Complexation of REE in Hydrothermal Fluids and Its Significance on REE Mineralization

Rare earth elements (REEs) have recently been classified as critical and strategic metals due to their importance in modern society. Research on the geochemical behaviors and mineralization of REEs not only provides essential guidance for mineral exploration but also holds great significance in enhancing our understanding of Earth’s origin and evolution. This paper reviews recent research on the occurrence characteristics, deposit types, and hydrothermal behaviors of REEs, with a particular focus on comparing the complexation and transport of REEs by F, Cl, S, C, P, OH, and organic ligands in fluids. Due to the very weak hydrolysis of REE ions, they predominantly exist as either hydrated ions or free ions in low-temperature and acidic to weakly basic fluids. As the ligand activity increases, the general order of transporting REEs is Cl⁻ ≈ ${\mathrm{S}\mathrm{O}}_4^{2-}$ > F⁻ ≈ ${\mathrm{P}\mathrm{O}}_4^{3-}$ > ${\mathrm{C}\mathrm{O}}_3^{2-}$ > OH⁻ under acidic conditions or OH⁻ > ${\mathrm{S}\mathrm{O}}_4^{2-}$ ≈ Cl⁻ > F⁻ under alkaline conditions. In acidic to neutral hydrothermal systems, the transport of REEs is primarily dominated by ${\mathrm{S}\mathrm{O}}_4^{2-}$ and Cl⁻ ions while the deposition of REEs could be influenced by F⁻, ${\mathrm{C}\mathrm{O}}_3^{2-}$, and ${\mathrm{P}\mathrm{O}}_4^{3-}$ ions. In neutral to alkaline hydrothermal systems, REEs mainly exist in fluids as hydroxyl complexes or other ligand-bearing hydroxyl complexes. Additionally suggested are further comprehensive investigations that will fill significant gaps in our understanding of mechanisms governing the transport and enrichment of REEs in hydrothermal fluids.