The western half of Quadra Island is underlain primarily by andesitic volcanic rocks of the Upper Triassic Karmutsen Formation which are overlain and bounded on the east by a northwest trending belt of Upper Triassic Quatsino Formation limestone, both of the Vancouver Group.

The area is underlain by highly fractured and sheared Upper Triassic Karmutsen Formation amygdaloidal andesitic flow rocks interlayered with dense, fine to medium-grained andesitic units and minor thin beds of sedimentary and tuffaceous material. The flow rocks dip gently south and southeast and range in thickness from 0.3 to 3.6 metres and more. Many of the flows are highly amygdaloidal with the amygdules filled with calcite, quartz, chlorite, actinolite or prehnite. The rocks are chloritized and cut by numerous stringers and veinlets of quartz, calcite and epidote.

Chalcocite is the most abundant mineral with native copper and chalcopyrite in lesser amounts. Bornite and pyrite are rare. Malachite, azurite and cuprite are confined to oxidized and weathered surfaces. The distribution of the mineralization is erratic. It is found along fracture plane surfaces and within irregular quartz- calcite veinlets, less commonly it occurs within amygdules or is otherwise locally disseminated. The mineralization tends to be more concentrated where fracture density is high.

The Copper Flat is comprised of chalcocite mineralization (similar to Pomeroy 2, 092K 119) in east trending fractures within chloritic amygdaloidal andesite flows.