evidence the magmatogenic nature of gold–antimony mineralization. The isotopic age of parent granitoids, obtained by LA-ICP-MS zircon dating, is Early Ordovician; age values are 491–486 Ma. 40Ar/39Ar dating of muscovite from ore veins shows a relatively young age value (439 Ma), which is due to the influence of late tectonic and thermal events associated with the Early Paleozoic orogeny occurred at that time throughout the territory of the present-day East Sayan.