Geological resources of scandium: a review from a Chinese perspective

Scandium (Sc) is a dispersed metal in Earth's lithosphere, with an average abundance of 16 to 22 ppm. In the meantime, it is widely considered as a critical metal because of its paramount significance in scientific research and technical innovation. With surging demands that are not backed-up by current supplies globally, the market price of Sc oxide is astonishingly five times more than the most expensive rare earth oxide of terbium (Tb). Production of Sc is significantly held back due to scarcity of economically viable grades at explorable depths within the crust, compared with other critical metals such as REE, Nb and Ta. Nevertheless, typical high- to intermediate-grade Sc deposits, as compiled in this review, consistently show close relationships to specific magmatic (e.g. ultramafic-mafic and carbonatitic), supergene and hydrothermal processes during Sc enrichment, especially the former two. Known potential Sc deposits are tentatively classified based on their host rocks and metallogeny, including those hosted in the ultramafic-mafic rocks and related laterites, in carbonatite and related laterites, in bauxite residue and processed coals, hydrothermal Sc deposits and Sc deposits related to syenite intrusions, pegmatites or marine sediments. We also discuss the Sc enrichment mechanism and associated tectonics and partition coefficients of Sc among diverse minerals and melts, which reveal the preference of Sc for clinopyroxene, garnet and iron oxides by isomorphic replacement or ion absorption during diverse magmatic and supergene Sc enrichment processes. Lastly, Sc resources in the world-class Bayan Obo deposit are discussed in detail as an illustrative benchmark example, where hydrothermal aegirine may host majority of carbonatite-derived Sc.