Stratiform Barite–Pb–Zn Deposits of the Zhairem Ore Field (Central Kazakhstan) as SEDEX-Type Deposits: Structure, Ore Composition, and Formation Conditions

The Zhairem ore field deposits—Western, Far Western, and Eastern Zhairem—are located in Central Kazakhstan, 300 km west of Karaganda. The studied deposits are hydrothermal-sedimentary objects of the SEDEX type. The ore field is confined to the axial part of the epicontinental paleorift filled with sedimentary rocks of the Upper Devonian age. The ore-bearing unit is a pack of carbonaceous carbonate–siliceous–clayey deposits containing stratiform deposits of barite–polymetallic ores. The deposits have a concentrically zonal structure: their axial parts (cores) are composed of lenses of massive barite ores, and the outer zones are layers of banded sulfide ores of lead and zinc. In turn, the composition of sulphide ores changes regularly from the center of the deposits to the periphery: near the barite “cores,” galena dominates in the composition of sulphides, while sphalerite does at a distance. At the Eastern Zhairem deposit, in addition to sulphide lead–zinc (galena–sphalerite) ores, oxide iron (magnetite-hematite) ores are widespread in the composition of the pack of banded ores. Data are presented on the geological conditions of localization, mineral composition, structural and textural characteristics of the ores, and the isotopic composition of sulfur in barite and sulphides. A model of deposit formation is proposed. According to it, barite and lead–zinc ores represent different parts of a single facies series of simultaneously forming hydrothermal deposits. Barite lenses are formed in the central part of paleohydrothermal fields, and sulphide deposits are formed on their flanks. The accumulation of ore matter occurred on the uneven surface of the sea basin bottom, in the bottom part of which stratification of the water column by oxidation–reduction characteristics (oxygen, sulfate, and sulfide ion concentrations) occurred. Barite accumulated in elevated areas in oxic and suboxic environments, and sulfides accumulated in depressions where anoxic and euxinic environments prevailed.