Semianthracite, anthracite and meta-anthracite in central Canadian Cordillera: their geology, characteristics and coalification history

Abstract

In the Jurassic and Early Cretaceous Bowser Basin and Late Cretaceous Sustut Basin in the north-central Canadian Cordillera, significant resources of coal ranging in rank from medium-volatile bituminous coals to meta-anthracite occur. Coal seams are up to 5 m thick, have calorific values (daf) ranging up to 39,400 kJ/kg and fixed carbon yields (daf) up to 99%. The coal seams are vitrinite rich, have moderate to high sulfur contents, and are variably argillaceous. In the Groundhog coal field of the Bowser Basin, the coals are mainly anthracites and meta-anthracites, which are commonly cut by carbonate and quartz veins, and are variably oxidized. The orientation of the optical indicating surface together with regional maturation patterns suggest that the coalification was pre-tectonic and, thus, pre-mid-Cretaceous. Extrapolation of the measured coalification gradients suggests that the coal measures were buried to depths of the order of 5000 m. Stratigraphic variation in the coalification gradients exist and are a product of varying thermal conductivity of the strata. Higher coalification gradients and levels of coalification in the southern part of the coal field reflect a combination of deeper depth of burial and higher paleoheat flow. Numerical modeling of the coalification gradients and burial histories suggests that paleoheat flows of the order of 80 mW/m 2 existed. Such values are consistent with deposition in a back-arc basin. In the Sustut Basin, the coal seams range in rank from medium-volatile bituminous coal to semi-anthracite and are up to about 2 m thick. Preliminary analysis indicate a good correlation between levels of coalification and stratigraphy which suggests that the coalification is pretectonic (pre-mid-Tertiary). The prominent variation in coal rank in the Sustut Basin is probably a result of the marked variation in depth of burial of the strata in isolated faulted-bounded basins.