The area is underlain mainly by limestone and argillite of the Jurassic to Triassic Kunga Group. These rocks are intruded by felsic sills related to the Middle to Late Jurassic Burnaby Island Plutonic Suite. Upper Triassic Vancouver Group, Karmutsen volcanics conformably underlie the Kunga Group limestones.

The argillaceous part of the Kunga Group (Sandilands Formation) is variable silicified and hornfelsed. Pyrite and pyrrhotite occur as disseminations and more rarely as fracture fillings, forming 2 to 5 per cent of rock volume and locally up to 15 per cent. Gold anomalies appear to be associated with sulphide veinlets within pyritic and silicified argillites.