Petrological characteristics of barakar coal seams, metamorphosed by lamprophyre sill in the Jharia coalfield, Bihar

Abstract

In the Jharia Coalfield, Bihar, dykes and sills of mica lamprophyre are common and have thermally metamorphosed the coal seams in contact into natural coke. Petrological study of the metamorphosed zones of the coal seams in the south-eastern extremity of the coalfield has been carried out. Four petrologically distinct zones designated as natural coke, coked coal, affected coal and unaffected coal are traced, when studied away from the contact. Chemically, a marked increase of volatiles and decrease of ash is recorded away from the seam-igneous contact. Phosphorus content is quite high in the natural coke ash.

The natural coke is characterised by numerous mineral-filled or empty vacuoles and strongly anisotropic, highly-reflecting nature of the fused matrix having physically unaltered fusinite embedded. The fused matrix is of mosaic appearance and consists of ill-defined units varying from one to fifty microns size. In the coked coal, the size and number of vacuoles has considerably reduced, and so also, the anisotropism and mosaic appearance of the matrix, accompanied by an increase in size of the units around fifty microns. The affected coal is distinctly folded, though minutely, and is micro-laminated, the lamination being due to occurrence of vitrinite layers and sheets with interbedaed fusinite, semifusinite, resinite and micrinite. The vesicles occur here and there. Resinite is seen in cleats and as coalesced films migrated along the bedding. The unaffected coal is a typical banded bituminous coal. The natural coke formation is discussed. Graphite is concluded to be absent.