Enrichment of rare earth elements during the weathering of alkaline igneous systems; insights from the Puxiong regolith-hosted rare earth element deposit, SW China

Regolith-hosted rare earth element (REE) deposits have been the focus of recent studies. Most studies concern deposits formed over granites and felsic volcanic rocks, but little is known about those deposits developed over silica-undersaturated alkaline igneous rocks. The recently discovered Puxiong REE deposit in Southwest China formed through the weathering of nepheline syenite that has REE concentrations ranging from 177 to 9,336 ppm. Hydrothermal processes partially enriched the parent nepheline syenite in REEs. About 60% of the REEs in the bedrock are hosted in britholite-(Ce), tritomite-(Ce), and cerite-(Ce) and approximately 21% in REE minerals that occur as inclusions in K-feldspar, with the rest in titanite, hiortdahlite, apatite, fluorite, and calcite. These minerals all can be easily decomposed to release REEs into soil solutions during weathering. The released REEs are adsorbed on clay minerals or precipitate as supergene rhabdophane and an Fe-Mn-REE oxyhydroxide phase. Nepheline syenite-derived regolith-hosted REE deposits are enriched in illite and halloysite, which have a higher ion exchange capacity than the parent granites. Illite formed through the weathering of primary alkali minerals in the nepheline syenite. In the strongly eroded midslope and valley, the regolith has the lowest total REE concentration (997 and 1,001 ppm on average, respectively) across the ore-bearing catchment, whereas the regolith in the hilltop and footslope has REE concentrations of up to 1,564 and 1,677 ppm, respectively. Moreover, regolith at the footslope has the highest heavy REE (HREE) concentration of 110 ppm on average. The light REEs (LREEs) tend to be concentrated in the B horizon and laterally across the hilltops, whereas the HREEs are mobilized by groundwater and soil solutions and accumulated in the upper C horizon vertically and the footslope profiles laterally. In conclusion, nepheline syenite was hydrothermally enriched in the REEs, and these elements were released to the weathering solution and then adsorbed onto clay minerals in sufficient concentrations to form economic regolith-hosted REE deposits. This process, which was controlled at Puxiong by the nature of clay minerals, pH, the redox conditions, the mobility of the REEs, and topography, led to maximum enrichment of the LREEs in the lower B horizon at the hilltop, and HREE enrichment in the upper C horizons vertically and in the footslope laterally.