Discovery of supergene REE-fluorocarbonate minerals in weathered spheres of Xiajialing regolith-hosted rare earth element deposit in Xiangshan, Jiangxi Province, South China

The Xiangshan ore field is developing not only a volcanic-related hydrothermal U deposit, but also a regolith-hosted rare earth elements (REE) deposit in the middle of the volcanic rock basin, where the spheroidal weathering layer has the highest REE content. Weathered spheres may play an important role in REE mineralization. To explore the enrichment and migration processes of REE in weathered spheres at different weathering stages, the mineralogy and geochemistry of the bedrock and weathered spheres with different weathering degrees of Xiajialing REE deposits were investigated. Results showed that the REE+Y concentration was 270-310 ppm in bedrock and 372-2377 ppm in weathered spheres, respectively. With an increase in weathering, the REE+Y content increased and then decreased, and the maximum value was observed in the core of the moderately weathered sphere sample. A large number of supergene REE-fluorocarbonate minerals were discovered in the weathered spheres (particularly in samples with a high REE content), and were mainly distributed in biotite and plagioclase. The REE2O3 (including Y2O3) content of the supergene REE minerals was 69.04-80.89 wt%. The supergene REE minerals were enriched in La2O3 (21.69-25.95 wt%), Nd2O3 (12.13-23.58 wt%), and Y2O3 (2.74-14.27 wt%), but lacked Ce2O3 (<1 wt%), and contained a certain amount of F (2.66-6.23 wt%). The Raman spectra of the supergene REE-fluorocarbonate showed several bands at 272, 741, 1103, and 3100-3700 cm(-1). These results indicate that REE are adsorbed on clay minerals that were formed by weathering of biotite and plagioclase; thus, the concentrations of REE around the clays were extremely high and may have exceeded the solubility product (K-sp) of the REE-fluorocarbonate, which contributed to the supergene REE-fluorocarbonate deposits. The formation of supergene REE-fluorocarbonate minerals may be one of the most important causes of the extreme enrichment of REE in some weathered sphere samples. However, the supergene mineral is easily weathered and would dissolved with weathering proceeding, leading to a decrease in the REE content in the sphere samples that have experienced relatively strong weathering. The supergene REE-fluorocarbonate may be an important intermediate product in the formation of the Xijialing regolith-hosted REE deposit.