Weathering alterations of coal mining wastes geochemistry, petrography, and mineralogy, a case study from the Janina and Marcel Coal Mines, Upper Silesian Coal Basin (Poland)

Weathering-caused alterations in coal mining waste geochemistry, petrography, and mineralogy occurring in the known time span of 1998–2018 in the temperate climate of Poland were investigated. A series of coal mining waste rocks representing two parts of the Upper Silesian Coal Basin (USCB), differing in coalification stage, i.e., corresponding to sub-bituminous and bituminous coals, have been selected to find how the level of organic matter conditionates the weathering. Differences in organic matter content, petrographic composition, rock mineralogy and inorganic geochemistry were also taken into account. Whereas organic petrography, including vitrinite reflectance values, do not show significant weathering signs, apart from irregular cracks and, in very rare cases, paler in color oxidation rims, organic geochemistry has been affected due to low-temperature secondary processes, including increased oxygen index values, particularly in less mature coal mining wastes. Biomarker distribution has been changed to the largest extent, with partial removal of n-alkanes possibly due to biodegradation of organic matter and water-washing of lighter polycyclic aromatic hydrocarbons, including alkyl naphthalenes. Organic matter of lower maturity in coal mining wastes from the USCB eastern part, as more reactive, showed a higher level of secondary transformations. Minerals formed during weathering due to time and storage on dumps were mainly oxidation products of iron sulphides: goethite and jarosite; however, most of the minerals have not been altered. Thus, despite physical changes in coal mining wastes, such as increased brittleness, decreasing fragment size and surface color change, there are only limited changes in inorganic chemistry and mineralogy. This indicates that low-temperature processes do not favour the inorganic elements release to the environment and points out that measures to prevent self-heating within the dumps are the most vital part of post-mining activity leading to the best scenario - slow low-temperature weathering of coal mining wastes.