The effect of fluorine in the genesis of carbonatite-hosted rare earth element and niobium deposits

Carbonatite is a rare but highly prospective rock that plays a significant role as a source of critical metals, notably rare earth elements (REE) and niobium (Nb). The widespread occurrence of fluorine-bearing minerals (e.g., fluorite, fluorapatite, and bastnäsite) in carbonatites indicates that their parent magmas are rich in fluorine. Nevertheless, the mechanisms by which fluorine controls REE and Nb mineralization remain unclear. Building on previous work, we systematically synthesize the regulatory role of fluorine in REE and Nb mineralization throughout the entire magmatic evolution of carbonatites. Our key findings are: (1) during partial melting, fluorine decreases the solidus of carbonated peridotite, thereby enhancing the generation and stability of carbonatitic melt. (2) In the mantle melting stage, it lowers the partition coefficients of REE and Nb between mantle minerals and melt, effectively promoting their extraction. (3) Throughout magmatic crystallization, fluorine significantly enriches REE and Nb in the residual melt. (4) During liquid immiscibility, fluorine exerts limited influence, as the partition behaviors of REE and Nb are primarily governed by structural differences between the conjugate melts. (5) In the hydrothermal stage, fluorine facilitates the mobilization, transport, and ultimate precipitation of REE- and Nb-bearing minerals, directly driving mineralization. The presented evidence implies that carbonatitic melts and fluoride-bearing minerals exert primary control over the spatial distribution and mineralization potential of REE and Nb. Future studies should focus on (1) clarifying how fluorine promotes REE and Nb enrichment in late-stage brine melts and (2) investigating its effect on the solubility and precipitation mechanisms of REE- and Nb-bearing minerals during the late stages of magma evolution.