Landscape-climatic conditions of searches
In the basis of landscape zoning of territories according to the conditions of conducting prospecting works, a set of characteristics characterizing the features of the geographical landscape is adopted. The main landscape complexes are relief, soil and vegetation cover, a layer of loose sediments, weathering crusts, bedrock, soil-ground, marsh and surface waters. All the components of the landscape are closely interrelated and depend on the geological structure, neotectonics and the climate of the area.
The smallest area, within which extremely homogeneous parts of the landscape are combined, is defined as an elementary landscape. This is an element of the relief, composed of one rock, one type of loose sediments and the development of a certain type of soil-vegetation cover. Taking into account geomorphological features and location relative to the level of groundwater, four main types of elementary landscapes are distinguished: watershed, slope, foot of slopes and local water bodies [Kazhdan, 1984]. Elementary watershed landscapes are autonomous & quot ;. They are covered with loose eluvial and eluvial-deluvial deposits formed by the destruction of local bedrock. Here, unbiased or almost unbiased residual halos of scattering of ore elements and their satellites are formed. Elementary landscapes of the slopes are characterized by loose deposits of the deluvial type. For them, biased halos and fluxes of scattering of the basic and accompanying metals of ores are inherent in varying degrees. Elementary landscapes of the slopes of the slopes are located on their lower parts or within the floodplain terraces of river valleys. Under these conditions, secondary accumulations of metals are usually formed in the form of superimposed salt halos, sometimes in combination with displaced halo remnants.
The elementary landscapes of local water bodies include swampy areas, riverbeds, ponds and lakes ponds. They develop water and salt haloes of ore components, which have a high migration capacity in the aquatic environment. The natural totality of elementary landscapes creates a common geochemical landscape. In its essence, this is a paragenetic association of conjugate elementary landscapes, connected by a commonality of the migration zone of elements. It should be borne in mind that the geologist often deals with relics of paleolandscape, with ancient weathering crusts and oxidation zones, which may not correspond to modern climatic and geographical zonation.
Elementary landscapes and their totality are manifested in various bio-climatic conditions. This determines the essential specificity of the dispersion aureoles and peculiarities of the method for searching for mineral deposits. Two types of bioclimatic regions are distinguished: arid and humid, in which fundamentally different conditions for the hypergenic migration of ore components operate.
Arid regions are characterized by a dry climate and a predominance of evaporation over the amount of precipitation. They are characterized by a lack of forest cover, often a weak development of grassy vegetation and a non-washable regime of hydrocarbonate-calcium slightly alkaline soil-groundwater. In such a landscape environment, there is a rapid decomposition and migration of organic substances. At the same time, in the aqueous environment, the ore elements have weak chemical mobility, and this leads to the formation of open and unshifted or weakly displaced metal haloes. Deserted, semi-desert, steppe landscapes and landscapes are located in arid regions. All of them are favorable for the formation of halos of ore components in the near-surface layer of loose sediments. They can be effectively detected by geochemical methods.
Humid areas are distinguished by a humid climate with a predominance of precipitating water precipitation over evaporation. They are characterized by rich vegetation, active accumulation of organic matter in depressions of the relief. washing regime and acid reaction of soil-groundwater. In tropical and subtropical humid regions with a hot climate, the washing regime of water leads to the formation of powerful weathering crusts and to the complete leaching of ore elements migrating in the water environment. In the moderately warm humid regions, the high migration capacity of these elements is maintained. This leads to the design of extensive lithochemical and hydrochemical haloes, sometimes detached from the deposits. In cold humid regions, the processes of physical weathering of rocks and ores predominate, with the formation of mechanical halos and ore streams.
Humid areas with hot and humid climates include tropical and subtropical landscapes. In temperate and extreme climates, the forest-steppe, forest, mountain-taiga, tundra and polar zones are distinguished in their composition, with a diverse combination of mechanical and geochemical halos and ore-matter fluxes.
Forest landscapes are divided into zones of tropical, northern coniferous and southern forests on silicate and carbonate rocks. They have the maximum intensity of the circulation of elements due to the high value of the hydrothermal index, the acid reaction of water and the year-round migration of the substance. Secondary changes in sulphide deposits are extremely pronounced here. Oxidation proceeds quickly and reaches hundreds of meters in depth. Metals are leached completely and empty iron hats remain. Only gold, platinum, cassiterite, tantalite, columbite, diamond, precious stones are preserved. In a young landscape, mineral deposits are manifested through contrasting secondary halos of dispersion. In the old landscape, mechanical halos of stable minerals are preserved. Biochemical halos here lose their contrast. Saline halos are contrasting, but they develop only in the lower horizons of the weathering crust. Hydrochemical and atmohimic halos are manifested in deep waters and large faults.
