Distribution and modes of occurrence of Nb in subbituminous coal: A case study from the Jungar Coalfield, Ordos Basin, China

Abstract

Niobium has been considered to be enriched in high-Al-Ga in north China coal and coal-hosted Nb(Ta)-Zr(Hf)-REY-Ga polymetallic deposits in the southwestern region of China. However, modes of occurrence and influencing factors of Nb in Al-Ga-rich coal in North China are rarely reported. This study investigated the distribution characteristics of Nb in the No.6 high-Al-Ga coal of the Jungar Coalfield in North China. The in-situ Nb concentration of selected minerals, including kaolinite, Ti-oxides, boehmite, and zircon, is further quantitatively characterized based on multiscale in-situ elemental analyses, including SEM-EDS and LA-ICP-MS spot and mapping analyses. The results showed that Nb is rich in the tonstein and mudstone partings among bulk samples and is highly concentrated in Ti-oxides, followed by zircon among the minerals. A certain amount of Nb is associated with kaolinite and boehmite with different modes of occurrence: vermicular kaolinite (65.94 ppm) > clastic kaolinite (25.43 ppm) > altered K-bearing kaolinite (18.11 ppm) > cryptocrystalline kaolinite (17.03 ppm) > clastic boehmite (9.08 ppm) > cryptocrystalline boehmite (7.97 ppm). The Nb-Nb/Ta and Zr/Hf-Nb/Ta ratios suggest that the primary source of Nb in the No.6 coal is the altered felsic volcanic ash and bauxite deposit. The high-Fe concentration in Ti-bearing minerals indicates that anatase may originate from the alteration of ilmenite with a process of Fe depletion and Nb enrichment. The enrichment of Nb in coal is attributed to the substitution of Nb for Ti in isomorphism in all Ti-oxides, high-Ti, and Ti-bearing minerals.