Abstract:
The coalfields of South Africa contain numerous dolerite intrusions, which affected the quality of
the surrounding coal through thermal processes, commonly believed to be 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, of which three intrusions occur as sills intruded close to the main
coal seam of the. A large database (>8000 boreholes) of coal quality data was 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 proximate analysis values, namely the ash, volatile content and dry
ash free volatile (DAFV) as defined in the coal industry. The resultant investigation showed no
correlation between the position and thickness of the dolerites, and changes 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. Firstly dolerite emplacement dynamics may influence the
size of the metamorphic aureole produced by an intrusion, invalidating intrusion size as a measure
of thermal output. Secondly, 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 and semi-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. However, the
perceived lack of a clear correlation between the coal quality parameters and the metamorphic
effects associated with dolerite intrusion may also reflect the inadequacies of proximate analysis
techniques in quantifying geological processes within the coal.
