Altered volcanic ashes in coal and coal-bearing sequences: A review of their nature and significance

Volcanic ashes in coal and coal-bearing sequences typically occur as persistent bands within coal seams (generally as tonsteins, but in a few cases as bentonites, K-bentonites, or as clay-free partings), as an intimate mixture with organic matter, as host rocks (such as roof and floor strata), or as thick layers in coal-bearing strata that are stratigraphically separated from coal seams, including those of thick, laterally persistent tuffs, and in the broader sense fragmental clay rocks and flint clays. Altered volcanic ashes have been found in numerous coals with rank ranging from lignite through various bituminous coals to anthracite, as well as in all the continents where coal beds are present. The main primary minerals in volcanic ash that survive post-depositional alteration include high-temperature quartz, plagioclase, sanidine, zircon, apatite, monazite, micas, rutile, and anatase. Alteration of volcanic glass and less stable primary minerals may result in the formation of kaolinite, smectite, illite, mixed-layer I/S and, in some cases, chlorite and zeolites. In addition to mineralogical and petrographic characteristics, identification of the parent magma type is commonly based on relatively immobile elements rather than the total alkali-silica contents, which are often affected by post-depositional alteration. Four types of volcanic ashes have been identified in coal and coal-bearing sequences, namely felsic, mafic, intermediate, and alkali. Altered mafic volcanic ashes are characteristically enriched in Sc, V, Cr, Co, and Ni; have positive Eu anomalies; and are of a medium-REE enrichment type. Altered alkali volcanic ashes are unique in their significant high concentrations of rare metals such as Nb, Ta, Zr, Hf, REE, and Ga, and are characterized by distinct negative Eu anomalies. Compared with altered alkali volcanic ashes, felsic tonsteins have relatively lower REE concentrations and less pronounced negative Eu anomalies, but a greater fractionation between light and heavy REEs. The compositional variation of different types of altered volcanic ashes is attributed to the tectonic framework and geodynamic controls. Volcanic ashes in coal may serve as chronostratigraphic markers to identify and correlate coal seams, and can also be used for radiometric age determination. The ashes may have provided terrigenous materials that served as substrates for peat development; and may have terminated peat accumulation if volcanic ash in large quantities fell into the peat swamp; they can also be used to indicate the geodynamic processes of coal formation; and perhaps explain mass extinction events. Admixed volcanic ash may lower the quality of the coal if not removed in the preparation plant. From a practical viewpoint, alkali volcanic ashes may be significantly enriched in rare earth elements, Y, Nb, Ta, Zr, Hf, and Ga, which have potential economic significance. This paper reviews the distribution, geochemical and mineralogical compositions, and the significance and applications of volcanic ashes in coal and coal-bearing sequences.