The Blackhorn Mountain occurrence refers to several significant showings of gold and silver-bearing quartz veins which outcrop in the mountainous terrain south of Blackhorn Mountain. Work on the showings took place in two phases: the first was from their discovery in 1936 until 1939, and the second phase was between 1979 and 1988. Detailed information on the early work is given in the Minister of Mines Annual Reports for 1937 and 1938.
The area lies in the Stikinia Terrane and the Gambier overlap assemblage near the northeastern margin of the Jurassic to Tertiary Coast Plutonic Complex, within a complex stack of recumbent folds and imbricated, gently southwest-dipping thrust sheets (Geological Survey of Canada Open File 1163, Map 1713A). The northeast-directed thrusting placed Upper Triassic (Carnian) and Lower Cretaceous volcanic and sedimentary rocks over Lower Cretaceous (Hauterivian) sedimentary rocks (Geological Survey of Canada Open File 1163, Papers 88-1E, 89-1E; Geology 1991). The thrusting took place in the Late Cretaceous because the thrusts are cut by a quartz diorite intrusion dated at 68 million years by the uranium-lead method on zircon (Geological Survey of Canada Paper 88-1E). The area of economic interest lies within the imbricated Upper Triassic and Lower Cretaceous rocks. The local geology probably involves more than one thrust sheet, but as thrusts were not recognized as such in the pertinent data sources, a structural interpretation of the local stratigraphy is not attempted here.
Most of the area of the occurrence is underlain by andesitic tuff and breccia, metamorphosed at greenschist grade and usually described as greenstone, or as chlorite schist where the rocks are sheared. Minor shale or argillite or sericitic schist is intercalated with the volcanics. Locally there are units of sedimentary rocks, including limestone, shale, greywacke and conglomerate. In general, the strata strike north to northeast and dip moderately to the northwest. The area is intruded by numerous felsic to intermediate porphyritic dykes and sills; the dykes are generally steep, and strike east.
Mineralization is associated with large quartz veins, which are generally concordant with bedding or foliation in the host rocks, although most are irregular and discontinuous. The most important vein is at the Homathko adit, in pyritic andesitic volcanics. The vein varies from 0.3 to 1.0 metre in thickness, and strikes 050 degrees and dips 80 degrees northwest. The vein has a strike length of at least 60 metres, but is locally offset by steep, northwest-striking faults, and by a vertical, 2 metre-thick quartz porphyry dyke. The vein also contains calcite, mainly along fractures. The main quartz vein pinches out into a quartz-veined, chlorite schist shear zone, but its actual limits have not been defined.
Chloritic and sericitic alteration of the host rock, or of inclusions in the quartz vein, is common, as is some silicification. The vein contains up to 10 per cent sulphides, averaging 2 per cent; in order of abundance, these are arsenopyrite, pyrite or pyrrhotite, sphalerite, chalcopyrite and galena. Pyrite is disseminated whereas the other sulphides tend to be in small masses. Visible gold occurs as fine grains on fracture surfaces, suggesting that it may have been introduced later than the other minerals (Minister of Mines Annual Report 1938). One such sample assayed 653 grams per tonne gold and 148 grams per tonne silver (Assessment Report 10654). An uncut average of 5 chip samples taken over 0.5 metre in sulphide-bearing quartz or quartz-carbonate vein material, locally with visible gold, was 71.7 grams per tonne gold and 56.2 grams per tonne silver (Assessment Report 10654). More typical values of samples from the adit are between 5 and 50 grams per tonne gold, and between 7 and 40 grams per tonne silver (Assessment Reports 9575, 12691; Minister of Mines Annual Reports 1937, 1938).
An area of about 1 square kilometre centred on the Homathko adit contains several other mineral showings in pyritic or limonitic (oxidized) sheared volcanics (Assessment Reports 9575, 12691), or in oxidized conglomerate (Assessment Report 17858).
Most of the development work that has been done was between 1936 and 1939, during which time the Homathko Gold Mines Limited company was formed. This work included the underground drifting, trenching, and 640 metres of diamond drilling over several holes. In addition, a mill was constructed locally, which processed 3.18 tonnes of high- grade ore from the Homathko adit and surface workings. This resulted in the recovery, by amalgamation, of 275 dollars of gold, from an average grade of milled ore of approximately 79 grams per tonne gold (Minister of Mines Annual Report 1937).
The more recent work has concentrated on prospecting the old workings, with some new, short diamond drilling and blasting. One verdict is that the mineralized quartz veins have restricted lengths and widths, limiting potential tonnage at high grades, although low- grade, bulk tonnage potential remains (Assessment Report 12691).