Dolerite intrusion morphology at Majuba Colliery, northeast Karoo Basin, Republic of South Africa

Dolerite sills, at times transgressive, and dykes are common at Majuba Colliery. Their behaviour within the Karoo stratigraphic pile limits and controls the effectiveness of extracting the deep-seated Gus coal seam. Due to the intrusion of dolerites, the coal seam elevation can vary by as much as 70 m. Data from 452 boreholes, 88 of which were drilled to granitic gneiss basement and data from the underground development were used to construct cross-sections through the colliery. Based on texture, geochemistry and mode of emplacement, there exist four different dolerite types (T1 to T4) at Majuba. These are intruded into sedimentary rocks of the Karoo Supergroup although one, the T3 dolerite, has been found intruded into basement gneiss. In the east, an ultramafic intrusion of pre-Karoo Supergroup age created a basement high. By far, the greatest number of dolerite sills intruded within a sandstone unit, and have identical sandstones in the footwall and hangingwall, rather than intruding along lithological boundaries.