The area of the Otter showing is underlain by the Eocene Metchosin Volcanics and the Eocene Sooke Gabbro; the latter forming the basement of the former. The Sooke Gabbro is composed of coarse- grained gabbro with about equal parts of bytownite and diopside and about 5 per cent olivine. Dykes of leucogabbro contain up to 80 per cent bytownite. The Metchosin Volcanics are divisible into a lower unit of pillow basalt, tuff and breccia and an upper unit of basaltic flows. Local agmatization, amphibolitization and mineralization of the gabbro occurred later along shear zones.

Chalcopyrite is reported to occur in both volcanics and gabbro (Geology, Exploration and Mining in British Columbia 1969, page 225).