The western half of Quadra Island lies within the Insular belt and is underlain primarily by andesitic volcanics of the Upper Triassic Karmutsen Formation, Vancouver Group. These are interbedded with and overlain to the east by a northwest trending belt of Upper Triassic Quatsino Formation limestone, also of the Vancouver Group.
The eastern half of Quadra Island lies within the Coast Crystalline belt and is mainly underlain by Jurassic to Tertiary intrusive rocks of the Coast Plutonic Complex. These granitic rocks are in fault and/or intrusive contact with the Insular rocks along a northwest trending zone from Open Bay to Granite Bay.
In the vicinity of the Geiler workings, fine-grained, porphyritic and amygdaloidal varieties of andesite predominate. Andesite and limestone occur in small lenses within the volcanic rocks. Both the lime rocks and the volcanics have been intruded by granitic rocks and outcrop approximately 1.2 kilometres northeastward from the Geiler workings.
The most abundant rock types comprise a group of greenstones. The predominant greenstone is a dark green, fine-grained andesite, some phases of which are porphyritic and contain phenocrysts of hornblende. One outcrop exposes amygdaloidal greenstone that strikes 140 degrees and dips northeast. Also in the vicinity, greenstone schist, containing some biotite, outcrops. This schist strikes 150 degrees and dips 40 degrees northeast.
Pods of white crystalline limestone occur occasionally in the greenstones. The most conspicuous band is in the vicinity of the silicified breccia-shear zone. In the vicinity of the main workings on the breccia-shear zone, irregular feldspar porphyry dykes are common. They intrude the greenstones and contain many angular xeno- liths of these rocks. The dykes are dark grey in color and contain small, but conspicuous phenocrysts of feldspar (Stevenson, J.S., 1938).
D.D. Cairnes (Geological Survey of Canada Summary Report 1913), describes three types of deposits on the Geiler. 1) Skarn-type mineralization, which was developed by a 9 metre shaft, (as of 1913) is composed mainly of garnet, amphibole, epidote, quartz and calcite, throughout which occurs sparsely disseminated pyrite, arsenopyrite, chalcopyrite, pyrrhotite and magnetite. Occasional particles of native gold were also reported. One sample assayed 1.0 gram per tonne gold (Minister of Mines Annual Report 1913). Elsewhere two shallow pits about 30 metres apart examined masses of ore material from 2 to 3 metres in width consisting domi- nantly of pyrrhotite with some disseminated chalcopyrite. The strike of the ore material appears to be the same in both pits, about 075 degrees, which indicates one continuous deposit. 2) A vein-like deposit of quartz and calcite as much as 0.9 metres thick within greenstone was explored by a 5.5 metre shaft. The vein material contained sparsely disseminated chalcopyrite, pyrrhotite and pyrite. 3) A silicified shear-breccia zone occurs traversing the andesitic volcanics and is at least 6 to 9 metres wide and traceable for over 150 metres. Throughout this zone the volcanics are extremely broken and shattered and the rock fragments are cemented mainly by quartz. Veinlets and stringers of quartz from 2 to 15 centimetres in width also cut the volcanic rocks. The quartz appears to constitute up to half of the rock mass in places and is sparsely mineralized showing only occasional particles of pyrite, chalcopyrite, native gold and a dark lustrous telluride identified as sylvanite. One 1.5 metre sample across the dip of numerous quartz stringers in greenstone assayed 8.9 grams per tonne gold (Stevenson, J.D., 1938). One chip across 30 centimetres of decomposed and oxidized shear assayed 48.0 grams per tonne gold and 0.4 per cent copper (Stevenson, J.D., 1938).
The Geiler produced (1940-1941) 1897 grams of gold, 497 grams of silver and 229 kilograms of copper from a total of 108 tonnes mined.
