Abstract—: The Ushkatyn-III deposit is located in Central Kazakhstan, 300 km west of the city of Karaganda. The ore member consists of Upper Devonian carbonate rocks containing stratiform layers of hydrothermal–sedimentary iron–manganese and hydrothermal barite–lead ores. The study objects include barite–lead (barite–galena) ores localized within a reef limestone member. The major ore minerals are calcite, barite, and galena; characteristic minor minerals include quartz, hematite, sphalerite, pyrite, muscovite–phengite, chamosite, K-feldspar, albite, fluorite, dolomite, rhodochrosite, and siderite; and accessory minerals include native silver, rutile, ilmenite, chalcocite, acanthite, chalcopyrite, pyrargyrite, tetrahedrite, zircon, pyrophyllite, and apatite. Cerussite, pyromorphite, kaolinite, montmorillonite, and malachite are identified as supergene minerals. Cerussite is one of the main minerals in the oxidation zone. Three major ore varieties are distinguished in terms of texture. They are related by layered–banded, pocketlike–net, and massive spotted mutual transitions. The ore structure is indicative of the fact that barite and galena were mostly deposited in open pores and fractures of incompletely lithified carbonate deposits. Based on δ³⁴S_barite = 10.9–15.3‰, the barite formation involved isotopically heavy sulfur of the sulfate ion dissolved in seawater, whereas the δ³⁴S_{sulfides (galena)} values from –25.7 to –12.6‰ record ³²S-enriched (light isotope) hydrogen sulfide generated in the course of bacterial sulfate reduction. A formation model is proposed for the Ushkatyn-III deposit. According to this model, its barite–galena, iron, and manganese ores were products of a single hydrothermal system that developed within a thick sedimentary sequence. Barite–galena ores formed near the seabed surface with the discharge of hydrothermal solutions in internal zones of the still-forming reef. The ore material was deposited in the area where the hydrothermal solutions carrying Ba, Pb, Zn, Fe, Mn, and other elements were mixed with near-surface waters filling pores and fractures inside the reef, characterized by bacterial reduction of seawater sulfate ion to hydrogen sulfide. Penetrating through the reef, the fluids lost most of the Ba and Pb, deposited as barite and galena, but retained Zn, Fe, and Mn in dissolved form. Subsequently, Fe and Mn were deposited as oxides: Fe, on the reef surface or at some distance from it, and Mn, at a considerable distance. Zn dispersed in the surrounding space without forming any orebodies.