RARE-EARTH GEOCHEMISTRY OF OLD PHOSPHORITES AND PROBABILITY OF SYNGENETIC PRECIPITATION AND ACCUMULATION OF PHOSPHATE

'Old' (Vendian-Early Cambrian) phosphorites of the World demonstrate ' seawater-like' patterns in their rare-earth element (REE) distribution s but are depleted in heavy REE. They also show a broad range of varia tion in total and elemental abundances. Primary bedded structureless p hosphorites contain only 15 ppm total REE (Sigma REE) 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 REE's. In primary bedded struc tureless phosphorites immediate burial of phosphate syngenetically pre cipitated and accumulated in situ at the sediment-water interface prev ented the later incorporation of the REE. Large deposits of Vendian-Ea rly Cambrian phosphorites have originated by both processes. Primary b edded varieties occur in deeper-water facies and granular phosphorites accumulated closer to the short. In terms of their REE geochemistry. old granular phosphorites are almost indistinguishable from their Phan erozoic counterparts. indicating that both were deposited by the same mechanisms. Primary bedded structureless phosphorites were only deposi ted on a large scale at the beginning of the Phanerozoic. (C) 1998 Els evier Science B.V.