Geology and Mineral Resources of the Cowichan Lake Area
Paper 1992 - 3
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Includes Geoscience Map 1991-2
The Cowichan Lake map area is centred on north-northwest trending Cowichan Lake, 27 kilometres west of Duncan, on Vancouver Island, British Columbia. The area lies at the southeastern end of the Vancouver Island Ranges and is characterized by fairly rugged topography with fault-line scarps and fault-controlled valleys, accentuated by glaciation.
The area is divided into two regions of differing structural style by a major thrust fault running along the north side of Cowichan Lake. The northern region is underlain by Paleozoic rocks forming the southwest limb of the Cowichan uplift, one of a series of major geanticlinal structures constituting the structural fabric of the Wrangellia terrane of Vancouver Island. South of Cowichan Lake, Mesozoic sequences form a syncline-anticline pair that parallels the Cowichan uplift and plunges to the northwest.
The oldest rocks in the Cowichan Lake area belong to the Paleozoic Sicker and Buttle Lake groups which contain volcanic and sedimentary units ranging from Middle Devonian to Lower Permian age. The Devonian Sicker Group is a thick package of lower greenschist facies, metavolcanic and volcaniclastic rocks that formed in an oceanic island-arc environment. The lowest unit is the Duck Lake Formation which, in the Alberni -Nanaimo Lakes area, comprises a suite of grey to maroon and green pillowed basalts and basaltic breccias with chert, jasper and cherty tuff interbeds near the top of the sequence. This unit has not been positively identified in the Cowichan Lake area, although a narrow band of green-grey pillowed basalt, about 1 kilometre southwest of Heather Lake, is tentatively assigned to the Duck Lake Formation.
Overlying the Duck Lake Formation is the Nitinat Formation, characterized by pyroxene-feldspar-porphyritic basalts and basaltic andesites. These typically occur as agglomerates, breccias, lapilli tuffs and crystal tuffs that formed as pyroclastic flows, debris flows and lahars. Pyroxene-phyric, amygdaloidal, pillowed and massive flows are also developed.
South of the Chemainus River fault in the Cowichan Lake map area, the Nitinat Formation is overlain by a sequence of interbedded volcaniclastic sediments and pyroclastic rocks of the McLaughlin Ridge Formation. A variety of lithologies are developed including thickly bedded, massive tuffite and lithic tuffite; interbedded, laminated sandstones, siltstones and argillites; heterolithic volcanic breccias and lapilli tuffs, commonly mafic to intermediate in composition; and feldspar crystal tuffs and lapilli tuffs. The sequence probably formed in the transition between the proximal volcanic-dominated facies and the distal volcaniclastic apron facies around a volcanic island. North of the Chemainus River fault, the mafic to felsic volcanics of the proximal volcanic facies dominate the McLaughlin Ridge Formation. The volcanics are predominantly intermediate pyroclastics, commonly feldspar crystal-lapilli tuffs, heterolithic lapilli tuffs and breccias, and minor pyroxene-phyric lapilli tuffs. Felsic crystal and dust-tuffs interfinger with andesitic lapilli tuffs and breccias at the eastern edge of the map. The felsic rocks appear to be stratigraphically high within the formation.
A distinctive maroon, schistose heterolithic breccia and lapilli tuff with minor jasper is the uppermost unit and is overlain conformably by thinly bedded cherty sediments of the Fourth Lake Formation of the Buttle Lake Group. A suite of greenstone dikes, informally called the "older dikes", intrudes the felsic volcanics and maroon breccia. The age of these dikes is unknown, but may be contemporaneous with minor volcanics in the lower Fourth Lake Formation. The Nitinat and McLaughlin Ridge formations form a coherent suite of medium-potassium calcalkaline chemistry typical of island arcs. In contrast, the older dikes are high iron-titanium tholeiites with an affinity to transitional or enriched ocean-floor basalts.
The Buttle Lake Group is made up of a dominantly epiclastic and bioclastic limestone sedimentary sequence ranging from Mississippian to Early Permian in age. Within the Cowichan Lake area, the Buttle Lake Group is either in fault contact with, or rests unconformably on the Sicker Group. The Fourth Lake Formation comprises mostly thin-bedded, often cherty sediments.
