Critical metals in the critical zone: controls, resources and future prospectivity of regolith-hosted rare earth elements

The rare earth elements (REE) are a group of 17 metals that include the lanthanides, Sc and Y, which are critical for many modern technologies including consumer electronics, medicine and communication. One of the major controls on the concentrations of the REE in regolith material (including soils) is the abundance of these elements in the parent material. It is known that REE concentrations are largely inherited from the protolith rather than acquired during pedogenic processes but our understanding of how pedogenesis affects fractionation and accumulation of REE to produce potentially economic deposits of these critical metals is limited. This study provides a review of (1) the biogeochemical controls on REE distribution and mobility during pedogenesis and (2) the potential for REE extraction from regolith material. Factors that control mobilisation of REE during weathering include (1) the initial distribution of the REE in protolith minerals and the resistivity of these phases to weathering, (2) adsorption and absorption of REE to Fe- and Mn oxides, clay minerals and organic matter and (3) variations in pH and Eh conditions. We also discuss the relative importance of biogeochemical controls on REE mobility in soils in southern Australia, where REE concentrations are demonstrated to be largely a function of weathering of REE-enriched protoliths, the sorption of REE to weathering products and the accumulation of resistant minerals in soils.