Concretionary phosphorite from SW Africa

Abstract

Phosphorite concretions, fish debris, quartz, rock fragments and minor amounts of molluscs occur along the eastern edge of a diatomaceous mud belt off the coast of SW Africa (Namibia). The concretions consist of light- and dark-coloured coprolites and dreikanters, predominantly of pebble and granule size; and irregular, tabular, and oval to spherical particles that predominante in the granule to very coarse sand fractions. Only the dark-coloured concretions were analysed geochemically, and these consist of francolite with minor amounts of Si, Al, Fe and K. The most abundant trace elements present are Sr, Zn, Ba and Th. The interstitial fluids of the diatomaceous mud are supersaturated in phosphate along the landward flank of the mud belt, and this causes slow precipitation of apatite on diatom frustules. Concurrently, the fluids enhance the elimination or replacement of allogenic impurities from the concretions through diagenesis. Adsorption of phosphate by slow-settling mica–illite is a possible mechanism by which the phosphate concentration of the interstitial fluids is raised.