The Copper Duke occurrence is underlain by the pre-Jurassic Twin Island Group which is comprised of medium to high grade metamorphic rocks whose contacts with the surrounding Jurassic to Tertiary Coast Plutonic rocks are commonly either complex migmatite zones or faults. The occurrence consists of magnetite and copper showings which were explored by several open cuts and four adits around 1908.

The host rocks are described as gneiss and schist which are cut by numerous diabase dykes. Mineralization consists mainly of magnetite, pyrrhotite and chalcopyrite in a gangue of epidote, garnet and hornblende. Grab samples from ore dumps in 1918 assayed from 3.8 to 16.4 per cent copper with traces of gold and about 68.6 grams per tonne silver (Minister of Mines Annual Report 1918, page 293).

In 1957, drilling in the magnetite-rich ore indicated that it was relatively free from impurities except for an appreciable amount of sulphur in the form of pyrrhotite (Minister of Mines Annual Report 1957, page 127).