Effect of dolerite intrusions on coal quality in the Secunda coal fields of South Africa

Abstract

The coalfields of South Africa contain numerous dolerite intrusions, which are believed to have affected the quality of the surrounding coal by a thermal process, controlled by the size of the magmatic body. Data gathered from a working coalfield in Secunda, South Africa, suggest that the relationship between intrusive sills and coal is complex and factors other than intrusion width must be considered in relation to the contact metamorphic effect. The study area contains multiple dolerite intrusions of Karoo age, separated according to their geometric relationship to the local coal seams. Three intrusions were selected for detailed study. The three dolerite bodies, namely the DO4, DO8 and DO10 dolerites, occur as sills intruded close to the main coal seam of the area. The dolerite sills have identical mineralogy and can only be distinguished through textural variations in thin section. Coal quality data was obtained from Sasol Mining Secunda and used to investigate the presence or absence of a change in coal quality relative to dolerite proximity. Reduction in coal quality was defined using three main proximate analysis values, termed Ash, Volatile content (Vols) and Dry Ash Free Volatile (DAFV) in the coal industry. These parameters were used to determine the extent of any effect deleterious to coal quality induced by the intrusion of the dolerite sills. The resultant investigation showed no correlation between the position and thickness of the dolerites, and a change in coal quality (as measured by proximate analysis). In the absence of a linear relationship between coal quality and dolerite proximity, two processes are proposed to explain the absence of the contact metamorphic effects expected from previous studies: -Dolerite emplacement dynamics may influence the size of the metamorphic aureole produced by an intrusion, and invalidating intrusion size as a measure of thermal output, - Hydrothermal fluids mobilised by the dolerite intrusions, either from the country rock or the intrusion itself may percolate through the coal and act as the metamorphic agent responsible for changing coal quality, by dissolving the volatile components of the coal and transporting them to other locations These two processes are sufficient to explain the lack of a clear “metamorphic effect” related to the dolerite intrusions. Copyright