THE ROLE OF TOPOGRAPHY IN GENERATING ECONOMIC CONCENTRATIONS OF ION-EXCHANGEABLE RARE EARTH ELEMENTS

Rare earth element (REE) resources are the focus of an expanding research effort in response to price fluctuations and market concerns over China’s dominance as the world’s supplier. Hard-rock resources receive much attention, but liberating REEs from host minerals can present economic and practical challenges to development. Another deposit type hosts REEs as exchangeable ions on surfaces of clay minerals. Such deposits form in laterites and highly weathered soils as weathering decomposes primary minerals. Extraction and processing are relatively simple as the unconsolidated deposit material can be leached with a mild salt solution to release the exchangeable REEs into solution as complexes or free ions. Approximately one third of China’s REE production comes from such deposits, testifying to their economic viability.

Virtually all soils and weathered materials contain exchangeable REEs, but intense weathering of chemically and mineralogically suitable parent material generates higher concentrations. A less-studied, but potentially important factor may be topography, as it could drive the lateral transfer of REEs in dissolved or colloidal forms and generate hot spots of preferential REE concentration. In order to better understand the importance of these processes, we are studying paired eluvial and illuvial weathering profiles derived from granites. Eluvial profiles, with no possible contributions of material from upslope, were collected from ridgecrests. Illuvial profiles, likely influenced by contributions of material from upslope, were collected at the base of planar hillslopes, but above beds of intermittent streams. One pair of profiles each, along with unweathered granite from nearby quarries, was collected from the Liberty Hill, Newberry, and Pacolet plutons of South Carolina. Elemental analyses of bulk material and extracts of exchangeable ions are underway, along with quantitative mineralogy and electron microprobe analyses. Elemental characterization of the colloidal size fraction from soil is being used to calibrate a recently developed mass balance model that distinguishes REE redistribution in colloidal versus solution form. The results will elucidate how topography and lateral transfers influence distributions of ion-exchangeable REEs in weathered materials.