Organic petrographical, geochemical, and mineralogical features of the Permian Coalbrook Sub-basin, South Africa: Reconstruction of paleo-depositional conditions

Abstract

Coal samples from four boreholes drilled as part of an exploration project in the Vereeniging-Sasolburg Coalfield, Coalbrook Sub-basin, South Africa, were studied to understand the depositional environments using organic petrography (macerals, microlithotypes, and vitrinite reflectance), inorganic geochemistry, and mineralogy. Given that South Africa is projected to remain reliant on coal for its energy needs in the short-to medium-term, the study aims to expand on the limited knowledge regarding this part of the Main Karoo Basin (MKB). The samples are classified as low-rank A sub-bituminous to medium-rank D bituminous, with variable maceral composition, with very low to moderate vitrinite (5.8–48.0 vol%, mineral matter free basis, mmf) although most samples are dominated by inertinite (from 42.0 to 90.6 vol%, mmf). The vitrinite to inertinite ratio (V/I) varies from 0.06 to 1.14, indicative of generally dry and oxidizing environments with frequent paleofires. The dominant minerals in the studied coals are kaolinite and quartz. Angular to rounded quartz grains suggest a detrital origin, whereas kaolinite is both detrital and authigenic, the latter also occurring within telovitrinite cell lumens. In contrast, detrital kaolinite mainly occurs in association with inertodetrinite, sporinite, and the detrital quartz. This indicates allochthonous deposition for the detrital minerals and the associated organic matter (including charcoal), likely reflecting flooding of the paleomire. The Al₂O₃/TiO₂ ratios indicate that the minerals were mostly derived from intermediate and felsic sources. Total sulphur (TS) values are generally low, suggesting freshwater conditions. The microlithotype facies plot suggests that the Coalbrook Sub-basin coals were deposited in lacustrine and lower deltaic environments, related to the undulating paleotopography at the base of the coalfield owing to the presence of karst features. This distinguishes the Vereeniging-Sasolburg from the other coalfields of the MKB (e.g., Witbank, Highveld, and Ermelo), which have the Dwyka diamictite at their base and formed in deltaic and fluvial environments. The paleotopography of the basement thus had a strong influence on peat accumulation in this coalfield, including in the Coalbrook Sub-basin.