South of the Chemainus River, the base of the formation is marked by a sequence of radiolarian ribbon cherts and cherty siltstones 100 to 200 metres thick, informally called the Shaw Creek member. This passes upwards into monotonous thinly bedded, sometimes cherty, turbiditic sandstone-siltstone-argillite intercalations. Thicker beds of sandstone, granule sandstone, breccia and conglomerate are exposed to the south of the Chemainus River. The ribbon cherts of the Shaw Creek member are absent north of the river, where thinly bedded turbiditic clastic sediments conformably overlie the McLaughlin Ridge volcanics and dominate the sequence. Minor volcanism was synchronous with early Fourth Lake Formation sedimentation in the Mount Whymper - Rheinhart Creek area. This ‘Mount Whymper suite’ consists of aphyric amygdaloidal basalt flows, interbedded with cherts and cherty sediments and olive-green, amygdaloidal, aphyric dacitic flows interbedded with maroon and green chert, jasper, magnetite-jasper and cherty sediments. The basalts are undersaturated and enriched in both compatible and incompatible trace elements, with a within-plate petrotectonic affinity.
The Mount Mark Formation conformably overlies the Fourth Lake Formation, although outcrops of the formation are not common in the Cowichan Lake area. It consists of well-bedded bioclastic calcarenite with porcellaneous micrite and tuffaceous limestone interbeds and thinly bedded black cherts.
Rocks of the Upper Triassic Vancouver Group outcrop both north and south of Cowichan Lake. They form the core of the Seymour Range anticline in the south and Karmutsen Formation basalts unconformably overlie the Paleozoic sequences north of Cowichan Lake. The group is subdivided into a thick lower basaltic volcanic package (Karmutsen Formation) and a thin upper sedimentary package (Quatsino and Parson flay formations). The lower Karmutsen Formation basalts rest unconformably on the underlying Paleozoic rocks. The basalts form pillowed flows, pillow breccias and hyaloclastite breccias interbedded with massive flows and sills. There is a tendency for the massive flows to dominate the sequence towards the top and the pillowed flows in the lower parts. Lithologically, the flows are dark grey, variably feldspar phyric basalts. Glomeroporphyritic flows and hyaloclastite breccia are commonly seen at the top of the pile. The Karmutsen basalts show amygdule infillings and alteration assemblages typical of the prehnite-pumpellyite facies. The mafic bodies of the Mount Hall gabbro, intrusive into the Paleozoic rocks, are coeval and consanguineous with the Karmutsen basalts. The basalts formed from an iron-titanium enriched tholeiitic magma, similar to continental tholeiite or enriched mid-ocean ridge basalt, probably in an oceanic flood-basalt province.
The Quatsino Formation is characterized by massive, thickly bedded, black micritic limestone, essentially. Unfossiliferous, though bioclastic micrite, oolitic limestone, calcirudite and calcarenite may occur locally. The formation is conformably overlain by thinly bedded sediments and tuffs, provisionally correlated with the Parson Bay Formation. This sequence comprises tuff and tuffaceous sandstone overlain by flaggy limestones and black limy argillites, which grade vertically into thinly bedded argillites with minor fossiliferous limestone interbeds. Flaggy, sandy limestone and bio thermal limestone ascribed to the Sutton limestone member of the Parson Bay Formation outcrop on the south shore of Cowichan Lake.
The Bonanza Group overlies the Vancouver Group sediments with a slight angular unconformity, only readily detectable from the regional distribution of rock units. The unconformity cuts down section and may result in thinning or elimination of the Parson Bay Formation. The bulk of the Bonanza Group consists of maroon to green-grey, feldspar-phyric basalt and andesite flows, lapilli and crystal tuffs, feldspar-hornblende andesite flows, dacite and felsic lapilli tuff, and various minor basalt, andesite and dacite dikes. Tuffaceous and epiclastic sediments are found interbedded with lapilli and crystal tuffs, within the basal part of the sequence. The Bonanza Group evolved in a convergent-margin setting as an arc on the Paleozoic and Triassic transitional crust.
