MECHANISMS OF FORMATION OF SULFATE LAKES IN SOUTHEASTERN TRANSBAIKALIA

We analyzed the mineral and chemical compositions of water and bottom sediments of lakes as well as the isotopic compositions of carbon and oxygen in dissolved and precipitated carbonates and of elemental, sulfate, and hydrogen sulfide sulfur. It has been established that the groundwater and surface waters feeding saline sulfate lakes belong to the sulfate–hydrocarbonate or hydrocarbonate–sulfate types with pH < 9 The research shows predominantly soda and sulfate lakes in the study region. It has been revealed that the sulfate coefficient of the water in such lakes exceeds 1 and that it decreases with increasing salinity as a result of the precipitation of thenardite and sulfate reduction. Changes in the hydrochemical types of lakes lead to changes in the mineral composition of their bottom sediments. During low-water periods, autochthonous sedimentation prevails, and the portions of dolomite and clay minerals increase: in sulfate lakes, kaolinite and hydromica accumulate; in soda lakes, montmorillonite is additionally prominent. Concurrently, heavy oxygen isotope accumulates in carbonates. Further increase in water salinity is accompanied by the precipitation of gaylussite, trona, and anhydrite and by the ware transition from soda to sulfate types. The formation of thenardite changes the lake water from the sulfate to the chloride type. During the period of lake desalinization, allochthonous sedimentogenesis prevails, leading to the accumulation of clastic material in lacustrine sediments, primarily plagioclases and feldspars; in addition, carbonates with an isotopically lighter composition of oxygen accumulate.