The geology of the region consists of: 1) a Mississippian to Triassic Cache Creek Group oceanic volcanic and sedimentary assemblage 2) the Upper Triassic dominantly mafic volcanic Takla Group 3) the Lower to Middle Jurassic Hazelton Group mafic to felsic volcanic and sedimentary rocks 4) the Upper Cretaceous to Lower Tertiary Ootsa Lake Group sedimentary and volcanic rocks and 5) the 0ligocene and Miocene Endako Group. The region has been intruded by the Lower Jurassic quartz monzonite to granodiorite Topley Intrusive Suite, Upper Jurassic plutons of the Francois Lake Suite and plugs and stocks related to Upper Cretaceous and Tertiary volcanism.
The Silver Island prospect is underlain by volcanic rocks and argillite of the Cache Creek Group, hornblende diorite of the Francois Lake Intrusive Suite and rhyolite considered to belong to the Oligocene to Miocene Endako Group.
Quartz-ankerite-barite veins occur in shear zones striking 120 degrees and dipping at about 45 degrees southwest. These veins occur mainly within diorite although at least one occurs in rhyolite. Mineralization consists of tetrahedrite and minor amounts of argentite, native silver, galena, sphalerite, chalcopyrite, pyrite, malachite and azurite. Numerous stringers of calcite cut the diorite. A selected sample over a width of 3.8 centimetres from No.2 tunnel assayed 23,780.6 grams per tonne silver, 8.0 per cent copper and 3.0 per cent zinc (Annual Report 1925, page 143).