Chemical and mineral composition of altered volcanic ash in coals of the Kharanorskoe Deposit, Eastern Siberia

Abstract

Relevance. The necessity to assess the role of altered volcanic ash on the formation of mineral and geochemical characteristics of coal-bearing deposits. Traces of volcanic material in coals are diagnosed mainly in the form of clay horizons – tonsteins. The study of the chemical and mineral composition of clay tuffaceous horizons and the host coals allows us to answer a number of questions concerning volcanic material impact on coal accumulation. Tonsteins are known in Paleozoic, Mesozoic and Cenozoic coal-bearing deposits. Finds of Mesozoic tonsteins are relatively rare. In this regard, the Kharanorskoe brown coal deposit is a unique object for the study of altered layers of volcanic ash in Cretaceous coal-bearing deposits. Aim. Comprehensive study of altered volcanic ash in Cretaceous coals of the Zabaikalsky Krai. Methods. Field studies; the mineral composition was studied by optical microscopy, scanning electron microscopy, X-ray diffractometry; chemical composition was determined by inductively coupled plasma mass spectrometry, inductively coupled plasma atomic emission spectrometry, and instrumental neutron activation analysis. Results and conclusions. The paper presents data on the mineral and chemical composition of tonstein in the Early Cretaceous Novy-1a coal seam of the Kharanorskoe deposit located in the southeast of the Eastern Siberia (Russia). The mineral composition of the tonstein is represented by kaolinite, quartz, and feldspar. Accessory minerals are zircon, ilmenite, monazite-(Ce), and xenotime-(Y). The tonstein is characterized by high concentrations of Ga, Nb, Hf, Ta, Hg, Th and U. The highest concentrations of rare trace elements in the coal, and especially in the coal ash of the Novy-1a seam, are localized in the areas of contact between the tonstein and the coal. The enrichment with REE, Zr, Nb, Y, Ta, Hf, Th and U is the most significant. Mineralogical and geochemical characteristics of the studied tonstein indicate its formation from volcanic ash of alkaline composition.

Keywords

• tonstein
• volcanic ash
• brown coal
• early Cretaceous
• Eastern Siberia