Search works of scales 1:50 000-1: 25 000
The most important methods of searching for deposits of rare and radioactive metals differ in technical methods for identifying search characteristics. Depending on this, all modern search methods are divided into three groups:
• geological and mineralogical,
• geophysical (radiometric),
In the early stages of research, these same methods, combined with special geological mapping methods, are used to create the geological basis for searches. In terms of forecasting and prospecting for uranium deposits, the effectiveness of using different methods is illustrated in Appendix 1.
Geological basis of searches
Effective research is possible only on a benign geological basis. For this purpose, geological, geophysical and other maps are compiled for promising areas, which ensure the possibility of confident forecasting of ore-bearing areas, the selection of effective methods and directions for prospecting. Unlike conventional geological mapping, specialized geological maps with the greatest completeness reflect those features of the geological structure that determine the leading search criteria for uranium and rare metal deposits of the expected industrial types. They are compiled on a scale from 1: 200 000 to 1:10 000 for geological and forecasting, geological survey and prospecting works. Specialized geological mapping includes:
• a complex of structural and geophysical works;
• compilation of aerogeological and landscape maps;
• special mapping of metasomatites and epigenetically altered rocks;
• water no-helium survey;
• geochemical survey, specialized in rare or radioactive elements with testing of rocks for the content of radiogenic lead. A complex of structural geophysical works (deep seismic sounding, gravimetric and magnetic surveys, electrical prospecting and other works) is carried out before or, at least, ahead of specialized geological mapping. On a scale of 1: 1 000 000, the DGS and gravimetry methods are of lesser importance, on a scale of 1: 200 000-1: 50 000 gravimetry and magnetometry, and on a scale of 1:10 000 and larger - high-precision gravimetry, magnetic and electrical reconnaissance.
When compiling geological maps of scales 1: 1 000 000 and smaller use of space television photographs of small and medium resolution. Small-scale images provide detection of blocky and large ring structures, suture zones and large deep faults, and from the middle resolution images, structural blocks, deep faults, volcanotectonic and ring structures, and many structural and material rock complexes are revealed. When mapping the scales of 1: 200,000 and 1: 50,000, high-resolution satellite images, high-altitude aerial and radar images are used, for which zones of increased permeability and tectonic disturbances, rock massifs, volcanic-tectonic, ring and other ore control structures are deciphered. When mapping on a scale of 1:10 000 and larger, various black-and-white, color, spectrozonal and other aerial and radar images are used.In combination with aerial and aerial photographic interpretation, maps of the present and paleolandscape conditions of exploration areas are drawn up, allowing to assess the features of the manifestation of search characteristics of uranium and rare metal mineralization in the surface of the Earth. Particular attention in specialized geological mapping is given to the study of metasomatic changes in host rocks, the separation of the formational and facies types of metasomatites, the delineation of the areas of development of megasomagic rocks that are promising to identify deposits of various uranium and rare metal formations. In the early stages of mapping metasomatites, an express petrophysical method developed by M.I. and V.I. The Pakhomovs . The method is based on the fact that the positive correlation relations between their density and magnetic susceptibility characteristic for unchanged rocks disappear or change their sign in metasomatically changed rocks. The work is reduced to a mass determination of these properties of rocks and the construction of maps of isolines of correlation coefficients. Later zones of metasomatically changed rocks are studied by conventional, petrographic and geochemical methods.
Water helium surveys that fix manifestations of helium anomalies along deep fault zones provide essential assistance in studying the block structures of ore-bearing regions.
The use of water-helium surveys is most effective in the early stages of research. Depending on the details of the water-helium surveys, one sample of water at a scale of 1: 1 000 000 is about 50-70 km, and at a scale of 1: 200 000 - by 10-15 km. Since the helium concentrations in the near-surface parts of the earth's crust are weakened in proportion to the square thickness of the sedimentary rock cover, sampling takes place at shallow depths of the basement (of the order of 100-200 m) from horizons 5-10 m below the day surface, and with a deep foundation (more than 500 m ) - from horizons of depth up to 100-150 m.
The helium content in the water is determined on the membrane indicators of helium mass-spectrometric (ICGM-1) or magnetically charged (INGEM-1) principles. The concentration of helium in the water is expressed in milliliters per liter of water. In near-surface horizons, the background content of helium in the waters usually does not exceed 1-10 " ml/l, and near the faults they reach 0.1-1 ml/l. At deeper horizons, background helium concentrations rise to 1-10 ml/l, and anomalous to 10-30 ml/l.
The principles and methods of geochemical mapping of radioactive elements (and their satellite elements) scattered in rocks have been developed and introduced into the practice of geological research by A.A. Smyslov and others .
Radio-geochemical maps are compiled on a geological-formational basis with the identification of those features of the geological structure and development of the area that contribute to the violation of the primary-constitutional scattering of radioactive elements. The study of radiogeochemical maps helps identify the main regularities of uranium migration and the conditions for the formation of its elevated concentrations.
Geochemical mapping of maps depends on their scale. Small-scale maps (from 1: 200,000 and smaller) reflect average uranium content in geological formations or in homogeneous geological fields, the degree of heterogeneity in the distribution of uranium and thorium, thorium-uranium ratios, the forms of finding these elements in rocks, the anomalous concentrations of uranium-satellite elements , anomalous ratios of radiogenic lead (especially elevated concentrations of Pb) and geochemical specialized rocks for uranium. According to the set of promising data, radiogeochemical provinces are allocated on the maps, and inside them are zones with a disturbed primary-constitutional distribution of elements, the most promising ones for detecting elevated concentrations of mobile uranium.
Medium-scale radiogeochemical maps (1:50 000-1: 25 000) reflect the uranium content in homogeneous geological fields in absolute values and in relation to the geochemical background, and within zones with a disturbed primary-constitutional distribution of elements, and removal of uranium and the most typical satellite elements.
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