Coal metamorphism in the upper portion of the Pennsylvanian Sturgis Formation in western Kentucky

Coals from the Pennsylvanian upper Sturgis Formation (Missourian and Virgilian) were sampled from a borehole in Union County, western Kentucky. The coals exhibited two discrete levels of metamorphism. The lower-rank coals with vitrinite reflectances indicative of high volatile C bituminous were assumed to represent the normal level of metamorphism. A second set of coals with vitrinite reflectances indicative of high volatile A bituminous was found to be associated with sphalerite, chlorite, and twinned calcite. The latter mineral assemblages indicate that hydrothermal metamorphism was responsible for the anomalous high rank. Consideration of the sphalerite fluid-inclusion temperatures from nearby ores and coals and the time-temperature aspects of the coal metamorphism suggests that the hydrothermal metamorphic event was in the 150 to 200 °C range for a brief time (10 ⁵ -10 ⁶ yr), as opposed to the longer term (25-50 m.y.) 60 to 75 °C ambient metamorphism.