All of the Paleozoic and Triassic sequences have been intruded by granodioritic stocks of the Early to Middle Jurassic Island Plutonic Suite. Stocks north of Cowichan Lake have an elongate outcrop pattern, often with different stratigraphic units on either side, as with the Mount Buttle-Meade Creek stock. Stocks intruded into the Mesozoic sequences south of Cowichan Lake are more rounded in outcrop shape. The intrusions are dominantly equigranular quartz diorite to granodiorite but show considerable lithological variation. Most of the large intrusive bodies are rich in mafic inclusions, especially in marginal agmatitic intrusive breccias. Contact metamorphic aureoles are developed around the intrusions, causing hornfelsing and skarning in Paleozoic rocks. A variety of dikes and small irregular intrusions, that are probably coeval with the Island Plutonic Suite, occur throughout the area. Lithologically they include intermediate feldspar porphyry, feldspar pyroxene porphyry, hornblende feldspar porphyry and minor diabase. The Jurassic intrusions are coeval with the Bonanza volcanics and form a metaluminous, medium to high-potassium calcalkaline suite typical of a convergent-margin environment.
Clastic sediments of the Upper Cretaceous Nanaimo Group lie unconformably on the older rocks. They outcrop mainly around the shores of Cowichan Lake, but are also preserved in fault-controlled valleys to the north of the lake. The lower Benson Formation comprises basal cobble and boulder conglomerates and overlying medium to coarse-grained sandstones. These are succeeded by black argillites and siltstones of the Haslam Formation. Younger formations of the Nanaimo Group are absent.
Southern Vancouver Island has a complex structural history with frequent rejuvenation of pre-existing structures. All Paleozoic rocks are affected by a series of southeast trending, upright to overturned, southwestverging folds. Penetrative axial planar foliation is absent throughout most of the area, except to the west of Mount Whymper and north of the Chemainus River fault, where foliation (schistosity in volcanics and cleavage in sediments) is well developed, trending north-northwest with generally steep northeasterly dips. Lineations plunge gently, up to 15 degrees, and may be to the west-northwest or east-southeast. Regional-scale warping of Vancouver Island occurred during the Early to Middle Jurassic, facilitating emplacement of the Island Plutonic Suite intrusions and producing the geanticlinal Cowichan uplift. North of Cowichan Lake, the present map pattern is dominated by the northwesterly trending contractional faults of the Tertiary Cowichan fold and thrust system. These are high-angle reverse faults, which become listric at mid-crustal levels. They generally place older rocks over younger. A footwall syncline is developed in the thicker Nanaimo Group section beneath the Cowichan fault at the east end of Cowichan Lake. The deformation probably took place during the crustal shortening accompanying formation and emplacement of the Pacific Rim and Crescent terranes outboard of Wrangellia.
The Cowichan Lake area has had a somewhat intermittent history of mineral exploration since about 1900. Production has been limited to small quantities of copper from the Blue Grouse mine and manganese and rhodonite from Hill 60. The localization of metal deposits in the area is controlled by the interplay of stratigraphy and spatial association with later intrusions and structures. Three major metallogenic epochs are recognized. Syngenetic mineralization occurred during the building of the Sicker arc. Oxide facies exhalites, such as the ‘900 zone’ of the Mineral Creek area, are found in the uppermost Duck Lake Formation in the adjacent Alberni – Nanaimo Lakes area. Sulphide facies equivalents are also found, though less commonly. Thin syngenetic manganese oxide beds and sulphidic argillites occur within the radiolarian cherts of the basal Fourth Lake Formation in the upper Shaw Creek area.
The Early to Middle Jurassic arc was characterized by epigenetic mineralization of various types and styles, spatially related to the Island Plutonic Suite intrusions. Copper-molybdenum veins and stockworks occur within intrusions and volcanic country rocks. Rhodonite forms by contact metamorphism of manganiferous chert. Iron-copper-gold skarns are developed in calcareous tuffs and limestones of the Karmutsen and Quatsino formations, though are rare in Mount Mark lithologies.
Mesothermal gold-bearing quartz-carbonate veins are located along Tertiary structures and have been one of the main exploration targets in the area.
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