Metamorphism and alteration near an intrusive-coal contact

A description is given of a mineralogical-petrological study of alteration and metamorphism on intrusion of a porphyrite into a coal seam of medium-volatile bituminous rank within the Collinsville Coal Measures, Eastern Queensland, Australia. The intrusive body was found to have been altered to a "white trap" consisting of albite, kaolinite, chalcedony, ankerite, siderite, calcite, and leucoxene. Its textural relations indicated that it had been partly crystalline at the time of its intrusion, containing plagioclase, pyroxene and perhaps amphibole, and ilmenite, and that it was of intermediate, probably andesitic, composition. The coal was affected up to 2\ feet below the lower contact. Near the contacts the coal had been converted to a natural coke with fine coke mosaic structure and inertinite-rich bands. A plastic mass, formed up to 2 feet from the lowest contact, was injected into this natural coke and into cracks in the porphyrite; this plastic material is characterized by a coarse mosaic structure. Liquid tarry products formed during carbonization of the coal had themselves been carbonized, giving rise to masses of spheres having a characteristic structure. The gases evolved had deposited coatings of pyrolytic carbon on walls of cavities and cracks. At a distance of 2-2 feet from the contact the vitrinite had become optically almost isotropic and spheres of a characteristic type had formed. Reflectances of the petrographic constituents of the metamorphosed coal are given. From these and other considerations it is concluded that temperatures attained during the metamorphism did not exceed 760° C.