The Potosa occurrence is underlain by amygdaloidal basalt of the Upper Triassic Karmutsen Formation (Vancouver Group). A fault zone hosts a small quartz vein mineralized with native gold. A small shipment of 0.9 tonnes was made in 1896 and reported to have yielded eight hundred dollars in gold (Geological Survey of Canada Memoir 58, page 95).

Recent mapping indicates a limestone lense and a hornblende porphyry dyke outcrop nearby.