Abstract

A column of coal from a seam thermally altered by an underlying lamprophyric sill has been studied petrographically, chemically, and mineralogically. Megascopically there are four major lithotypes present within the limits of the seam. They are: carbonaceous shale, natural coke, altered coal, and a layer of quartz grains. Megascopic evidence indicative of thermal alteration is the highly contorted bedding, natural coke with evidence of flowage, and prismatic fracturing within the highly altered coal zones.

Micrinoids and fusinoids are common, with the cell cavities of the fusinoids commonly filled with calcite. Vitrinoids are all altered to a dark-brown color in thin section. Fungal derived bodies are common in the upper part of the seam and appear to be the most thermally resistant petrographic entities. No evidence of spore or cuticular material remains. Mosaic structure and vesiculation are additional common microscopic evidences for the thermal alteration.

Sulfur distribution apparently has no relation to the presence of the intrusion. Ash values are highest at the top and bottom of the seam and are related to shale-rich zones. Volatile matter (d.a.f.) is high at the seam margins and increases as the distance from the intrusion decreases. This is attributed to one, or both, of two factors. Either the volatiles were released and readsorbed or they were never released.

Only quartz, kaolinite, siderite, calcite, and Fe2(SO4)3 were identified by X-ray examination. No evidence of the presence of cristobalite, mullite, or pyprophyllite was detected. Mean maximum reflectance values range from 1.00 percent to 3.00 percent and increase toward the intrusion.

Evidence based on mineralogy, chemistry, reflectance, melting point, and electron spin resonance indicates that a temperature gradient did exist across the seam from greater than 375° C at the top to less than 600° C at the base, and also that the temperature of the underlying shale at the contact with the sill was not more than 600° C.

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