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Geology and manganese deposits of the north shore of Cowichan Lake, Vancouver Island, B.C. Fyles, James Thomas


The north side of Cowichan Lake, Vancouver Island B.C. is underlain by volcanic rocks, sediments, and intrusives of Mesozoic age. The oldest rocks, Jurassic or Triassic flows known as the Sicker andesites, are conformably overlain by about 3000 feet of cherty tuffs, coarser pyroclastics, and small lenses of limestone known as the Sicker sediments. Cherty tuffs form the lower members of the Sicker sediments, felspathic tuffs the central members, and coarser pyroclastics the upper members. About 2000 or 3000 feet of basaltic and andesitic flows conformably overly the Sicker sediments. The Sicker andesites and sediments and younger flows are intruded by large dyke-like bodies of granodiorite or quartz monzonite, known as the Saanich granodlorite and correlated with the Coast Range intrusives. Upper Cretaceous shales sandstones and conglomerate unconformably overlie the volcanics and Intrusives. The Sicker series and overlying flows are tightly folded into overturned and asymmetrical northwesterly trending synclines and anticlines. The Cretaceous rocks are gently folded and dip north along a narrow belt on the north side of a down-faulted block. Manganese deposits occur in the lower cherty beds of the Sicker sediments as lens shaped bodies parallel to the bedding of the sediments. They are commonly in chert free from felspatnic material, and are always associated with jasper or jaspery sediments. The main manganese minerals are rhodonite, spessartite, an unidentified yellow manganese silicate, and small amounts of rhoaochrosite. Residual manganese oxides coat the surfaces of the deposits. Several features of the deposits, such as the fact that rhodonite commonly cross cuts and replaces the chert suggest that the deposits are of replacement origin. Other features such as their bedded appearance and the fact that they occur, at about the same horizon In the Sicker sediments indicate a sedimentary origin. Theoretical considerations support the view that the deposits are sedimentary and suggest that the replacement features were formed by metamorphism.

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