Organic petrology of coals from Botswana: Evidence of variations in depositional environments

Permian coal samples from five major coalfields in Botswana—Lechana, South Orapa, Morupule, Takatokwane, and Tuli—were systematically characterized using coal petrography (maceral and mineral composition, microlithotype, and vitrinite reflectance) and geochemical analyses (inherent moisture, volatile matter, ash, and fixed carbon contents, total sulphur (TS), and gross calorific value (GCV)) to assess coal quality and enable the consideration of the depositional environments at the time of coal formation in these coalfields. Lechana and Morupule coals are inertinite-rich (avg. 54.5 and 84.4 vol.%, respectively; mmf basis), indicative of oxidizing conditions, whereas South Orapa, Takatokwane, and Tuli coals are vitrinite-dominated (avg. 78.1, 67.3, and 82.5 vol.%, respectively), reflecting deposition under oxygen-depleted (oxygen-lean), waterlogged peat-forming environments. Vitrinite reflectance indicates medium-rank D-C bituminous coals at Lechana, Morupule, and Tuli; low-rank sub-bituminous coals with localized medium-rank intervals at South Orapa; and a broader range of reflectance readings obtained for the Takatokwane samples, likely influenced by igneous intrusions and tectonic control. Elevated ash contents in all samples reflect detrital input of silicates and epigenetic mineralization of carbonates and sulphides, overall impacting the calorific value. The TS contents, up to 4.5 wt.% in some samples, will require selective mining and/or beneficiation prior to use in industrial applications. Selected samples, notably from Morupule Coalfield, might have potential to meet thermal power specifications, and certain studied coal samples (no. 12106, 12110, 12111, 1287, 1298, and 1471) from different coalfields shows potential for synthetic fuel production. Therefore, further detailed and systematic investigations are recommended. Microlithotype and maceral analyses enable the elucidation of palaeomire depositional conditions. The peat-producing plants in the Lechana Coalfield accumulated in palaeomires evolving from lacustrine–deltaic to fluvial and back to lacustrine facies conditions. The occurrence of clay minerals, individual quartz grains, sporinite, and inertodetrinite highlights water table fluctuations, sediment influx, and periodic oxidation. The Morupule coal samples, dominated by durite and inertite, reveal more stable lacustrine deposition interrupted by episodic flooding, oxidation, and palaeowildfire events. The association of inertodetrinite, semifusinite, fusinite, and detrital clay infillings within cell lumens provides further evidence of episodic fluvial incursions into the mire. In contrast, the Takatokwane, South Orapa, and Tuli coals are vitrite and clarite rich, consistent with fluvial peat mire formation under waterlogged, oxygen-depleted (oxygen-lean) conditions. Variations in mineral and sulphur content further highlight localized geochemical controls. A regional gradient from fluvial to deltaic–lacustrine facies reflect changes in subsidence, hydrology, and water table across the Kalahari-Karoo Basin. These findings demonstrate that microlithotype composition provides critical insights into the complex depositional history of Permian coal-bearing palaeomires in Botswana, with important implications in understanding coal basin evolution and potential for coal conversion. Nevertheless, detailed mineralogical, palynological and organic geochemistry studies should also be taken place in future to have better understanding about peat-forming environments of Botswana coals.