Geology of Northern Vancouver Island: Preliminary Compilation
(NTS 92L/5, 6, 11, 12; 102I/8, 9)

BCMEMPR Open File 1997-13

Compiled by: G.T. Nixon, J.L. Hammack, G.J. Payie, L.D. Snyder, V.M. Koyanagi, J.V. Hamilton, A. Panteleyev, N.W.D. Massey, J.W. Haggart, D.A. Archibald

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Open File 1997-13 is a coloured geological map of northern Vancouver Island (92L/5,6,11,12; 102I/8, 9), British Columbia. The map is at 1:100 000 scale and includes a legend and an inset map of the geology of the Merry Widow camp, and a table of formations and correlation chart.

The oldest rocks in the area are Upper Triassic and comprise the Vancouver Group. The lowest unit is the Carnian Karmutsen Formation, which consists of coarse plagioclase phyric basaltic lavas and aphanitic to plagioclase phyric subaerial basaltic flows, minor pillow lava, pillow breccia and hyaloclastite and thin layers of limestone. The overlying Carnian to Lower Norian Quatsino Formation is predominantly thin bedded to massive lime mudstone. The Carnian to Upper Norian Parson Bay Formation consists of a series of units: a volcanic member of mafic lapilli tuff and tuff-breccia; shale; mixed sedimentary-volcaniclastic lithologies; mainly calcareous lithologies with interbedded tuff; and argillaceous to silty lime mudstone, calcareous siltstone and mudstone. The overlying Upper Carnian to Upper Pleinsbachian Harbledown Formation is predominantly submarine intercalated pyroclastic, epiclastic and fine sedimentary rocks including lithic tuff, feldspathic tuff/wacke, volcanic breccia, tuffaceous sandstone, and siltstone. Bonanza Volcanics are of Sinemurian to Bajocian age. The volcanics are basaltic to rhyolitic, mainly subaerial flows and pyroclastic rocks with intercalated non-marine to marine pyroclastic, epiclastic and sedimentary rocks. Mappable units of mainly intermediate to felsic or predominantly felsic lavas and/or pyroclastic rocks occur. Lower Cretaceous Longarm Formation equivalents comprise thinly bedded to massive sandstone, siltstone, mudstone, pebble conglomerate, and minor coal. And lower conglomeratic facies and an upper sandstone facies comprise the overlying Coal Harbor Group. The Upper Cretaceous Blumberg Formation also has a lower conglomerate unit but has an upper dominantly lithic to arkosic wacke unit. Upper Cretaceous Nanaimo Group equivalents are arkosic to lithic wackes, conglomerate, siltstone, and minor coal. Tertiary felsic volcanic rocks are mainly andesitic to rhyolitic flows and volcanic breccia.

In the Benson River area Upper Triassic or Lower Jurassic basalt intrusions are columnar-jointed. Rocks of the Early to Middle Jurassic Island Plutonic suite are mainly diorite, quartz diorite, monzodiorite, monzonite, quartz monzonite, granodiorite, and quartz-hornblende porphyry.

Three phases of deformation have been recognized: a pre-Cretaceous event; Late Cretaceous to Tertiary transpression; and Tertiary extension. Faulting is the predominant strain-release mechanism. On the map, northwest trending through-going faults define areas that are internally segmented by northeast trending faults. Block faulting dominates the structural pattern in the map-area.

The area contains economically important porphyry copper-molybdenum-gold deposits, including the Island Copper mine, base and precious metal skarns, like those in the Merry Widow camp, and precious metal mineralization associated with epithermal acid sulphate alteration zones, like that at Mount McIntosh.