Mineralogy and Genesis of the Oxidation Zone of Barite–Lead Ores of the Ushkatyn-III Deposit, Central Kazakhstan

Abstract—The Ushkatyn-III deposit in Central Kazakhstan is a complex object that combines industrially significant deposits of (a) hydrothermal lead and barite ores, (b) hydrothermal-sedimentary iron and manganese ores, (c) supergene (oxidized) lead and barite ores. The mineralogy of the oxidation zone of barite–lead ores was studied. The non-oxidized hydrothermal barite–lead ores are almost entirely composed of calcite, barite, and galena; typical secondary minerals are quartz, pyrite, muscovite–phengite, potassic feldspar, albite, and fluorite; the most characteristic accessory minerals are hematite, sphalerite, chalcopyrite, chamosite, and apatite. Fe–Mn carbonates (rhodochrosite and Mn-rich siderite) occur sporadically in ores. The following processes occur in the oxidation zone: (1) dissolution of calcite; (2) replacement of galena by cerussite and lead phosphates (pyromorphite and phosphohedyphane); (3) formation of montmorillonite and kaolinite due to decomposition of feldspars, chamosite, and mica; and (4) formation of goethite due to the alteration of pyrite and, in part, hematite. Barite is stable in the oxidation zone but it was subject to local recrystallization and redistribution. The sequence of main mineralogical changes in ores was as follows: (1) assemblages of primary ores: calcite + barite + galena ± quartz ± pyrite ± hematite ± feldspars ± muscovite ± chamosite; (2) assemblages of weakly oxidized ores: calcite + barite + cerussite + pyromorphite (phosphohedyphane) + quartz ± galena ± pyrite ± hematite ± goethite ± muscovite ± chamosite ± montmorillonite; and (3) assemblages of highly oxidized ores: barite + cerussite + pyromorphite (phosphohedyphane) + quartz + montmorillonite + kaolinite ± galena ± hematite ± goethite ± muscovite. The bulk of the oxidation zone is composed of highly oxidized ores. Mn–Fe carbonates are less soluble than calcite. Therefore, at the early stages of the development of the oxidation zone, they occur in association with galena, barite, cerussite, phosphohedyphane, quartz, chamosite, and some other minerals. Rhodochrosite and Mn-rich siderite are completely replaced at late stages by oxides of tri- and tetravalent manganese.