Distribution and enrichment of scandium in minerals of the Bayan Obo REE-Nb-Fe deposit, Inner Mongolia, north China

Abstract

Scandium is a critical metal, and its mineralization mechanisms have attracted widespread attention. The Bayan Obo REE-Nb-Fe deposit in northern China is one of the most important Sc resources. However, the distribution and enrichment of Sc in minerals remain poorly understood. This study systematically investigates the Sc contents in over 20 minerals using laser ablation ICP-MS. Results show that columbite (average Sc: 9100 ppm), samarskite (2200 ppm), aegirine [NaFe³⁺(Si₂O₆)] (1250 ppm), and zircon (350 ppm) are significant host minerals for Sc. Scandium is variably enriched in hematite, diopside, and riebeckite [Na₂(Mg, Fe)₃Fe²⁺(Si₈O₂₂)(OH)₂], with contents ranging from 20 to 200 ppm. The other 17 minerals, including aeschynite, fergusonite, dolomite, albite, fersmite, magnetite, pyrrhotite, ilmenite, monazite, rhodochrosite, parisite, rutile, barite, apatite, calcite, pyrite, and cordylite, have average Sc contents below 20 ppm. Scandium can be incorporated into aegirine via Sc³⁺ + Al³⁺ ↔ Mg²⁺ + Ti⁴⁺ and into riebeckite via Sc³⁺ + Mg²⁺ ↔ Al³⁺ + Fe²⁺. The Sc-rich rim and Sc-depleted core in aegirine indicate heterogeneous Sc distribution, likely caused by dissolution-reprecipitation processes. Contrasting enrichment patterns of Sc and REE in aegirine are due to various F/Cl ratios in fluids. Scandium preferentially incorporates into mafic silicate minerals due to its similar ionic radius to those of Fe and Mg. Oxidation of magnetite to hematite cannot lead to Sc enrichment. Elevated Sc in studied hematite suggests a direct precipitation from Sc-bearing fluids rather than a transformation from magnetite.