RARE EARTH ELEMENT FATE AND TRANSPORT IN FLUIDS: INSIGHT INTO ORE-GRADE ENRICHMENT

Rare earth element (REE) enrichment in major REE deposits is in a large part controlled by their transport in fluids. The deposits not only include low temperature (<50°C) deposits (ion-adsorption clays, laterites, and phosphorites), but also higher temperature systems (~150-500°C) including late-stage orthomagmatic fluids derived from carbonatite and peralkaline magmas. The fate and transport of REEs in mineralizing fluids is poorly understood because a complex set of variables affect REE solubility including solution chemistry, pH, redox conditions, solid phase mineralogy and composition, temperature, and pressure. Depending on conditions, the REEs can act nonuniformly leading to greater transport of light (L) or heavy (H) REEs. Insight into REE fractionation in mineralized systems can be gained by evaluating REE behavior in surface waters because sites can be sampled that have distinct solution chemistries, dissolved and colloidal fractions can be sampled, and complete water chemistry can be determined. In aqueous solutions, REEs occur as free ions or as stable aqueous complexes. Trivalent REEs form stable aqueous complexes with inorganic ligands including carbonate, chloride, fluoride, hydroxide, phosphate, and sulfate. The behavior of REEs in aqueous solutions can be described as a competition between the formation of stable aqueous complexes and the tendency of REEs to partition to solid phases, either by adsorption or (co)precipitation. This study evaluates the partitioning of REEs between aqueous solutions and colloidal material over a range in pH (3.2-8.0) and solid and aqueous compositions with data from filtrations studies at 4 different sites. Sample aliquots include an unfiltered fraction, 0.45 µm filtrate, and ultrafiltrate (~<0.005µm). Results show that the REEs remain in solution in waters with pH below ~5.0. REEs partition onto solid phases in circumneutral waters that have colloidal material, but the nature of the fractionation can vary from LREE enriched, HREE enriched, or no fractionation. Inorganic solution speciation of REEs at low temperature is well constrained, such that REE speciation can be determined. Solid phase properties including composition, size, and degree of crystallinity play an important role the nature of REE fraction in surface waters.