Bitumens in Uranium Deposits

The association of uranium mineralization with solid bitumens is widely recorded. Uranium ore-related bitumens are generally the altered residues of crude oils. Even if petroleum cannot be considered either as a source or as a carrier fluid for uranium, spatial relationships between source-rocks or oil fields with uranium deposits are evident. Uraniferous bitumens occur in various reservoir facies (sandstones, faults, breccias) and have been recognized in Precambrian, Cambrian, Permian, Triassic, and Tertiary sedimentary deposits. They are generally insoluble in the usual organic solvents and display quite high aromaticity factors. Such characteristics are frequently induced by radiolytic degradation, which is known to drastically transform petroleum by dehydrogenation and polymerization. Microspectroscopic and isotopic investigations on mineralized and unmineralized bitumens from Precambrian deposits are presented in this chapter.Oil-to-source rock correlations have been established down to the molecular level and, when combined with geological and thermal information, provide complementary data on the age of uranium mineralization.Biodegradation has also been quoted as a major phenomenon for the formation of solid bitumen in uranium deposits. Two different alteration pathways have been recorded and respectively correspond to an oxidation and a CH₂ loss.The importance of bitumens in the mineralizing process is not precisely known. Nevertheless, most of the studies dealing with this subject emphasize the role of a reducing substrate played by bitumens when interacting with uranium-carrying aqueous solutions.In this chapter, several examples of uranium-bitumen associations are discussed with special emphasis on the geochemical modifications undergone by bitumen during alteration processes.