Redox constraints on clay-sized rare earth element fractionation in weathered granite crust in the Chahe deposit, China

Clay minerals are major carriers of rare earth elements (REEs) in ion-adsorption-type REE deposits. Recent studies have revealed that REEs are hosted predominantly in exchangeable clay-adsorbed components; however, REEs occur within numerous fractions in clay-sized components. The comprehensive characterization of REEs is derived from the distribution patterns of these fractions. Ion-exchangeable REEs had been paid more attention to and other fractions were ignored in previous studies, which results in incomplete understanding of REEs enrichment in the weathered crust. This study aimed to investigate the fractionation and enrichment of REEs in different clay-sized fractions in weathered granite. We investigated weathered granite crust in four drill cores from the Chahe ion-adsorption-type REE deposit in Yunnan Province, China. Our study involved several steps: extraction of the clay-sized components, chemical sequential extraction, and X-ray diffraction and inductively coupled plasma mass spectrometry analysis. Our results indicate that the REEs occur primarily in four distinct fractions; i.e., ion-exchangeable, amorphous FeMn oxides, crystalline FeMn oxides, and layered silicate-bound fractions. Ion-exchangeable REEs occur primarily in vermiculite and illite; however, at high ionic strengths, the REEs may undergo partial transformation and enter the layered silicate-bound fraction. Cerium occurs mainly in the FeMn oxides fractions, and fractionation of Ce and LREE − Ce (i.e., light REEs except for Ce) occurs under relatively oxidizing conditions, resulting in Ce enrichment and light REE − Ce depletion. In addition, the Ce anomalies in the various fractions are not controlled solely by the redox environment. Our study also quantified the proportions and fractions of clay-sized REE and the results could provide new insights into the distribution of REEs in weathered granite crust.