Formation of boehmite through desilication of volcanic-ash-altered kaolinite and its retention for gallium: Contribution to enrichment of aluminum and gallium in coal

Multiple parting bands within Permo–Carboniferous coal that are generally enriched in critical metals in the northern North China have been confirmed to be altered from volcanic ash (tonsteins). The tonsteins and adjacent coal in some areas contain boehmite with varying contents. Whether or not the boehmite has a relationship with the volcanic-ash-altered kaolinite remains poorly understood. Herein, petrographical, mineralogical, whole-rock geochemical, and in-situ elemental analyses, combined with U–Pb–Hf isotopic analysis of zircon, were performed for the boehmite-bearing/rich partings and coal collected from the eastern edge of the Datong and Jungar coalfields. Except for the boehmite, well-ordered kaolinite is predominant in the parting samples, where the columnar/tabular and vermicular kaolinite are commonly present. Additionally, the zircons separated from the representative parting and coal samples are mostly igneous origin with only one age population of ca. 300 Ma, almost same with the depositional age of the coal, suggesting that the kaolinite in the inorganic constitutes of the investigated coal seams were mainly altered from volcanic ash. Numerous fine-grained boehmite exhibit textural features of replacement for columnar/tabular and vermicular kaolinite along the cleavage cracks in the parting samples, suggesting that such boehmite were formed from desilication of volcanic-ash-altered kaolinite. The tonsteins and coal along the eastern edge of the coalfields were uplifted and influenced by fault activities due to Indosinian and Yanshan orogeny, which were probably experienced intensive leaching by silica-undersaturated percolating water and groundwater, resulting in desilication of volcanic-ash-altered kaolinite and precipitation of residual Al as boehmite. The Ga, preserved in the alumina octahedral sheets of volcanic-ash-altered kaolinite, were retained by boehmite due to its very similar geochemical behavior with Al. The formation of boehmite and its retention for Ga made a contribution to the genesis of high–Al–Ga coal in the northern North China.