Scandium distribution in the Bayan Obo REE-Nb-Fe deposit, China: A multi-scale geochemical perspective

Scandium (Sc) is a dispersed element in average continental crust but can accumulate in carbonatite systems. In the Bayan Obo carbonatite deposit, China, substantial Sc2O3 resources have been estimated; however, the occurrence and spatial distribution of Sc remain poorly understood, impeding efforts to explore and extract these resources. This study presents novel results of geochemical, mineral and textural mapping from 270 systematically collected field samples and 13 representative laboratory samples, revealing the Sc distribution at various scales, from deposit down to mineral, within this complex carbonatite system. Whole-rock analyses identified Sc anomalies (over 100 ppm) along the lithological contacts between the orehosting dolomite intrusion and the surrounding slate/schist. The banded/massive ores and vein-type ores exhibit variable yet relatively high Sc contents, ranging from 2 to 378 ppm (average 67.6 ppm) and 91 to 273 ppm (average 153 ppm), respectively. In contrast, slate/schist shows lower Sc levels (1-218 ppm, average 38.2 ppm), while the ore-hosting dolomite has consistently medium to low Sc contents (17-77 ppm, average 34.0 ppm). Micro-scale analyses reveal that aegirine and Na-amphibole are common Sc carriers in all rock types except slate/ schist, in which mica and ilmenite are the major Sc-bearing minerals. Sc-rich domains (over 0.3 wt% Sc2O3) are detected in polished thin sections, in which three individual Sc minerals, i.e., thortveitite, aegirine-jervisite solid solution, and bazzite, are documented for the first time in this deposit. Variations in chemical composition and microtexture in these Sc minerals indicate a multi-stage Sc mineralization process. These findings suggest that hydrothermal alteration is the primary mechanism for Sc enrichment in this carbonatite system, laying a solid foundation for future resource utilization of the Bayan Obo deposit.