The Big Boy North occurrence area is underlain by andesite of the Upper Triassic Karmutsen Formation, Vancouver Group. The area to the west of the volcanics is intruded by a stock of the Early to Middle Jurassic Island Intrusions. These plutonic rocks on Vancouver Island vary in composition from gabbro to quartz monzonite but are mainly granodiorite and quartz diorite. Dykes of quartz porphyry are also reported to cut the andesite.

A quartz vein, from 5 to 10 centimetres in width, occurs in a shear that is exposed on the face of a 7.6 metre high bluff. The vein carries a little chalcopyrite and free gold.