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Geology of Britannia Ridge, east section, southwest British Columbia McColl, Kathryn Margaret


A stratigraphic framework for altered volcanic rocks assigned to the Late Jurassic - Early Cretaceous Gambier Group (Heah et al., 1986) is proposed that correlates units from the abandoned Britannia Mine with strata along the eastern section of Britannia Ridge and the Furry Creek valley. Geological mapping defines three packages, interpreted as volcanic sequences, that represent a continuous succession of mainly mafic to felsic volcanics. From north to south, packages 1, 2 and 3 dip moderately and face consistently southwest. Package 1 is characterized by a mafic flow sequence intruded by rhyolite domes. Package 2 is dominated-by a felsic dome complex and a sedimentary basin, both blanketed by a thick pyroclastic flow unit. Package 3 is characterized by felsic lavas and ash flow tuffs gradational upwards to a thick argillite unit. Developed orebodies occur at two major horizons that are close to the lower and upper contacts of package 2. Sulphides associated with chert horizons occur within altered sheared equivalents of crystal lithic tuff and felsic breccia related to felsic dome complexes. Whole rock analyses define an altered calc-alkaline volcanic suite. Alteration, attributed to hydrothermal solutions accompanying massive sulphides, is most intense in packages 1 and 2 where rocks are MgO-rich, CaO-poor and enriched or depleted in alkalis compared to normal subalkaline rocks. Rocks in package 3 and from later crosscutting dykes are relatively unaltered. Rb-Sr values tentatively define a Jurassic to Early Cretaceous age of 167 ± 37 Ma for the altered volcanic suite. K-Ar dates of 90.5 ± 3.2 Ma and 81.4 ± 3 Ma are interpreted as reset by Coast Plutonic Intrusions surrounding the volcanic pendant and a later deformational or undocumented intrusive event. Massive sulphides associated with felsic volcanic rocks on Britannia Ridge are similar to volcanogenic massive sulphide deposits described in the Canadian Archean greenstone belts and the Miocene Kuroko deposits of Japan.

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