Foghorn Mountain is underlain by metavolcanics derived largely from intermediate crystal-lithic tuffs and porphyritic flows of the Devonian to Mississippian part of the Eagle Bay Formation. The rocks are pale to medium green chlorite-sericite schist, quartz- sericite schist, chert, phyllite and sericitic quartzite. These comprise a relatively flat lying plate, occurring as a gentle north- plunging synform. To the west, separated by an east-dipping thrust fault, are basalt, gabbro, chert and minor sediments of the Fennell Formation. The Middle Cretaceous Baldy Batholith lies to the south.

Several narrow, discontinuous quartz veins contain galena, sphalerite and minor pyrite and chalcopyrite. A grab sample from an adit dump assayed 12.48 per cent lead, 2.02 per cent zinc, 0.25 per cent copper, 204.7 grams per tonne silver and 0.07 grams per tonne gold (Assessment Report 4876).