Intrusive Rocks and their Influences on Coal-Seams, Chikuhō Coal-Field, Japan

Abstract

The author treats of the occurrences and the petrographical properties of the intrusives, their influences on coal-seams and some suggestions on the utilization of the metamorphosed coal in Chikuhō coal-field in this paper. Several thick coal-seams in the lower part of the coal-bearing Tertiary in the field are intruded with the igneous dykes and sheets of pyroxene-andesite or olivine-basalt. The coal, which is strongly influenced by these intrusives, is lower in volatile matters and higher in carbon and ash than are such as those of anthracite, and has been changed into graphite in the extreme case. A property of the decrepitation on heating is one of the characteristics of the coal. Macerations can be observed near the contact, and materials from the intrusives have been supplied to the coal. Abundant micro-pores and veins, containing quarts, chalcedony, opal, calcite and zeolites, also can be observed in the thin sections of such metamorphosed coal. Although oxygen and hydrogen in the coal decrease in inverse proportion to the proximity of the intrusives, those elements rather increase a little in amount instead of reaching a minimum when the coal is in direct contact with the rocks. The author concluded that the phenomenon of the contact-metamorphism of coal by intrusives in this region can not be explained completely by a simple thermal effect only, but some substances such as water-vapor, gases and hydrothermal solutions from the intrusives have been supplied to the coal in the high pressured conditions underground.