Geochemistry of tonsteins and its implications for Li-Zr (Hf)-Nb (Ta) enrichments in Cisuralian coal seams, Shanxi Province, North China

Abstract

Altered volcanic ashes within coal and coal-bearing sequences, often occurring as persistent bands embedded within coal seams (generally as tonsteins), can characterize the geochemical features and hence quality of coal. This study investigates the mineralogical and geochemical features of tonsteins within the coal seams from Shanxi Formation, North China in order to define the critical elements enrichments in coal benches. Mineralogical analysis indicates that the tonsteins are predominantly composed of kaolinite and quartz, while illite was identified in one tonstein sample and some parting/floor samples. The geochemical data indicate enrichments in SiO₂, Al₂O₃, and CaO, while tonsteins display higher SiO₂/Al₂O₃ ratios than partings/floor and coals. Compared with world hard coals, the coal benches are enriched in Zr, Hf, Pb, and Th. These critical metals enrichments in coal benches are attributed to the volcanic ash input during peat accumulation. Hierarchical clustering of trace, major, and REY elements shows distinct grouping patterns between coal and non-coal samples. Group A3 clusters trace elements Li, Zr, Hf, Nb, and Ta together with major element oxides Al₂O₃ and SiO₂, and is primarily related to ash yield. The TiO₂/Al₂O₃ ratios, discrimination diagrams of Zr/TiO₂ vs. Nb/Y and Al₂O₃/TiO₂ vs. Nb/Yb ratios as well as REY distribution patterns, indicated that the tonsteins, transformed from air-fall volcanic ash, were likely derived from volcanic eruptions in Inner Mongolia Paleo-Uplift (IMPU), with volcanic ash of intermediate rock composition. The enrichment of critical metals, particularly Li-Zr (Hf)-Nb (Ta) assemblage, is primarily influenced by the input of multiple layers of intermediate volcanic ash during peat accumulation in the Cisuralian coal, North China.