A GLOBAL ASSESSMENT OF REE’S IN SEDIMENTARY PHOSPHATE DEPOSITS

Our evaluation of the worlds sedimentary phosphate deposits suggests that each year an estimated 56,000 metric tons (t) of rare earth elements (REEs), including 23,000 t of heavy REEs (HREEs), are mined, beneficiated, and put into solution, but not recovered, by operations associated with the global phosphate fertilizer industry. Leaching experiments using dilute H2SO4 and HCl demonstrate that REEs in phosphorites are nearly 100% extractable using technologies currently employed to meet global phosphate fertilizer needs, and thus, not subject to the many technological and environmental challenges that vex the exploitation of many identified REE deposits. Importantly, our assessment suggests that by-product REE production from these phosphate mines could meet global REE requirements. For example, the calculated REE flux accompanying phosphate production in the United States is approximately 40% of the world’s total and, alone, could supply 65% of global HREEs needs.

Published values for known horizons and/or published values for equivalent-age horizons, combined with the supposition that REE concentrations in francolite from specific phosphate horizons are relatively consistent, can be used to approximate REE grades and tonnages of currently mined phosphorites. These speculative estimates provide approximations of REE flux through mining, beneficiation, and dissolution operations associated with ongoing phosphate fertilizer production.

Moreover, recognition that the tonnages and HREE concentrations of some unmined phosphorite deposits dwarf the world’s richest REE deposits suggests that these deposits might constitute standalone REE deposits. Documented global-secular REE abundance variations identify phosphorite occurrences deposited during periods that were favorable for deposition of high-REE containing strata. The hypothesized genesis of these REE-rich occurrences strongly supports the long-debated suggestion that oceanic REE contents vary in a secular fashion and that associated high-grade REE abundances reflect oceanic redox state transitions during specific time periods. This new process-based model, based on observed variations in global-secular REE abundances, identifies phosphorite horizons deposited during periods favourable for high-grade REE accumulation.