HIGH PROPORTIONS OF ION-ADSORBED REES ON THE LIBERTY HILL PLUTON, SOUTH CAROLINA, A POSSIBLE LINK TO HYDROTHERMAL ALTERATION

Ion-adsorbed rare earth element (REE) deposits in southern China supply the majority of world heavy REE production and substantial light REE production. The basic deposit model includes primarily granitic rocks that have experienced deep and extensive weathering in a setting with low erosion rates leading to redistribution and enrichment of REE within the weathering profile. Additionally, predominance of easily weathered REE-silicate and –fluorocarbonate minerals in parent rock, as opposed to refractory REE-phases, is conducive to liberating REEs from minerals for subsequent adsorption onto clay surfaces. We examined the distribution and forms of REEs in the Hercynian Liberty Hill pluton. Located in the highly weathered and slowly eroding South Carolina Piedmont, Liberty Hill receives ~1500 mm annual rainfall with a mean annual temperature of 17 °C. The pluton is medium to coarse-grained biotite-amphibole granite with minor biotite granite facies. REE-bearing phases are diverse and include monazite zircon, titanite, allanite, apatite and bastnäsite. Nine weathered profiles were sampled up to 7 m-deep across the ~400 km² pluton. In one profile, ion-adsorbed REEs plus yttrium (REE+Y) ranged up to 580 ppm and accounted for up to 77% of total REE+Y in samples. In the rest, ion-adsorbed REE+Y ranged 8–194 ppm and only accounted for 3–39% of totals. The highest ion-adsorbed REE profile was located along the mapped boundary of two granite facies and also contained abundant (up to 14%) smectite. Those data are suggestive of hydrothermal alteration in that location. Post-emplacement deuteric alteration favors generation of easily weathered REE phases, particularly fluorocarbonates. In the case of Liberty Hill, hydrothermal alteration along the facies boundary may have had the effect of converting less soluble to more soluble REE minerals. Laterally extensive deuteric alteration is more likely to generate larger scale, economic deposits of ion-adsorbed REEs. The potential link to hydrothermal alteration at Liberty Hill suggests a potential pathway to localized formation of such ores.