Uranium and organic matter in a paleodeltaic environment; the Coutras Deposit (Gironde, France)

Abstract

The Coutras uranium deposit located north of the Aquitaine basin (France) was discovered by COGEMA in 1980. It is hosted by middle Eocene arenaceous unconsolidated sediments, at the surface (the Jean Vincent area) and down to 100 m depth (the Le Fieu area). The detrital components of the sediments are mainly quartz, feldspars, and granitic and metamorphic rock fragments. The host sediments originated from the French Massif Central to the east. Orebodies are black tabular lenses occurring in sediments deposited in a delta plain environment where plant debris is abundant. Preservation of ligneous vegetation during burial established a reducing environment capable of concentrating uranium.There is a good correlation between uranium, total sulfur, and organic carbon. Uranium is mainly fixed in organics as organo-uranyl compounds. Uranium minerals detected include coffinite, pitchblende, and secondary hexavalent U minerals. Accumulation of uranium was favored by a permeable, organic-rich, and slightly acid environment. Adsorption processes probably enhanced ore formation. The lack of oxidized tongues may be the result of oxygen-depleted uranium-bearing solutions when they reached the organic-rich zone. The Coutras uranium deposit is then a good example of a tabular-type deposit which was deposited during early diagenesis.