The coalification of South African coal

Abstract

It has been generally accepted that the variation in rank of South African coal is essentially due to the metamorphic effect of dolerite dykes and sills. However, no satisfactory explanation could be given for the fact that true anthracite has not been formed adjacent to dykes and transgressive sills in those South African coal fields where the rank of the coal is normally low. In these areas, the coal close to the intrusives is often referred to as “burnt”. A detailed examination of the contact metamorphism of South African coal by dolerite intrusives shows that a dyke or sill affects the coal to variable distances, generally from 0.6 to 2 times the thickness of the intrusive, and that this distance is independent of the rank of the coal outside the contact aureole. This is explained by an initial episode of contact metamorphism while the coal was still in the lignite stage of coalification, followed by burial metamorphism during which the paleogeothermal gradient increased in an easterly direction. This regional increase in paleogeothermal gradient is probably related to the large-scale magmatic activity that culminated in the extrusion of the Drakensberg basalts and the break-up of Gondwanaland.