A case study on the apparent absence of thermal effects of an andesitic intrusion on the rank of adjacent coal

Abstract

The intrusion of the Jumbo Dome andesitic plug into Miocene coal-bearing rocks of the Hosanna Creek field, Nenana basin, interior Alaska caused dramatic structural adjustments and attitude changes adjacent to its flanks. The area contains major commercially minable subbituminous coal resources. Jumbo Dome andesite has been radiometrically dated by KAr methods as late Pliocene to Pleistocene. Proximate analysis and vitrinite reflectance data on samples from two coals on the southwest flank of the dome indicate no significant difference in rank or character from other coal beds in the Nenana basin. Petrologic and geochemical evidence suggests that the Jumbo Dome andesitic magma did not cool slowly in equilibrium but experienced several stages of crytallization under various temperature and pressure regimes. The Jumbo Dome intrusive body was evidently shallowly emplaced within a few hundred meters of the surface. The pluton cooled relatively rapidly, exposing the coal-bearing rocks to only a minor temperature difference of short duration.