Search

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 δ34Sbarite = 10.9–15.3‰, the barite formation involved isotopically heavy sulfur of the sulfate ion dissolved in seawater, whereas the δ34Ssulfides (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. [ABSTRACT FROM AUTHOR]

The Ushkatyn-III deposit is located 300 km west of Karaganda (Central Kazakhstan). It is classified as a weakly metamorphosed Atasu-type hydrothermal-sedimentary ore deposit. Stratiform iron, manganese, and barite–lead orebodies occur in the Upper Devonian (D3fm2) carbonate sequence of this deposit. The Fe- and Mn-bearing rocks were studied. Iron ores are composed of hematite, calcite, and quartz. Manganese ores are divided into two (braunite and hausmannite) types. Braunite ores are composed of braunite, calcite, quartz, and albite; hausmannite ores are composed of hausmannite, rhodochrosite, Mn-calcite, tephroite (sometimes with sonolite and alleghanyite), and friedelite. The mineral composition of ores was formed during the burial of metal-bearing sediments containing oxides of Fe3+ and Mn3+/Mn4+. The nature and intensity of the postdepositional processes was controlled by the content of organic matter (OM) in initial rocks. Braunite ores formed under oxidizing conditions; hausmannite ores, under reducing conditions. The initial presence of OM is indicated by the carbon isotope composition in carbonates. High δ13Ccarb values (from –0.3 to 2.1‰) in hematite ores, braunite ores, and host limestones are close to those in the seawater bicarbonate precipitated as a part of biogenic calcite (e.g., shell fragments). At the same time, low δ13Ccarb values (average –12.3‰) in rhodochrosite and Mn-calcite from hausmannite ores indicate the influence of carbon from the OM buried in sediments. The general oxygen isotope composition in iron ores, manganese ores, and host rocks was determined by variations of the isotope ratio in the initial precipitates of 18O-rich calcite and 16O-rich Fe/Mn oxides. Conversion of the primary sedimentary Fe3+ and Mn3+/Mn4+ oxides to hematite, braunite, and hausmannite slightly changes the oxygen isotope composition of Fe–Mn minerals. Average values of δ18Otot determined in the hematite (1.7‰), braunite (14.8‰), and hausmannite (12.7‰) ores are close to those in the sedimentogenic iron (0‰) and manganese oxides (11.5‰). The oxygen isotope composition suggests that initial manganese oxides did not participate in the formation of carbonates in braunite ores. Average oxygen isotope composition of carbonates in these ores (δ18Ocarb 19.1‰) is close to that in host limestones (20.8‰). During the formation of hausmannite ores, in contrast, manganese oxides reacted with OM leading to the formation of rhodochrosite and silicates. Consequently, the light isotope oxygen in the starting oxides was fixed in carbonates (δ18Ocarb 15.9‰) and rocks in general (δ18Ototal 12.7‰). The oxygen isotope composition was not averaged during the formation of hausmannite ores even in the mineralogically similar aggregates in the adjacent rock layers. [ABSTRACT FROM AUTHOR]