Foghorn Mountain is underlain by metavolcanics derived largely from intermediate crystal-lithic tuffs and porphyritic flows of the Devonian to Mississippian Eagle Bay Formation. The rocks are pale to medium green chlorite-sericite schists, quartz-sericite schist, chert phyllite and sericitic quartzite. These comprise a relati- vely flat lying plate, occurring as a gentle north plunging syn- form. 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.

A large hydrothermally altered zone (gossan) of heavily lim- onitic sericite schist contains disseminated and massive pyrite and minor galena, sphalerite and chalcopyrite. A sample of selected pieces of white quartz with minor pyrite assayed 0.10 per cent copper and 18 grams per tonne silver (Assessment Report 7813). A drill hole intersected 5 metres assaying .02 per cent lead and 0.02 per cent zinc (Assessment Report 7757). Another hole, 200 metres to the south, intersected 10.5 metres of 27.5 grams per tonne silver, 0.11 per cent lead, 0.03 per cent zinc, 0.25 per cent copper and 0.14 grams per tonne gold (Assessment Report 8530).