Allanite Alteration‐Driven Fractionation and Enrichment of Rare‐Earth Elements (REEs) in Regolith‐Hosted Deposits: Example From the Tengchong Block, SW China

Magmatic–hydrothermal alteration of rare‐earth element (REE)‐rich accessory minerals in granitoids is critical for understanding light REE (LREE) and heavy REE (HREE) fractionation in regolith‐hosted ion‐adsorption REE (iREE) deposits. However, whether Cl − ‐rich and F − –‐rich fluids exert a key control on this fractionation during the magmatic–hydrothermal stage remains poorly constrained in the Tengchong Block of western Yunnan, China. This study integrates petrography, Raman spectroscopy and in situ elemental/isotopic (U‒Pb, Nd) analyses to compare allanite from altered bedrocks of the newly discovered Jueyeba (LREE‐dominated) and Tuguanzhai (HREE‐dominated) iREE deposits in Tengchong. The results show that magmatic (Aln‐1) and altered (Aln‐2/3) allanites share the same U‒Pb age (42.8 ± 0.9 Ma) and have similar ε Nd ( t ) values but different ranges (Jueyeba −10.7 to −7.5; Tuguanzhai −6.9 to −5.4), implying different alteration fluids between the two localities within a relatively closed magmatic–hydrothermal autometasomatic system. At Jueyeba, altered allanite shows F and Th depletion but U enrichment, indicating relatively Cl − ‐rich, acidic, and higher‐temperature fluids that mobilise LREEs. At Tuguanzhai, higher F and Th contents with biotite chloritisation and calcite indicate relatively F − –‐rich, alkaline, and lower‐temperature fluids that efficiently mobilise HREEs. Thus, allanite fingerprint fluid properties affect LREE/HREE fractionation and iREE deposit type.