The area of the Penton occurrence is underlain by unnamed volcanics and limestone of Permian and/or Triassic age and metavolcanics of the Lower Jurassic Bonanza Group. The strata are intruded by granodiorite to quartz diorite of the Early to Middle Jurassic Island Plutonic Suite (formerly known as the Island Intrusions).

Bonanza volcanics are irregularly mineralized with pyrite and chalcopyrite, chiefly along shear zones, near the contact with granodiorite. A mass of magnetite, 3.7 metres wide, with minor chalcopyrite occurs with garnet-epidote-diopside skarn along the contact.