Abstract

The purpose of this study is to characterize sedimentologically and isotopically authigenic mineralization and low-rank coals (lignite) from the Velenje basin, Slovenia. Six structural types of calcite mineralization in a fine lignite gelified matrix were recognized, namely: (A) laminae and thin beds, (B) small lenses and lenticular bodies, (C) dispersed mineral matter in minute detritus, (D) calcite-substituted xylite fragments, (E) encrusted xylite fragments, and (F) calcite-coated and calcite-filled pores. Scanning electron microscopy revealed that in parts the calcified lignite is almost pure calcite. The carbon (–4.3 to +16.4‰) and oxygen (–14.5‰ to –6.6‰) isotopic composition indicates a carbonate precipitation temperature of between 5.7 and 45.5°C. The δ¹³C_org. values and δ¹⁵N_bulk of organic compounds were from –29.4‰ to –23.7‰ and from +1.8‰ to +5.9‰, respectively. The lower δ¹³C_org. values (up to –29.4‰) in the Velenje lignite indicate a high degree of gelification, while the δ¹⁵N_bulk values (up to +5.9‰) are indicative of intense mineralization. Furthermore, a carbon isotope fractionation factor (ε_TOC) of more than 32% suggests that unlike in recent lake sediments from Slovenia, Croatia, and Argentina, where oxic processes dominate, there was intense microbial activity (methane formation in anoxic conditions). In addition, the ε_TOC values in the Velenje samples are unrelated to mineralization type but are related to the degree of gelification (bacterial activity). The investigation of mineral occurrence is important since cleat-filling minerals reduce coal permeability, which in turn can trigger gas outbursts and inhibit gas extraction.