The Mount Washington area is underlain by sediments of the Upper Cretaceous Nanaimo Group (Comox Formation), which unconformably overlie mafic volcanic rocks of the Upper Triassic Vancouver Group (Karmutsen Formation). Intruding both formations is an Oligocene quartz diorite stock of the Mount Washington Intrusive Suite, dated at 35 million years (+/- 6 million years). Several later breccias zones follow the Karmutsen-Comox unconformity and cut all rock units. They have recently been interpreted as stacked thrust faults or de- collements and have been mineralized by a later Tertiary epithermal gold-copper-arsenic event (see 092F 116).

The Karmutsen Formation comprises basaltic, massive and pillow lavas that are commonly porphyritic. The lavas grade into pillow breccias and aquagene tuffs. The overlying Comox Formation comprises fine-grained sandstone and greywacke, with interbedded siltstone. A basal conglomerate of the Comox Formation known as the Benson member consists of rounded clasts of Karmutsen Formation rocks.

The Tertiary quartz diorite stock is variably porphyritic and forms the core of Mount Washington. Several sills and dykes of quartz diorite and quartz diorite porphyry are related to the stock.

The Oyster breccia is a roughly circular feature that measures in excess of 350 metres in diameter and is thought to be a collapse breccia. The rocks surrounding the breccia "pipe" are mainly massive and/or amygdaloidal Karmutsen basalts. The breccia structure outcrops are composed mainly of Comox quartzite and minor porphyritic diorite. However, the fragment composition changes with depth from sediments to Karmutsen volcanics and intrusives. The matrix is often vuggy, the interstices lined with crystalline quartz and filled with limonite and fine fragments of limonitic quartzite.

A diamond-drill hole was put down on the centre of the zone in 1975 to a depth of 184 metres. This hole was deepened in 1988 to a depth of 542 metres. In the drill hole above 212 metres, the breccia is vuggy, bleached and intensely kaolinized. Below 212 metres, the breccia is very siliceous, glassy over short sections, with all interstices filled with quartz crystals and/or carbonate. The quartz matrix may also be lined with chlorite. The hole intersected small amounts of vein or matrix hosted molybdenite, arsenopyrite, and chalcopyrite, pyrite, realgar and orpiment is indicative of an epithermal overprint. Gold and silver content of core below 184 metres was negligible; assays for core above 184 metres were not reported.

The Pyrrhotite Creek breccia zone is an epithermal-type structure that occurs along a creek near the southern perimeter of the Oyster breccia. The structure consists of a lenticular zone of partially silicified and kaolinized bleached and brecciated basalt, mineralized across a width of about 1 metre and dipping toward the Oyster zone. Sulphide minerals present in decreasing order of abundance are pyrite, arsenopyrite, chalcopyrite, orpiment and realgar. The chalcopyrite sections of drill core were coincident with quartz veining and visible arsenopyrite.

A grab sample of silicified fault breccia assayed 14.50 grams per tonne gold, 31.88 grams per tonne silver, 1.04 per cent lead and 0.05 per cent zinc (Assessment Report 17193). A 43 centimetre section of core assayed 2.78 grams per tonne gold, 6.86 grams per tonne silver, 0.07 per cent copper and 3.67 per cent arsenic (Assessment Report 17193).