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In this paper data on the mineral and chemical composition of tonsteins from the Jurassic Azeisk deposit located in the Irkutsk Basin (South Siberia, Russia) are presented. The mineral composition includes kaolinite (87–89%); quartz (3.0–4.0%), cristobalite (4.9–5.3%), illite (1.9–2.1%) and feldspar (1.0–2.0%). Accessory minerals comprise zircon, monazite, uranium oxides, native minerals (gold, silver, nickel, zinc, tungsten, and silicon), sulfides and intermetallic compounds. The tonsteins and coals of the Azeisk deposit have high concentrations of REE, Y, Zr, Hf, U, Th, Ta, Sn, Ga, Cu, Pb, Se, Hg, Sb, and Te. The highest concentrations of these elements were found at the contact with coals. The enrichment of the coal with lithophile rare metals is possibly caused by their leaching from the tonsteins. The accumulation of these trace elements is a characteristic feature in coals both above and below the tonsteins. The width of the enrichment in the coal zone depends on the properties of chemical elements and features of the coal properties and character of interaction between the coals and tonsteins. The authors verify the pyroclastic nature of the initial matter of the tonsteins of the Azeisk deposit. Mineralogical and geochemical data from the tonsteins suggest a felsic (rhyolite) composition for the original volcanic matter that was altered in the aggressive acidic environments of the peat bogs. As a result of the burial of this material in a paleo-peatland some of the elements typical of felsic pyroclastics accumulated.