Distribution patterns of rare-earth elements and uranium in tertiary sedimentary phosphorites of Hahotoé–Kpogamé, Togo

Abstract

Seventeen samples have been used to study the distribution behaviour of rare-earth elements and uranium in four profiles of the exploited phosphatic layer C1 of Hahotoé–Kpogamé deposits in Togo. Compared with shale values (NASC: North American Shale Composite) these phosphorites are enriched with REE by factors of 4–7 and with uranium by a factor of 43. REE display seawater-like distribution patterns with a pronounced negative cerium anomaly and an enrichment of light REE relative to shales. The phosphorite deposits of Togo are altered in a tropical climate that led to the formation of crandallite, kaolinite and goethite at the expense of francolite and clay minerals. Horizons rich in crandallite, kaolinite and goethite accumulate high amounts of REE and uranium released during the weathering of phosphorites. The alteration does not considerably affect the Ce-anomalies which remain negative for all samples but does affect the REE fractionation whereby crandallite, kaolinite and goethite rich samples show an enrichment of heavy REE compared with the light ones.