Kerogens and Bitumens in Precambrian Uraniferous Ore Deposits: Witwatersrand, South Africa, Elliot Lake, Canada, and the Natural Fission Reactors, Oklo, Gabon

Abstract

Org. matter-mineral assocns. in the Witwatersrand, Elliot Lake, and Oklo ore deposits help to elucidate the various roles of kerogens and bitumens in uranium metallogeny. Sedimentol. observations imply that at least in the Witwatersrand, ancient mats of cyanobacteria helped initially to trap and conc. detrital uranium minerals from low-energy water transport close to 3 Ga ago. These microbial mats were converted to kerogen during diagenesis while retaining their acquired minerals. In the 2.15-2.37 Ga old Elliot Lake ores, long after deposition apparent hydrothermal events helped bitumen to evolve in, and then move away from, thin layers of syngenetic kerogen. However, bitumen mobilization did not allow uranium migration. At the 1.8-2.1-Ga-old uranium deposits in the vicinity of Oklo, kerogen abetted the post-depositional concn. of uranium in high grade ores and the natural fission reactors. Nuclear criticality was maintained for more than a hundred thousand years when the contained water, acting as moderator, was expelled. The Oklo natural reactors are time-tested analogs of radioactive waste containment models. Consideration of the elemental, mol. compns., maturity, graphite cryptocrystallinity and the mineral assocns. of Witwatersrand, Elliot Lake, and Oklo kerogens and bitumens together, helps to clarify a no. of common and distinct effects of org. matter on uranium mineralization in these Precambrian ore deposits.