Rare-earth geochemistry of `oldʹ phosphorites and probability of syngenetic precipitation and accumulation of phosphate1

`Oldʹ (Vendian–Early Cambrian) phosphorites of the World demonstrate `seawater-likeʹ patterns in their rare-earth element (REE) distributions but are depleted in heavy REE. They also show a broad range of variation in total and elemental abundances. Primary bedded structureless phosphorites contain only 15 ppm total REE (ΣREE) while granular (pelletal) varieties contain up to 650 ppm. REE were incorporated into phosphate grains diagenetically during prolonged periods of transport and redeposition. A higher surface/mass ratio in pelletal phosphorites contributed to their enrichment in the REEʹs. In primary bedded structureless phosphorites immediate burial of phosphate syngenetically precipitated and accumulated in situ at the sediment–water interface prevented the later incorporation of the REE. Large deposits of Vendian–Early Cambrian phosphorites have originated by both processes. Primary bedded varieties occur in deeper-water facies and granular phosphorites accumulated closer to the shore. In terms of their REE geochemistry, old granular phosphorites are almost indistinguishable from their Phanerozoic counterparts, indicating that both were deposited by the same mechanisms. Primary bedded structureless phosphorites were only deposited on a large scale at the beginning of the Phanerozoic.