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Ministry of Energy Mines and Responsible for Core Review

Geology and Mineral Resources of the Duncan Area (NTS 092B/13)

Paper 1992 - 4

by N.W.D. Massey, 1995

 

View PDF (52 MB)

Includes Geoscience Map 1991-3

 

The Duncan map area lies about 50 kilometres northwest of Victoria on southern Vancouver Island and includes parts of the Gulf Islands.  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 straddles the eastern end of the Cowichan uplift, one of a series of major geanticlinal structures constituting the structural fabric of Vancouver Island.

 

The oldest rocks in the Duncan 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 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.

 

In the Duncan map area, this unit is only found in the Bear Creek area, south of the Cowichan River.

 

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 tufts 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 is overlain, apparently conformably, by a heterogeneous sequence of mafic to felsic volcanics and volcaniclastic sediments of the McLaughlin Ridge Formation.  The volcanics are predominantly intermediate pyroclastics, commonly feldspar crystal-lapilli tufts, heterolithic lapilli tufts and breccias and minor pyroxene-phyric lapilli tuffs.  A thick package of felsic quartz-crystal, quartz-feldspar-crystal and fine dust-tuffs is developed in the Chapman Creek-Mount Sicker-Mount Richards area.  It thins to the west, where it interfingers with andesitic lapilli tufts and breccias.  The uppermost unit is a distinctive maroon schistose heterolithic breccia and lapilli tuff with minor jasper, exposed in the Chapman Creek area.  

 

Coeval, and probably consanguineous, with the felsic volcanics in the McLaughlin Ridge Formation is a suite of granodiorite stocks and quartz porphyry dikes collectively known as the Saltspring Intrusive Suite.  The Sicker Group volcanic rocks and associated intrusions form a coherent suite of medium-potassium calcalkaline chemistry typical of an island-arc.

 

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 Duncan area, the Buttle Lake Group is most often found in fault contact with the lower volcanic units of the Sicker Group.  However, it is conformable on the McLaughlin Ridge Formation south of Sansum Point and in the Chapman Creek area and on the southwest limb of the Cowichan uplift, is unconformable on the lower volcanics.

 

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.  The ribbon cherts of the Shaw Creek member are absent north of the Chemainus 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 Coronation Mountain area. This 'Coronation Mountain suite' consists of hornfelsed, amygdaloidal diabasic flows, which are generally massive but occasionally pillowed, and interbedded cherty tufts and sediments.  The basalts are slightly undersaturated olivine tholeiites or transitional basalts, with somewhat enriched incompatible trace-element contents akin to ocean-island tholeiite or enriched ocean-floor basalt.

 

The Mount Mark Formation conformably overlies the Fourth Lake Formation, although outcrops of the formation are restricted to the Fairservice Mountain -Bear Creek area, south of the Cowichan River. It comprises massive and laminated crinoidal calcarenites with chert and argillite interbeds.

 

Basalts of the Karmutsen Formation are only exposed in the extreme southwest corner of the map area, on the west side of the Chanlog fault.  They are comagmatic with the extensive mafic bodies of the Mount Hall gabbro, intrusive into the Paleozoic rocks.  These intrusions are medium to coarse-grained diabase, gabbro and leucogabbro with minor diorite, commonly porphyritic with feldspar phenocrysts often forming glomeroporphyritic clusters.  Thick gabbro bodies under Mount Hall and Coronation Mountain show layering of porphyritic and nonporphyritic lithologies.  The basalts, diabases and gabbros formed from an iron-titanium enriched tholeiitic magma, similar to continental tholeiite or enriched midocean ridge basalt, probably in an oceanic flood-basalt province.

 

The Paleozoic and Triassic rocks have been intruded by granodioritic bodies of the Early to Middle Jurassic Island Plutonic Suite.  With the exception of the large Ladysmith pluton, these granodiorite intrusions are elongate in surface outcrop shape, paralleling the overall structural grain of the area.  The dominant lithology is a medium to coarse-gained, equigranular granodiorite to quartz diorite with a characteristic 'salt-and-pepper' texture.  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, hornblende feldspar porphyry and minor diabase.  The Jurassic intrusions are coeval with the Bonanza volcanics, which outcrop in the Cowichan Lake area to the west.  They 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 Maple Bay to Mount Prevost area, the Cowichan and Chemainus River valleys and the shoreline from Crofton to Ladysmith.  The lower Benson Formation comprises basal cobble and boulder conglomerates and overlying medium to coarse-grained sandstones.  These are succeeded by the black argillites and siltstones of the Haslam Formation.  

 

Pebble and cobble conglomerates of the Extension Formation conformably overlie Haslam Formation argillites on top of Mount Prevost and along the Chemainus River.  Grey, medium to coarse-grained sandstones are interbedded with and overlie the conglomerates.  Argillites of the Fender Formation, overlying the Extension Formation, are exposed in the incised gorge along the Chemainus River just below its confluence with Chipman Creek.  Younger formations of the Nanaimo Group are absent.

 

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. Penetrative axial planar foliation is absent throughout most of the area, except north of the Fulford fault, where foliation (schistosity in volcanics and cleavage in sediments) is well developed and trends north-northwest with generally steep northeasterly dips.  Lineations plunge gently, up to 15 degrees, to the west-northwest or east-southeast.  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.  Mesoscale footwall folds and minor imbricate faults are developed along most of the thrusts, particularly where thicker sections of Nanaimo sediments occur in the footwall, as in the Mount Prevost area where a deformational zone up to 1.5 kilometres wide occurs beneath the Fulford fault.  The deformation probably took place during the crustal shortening accompanying the formation and emplacement of the Rim and Crescent terranes outboard of Wrangellia.

 

The Duncan area has had a somewhat intermittent history of mineral exploration since the late nineteenth century.  'Three mines on Mount Sicker (Lenora, Tyee and Richard III) formed the foci for one of the largest townsites in the area at the turn of the century.  However, only sporadic production has ensued since then.  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. Kuroko-style massive sulphides are associated with felsic volcanics in the upper part of the McLaughlin Ridge Formation.

 

They occur in a belt extending from Saltspring Island to Rheinhart Creek, bounded to the south by the Fulford fault and appear to have formed close to the volcanic centre located in the Duncan - Saltspring Island area.  Jasper and oxide-rich cherts occur within the volcanics of the Nitinat and McLaughlin Ridge formations but appear to have negligible economic mineralization.  Thin syngenetic manganese oxide beds and sulphidic argillites occur within the radiolarian cherts of the basal Fourth Lake Formation.

 

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.  Rhodonite forms by contact metamorphism of manganiferous chert.

 

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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