REVISITING INTRUDED COALS AND SHALES ALONG THE PURGATOIRE RIVER VALLEY, CO: AN OCCURRENCE OF NATURAL PITCH COKE

Numerous examples of coke produced by the igneous intrusion of coal have been reported in the Spanish Peaks region of south central Colorado. However, in a recent study of an intruded section of the Raton Formation (Upper Cretaceous-Paleocene) along the Purgatoire River near Medina Plaza, CO, coke derived from pitch has been observed. This material (herein referred to as pitch coke) occurs in "fingers" (showing polygonal joints) directly above a lamprophyre sill and in carbonaceous Type III shale directly below the sill. This pitch coke is characterized by a remarkable flow mosaic texture, high reflectance (Ro > 7.0%) and anisotropy, abundant devolitilization vesicles, and an absence of inertinite inclusions. Only rare shale inclusions were noted. Geochemically, raw samples of the pitch coke "fingers" have low HI and OI values (<10), high %C (~ 70%), low N contents (~ 0.6%), C/N ratios of ~120, and d¹³C of -26‰. Coke textures range from coarse lenticular mosaic to medium ribbon mosaic. Along boundaries of the "fingers" there is evidence for multiple stages of accumulation, including coarse or ribbon coke edged by vapor-deposited carbon (pyrolytic carbon), secondary layers of highly porous coke (not unlike sponge coke), and small areas of mesophase spheres.

This pitch coke is quite different from coked coal from the same locality. The coked coal has a medium grained circular mosaic texture that is consistent with the high volatile bituminous rank of the unaltered coal in the area. The coal-derived coke has similar devolitilization vesicles but also has numerous inclusions of inertinite macerals such as fusinite and funginite. A previously reported coal "dike" that had flowed through a fracture in a sill in the same area showed coarse circular mosaic texture.

These observations suggest that the coke found in this study was not formed by the direct coking of coal, but from a liquid phase (pitch) that was subsequently coked by the intrusion. This pitch could have been derived from coal or from the organic matter contained within the carbonaceous shales that lie above the sill (in the latter case, this could then be considered a natural petroleum coke), or both. Unaltered Type III shale sampled at this site has a HI of ~350 suggesting some capacity to generate petroleum.