THE URANIUM BLACKS: GENETIC PROBLEMS AND MINERAL COMPOSITION

Abstract

The mineral composition of uranium blacks was studied using analytical transmission electron microscopy based on selected-area electron diffraction. The stages of formation of mineralogical zoning in supergene profiles of uranium deposits are characterized. The mineralization of the reduction stage is represented by so-called uranium black mineralization in these profiles. Uranium blacks occur in the secondary enrichment zones of hydrothermal deposits. In infiltrational deposits, they are concentrated in a specific zone of cementation with reduced conditions and the richest ore mineralization, which follows the oxidized zone. Some examples of uranium deposits of various genetic types with uranium black mineralization of economic interest are considered. Loose uranium mineral aggregates are studied in various infiltrational (hydrogenic or exogenic epigenetic) deposits, in secondary enrichment zones of hydrothermal deposits, and in deposits completely contained in the weathering mantle. According to mineralogical studies, at least three mineral forms of uranium compose uranium blacks: oxides (uraninite), phosphates (ningyoite), and silicates (coffinite). The results of this work specify the sequence of the processes of uranium mineral deposition in sandstone-hosted ores. The genetic analysis of crystallochemical data on the basis of the elemental composition of isomorphic admixtures in ore minerals enables one to estimate the geochemical conditions of the reducing processes in the supergene zone.