Scandium mineralogy in Bayan Obo deposit, north China: Tracing magmatic to hydrothermal enrichment pathways

As a rare metal essential for aerospace and clean energy technologies, scandium (Sc) has a low abundance and dispersed occurrence in the Earth’s crust. The Bayan Obo carbonatite-hosted deposit, the world’s largest REE deposit, hosts significant Sc resource distributed across distinct types of Fe-Nb-REE ore, primarily disseminated, banded, massive, and vein-type ores. Previous studies attributed Sc enrichment at Bayan Obo dominantly to hydrothermal processes, as direct mineralogical evidence for magmatic-stage enrichment remained elusive. This study integrates whole-rock geochemistry, mineralogical characterization, and Th-Pb isotopic dating of these types of ore to unravel Sc speciation and enrichment mechanisms during magmatic to hydrothermal stages. Results demonstrate that Sc is primarily hosted in magmatic Sc-rich columbite-(Fe) and ixiolite (max. 22.59 wt% Sc2O3), and aegirine (max. 2.46 wt% Sc2O3), while hydrothermal processes generated thortveitite (max. 50.76 wt% Sc2O3) in fluorite-banded ores. Moreover, metasomatism associated with F-bearing fluids facilitated Sc enrichment in secondary aegirine-jervisite assemblages and altered columbite-(Fe). Late-stage vein-type mineralization incorporates minor Sc (∼1 wt% Sc2O3) in magnesio-arfvedsonite, chevkinite-(Ce), and samarskite-(Y). We propose a two-stage Sc mineralization model, where initial magmatic fractionation partitioned Sc into Nb-Fe oxides and primary aegirine, followed by the hydrothermal remobilization of Sc into discrete silicate phases at ca. 421 Ma. This work systematically establishes the foundational role of magmatism in Sc enrichment within carbonatite-hosted deposit. The genetic framework advances global understanding of Sc mineralizing processes in carbonatite-hosted deposits, providing a refined basis for exploration targeting in similar systems.