Under conditions of northern coniferous forests developing on carbonate rocks that create an alkaline reaction of waters, lithochemical haloes acquire special significance for searches. Sampling is recommended to be carried out directly from the surface of the earth, because metals form soil colloids and lose their mobility. If coniferous forests are located on moraine sands, the lithochemical haloes are buried and the search for deposits is possible only by testing the boundary horizons of glacial deposits with bedrock. In the conditions of forests confined to permafrost, physical weathering will sharply prevail over the chemical. Here flows of scattering of gold, platinum, cassiterite, wolframite, scheelite, columbite, diamond and elements of Cu, Ni, Co, Cr, Ti are developing. Saline haloes are characteristic of Ni, Co, Cu, and high contents of Cu, Ni, Co, Cr, Ti are found in the needles and bark of larch.
Tundra landscapes are formed under the condition of low temperatures at high humidity and low volatility. Physical weathering prevails and large-scale crumbling and scree debris are formed. Soil and groundwater are not highly mineralized. Tundra peatlands develop in places. In such areas, only deep geochemical searches should be used. For example, on the Kola Peninsula and Finnish Lapland, sulfide copper-nickel deposits at the exits are accompanied by biochemical halos of Cu, Ni, Li, Rb, Cs and hydrochemical haloes of Mo, Cu, Ni, Zn, Li, Rb, Cs. In such conditions, one can use the search for gold deposits, cassiterite, wolframite, ilmenite, cinnabar by their mechanical dispersion halos, and sulfide ores by litho- and hydrochemical methods.
Steppe landscapes are subdivided into zones of chernozem and chestnut steppes. For the chernozem steppes, there is an increased amount of precipitation with their high evaporation. Chestnut landscapes are formed in conditions of a drier climate when soil is enriched with calcium carbonate. The formation of halos of secondary scattering (and especially lithochemical) makes it possible to perform searches with lithochemical, hydrochemical, biochemical, and atmo- chemical methods. Testing is recommended to be carried out from the ground.
Desert landscapes occur in conditions of lack of moisture and high temperatures. Here the influence of bioclimatic factors is insignificant. The first place is geological substratum. Therefore clay, sand, gypsum, salt-karst, stony deserts are widespread. In the subordinate landscapes are developed takyrs, salt lakes, solonchaks. Water migration of elements, except Na, Cl, CO, is weakened. Therefore, halos of secondary scattering of metals of any deposits almost do not arise, and therefore the use of lithochemical searches is inexpedient. But successful are the schlich and superficial geochemical searches.
Sediment thickness and area nudity
Quaternary cover in the search plays a dual role. The loose cover may contain placers of gold, cassiterite, diamond and other useful minerals, or it acts as a medium in which secondary aureoles and scattering flows of ore components are formed. Then, loose sediments are a zone of prospecting for deposits, and secondary aureoles and scattering flows located in them contribute to the identification of overlapped and hidden-overlapped ore deposits in the underlying bedrock.
On the other hand, loose deposits greatly impede the search for ore concentrations in the bedrock. When the thickness of the allochthonous sediments increases, the indigenous ores and their halos are buried. Then the signs of these ores and their aureoles do not fully manifest.
According to the capacity of overlapping loose rocks and nudity, the territories are divided into the following categories:
1. Area of the first category. Naked or covered with thin (1-2 m) eluvial-deluvial deposits with the appearance of open halos of normal intensity and with other distinct search characteristics.
2. Area of the second category. They are blocked by loose sediments (loams) of medium thickness (up to 10-20 m). Here, the aureoles of the scattering of ore matter are sharply attenuated in the daytime surface. Other search signs are poorly developed.
3. Areas of the fourth category are areas covered by cover loams and other allochthonous deposits of considerable thickness (up to 20-30 m). They are characterized by the manifestation of shallow buried halos of scattering of ore matter and the absence of other geological search features.
4. Area of the fourth category. This is a square that is covered by a powerful cover of allochthonous deposits with a capacity of many hundreds of meters. They are characterized by deeply buried halos of scattering of ore material and the absence of other search signs.
The categories of areas are taken into account when designing and performing forecasting and prospecting works in a particular region. Most traditional search methods give a good effect only on the area of the first category. When carrying out prospecting works in areas covered by a significant loose cover, a complex of deep geophysical, aerospace studies with drilling of structural and search wells is necessary.
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