Distribution de l'uranium, du thorium, du zirconium, du hafnium et des terres rares (TR) dans des grains de phosphates sédimentaires

Abstract U, Th, Zr, Hf and REE contents have been determined in phosphate grains from central Tunisia, southern France (Montagne Noire) and the southeastern Pacific, and in organic fractions (humic acids, kerogens and stable residues) separated in Paleocene-Eocene phosphates of the Gafsa-Metlaoui Basin (central Tunisia). Due to adsorption, phosphatic pellets concentrate high Zr, REE and Th contents, and fish debris takes up considerable amounts of Zr but not of Hf. A separation of Zr from Hf, related to the adsorption of Zr preferred to Hf in apatite, and a clear positive correlation between Zr and REE contents are found. We also note a more or less pronounced fractionation between light and heavy REE, which is influenced mainly by organic complexes, as shown from the clear enrichment of the heavy REE in each of all of the organic fractions.