An unusual occurrence of ferroan magnesite in a tonstein from the Minusinsk Basin in Siberia, Russia

Abstract Altered volcanic ash layers are common constituents of coal beds throughout the world and are typically preserved as thinly bedded, kaolinite-rich layers called tonsteins. Two tonsteins from the Minusinsk coal basin in southern Siberia, Russia, were selected for bulk chemistry analysis and the lower tonstein was selected for analysis by X-ray diffraction (XRD) and scanning electron microscopy with an energy dispersive detector (SEM/EDS). In addition to minerals commonly found in tonsteins such as kaolinite, K-feldspar, quartz, apatite, zircon, and sulfide minerals, the mineralogical profile of this tonstein includes a unique groundmass component of ferroan magnesite, confirmed by XRD. Ferroan magnesite has never before been reported in tonsteins and has been only briefly mentioned in one paper as a rare mineral found in freshwater sediments associated with coal. The probable parent magma type of the ash was evaluated using TiO2/Al2O3 and Zr/TiO2 vs. Nb/Y classification plots as well as chondrite-normalized REE distribution profiles. The results indicate a probable magmatic source of rhyolitic to slightly andesitic composition. The magnesium source is enigmatic: felsic to intermediate magmas are typically Mg-poor. Furthermore, a Mg-rich environment during initial clay mineral formation would typically result in a smectite or illite clay mineral assemblage instead of the dominant kaolinite present. Several potential external sources of Mg for ferroan magnesite formation were examined, including marine incursion, hydrothermal activity, and influx of non-marine Mg-rich porewaters. Low total sulfur content and absence of illite make marine incursion improbable, and there is no evidence of hydrothermal influence. Total MgO levels fall well within expected limits for a felsic magmatic source, so there is no clear indication of a large influx of Mg into the system post-depositionally. The most likely explanation of this unusual occurrence of ferroan magnesite is that magnesium and iron-bearing minerals in the volcanic ash and underlying rock layers were dissolved and mobilized by HCO3-bearing solutions from organic material in the peat swamp, which in turn allowed ferroan magnesite to precipitate in the subsurface during diagenesis.