COKED COAL MARGINAL TO KIMBERLITE INTRUSIONS IN THE TANOMA MINE, INDIANA COUNTY, PENNSYLVANIA

Coked coal exposed underground in the Lower Kittanning seam adjacent to thin kimberlite intrusions, of probable Jurassic age, revealed contact metamorphic temperatures in the 500°C range. Thin dikes, bulbous sills and unusual stratabound en echelon dike/sills intrude the 5 feet thick coal seam for over 2 km in an east-west direction. A hydrofracturing mechanism is proposed to account for the highly directional en echelon ”bladed” dikes in the coal seam.

The kimberlite is porphyritic with phenocrysts of olivine, Cr-diopside, picroilmenite, garnet and phlogopite in carbonate serpentine-rich matrix with microcrysts of perovskite, ilmenite, spinel, monticellite, apatite and phlogopite. The coal reached an apparent rank of medium volatile bituminous and, based upon coke texture, probably did so before intrusion. Coke adjacent to the contact zone was dominated by anisotropic carbon having individual isochromatic areas up to 10 times the size of the textures produced during laboratory carbonization of the coal. In one area beyond about 10 cm from the dike, competent coal bedding structures were observed. Samples of the natural coke adjacent to the contact zone were subjected to laboratory carbonization at 100°C intervals between 400° - 700°C and maximum reflectance measurements performed in order to identify the temperature below which thermal treatment failed to induce a change in the measured reflectance. From this experiment a carbonization temperature of 494 ±5°C was determined for the coke adjacent to the contact zone and 514 ±9°C was estimated for the carbon material intimately mixed with the kimberlite matrix.