RARE EARTH ELEMENTS IN CLAYS: A DOMESTIC RESOURCE FOR THE FUTURE?

Rare earth element (REE) ion-adsorption clay deposits, which form by deep weathering of granite, are of global economic importance because they currently supply virtually all global production of heavy REE, Y, and a large proportion of light REE [1]. Prospective source rocks in the SE-US for this deposit type are mainly REE-enriched, highly fractionated granitic rocks.

We identified a group of Neoproterozoic granites and regoliths (e.g., Stewartsville and Striped Rock, VA) that have enriched REE contents and chondrite-normalized patterns (n=148), which are similar to source rocks for granite-derived REE clay deposits of China [2]. Of these, the Stewartsville regolith has REE_total concentrations (~500-2800 ppm) and fractions of extractable adsorbed REE (30-70%; av. ~900 ppm) that approximate grades of REE clay deposits at Heling and Guposhan, China. Striped Rock regolith deposits also have high REE_total concentrations, but contain a lower percent extractable RE ions (15-40%). These regoliths contain a diverse suite of REE minerals, including refractory igneous phases, remnants of soluble minerals, adsorbed RE ions, and deuteric, secondary, and authigenic REE-phases. We find that for these granites REE enrichment in the regolith results from removal of alkalis and other mobile elements by weathering of igneous phases. REE released by the breakdown of minerals such as allanite and apatite are fixed as hydroxide ions on clay edges and in interlayer sites of clay minerals. REE are also converted to secondary minerals, such as cerianite, mixed RE-oxides, fluoro-carbonates (e.g., synchysite, doverite), and phosphates (e.g. monazite), which precipitate as cements and open space fill in the regolith.

Accessory mineralogy, modal contents, and micro-chemical scale REE redistribution processes are key factors in the retention of REE as adsorbed ions on clay. Thus, the resource potential of such deposits is a function of the relative proportions of light to heavy REE (La_N/Yb_N) in the source rocks and the amount of readily extractable adsorbed REE compared to the quantity of REE locked up in insoluble secondary and residual minerals.

[1] Bao & Zhou (2008) Ore Geology Reviews 33: 519–535.

[2] Foley, Ayuso, Bern, Hubbard, & Vazquez (2014) Acta Geologica Sinica 88: 428-430.