Preferential enrichment of Ce-dominated-rare earth elements in manganese oxides from granitoid regolith: Nature, origin, and mineralization implication

Manganese oxides are considered among the most effective oxidants and sorbents in natural systems. In granitoid regoliths with rare earth elements (REEs) mineralization， manganese oxides can form important carrier minerals of REEs together with clay minerals. However， the occurrence state of REE in manganese oxide remains unclear， and the role of manganese oxide in REE mineralization is not currently well established. We report detailed mineralogical， geochemical， and X-ray photoelectron spectroscopy data on manganese oxides from the Nan'an heavy REE-type and Jiaping light REE-type deposits. Manganese oxides (mainly hollandite， jacobsite， and ramsdellite) are mainly concentrated in the fully weathered layer of the granitoid regolith. These show higher total REE contents and strong positive Ce anomalies， but the ion-exchangeable REE only accounts for 35% of total， with fewer than 10% of Ce being ion-exchangeable. Our study indicated that manganese oxides are often symbiotic with nanosized cerianite (Ce) particles in space， with Ce mainly being Ce(IV) distributed on the surface of manganese oxides. These findings suggest that manganese oxides act as catalysts or oxidants， oxidizing Ce(III) to insoluble Ce(IV) and thereby driving secondary REE mineral precipitation. This constitutes a critical factor driving REEs enrichment (particularly Ce) in the upper layer of granitoid regolith. This pattern of REE enrichment improves our understanding of REE occurrence in granitoid regolith-type REE deposits and provides a scientific explanation for the low leaching rate of Mn-rich REE ore during the REE recovery process.