Late Quaternary geyserites in the Ol'khon area, Baikal rift zone, are a geologically and petrolog ically unique rocks, which may provide information on the character of hydrothermal processes during the development of modern rift zones. The geyserites of this area are characterized by the presence of minerals atypical of such rocks: highly crystalline graphite (which is usually formed at high temperatures) and α car byne (which is a natural carbon modification that has been previously found only among products of exper iments). Other newly formed minerals of the geyserites are equilibrium chalcedony, talc, and tremolite. This suggests that the rocks were formed under conditions significantly different from those under which classic geyserites are produced. Geyserites in the Ol'khon area were most probably produced by silica saturated solutions rich in certain ore components (Fe, Mn, Sb, As, and Y) and heated to temperatures of about 400°C.
We present results of a complex study of evaporite sediments from one of the small saline lakes with carbonate sedimentation in the Ol’khon area and substantiate their high significance for paleoclimatic reconstructions. The mineral composition of the bottom sediments was studied by XRD analysis, IR spectroscopy, scanning electron microscopy, elemental analysis (SR-XFA), etc. By decomposition of the complex XRD profiles of carbonate minerals into individual peaks by Pearson VII function, we identified carbonate phases in each sample and determined their proportions. A high-resolution carbonate record has been obtained for the first time for the lacustrine sediments. It bears the information about the stratigraphic distribution of Mg-calcites (a continuous series of structurally disordered low- to high-Mg calcites, up to Ca-dolomites), in which the amount and proportions of phases with different Mg contents are controlled by Mg/Ca, salinity, and total alkalinity of the lake water changing depending on the climatic cycles and lake level fluctuations. Comparison of the carbonate record for the Holocene section dated by the radiocarbon (14C) method with results of lithological, diatom, and palynological analyses, data on stable isotopes (δ18O and δ13C), and the distribution of some geochemical indicators of climatic changes permitted the reconstruction of the intricate evolution of the Lake Tsagan-Tyrm basin, which was controlled by the regional climate from the Atlantic period to the present time. The directed change in various characteristics of essentially carbonate sediments evidences that the Ol’khon regional climate has become more arid in recent 6.5 kyr. Moreover, drastic frequent changes of climate and, correspondingly, the Lake Tsagan-Tyrm water level in different periods of its existence have been revealed. The widespread saline and brackish lakes in southern East Siberia, Mongolia, and North China and the highly informative (in terms of paleoclimate and paleolimnology) carbonate sediments might help to recognize the general tendencies of paleoclimate changes and local fluctuations in Central Asia.
The paper presents a review of shallow lakes on two neighboring territories, Transbaikalia in Russia and Northeastern Mongolia. Five systems of shallow lakes are distinguished in the regions under study: Barguzin, Eravna-Gusinoozersk, Ingoda, Onon-Borzya, and Eastern-Mongolian systems. The shallow lakes are sub-grouped according to common chemical compositions and geochemical evolution types. Characteristics of ground-and surface waters feeding the lakes are considered. Based on results of trace element analyses of water samples from about 200 lakes and more than 100 springs, wells and boreholes, elements that manifest higher concentrations, resulting from evaporation of lacustrine waters, are determined. Prospects of assessment of ore-generating potential of the shallow lakes as 'liquid ore' sources in view of commercial extraction of Li, U, REE, and others metals are discussed.
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