THE INCORPORATION OF URANIUM INTO DIAGENETIC PHOSPHORITE

Abstract

The behavior of U during the diagenetic formation of marine phosphorite has been modelled. The model examines a dissolution-reprecipitation replacement of skeletal hydroxyapatite, calcium carbonate and earlier generated francolite by francolite. The amount of organic matter consumed relative to the mass of francolite formed, the replacement reaction progress, and the concentration of U in the replaced phases are the important parameters which dictate the concentration of U in the phosphate rock. A partition coefficient between apatite and interstitial solution was calculated, and is U F = 0.57. Natural phosphorites have been examined and are discussed in the light of the proposed model. The U mass-balance in a Recent phosphorite is in good accord with theoretical predictions. Differences in U concentrations between sea-floor phosphorites are explained either by the (original) variation in the organic matter distribution in the corresponding sediments and/or by mineralogical differences (CaCO 3 vs . hydroxyapatite) therein. Senonian phosphate rocks which were formed via the francolite francolite transformation, demonstrate that during that process the organic matter content in the sediment was approximately 50%. The model supports the idea that phosphorite rock formation is a multistage process.