RARE-EARTH ELEMENTS IN PHOSPHORITE AND ASSOCIATED SEDIMENT FROM THE NAMIBIAN AND SOUTH-AFRICAN CONTINENTAL SHELVES

Rare earth elements (REE) and major elements have been determined in t wenty-nine phosphorite and associated sediment samples from the southw estern coast of Africa, including thirteen samples of unconsolidated s ediment collected during recent cruises to phosphate-rich areas on the Namibian shelf. The concentrations of REE in various phosphorite type s from southwestern Africa differ by more than two orders-of-magnitude . The samples mostly exhibit REE patterns which are similar to that of shale, having no Ce anomaly. The Ce content appears to have been cont rolled by the palaeoredox conditions of the localised environment of d eposition or diagenesis and not to have been influenced by marine upwe lling, or by the general palaeoredox conditions of the water column. T he environments in which REE were incorporated into the francolite app ear to have been anoxic, or sub-oxic, in the case of both Namibian unc onsolidated authigenic phosphorites, and the predominantly 'diagenetic ' phosphorites off the South African coast. The ultimate REE source in both cases appears to have been (mainly eolic) continental detritus. Pelletal phosphorites are significantly enriched in REE relative to no n-pelletal varieties. If sequestration of REE dissolved in the water c olumn contributed to the high concentrations of REE in pelletal phosph orites, then the main carrier phase appears to have been inorganic det ritus or biogenic silica equilibrated with oxygen-depleted bottom wate rs but not fish debris. The contention that onland phosphorites are en riched in REE relative to their offshore counterparts is, however, ref uted in the case of southwestern Africa, and the effects of groundwate r REE enrichment are not obvious in the majority of onland phosphorite s analysed in this study.