CONTRASTING MECHANISMS OF REE ENRICHMENT IN TRIASSIC AND JURASSIC PHOSPHORITES OF WESTERN CANADA

High concentrations of rare earth elements (REEs) in phosphorites have led to significant interest in phosphorites as a potentially economical source of REEs. This has spurred renewed research into the mechanisms that govern REE enrichment and relative abundances in phosphorites. Potential controls on phosphorite REE contents include secular changes in seawater chemistry, extent of diagenetic recrystallization, Fe redox cycling, and phosphorite morphology. Samples of phosphorites from the Ursula Creek exposure of the Toad Formation and Ne Parle Pas and Crowsnest exposures of the Early Jurassic Fernie Formation in the Western Canada Sedimentary Basin have been analyzed. Total REE abundances are up to 274 ppm for Ne Parle Pas, 883 ppm for Ursula Creek, and 2917 ppm in the Crowsnest phosphorites. Patterns of REE distribution in the Ursula Creek phosphorites display MREE enrichment, consistent with a diagenetic control. By contrast, the Ne Parle Pas and Crowsnest phosphorites display seawater-like REE patterns, suggesting that the extremely high REE concentrations at Crowsnest do not result from diagenetic recrystallization. Scanning electron microscope images of phosphorite thin sections show that the Ursula Creek and Crowsnest phosphorites are morphologically distinct, with much higher concentrations of REEs in the Crowsnest grainstones compared to the microcrystalline Ursula Creek phosphorites. Pyrite grain morphology is also consistent with bottom water anoxia during deposition of the Crowsnest phosphorites, as opposed to more oxic conditions during deposition of the Ursula Creek phosphorites, suggesting a possible redox control on phosphorite REE contents. Elemental mapping of REE concentrations indicates that REEs are homogeneously distributed throughout phosphate and that concentrations of REEs within carbonate minerals in phosphorites are comparable to or sometimes exceed those in phosphate.