The mineral character and geomechanical properties of the transitional rocks from the mesozoic – neogene contact zone in the Bełchatów lignite deposit

Purpose
Beginning more than 30 years ago, opencast lignite mining in the "Bełchatów" area is an important source of accompanying minerals. Lignite mining in the "Bełchatów" mine yields, on average, more than 35 million tonnes per annum and requires removing more than 110 million cubic meters of overburden. Therefore the mine outside of the main mineral exploitation of lignite, leads to a large-scale economy of accompanying minerals. Part of the minerals are present in the overburden and are exposed on the slopes of the opencast mine; these minerals are selectively exploited in the event of the absence of recipients stored on anthropogenic deposits. The object of this mineralogical-geochemical study is a group of transitional rocks such as opoka-rocks, gaize and marls exposed when contact occurs between Neogene sediments and Mesozoic basement rocks in the "Bełchatów" lignite deposit. In the case of these rocks, during preliminary geological research carried out on the mine, doubt often appear as to their explicit petrographic character and hence their practical use. Advanced mineralogical methods allow mistakes in their identification to be avoided and a geomechanical study indicates possible direction of their practical use.
Methods
The heterogeneous petrographic character of the examined rocks required the use of a broad research spectrum. The fol-lowing microscopes were used in the framework of the mineralogical research: Polarizing Olimpus BX51 and electron (SEM) FEI Quanta 200FEG equipped with an X-ray spectrometer (EDX Genesis) and backscattered electron detector (BSE). In addition, observations were carried out using a cathodoluminescence apparatus, the Cambridge Image Technolgy Ltd CCL 8200 mk3 model, and a polarizing microscope, type Nikon Optiphot 2. Determination of the phase composition (qualitative and quantitative) was made using X-ray diffraction and utilizing the powder method of Debye-Sherrer. An X-ray diffractometer, a Philips PW 3020 X'PERT, was also used in the study. Analyses by Fourier transmission infrared spectroscopy were performed using the production apparatus BIO-RAD, model FTS 165, equipped with a package of programs for the digital processing of results. The chemical composition was determined by atomic absorption spectroscopy (AAS) using a spectrophotometer Philips PU 9100Xi Camera SX-100 and atomic emission spectroscopy with inductively coupled plasma (ICP AES) using spectrometer 40 PLASMA. The geomechanical properties were determined in accordance with the following standards: open and total porosity (PN-EN 1936:2001) compressive strength in air – dried state (PN-EN 1926:2001), bulk density and density (PN-EN 1936:2001), relative humidity (PN-EN 1925:2001); absorbability of stone material (PN-EN 13755:2002) and abrasion on the face of Boehme (PN-84/B-04111).
Results
Mineralogical-petrographic studies of the transitional rocks showed that the dominant component was SiO₂ which was presented in the form of opal type A and CT, chalcedony, quartz and microquartz. In addition, it was found that the rocks studied had been covered by secondary mineralization processes. The observations carried out showed that these rocks underwent primary silicification and decalcination processes, which contributed to the diverse petrographic nature of the rocks studied and secondarily experienced a change in their physico-mechanical properties. The silification process was the result of diagenetic processes taking place within the Neogene argillaceous rocks occurring when a sub-coal series is situated in the immediate vicinity of the bedrock. There was a release of extruded silica-rich water from the clays during the mechanical compaction process. Some amounts of silica may also be derived from plagioclase dissolution and the transformation of terrigenous material, mainly grains of potassium feldspar. The precipitation of silica from porous solutions occurred most when coinciding with the presence of carbonic acid formed by the decomposition of organic substances of vegetable origin.
Practical implications
The studies carried out have a significant practical implication, as the transitional sediments from the "Bełchatów" lignite de-posit can be used in the production of building materials.
Originality/ value
Against the background of a number of published papers on the rocks accompanying lignite seams there is a lack of the mineralogical-petrographic studies of the transitional sediments in the Mesozoic-Neogene contact zone in the "Bełchatów" lignite deposit taking into account the aspect of raw materials. This paper has been produced to fill the void in this area.