Influence of a basic intrusion on the vitrinite reflectance and chemistry of the Springfield (No. 5) coal, Harrisburg, Illinois

The Springfield (No.5) coal, a Pennsylvanian-age high volatile B bituminous rank coal in the Carbondale Formation, is mined in the southeastern section of the Illinois Basin. Intruded into this coal is a 10.1-m-wide basic dike that is near vertical through the extent of the mined coal bed, and strikes N 22° W. The intrusion and the effects it had on the coal were studied petrographically and geochemically. Using a published heat flow model by Carslaw and Jaeger (1959) [Carslaw, H.S., Jaeger, J.C., 1959. Conduction of Heats in Solids. Oxford University Press] in conjunction with thin-section microscope analysis, a contact temperature of ∼600 °C was determined. Vitrinite reflectance was measured for a suite of coal samples collected distal to the intrusion. Vitrinite reflectance values rose uniformly from the ambient reflectance values of ∼0.69%, at just greater than one-dike thickness away, to ∼5% at the dike/coal contact. The increase in reflectance values for samples within ∼1.2 times dike thickness is concordant with data published by Bostick and Pawlewicz (1984) [Bostick, N.H., Pawlewicz, M.J., 1984. Paleotemperatures based on vitrinite reflectance of shales and limestones in igneous dike aureoles in the Upper Cretaceous Pierre shale, Walsenburg, Colorado. In: Woodward, J.G., Meissner, F.F., Clayton, J.L. (Eds.). Hydrocarbon Source Rocks of the Greater Rocky Mountain Region. Rocky Mountain Association of Geologists, pp. 387-392] on the effects of mafic intrusions into Cretaceous-age shales in Walsenburg, Colorado. Comparison of Colorado data with data from these sites indicates that, despite differences in intrusion temperature, timing, or coal rank at time of the intrusion, ambient mean vitrinite reflectance values remain unchanged at just greater than one-dike width from an intrusion.