Uranium deposits of Franceville basin (Gabon): Role of organic matter and oil cracking on uranium mineralization

Abstract The lower Proterozoic uranium deposits in the Franceville basin (Gabon) are mainly known to have hosted natural nuclear reactors. However, this basin also witnessed a former petroleum system which shows major occurrences of uranium ores associated with organic matter. The uranium mineralization is the result of several episodes of oil generation and migration, fluid mixing and hydrofracturing. During burial, at around 2.0 Ga, black shales were heated and expelled hydrocarbons which migrated in underlying sandstones. The first episode of oil migration was synchronous with a silicification event produced by a hot low salinity fluid, as shown by fluid inclusions trapped in quartz overgrowths. The second episode was contemporaneous with quartz dissolution and is characterized by the mixing of hydrocarbons and oxidizing U-bearing brine which resulted in the precipitation of UO2 inclusions in oil. Then, oil-to-gas conversion generated overpressured gas trapped in pores with uraniferous bitumen nodules. Changes in the stress regime probably favored radial hydrofracturing around bitumen nodules and subsequently the development of a pervasive micro-fracture network. There, UO2 precipitated together with Cu-Fe sulfides through thermochemical sulfate reduction favored by the presence of hydrocarbons and methane.