Retention of rare earth elements in authigenic phases following biogeochemical dissolution of ore from Elliot Lake, Ontario

Abstract

The aim of this pilot study is to investigate the retention of uranium (U) and rare earth elements (REEs) by secondary phases following the biogeochemical mineral dissolution of quartz-pebble conglomerate originating from Elliot Lake, Ontario. An understanding of the geochemical controls of U and REE dissolution and subsequent retention in authigenic secondary phases following biogeochemical ore dissolution could provide information relevant to the environmental management of mine waste materials and economic element recovery. The retention of U, thorium (Th), REE, and base metals in secondary phases following biogeochemical ore dissolution formed on mineral surfaces, both in terms of their concentration and association with other elements present in the ore, is characterized in this study. The loss of mass of U (87%) from the quartz conglomerate ore is significantly higher than for the associated REE's, reflecting the mobile character of UO22+. A small but significant portion of the uranyl ions (13%) is retained in the ore sample, either as residual uraninite or as uranyl-phosphate complexes and minerals within Fe- and Al-rich surface coatings. The preliminary adsorption of phosphate (PO43−) ions on the surfaces of Al-Fe-(hydr)oxide mineral surface coatings forms negatively-charged surface sites within the porous media of the leached mineral material, providing adsorption sites for positively-charged metal species on the phosphate-bearing surface sites, allowing the formation of either ternary metal-phosphate-Al-Fe-(hydr)oxides complexes or nucleation of metal-phosphates, thus leading to the retention of U, REE, Th, Zn and Pb. Similarly, the observed retention of Pb, Zn, Cu and Ni is explained by the neoformation of authigenic minerals of the jarosite group within the pore spaces of Fe-, Al- and Si-rich coatings.