Marine Phosphorites as Potential Resources for Heavy Rare Earth Elements and Yttrium
Marine phosphorites are known to concentrate rare earth elements and yttrium (REY) during early diagenetic formation. Much of the REY data available are decades old and incomplete, and there has not been a systematic study of REY distributions in marine phosphorite deposits that formed over a range of oceanic environments. Consequently, we initiated this study to determine if marine phosphorite deposits found in the global ocean host REY concentrations of high enough grade to be of economic
... be of economic interest. This paper addresses continental-margin (CM) and open-ocean seamount phosphorites. All 75 samples analyzed are composed predominantly of carbonate fluorapatite and minor detrital and authigenic minerals. CM phosphorites have low total REY contents (mean 161 ppm) and high heavy REY (HREY) complements (mean 49%), while seamount phosphorites have 4-6 times higher individual REY contents (except for Ce, which is subequal; mean ΣREY 727 ppm), and very high HREY complements (mean 60%). The predominant causes of higher concentrations and larger HREY complements in seamount phosphorites compared to CM phosphorites are age, changes in seawater REY concentrations over time, water depth of formation, changes in pH and complexing ligands, and differences in organic carbon content in the depositional environments. Potential ore deposits with high HREY complements, like the marine phosphorites analyzed here, could help supply the HREY needed for high-tech and green-tech applications without creating an oversupply of the LREY. Minerals 2016, 6, 88 2 of 22 the range of REY concentrations hosted by marine phosphorite deposits from the modern global ocean and whether the deposits are of high enough grades to be of economic interest. Phosphorites in the global ocean occur in three general environments, continental margins (CM: shelf, slope, banks, and plateaus), seamounts, especially the old (Cretaceous) seamounts in the NW Pacific, and lagoon/insular deposits (e.g., [4, 5] ). Research presented here addresses only the CM and seamount phosphorites. Our global dataset includes typical shallow-water continental-margin, upwelling-zone phosphorites, and deep-water seamount phosphorites collected from the Pacific and Atlantic Oceans (Table S1, Figure 1 ). Similar studies have been completed for land-based phosphorite deposits, with positive results as to the resource potential of REY as a byproduct or co-product of the focus phosphate mining (e.g., [6, 7] ). However, recovery of these land-based REY would require the addition of costly infrastructure and changes in extractive processing to the existing phosphate mining operations. For example, it would be beneficial to change the dissolution acid from sulfuric acid, which produces phosphogypsum as a waste product, to hydrochloric acid. This would alleviate processing a second solid for recovery of the REY partitioned into the phosphogypsum phase (e.g., [6, 8] ). Production of REY as a byproduct or co-product of phosphate mining needs to be considered in the early stages of planning for a new terrestrial or marine mining operation. With this in mind, three CM areas have been leased in the global ocean for exploration for marine phosphorites: Chatham Rise off New Zealand by Chatham Rock Phosphate Ltd. (Wellington, New Zealand); offshore Baja California, Mexico by Odyssey Marine Exploration and Exploraciones Oceanicas; and on the shelf off Namibia by Namibian Marine Phosphate Ltd. Again, consideration of REY as a byproduct at an early planning stage for a marine phosphorite operation would be warranted should extraction of the REY be determined to be economic.