Summary:
All of the known coal deposits in South Africa are hosted in sedimentary rocks of the Karoo Basin which covers approximately 60% of South Africa and is filled with units of the Karoo Supergroup.
The Karoo Supergroup is lithostratigraphically subdivided into the Dwyka, Ecca, Beaufort and Stormberg groups, succeeded by the Drakensburg Formation. The lithologies are representative of depositional environments including glacial, deep marine (including turbidite), shallow marine, deltaic, fluvial, lacustrine through to aeolian deposits. Within the Main Karoo Basin, the Karoo Supergroup as depicted in the figure below, comprises the Dwyka Formation, Ecca Group and Beaufort Group overlain in turn by the Stormberg Group and Drakensberg Group. Sediments reach 12,000 m thickness, overlain by thick flood basalts of the Drakensberg Group.
Within the Karoo Basin, 19 coalfields have been defined based on variations in sedimentation, origin, formation, distribution and quality of the coals. The coals range in age from Early Permian (Ecca Group) through to Late Triassic (Molteno Formation, Stormberg Group) and are predominantly bituminous to anthracite in rank, which is a classification in terms of metamorphism under the influence of temperature and pressure. The Gus Seam is stratigraphically located in the Vryheid Formation within the Ecca Group.
Uitkomst falls within the Utrecht Coalfield, which has a total of nine recognised coal seams which are developed to varying extents across the coalfield. At Uitkomst, only two seams are developed, namely the Alfred and Gus Seams. The Gus Seam is however the only seam of economic importance at Uitkomst Colliery, this due to the Alfred Seam being poorly developed and sporadic or too thin for economic extraction on the Uitkomst property.
The Gus Seam comprises banded bright, dull and lustrous coal in varying proportions with the lower portion of the seam invariably being of better quality than the upper part. The seam usually contains one or more fine- to medium-grained sandstone partings towards the top which may be up to 20 cm in thickness.
The seam thickness varies between 0.8 cm and 1.9 m in a north-south trending zone in the central area of the mining lease area. To the south, the seam outcrops into the Dorpspruit and Kweekspruit Valleys. The seam extends north under the escarpment where the depth of seam is approximately 300 m below surface. The seam depth rapidly increases to the north to approximately 800 m below surface.