Rare earth element mobility in and around carbonatites controlled by sodium, potassium and silica

Carbonatites and associated rocks are the main source of rare earth elements (REE), metals essential to modern technologies. REE mineralization occurs in hydrothermal assemblages within or near carbonatites, suggesting aqueous transport of REE. We conducted experiments from 1200 °C and 1.5 GPa to 200 °C and 0.2 GPa using light (La) and heavy (Dy) REE, crystallizing of fluorapatite intergrown with calcite through dolomite to ankerite. All experiments contained solutions with anions previously thought to mobilize REE (chloride, fluoride and carbonate), but REE were only significantly soluble when sodium or potassium were present. Dysprosium was more soluble than lanthanum when alkali-complexed, an effect magnified by potassic fluids. Addition of silica either traps REE in early crystallizing apatite or negates solubility increases by immobilizing alkalis in silicates. Anionic species such as halogens and carbonates are not sufficient for REE mobility, and additional complexing with alkalis is required for substantial REE transport in and around carbonatites as a precursor for economic grade mineralization.