The occurrence and distribution of Scandium resources in the world-class Bayan Obo REE-Nb-Fe deposit, China

Scandium (Sc) is famous as a rare-scattered element in nature but can be anomalously enriched by the carbonatite system, especially in the Bayan Obo deposit (~0.14 Mt). However, the Sc distribution of Bayan Obo deposit is rarely revealed, which hinders resource utilization and the understanding of Sc mineralization. Thus, we comprehensively analyze the occurrence and distribution characteristics of Sc resources of Bayan Obo deposit, from deposit scale to mineral scale, based on 236 samples and 13 representatives. In the deposit scale, the Sc contents are 301-2 ppm (banded/massive type ores), 273-91 ppm (vein-type ores), 77-17 ppm (ore-hosting dolomite), and 281-1 ppm (slate/schist) for different rock types, respectively. Among them, the high-Sc rocks usually occur in the vicinity of the contact zone between the H9 slate/schist and the ore-hosting dolomite, which represents the most intently fenitization area in Bayan Obo. In the rock scale, Sc is heterogeneously scattered in rock with a small volume ratio (less than 0.1‰) of extremely high-Sc domains (Sc contents >0.3 wt. %). In contrast, Sc resources are more commonly found in aegirine and Na-amphibole, ranging from hundreds to thousands of ppm, sometimes reaching several weight percent, of banded/massive type ores and vein-type ores, and in biotite and titanite of altered slate/schist, typically in dozens of ppm. Notably, in the mineral scale, three types of individual Sc minerals are first distinguished in the Bayan Obo carbonatite deposit, that is thortveitite, aegirine-jervisite solid solution, and bazzite. Their metasomatism texture and mineral associations record two Sc migration and precipitation events in this deposit. These new reports reveal that the hydrothermal process is the main controlling factor of Sc resources in the Bayan Obo carbonatite system, and provide a scientific basis for the rational and effective utilization of Sc resources in this deposit.