Paper 1992 - 2: Geology and Mineralization of the Alberni - Nanaimo Lakes Area
(092F/01W , 02E)
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Includes Geoscience Map 1991-1
Paper 1992-2 covers the geology and mineral potential of the Alberni - Nanaimo Lakes map area, which is located on Vancouver island, to the east of Alberni Inlet and to the southeast of Port Alberni in southwestern 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 that were accentuated by glaciation. The area covers the northwestern end of the Cowichan uplift, one of a series of major geanticlinal structures constituting the structural fabric of the Wrangellia terrane of Vancouver Island.
The oldest rocks in the Alberni - Nanaimo Lakes area belong to the Paleozoic Sicker and Buttle Lake groups which contain volcanic and sedimentary units ranging from Middle Devonian to Early Permian age. The Devonian Sicker Group is a thick package of lower greenschist metavolcanic and volcaniclastic rocks that formed in an oceanic island-arc environment. The lowest unit is the Duck Lake Formation which 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. Well-bedded felsic tuffs and lapilli tuffs are associated with the cherts and jaspers. Massive dacite-rhyolite dikes and sills intrude the pillowed basalts. The pillowed basalts can be divided into two subunits on the basis of geochemistry. The apparently lowermost flows are tholeiitic with an affinity to enriched-type mid-ocean ridge basalts and probably represent the oceanic substrate upon which the Sicker arc developed. The uppermost lavas, and dacite intrusions, are of high-potassium calcalkaline chemistry and mark the initiation of arc construction. These two suites were not recognized nor distinguished in the field.
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.
The Nitinat Formation passes upwards transitionally (over a thickness of about 150 metres ) into the McLaughlin Ridge Formation, a sequence of volcaniclastic sediments dominated by thickly bedded, massive tuffites and lithic tuffites, interbedded with thinly bedded tuffites and laminated tuffaceous sandstone, siltstone and argillite. The beds tend to form fining-upward cycles from tuffite to argillite, but overall the sequence becomes coarser towards the top with more frequent development of lithic tuffite and coarse pyroclastic horizons. The sequence probably formed as a volcaniclastic apron around a volcanic island and grades eastwards into more proximal volcanic-dominated facies in the Duncan area. The Nitinat and McLaughlin Ridge formations form a coherent suite of sodium-potassium calcalkaline chemistry typical of island arcs.
The Buttle Lake Group is made up of a dominantly epiclastic and bioclastic limestone sedimentary sequence ranging from Mississippian to Early Permian in age. This sedimentary package is apparently conformable on the underlying volcanics along the northeastern limb of the Cowichan uplift, for example, in the upper Cameron River valley and St Mary's Lake area, but is unconformable along the southwestern limb and in the Fourth Lake area.
The Fourth Lake Formation comprises mostly thin-bedded often cherty sediments. These vary from green and red ribbon cherts, black cherty argillites, green and white cherty tuffs, grey and green siltstones and argillites, to thicker bedded green volcanic sandstones. The upper part of the formation is characterized by thinly bedded, turbiditic sandstone-siltstone-argillite intercalations, with some thicker beds of volcanic sandstone. These pass upwards into argillite-calcarenite interbeds at the top of the sequence.
The Mount Mark Formation conformably overlies and laterally interfingers with the Fourth Lake Formation. It consists of well-bedded bioclastic calcarenite and calcirudite with minor argillite and chert interbeds. The overlying St Mary's Lake Formation is sporadically preserved beneath the Triassic unconformity. It comprises clastic sediments varying from polymictic conglomerates to volcanic sandstones and argillites.
Rocks of the Upper Triassic Vancouver Group are exposed throughout the map area, flanking the Paleozoic core of the Cowichan uplift. The group is subdivided into a thick lower basaltic volcanic package (Karmutsen Formation) and a thin upper sedimentary package (Quatsino and Parson Bay 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 the lower parts. The Karmutsen Formation basalts show amydgule 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 Formation 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. Succeeding limestones, argillites and tuffaceous sediments of the Quatsino and Parson Bay formations are poorly developed in the map area.
All of the Paleozoic and Triassic sequences have been intruded by granodioritic stocks of the Early to Middle Jurassic Island Plutonic Suite. These bodies are usually elongate in shape, although the Fourth Lake stock is roughly circular. The intrusions are dominantly equigranular quartz diorite to granodiorite but show considerable lithological variation. The Corrigan pluton in particular is heterogeneous and composite, comprising a mix of diorite, quartz diorite, granodiorite and monzogranite phases with abundant minor intrusive dikes. Most of the large intrusive bodies are rich in 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, hornblende feldspar porphyry and minor diabase. The Jurassic intrusions 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 are most thickly developed in the Alberni Valley, though only exposed around the margins due to Quaternary cover. The lower Benson Formation comprises basal conglomerates and overlying medium to coarse-grained sandstones. These are succeeded by the black argillites and siltstones of the Haslam Formation. Younger formations of the Nanaimo Group are absent.
Tertiary dacite porphyries of the Mount Washington Intrusive Suite occur throughout the area. Where the magma has penetrated the Nanaimo Group sediments, it has spread out laterally to form thick sills.
Southern Vancouver Island has a complex structural history with frequent rejuvenation of previous structures. All Paleozoic rocks are affected by a series of southeast-trending, upright to overturned, southwest-verging folds. Associated schistosity and lineation are absent from most of the area, only occurring to the west of the Mineral Creek fault. Regional-scale warping of Vancouver Island occurred during the Early to Middle Jurassic, facilitating the emplacement of the Island Plutonic Suite intrusions and producing the geanticlinal Cowichan uplift. 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 that become listric at mid-crustal levels. They generally place older rocks over younger.
The deformation probably took place during the crustal shortening accompanying the formation and emplacement of the Pacific Rim and Crescent terranes out-board of Wrangellia. The north-trending Mineral Creek fault and associated northwest-trending faults, such as the Stokes fault, are subvertical with small, apparently sinistral offsets.
They may have formed during minor extension accompanying late-stage post-contractional relaxation.
The Alberni - Nanaimo Lakes area has had a long history of mineral exploration and production, starting with small-scale placer-gold mining on China Creek in 1862. 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. Sulphide facies equivalents are also found, although 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 rock. Production has been minor from these deposits but came from the Havilah and WWW mines.
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. A stratiform auriferous hematite cap has developed on the top of the skarn on the Villalta property, probably formed by residual weathering during the middle Cretaceous.
Mesothermal gold-bearing quartz-carbonate veins are located along Tertiary structures and have been one of the main exploration targets in the area. Historic production has ensued from the Victoria, Thistle and Black Panther mines. Tertiary epigenetic quartz-arsenic-(antimony) veins are variably developed in dacite porphyry sills and Haslam Formation argillites on the Coal and Grizzly properties.
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