Geochemical Prospecting for Minerals

methods based on the study of the principles of distribution of chemical elements in the lithosphere, hydrosphere, atmosphere, and biosphere for the purpose of discovering mineral deposits. Lithochemical, hydrochemical, atmochemical (gas), and biogeochemical methods are differentiated, according to the nature of the substances to be studied for geological-prospecting purposes.

The local average content of chemical elements in rock, soils, natural waters, and the surface atmosphere and plants far from deposits is characterized by the so-called geochemical background (C_b), which is similar to the Clarke (abundance ratio) figures for the elements. The content of chemical elements changes regularly in the vicinity of mineral deposits, forming geochemical anomalies—indications of the probable location of commercial deposits. These anomalies are primary and secondary dispersion halos and flows of a mineral substance, which originate during the formation process of deposits or as a result of the subsequent migration of chemical elements. The geochemical halos of deposits greatly exceed the size of the beds and are often confined to the cap rocks—that is, they are located near the surface, which facilitates their discovery and, under favorable conditions, makes for high efficiency of geochemical prospecting. Unlike the pay content of useful components in deposits, the content of these same chemical elements in anomalies often differs only slightly from the local background, which requires highly sensitive methods for their discovery. For example, during geochemical prospecting for mercury deposits, the analysis of rock is conducted with a sensitivity of 1 x 10^-8 percent Hg (1 x 10^-7 percent for gold), which is 10 million times (3,000 times for gold) less than the pay content of these metals. The content of a chemical element, which depends on the normal or log-normal law of background content distribution, serves as the criterion for distinguishing anomalies.

Geochemical prospecting is conducted by the systematic determination of the contents of chemical elements within a region under study by taking samples in a specific prospecting grid for subsequent analysis of their composition. The content of chemical elements of the desired mineral—the principal valuable components of the deposit or their accessory minerals—is determined in the test samples. There are more advanced geochemical prospecting methods, such as aerial and motor-vehicle methods that do not require sampling and that have continuous automatic recording, or surveys on foot with instrument readings taken at observation points. Such instruments are presently designed for determining the content of a limited number of chemical elements (for example, radiometers and berylometers).

Geochemical prospecting is most widely performed in the search for ore deposits; the most important method used is the lithochemical survey, which is based on the massive sampling of rocks and their weathering products. Numerous deposits of nonferrous metals, rare metals, and gold, including deposits that are located in concealed beds and are inaccessible to conventional geological methods of exposure, have been discovered by means of this method. The hydro-chemical method is based on the study of the composition of natural surface and underground waters by producing the dry residue or by coprecipitation or extraction of ore elements, with subsequent spectral or chemical analysis. Low pH values and high sulfate-ion (S0₄^-) content in waters can serve as mineralization indicators during exploration for sulfide deposits. Geochemical prospecting for oil and gas deposits is based on the determination of hydrocarbon-gas content in soil air or in rock samples. The biogeochemical method is based on the study of the chemical composition of plants, generally by means of preliminary ashing and subsequent spectral analysis. The use of hydrochemical and biogeochemical methods is expedient in conditions that are unfavorable for conducting lithochemical surveys.

As a result of geochemical prospecting, maps and graphs of mineral element-indicator content are compiled, from which an interpretation is made of the exposed geochemical anomalies with consideration for geological and other data; as a rule, few of them correspond to commercial deposits. Consequently, the evaluation of geochemical anomalies demands accurate analysis of the conditions of the dispersion and concentration of the chemical elements based on the theoretical laws of geochemistry. Mathematical methods, in which digital computers are used to process the results of geochemical prospecting, are growing in importance. The efficiency of geochemical prospecting is ensured when it is performed together with geological and geophysical investigations and in conjunction with the sinking of mine shafts and the drilling of wells.

The theoretical foundations of geochemical prospecting were laid down in the works of V. I. Vernadskii; these methods were first adopted in the USSR (N. I. Safronov, A. P. Solovov, and V. A. Sokolov).