Halos of Dispersion

zones of increased (less frequently, reduced) content of chemical elements in natural formations associated with deposits of useful minerals. Primary halos of dispersion of deposits arise in the surrounding rock simultaneously with the formation of the mineral body. Secondary halos of dispersion form in the products of decomposition of rock, and also in soil, water, plants, and the surface atmosphere as a result of supergene processes taking place on the land surface.

Primary halos of dispersion are typical of endogenic deposits; around exogenic deposits they are less distinct. The primary halos of dispersion of valuable components repeat on an expanded scale the outlines of the deposit and sometimes extend for many hundreds of meters beyond it; they are governed by the magmatic, tectonic, lithologic-facial, stratigraphic, and structural characteristics that determine the conditions of formation of deposits. A distinctive feature of primary halos of dispersion is their zonal structure, a directed and regular change in the ratios among the contents of elements; this feature is used extensively in lithochemical surveying.

Secondary (supergene) halos of dispersion are subdivided into mechanical halos (dispersed in the solid phase), saline halos (dispersed in the form of soluble compounds), gaseous halos, and biogeochemical halos. Mechanical halos of dispersion form during decomposition of deposits with stable primary or secondary minerals in the zone of weathering, as their fragments progressively disintegrate and particles of the useful mineral are diffused with eluvial and talus deposits. Saline halos of dispersion in capillary solutions of rock and subterranean water form deposits of mineral salts, certain metal sulfates, and other easily soluble primary and secondary minerals as the result of diffusion, capillary elevation, evaporation of mineralized solutions, sorption, and biogenic accumulation of the chemical elements of the mineral. Gaseous halos of dispersion are typical for deposits of petroleum and natural fuel gases, helium, and radioactive ores; many ore deposits are associated with halos of dispersion of mercury vapor and other gases. Biogeochemical halos of dispersion form in plants in the region of the deposit, which contain increased quantities of the valuable elements and their associated minerals.

Secondary halos of dispersion are subdivided according to genesis into sedimentary halos (in present-day eluvial and talus deposits and ancient crusts of weathering of ore-encompassing rock) and superimposed halos (in deposits carried from remote places that cover the rocks of the ore-bearing substrate). In terms of the degree of accessibility for study, a distinction is made between open halos (forming outcroppings on the surface) and closed halos (found, with existing equipment, only at a certain depth beneath the surface). Geochemical prospecting for useful minerals is based on identification of the halos of dispersion of deposits.