Coalification levels and their significance in the groundhog coalfield, North-Central British Columbia

Abstract

The only significant deposits of anthracite and meta-anthracite in Canada occur in Upper Jurassic-Lower Cretaceous strata of the Groundhog coalfield in northcentral British Columbia. The coal rank in the coalfield varies from low volatile bituminous (1.70% R 0 max) to meta-anthracite (5.8% R 0 max). The main coal bearing unit, the Currier, includes up to 17 seams of anthracite and meta-anthracite most of which are less than 1 m thick. In the McEvoy unit, which overlies the Currier, up to 9 coal seams, mainly of semi-anthracite, occur that are up to 0.8 m thick. The coals are variably argillaceous, locally sheared and cut by quartz and less commonly, by carbonate veins. Coalification gradients in the coalfield vary from 0.8% to 3.0% R 0 max km −1. The rank of coal within both the McEvoy and Currier units appears to increase towards the eastern edge of the coalfield. The level of coalification and the coalification gradients in the coalfield are anomalously high considering an indicated maximum depth of burial of 3500 m. From comparison with coalification models it appears that geothermal gradients in the order of 50° to 70°C/km must have existed for a period of time measured in millions of years. Studies to date suggest the coalification is pre-tectonic and thus pre-Late Cretaceous although there is some evidence for high heat flow in the Tertiary. The origin of the high heat flow may be related to intrusion accompanying collision of the Stikine terrain with the early Mesozoic margin of North America and/or high heat flux over an easterly dipping subduction zone below the Coastal volcanic-plutonic arc to the west.