The region of the occurrence is underlain by Cenozoic lavas and pyroclastics of basaltic to rhyolitic composition. The Mount Edziza and Spectrum Range volcanic piles cover an area of nearly 1600 square kilometres. Many of the flows are reported to contain layers of obsidian up to 9 metres thick.

J.G. Souther (GSC Paper 71-44) best describes the specific geology of Mount Edziza as follows: "The basal, shield portion of Mt. Edziza is exposed in an escarpment along its western side and cross-sections of the entire pile may be seen in steep-walled radial valleys cut deeply into the eastern side. The lower part comprises a succession of very uniform, dark grey, fine-grained basaltic flows containing sparse phenocrysts of plagioclase, pyroxene and olivine. Individual columnar-jointed flows from 1.5 to 6 metres thick are separated by layers of loose reddish brown scoria and ash. The ab- sence of interlayered colluvium indicates that these lower flows were extruded in rapid succession. They are overlain by a thick bed of unconsolidated, rhyolitic, air-fall pumice above which the section is extremely complex and variable. Basalt, rhyolite and dacite flows, domes, and pyroclastic deposits are interlayered with fluvial and glacial deposits. The acid phases occur as bulbous lava domes, thick stubby flows and both welded and unwelded ash-flow sheets. Basaltic phases, erupted during non-glacial periods, form relatively thin flows that surround and engulf the piles of acid ejecta. During the Pleistocene, subglacial eruptions of basalt formed thick piles of sideromelane tuff-breccia and pillow lava. Rocks within the central conduit have been intensely altered by late solfa- taric action to a bleached, white, carbonitized rock containing from 5 to 50 per cent finely disseminated pyrite."