Characteristics of landscapes
Landscapes of arid and semi-arid regions
Typical desert and steppe landscapes are characterized by high summer temperatures, moisture deficit, limited biological cycle of substances and the intensity of decomposition of dead plant (mainly grassy) remains. In this regard, the biosphere has no significant effect on the water migration of uranium and its satellite elements. The eluvial soils and weathering crusts almost do not contain organic substances, they have light colors and are characterized by sharply oxidizing conditions. The non-flushing regime predetermines the weak removal of weathering products. In the desert climate only sparingly soluble salts (sulfates and chlorides) are mobile, so that the soils, weathering crusts and near-surface layers of loose sediments are enriched in carbonates, uranium and radium, and have a slightly alkaline reaction. In the formation of various loose continental deposits, due to a lack of moisture, uranium does not migrate, but accumulates in the most fine-grained (clay) fractions.
In takyrs flat clayey depressions, which are formed due to the demolition of the most fine-grained fractions of eluvial deposits, uranium concentrations are often high.
Under the conditions of the dissected relief, horizons of groundwater are usually formed, which have low mineralization (less than 1 g/l), weakly alkaline reaction, sulfate or hydrocarbonate composition and elevated uranium contents (up to n - KG g/l). In some areas, the content of uranium in the waters varies greatly depending on the season. During periods of precipitation, it increases several times, because the sediments are enriched with uranium, seeping through the near-surface layers of loose sediments. In dry seasons, groundwaters do not come into contact with enriched uranium near-surface layers of loose sediments and are characterized by markedly lower contents.
Moving to unloading areas, fresh groundwater is exposed to evaporation, especially in areas of their near-surface occurrence. As a result, bicarbonate-calcium waters gradually become sulfate-calcium, chloride-sulfate and, finally, sulfate-chloride waters. In them, the content of SG, SOj and Na * ions increases, and calcium and gypsum carbonates are first deposited in the soils, and then easily soluble chlorides and sodium sulfates, forming various underwater (superaqual) soils, meadow solonets, solonchak and typical solonchaks. With a depth of groundwater 2-3 m in the soil formed a complete salt profile: at the level of groundwater - calcium carbonate, higher - gypsum and even higher - chlorides and sulfates of sodium and magnesium. Uranium evaporates with carbonate and gypsum during evaporation, and the upper salt crusts are usually depleted in uranium. Concentrations of uranium in saline nights can also be related to its deposition on reducing hydrogen sulphide and gley barriers.
If, when the basis of erosion is lowered, the saline soils fall on river or lake terraces, then the reducing environment is replaced by an oxidizing one, and the evaporation concentration is replaced by eluvial leaching. In a dry climate, only sparingly soluble salts leach out of them, and the gypsum and uranium compounds remain in the form of relics of the superacquisition stage and form the so-called & quot; evaporation & quot; anomalies in the form of yellow films of secondary uranium minerals, mainly silicates and carbonates.
The waters of rivers and lakes within desert and steppe landscapes also differ in elevated uranium content of the order n 1 (G g/l, which is associated with the influence of evaporation concentration processes.
Thus, landscapes of deserts, semi-deserts and dry steppes are favorable for the formation of elevated uranium accumulations in near-surface natural formations. Outputs of uranium mineralization under these conditions practically do not leach out, uranium haloes are formed and stored in the near-surface layer of loose sediments with uranium concentration in soils, waters and in some plants.For uranium-bearing provinces and areas located in desert landscapes, near-surface uranium accumulations in the form of bright yellow films of secondary uranium minerals are also characteristic, since in arid climate the phenomena of sorption of uranium are hampered, and uranium is contained in waters in anionic form, which promotes formation independent uranium minerals. Yellow minerals in soils appear when the content of uranium in clays n • 0.001%, ie, at relatively low concentrations. Such manifestations of uranium mineralization have repeatedly misled geologists who regard them as search traces of uranium deposits. Obviously, they should be considered as assessment signs of uranium-bearing regions and provinces, and in some cases - as predictive signs of uranium ore sites or fields.
Landscapes of the northern steppes and forest-steppe are located in bioclimatic zones, transition from typically arid to humid. They are characterized by an increase in the annual amount of precipitation in comparison with the arid zones, a greater moistening of the earth's surface and a less intense manifestation of evaporative concentration processes. In this regard, the evaporative concentration of water is completed mainly at the stage of precipitation of calcite, which contributes to its accumulation in soils and loose sediments. Groundwater has a hydrocarbonate composition, a neutral or slightly alkaline reaction, and eluvial soils are of the chernozem type.
Neutral hydrocarbonate leaching in soils is very favorable for the migration of uranium in the form of carbonate complexes. From the upper horizons of the soil, uranium can be carried away, but it concentrates shallowly from the surface, and often in the chernozem steppes the content of uranium in soils and loose sediments is almost unchanged. Groundwater and the forest-steppe rivers usually contain elevated uranium concentrations of the order of n KG g/l, which is associated with a weak development of uranium sorption processes and indicates favorable conditions for its migration in natural waters.
In the forest-steppe zone, conditions favorable for the concentration of uranium on gley and hydrogen sulfide barriers in peatlands and alluvial deposits occur locally. In the transitional conditions of forest-steppe landscapes, water still retains a slightly alkaline reaction favorable for the accumulation of uranium, and on the other hand, restoration barriers are formed in the accumulation of peat and organic residues in alluvial deposits.
Thus, in the landscapes of the northern steppes and forest-steppes, the water migration of uranium is much more intensive than in the deserts, but is limited by distances of the order of hundreds and first thousands of meters from the mother sources. In this connection, all forms of scattering of uranium and its satellites in the form of areals, streams and halos in various natural formations are clearly manifested in steppe landscapes.
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