The occurrence and distribution of thorium in the giant Bayan Obo REE-Nb-Fe polymetallic deposit, China

Abstract

Thorium (Th) is increasingly regarded as a critical element for next-generation nuclear energy systems due to its status as the most abundant fissile material on Earth and its advantageous safety properties. The Bayan Obo deposit in Inner Mongolia (China), contains the largest known reserves of rare earth elements (REE) globally, along with significant amounts of niobium (Nb), iron (Fe), and Th. Despite the considerable Th resources within this deposit, the spatial distribution, occurrence state, and genetic relationship with the REE-Nb-Fe mineralized systems are still poorly understood.

In this study, Th distribution and the intrinsic relationship with REE-Nb-Fe mineralization were investigated by a systematic mineral-geochemical composition analysis (XRF, TIMA, and EPMA) of various rocks and ores in the Bayan Obo deposit. Spatially, Th concentrates along the contact zone (fluorite-rich banded/massive ores) between ore-hosting dolomite and K-rich slate, with four high-Th zones (>200 ppm) in the Main and East open pits. The abundance of Th in ores and rocks exhibits considerable variability, ranging from 1.1 to 587 ppm. This significant heterogeneity in the distribution of Th within the deposit is primarily attributed to Th migration induced by late thermal events, thereby creating scattered Th-Pb ages in the REE minerals. Chemical and mineralogical mapping indicates that Th in the Bayan Obo deposit is primarily hosted in REE minerals, Nb-bearing minerals and thorite. Although Nb-bearing minerals exhibit higher Th concentrations (e.g., aeschynite: 4.1 wt% ThO₂), REE minerals (e.g., monazite and fluorocarbonate) dominate Th resource volume due to their extensive distribution. Among these REE minerals, monazite in the disseminated, banded and vein ores of the West Open Pit displays the highest Th content (median >1 wt%). Additionally, the biotite-altered disseminated ore in the Main Open Pit contains elevated Th concentrations, present as thorite and Th-rich monazite. For efficient, resource-specific mining strategies, optimal ore types include: biotite-altered disseminated ores in the Main Open Pit and the disseminated and massive ores in the West Open Pit for Th-REE-Nb, and massive ores in the Main and East open pits for Th-Fe-REE.

Moreover, Th concentrations closely align with REE and Fe but diverge sharply from Nb. Thorium-REE-rich minerals mainly occur in fluorite-rich banded and massive ores, while Nb-bearing minerals concentrate in ores dominated by aegirine and riebeckite, linked to alkaline metasomatism. Geochemical analyses reveal a strong Th-F correlation but no association with K₂O or Na₂O, which highlighting distinct enrichment-precipitation pathways: the Th-REE-Fe systems rely on fluorination-driven, contrasting sharply with Nb’s alkaline metasomatic origins. These insights guide targeted exploitation strategies and deepen understanding of the deposit’s complex metallogenic history.