The Keystone occurrence is located about 8.0 kilometres south of Alice Arm in the valley of Roundy Creek. The area has been explored numerous times between 1916 and 1968 for base and precious metals.
The region is underlain by Middle Jurassic to Upper Cretaceous Bowser Lake Group argillite, shale, siltstone, greywacke and conglomerate. These are intruded by Lower Tertiary granodiorite and diorite of the Coast Plutonic Complex. These sediments have been folded and contact metamorphosed to biotite hornfels.
Quartz veins are found along or near the contact of a 30 to 90 metre wide granitic spur that extends northeastward into the argillite. The veins are localized along a shear zone developed along the eastern edge of the spur, with the intrusion forming the footwall. The shear zone extends into and parallels the argillite, dipping 45 to 60 degrees west, which lies on the western flank of a gentle north-northwest trending anticline. The shear zone strikes 032 degrees, dips 50 degrees west, is up to 15 metres wide and locally extends into the intrusive body. The zone can be traced for 200 metres along Snow Creek, a tributary of Roundy Creek, where it contains sporadic variably sulphidic quartz veins.
The veins are lenticular in nature and from 5 to 46 centimetres in width. A quartz vein, exposed in a 14.6 metre long adit along the shear zone, strikes 178 degrees and dips 50 degrees west. This vein is 25 to 30 centimetres wide and is enclosed in sheared granite. A 36 centimetre quartz vein developed in sheared argillite and granite is exposed in a trench just above the adit. In a 213 metre long, north-trending adit (the Bowyer Tunnel), 69 metres below the upper adit, the shear zone is intersected at about 122 metres from the entrance, where quartz diorite contacts the argillite.
Mineralization in the veins generally consists of galena, sphalerite, pyrite and pyrrhotite in a gangue of quartz with minor carbonate. The vein exposed in the upper adit contains fine-grained galena, sphalerite, pyrrhotite and pyrite. A grab sample from the adit assayed 13.0 grams per tonne gold, 116.0 grams per tonne silver, 4.9 per cent zinc, 6.8 per cent lead and 0.06 per cent cadmium (Property File - Marshall Creek Copper Company Ltd. 1965 Annual Report, page 5).
