The area is underlain by Jurassic to Tertiary Coast Plutonic Complex rocks consisting of diorite and tonalite. The intrusives host roof pendants comprising sedimentary and volcanic rocks. The sedi- mentary pendant rocks are commonly limestones, which host numerous dykes. Sulphide mineralization occurs in skarn developed in the limestone near intrusive contacts.

The Royal Arch occurrence comprises an intensely sheared and brecciated, northwest trending crystalline limestone pendant within tonalite. The pendant is generally 30 to 60 metres wide but locally is 106 metres wide and appears to be caught up in a fault zone striking 320 degrees. Banding in the limestone indicates a northerly strike with an 85 degree east dip.

Minor, irregular siliceous epidote-garnet skarn is developed in limestone near the intrusive contacts. A shallow shaft explores a 60 centimetre wide skarn zone mineralized with disseminated chalcopyrite, sphalerite and pyrite. Best assays from several samples were 6.9 per cent copper, 17.5 per cent zinc and 75.41 grams per tonne silver (Minister of Mines Annual Report 1926, page A312).