Distribution of rare-earth and some trace elements in Tertiary phosphorites from the Senegal Basin and their weathering products

Abstract

The distribution of rare-earth and some trace elements is studied in the Eocene phosphate deposits of Farim-Saliquinhe Lam-Lam and N'Diendouri-Ouali Diala from the Mauritania-Senegal-Guinea basin. The depth of weathering varies between the three deposits. The strong weathering of Lam-Lam is marked by the leaching of apatites (Ca-phosphate) and the appearance of epigenetic crandallites (AlCa-phosphate). In the void spaces that have resulted from leaching of apatites and then of crandallites, ferruginous and kaolinitic fillings are epigenised in millisites (AlCa-phosphate). All the studied phosphates have high REE, Ba and Sr abundances and present shale-normalized patterns with a negative Ce anomaly of variable intensity. Clay minerals of sediments (smectites) and of weathered rocks (kaolinites) are characterized by low REE and high V, Zn, Cu, Cr, Sc and Zr abundances; they have no Ce anomaly. The comparison of the REE patterns and of the trace-element abundances lead to three important conclusions. At first, the genesis of AlCa-phosphates is marked by specific leaching of heavy REE relative to the light ones. Then, millisites have a REE abundance and a Ce anomaly less pronounced than those of apatites and crandallites; they show also V, Zn, Cu, Cr, Sc and Zr abundances like clay minerals. These facts can be related to their epigenetic formation from clay minerals. Last but not least, the negative Ce anomaly of epigenetic crandallites follows the same trend as in apatites. Therefore, this anomaly cannot constitute a proof of the primary character for apatites. If such variations should be related to the paleogeographic setting of studied deposits they must be considered with prudence.