Mineralogy and organic petrography of the No. 6 coal seam, Soutpansberg Coalfield, South Africa: Evidence for hydrothermal activity.

The Soutpansberg Coalfield is a Karoo-aged intracratonic rift basin in South Africa's Limpopo Province. This study aims to assess samples from the No. 6 coal seam in the Makhado and Voorburg south area of the Soutpansberg Coalfield to determine the peat-depositional conditions, organic matter provenance, and their relation to paleoclimate and tectonic setting based on coal-petrography, mineralogical and SEM-EDX analyses. The coals are classified as medium-rank bituminous coal, with high ash yield (avg. 36.08 wt%; db). Ash yields display increasing trends from the middle upper (MU) to bottom lower (BL) part of the seam, while fixed carbon content values decrease. The maceral composition indicates that most samples are rich in vitrinite (-62%), with some inertinite (-10%) and very low liptinite (~1.4%). However, BL and BU in one core from the Makhado valley area are marked by very high inertinite (avg. 64%) and low vitrinite (avg. 7.3%). The patchy occurrence of inertinite macerals indicates high oxidation conditions or forest fires in peatmire at certain times. The coal facies diagrams indicate that precursor peat of the No. 6 seam started accumulating under mesotrophic hydrological conditions with a limited oxygen supply and low degree of degradation. Megathermal terrestrial plants might be the major component of the vegetation and the peatmire was formed under mildly oxic-to-anoxic wet-forest conditions. The mineralogical composition indicates that inorganic matter in coal is mostly composed of quartz, kaolinite, siderite, muscovite, dolomite, calcite, and pyrite. The Al₂O₃/TiO₂ ratio shows that clay minerals are derived from felsic igneous rocks of granite or granodiorite composition. The Al-Fe-Mn ternary plot suggests some influence of hydrothermal activity in the post mire as the sample has an A1/(A1 + Fe + Mn) value of < 0.4 and (Fe + Mn)/Ti value > 15. The presence of massive porphyritic-type pyrite in the coal also indicates they may be formed by hydrothermal mineralization instead of sedimentation, strongly indicating the occurrence of hydrothermal activity. The SEM-EDX analyses denoted that As, Ni, Pb. and Co are associated with pyrite. Enriching these elements in coal might be terrigenous during sedimentation/peatification, and/or hydrothermal fluids during igneous intrusion emplacement. The chemical index of weathering (CIW) and chemical index of alteration (CIA) range from 92.04 to 97.66 (avg. 95.40) and 98.25 to 99.67 (avg. 99.26), respectively. The geochemical indices like CIA, CIW, A-CN-K ternary diagram, and SiCL vs log (K₂O/Na₂O) plot show that the paleoclimate conditions were warm and the coal component suffered strong chemical weathering, under passive continental margin depositional environment supporting luxurious vegetation and organic matter preservation. The inferred tectonic setting is consistent with tectonic events seen in East and Central Africa during the Carboniferous to the Permian period. [ABSTRACT FROM AUTHOR]