Abstract
Coal remains central to South Africa’s energy sector, with the Witbank Coalfield playing a pivotal
role due to its extensive reserves. Amongst many others in the country, Belfast Coal Mine
contributes significantly to thermal coal production. The mine has experienced discrepancies
between geological model predictions and measured (in pit-derived) total sulphur (TS) values
leading to costly quality control challenges, particularly for the crush and screen plant, which is
supplied as a run-of-mine product. The purpose of the research was to examine variability in the
quality of the No. 2 Seam within the mine in the Witbank Coalfield, Main Karoo Basin.
Five mining blocks within the Belfast Mine were selected for sampling to capture the variability
in the quality parameters. Firstly, samples were collected from blocks predicted to have low (<1
wt%) and high TS contents (>1 wt%) by the geological model. Additional samples were collected
from blocks affected by geological features and processes commonly observed within the mine
including dolerite intrusions, in-seam clastic partings, and weathering. Petrographic analysis was
used to determine maceral composition and coal rank. In addition, petrography enabled description
of the modes of occurrence of clays, quartz, carbonates, and sulphides (i.e., pyrite). X-ray
diffraction (XRD) was utilised to identify and quantify mineral phases and X-ray fluorescence
(XRF) complemented this by providing elemental compositions. Proximate, ultimate and calorific
value analysis established the quality of the coal samples. Finally, sulphur form analysis was
conducted to understand the different forms of sulphur. Using the results obtained, a reconstruction
of the depositional environment was conducted, and the impact of post-depositional processes on
the coal seam were also assessed.
Mean random vitrinite reflectance measurements (between 0.633 and 0.742 %RoVmr) classified
all samples as medium rank C bituminous. Inertinite, ranging between 31.8 and 89.8 vol%, was
the dominant maceral group in most of the samples from the No. 2 Seam. Fusinite, semifusinite,
and inertodetrinite were predominant. Vitrinite (2.0 to 24.0 vol%) primarily composed of
collotelinite and collodetrinite, whereas liptinite occurred mostly as sporinite (at a maximum of
5.9 vol%). The general dominance of inertinite suggests low water-level and fire-prone
paleoenvironmental conditions, periodically interrupted by higher water level and reducing
periods that favoured vitrinite formation.
iv
Quartz and kaolinite dominated the mineral composition of the coal samples from the No. 2 Seam.
Kaolinite, attributed to detrital input, primarily infilled cell lumens in organic matter. Quartz
appeared rounded and angular, mainly associated with inertodetrinite, supporting a detrital origin.
Carbonate minerals were mostly epigenetic, infilling cleats in macerals. The Chemical Index of
Alteration (CIA = 64.0 to 145.76) indicates that moderate to intense chemical weathering produced
the detrital silicate minerals under warm and humid conditions. The Al₂O₃/TiO₂ ratio, ranging
between 14.92 and 25.3, indicated detrital input from felsic to intermediate igneous rocks likely
originating from the felsic phase of the Bushveld Complex. The TS values ranged from 0.13 to
5.04 wt% with most samples classified as moderate sulphur coals, and a few at the base and the
top of the No. 2 Seam, as high sulphur. Pyritic sulphur was the dominant form, with the pyrite
occurring as both syngenetic and epigenetic forms.
The in-seam clastic parting reflects flooding during peat accumulation, introducing significant
clastic material into the paleomire, resulting in elevated inertodetrinite, ash yields (related to quartz
and kaolinite), and sulphur contents. Additionally, samples underlain or overlain by carbonaceous
siltstone seem to be associated with elevated TS contents compared to those bound by sandstone.
The dolerite intrusion also seems to have had a substantial impact on the quality of the coal, leading
to increased levels of sulphur (pyrite) and carbonates. This was attributed to the migration of
hydrothermal fluids along cleats and fractures. Weathering resulted in a friable coal texture,
reducing volatile matter and fixed carbon contents, while increasing inherent moisture. Signs of
mineral alteration were observed in the weathered samples; however, the sulphur remains mostly
pyritic, suggesting limited oxidative transformation had occurred. Discrepancies between the
predicted and measured TS values are likely due to variations in sulphur distribution with the No.
2 Seam, primarily related to the enrichment of pyrite by multiple sources.