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Cariboo Regional Geology
Barkerville Terrane (Kootenay / Yukon Tanana Terranes)
Most of the property area is underlain by marine strata of the Barkerville terrane, whose age is classified broadly as Late Proterozoic to Mid-Paleozoic. It is categorized by the Geological Survey of Canada as a subdivision of the Kootenay terrane. The region was deformed by intense, complex, in part isoclinal folding and overturning that produced an intimate interlensing of impure quartzite, siltstone, ankeritic dolomite, pelite and amphibolite. These rocks are cut by dikes and sills of metamorphosed diorite. Locally, stronger shear deformation produced mylonitic textures.
The northeastern third of this terrane is the main zone of economic interest in the Cariboo district. Struik described it as "gold-enriched", because it contains the historic Wells and Barkerville mines and the Cariboo Hudson deposit, 39 km and 18 km northwest of the Ace project area, respectively. This zone contains olive and grey micaceous quartzite and phyllite, amphibolite, marble, meta-tuff and meta-diorite sheets or sills. These descriptions are compatible with the rock types on the Ace project area, although the latter contains more metamorphosed felsic/intermediate volcanic rocks. Stratigraphic tops are unknown. The Barkerville terrane is cut by the Mid-Devonian Quesnel Lake gneiss (350 Ma), a coarse grained, leucocratic, biotite granitic gneiss with megacrysts of potassium feldspar. The main body of gneiss is 30 km long by 3 km wide and is elongated parallel to the eastern border of the Intermontane belt. Its contacts are in part concordant with, and in part perpendicular to, metamorphic layering. The Barkerville terrane hosts folded, sill-like masses up to 300 m thick of gneissic meta-diorite (400 Ma) and contains post-metamorphic anatectic pegmatite (86 Ma), particularly in a high-grade metamorphic aureole northwest of the North Arm of Quesnel Lake.
Cariboo Terrane
The northeastern part of the Little River area is underlain by marine peri-cratonic sedimentary strata of the Cariboo terrane. The Cariboo terrane consists mainly of limestone and dolomite with lesser siliceous, clastic, sedimentary rocks and argillite. Some geologists believe that the Cariboo terrane is a shallow, near-shore facies and the Barkerville is a deeper, offshore facies of the same erosion-deposition system. No rifting is suspected between the Cariboo terrane and the North American continent, in contrast to that between the Barkerville terrane and the North American continent.
The Cariboo and Barkerville terranes are separated by the regional Pleasant Valley thrust fault, which dips northeast moderately to steeply. It is reported by Struik (1988) to have moved the Cariboo block from the east over the Barkerville block along a strike length of over 100 km. In the map area, the fault cannot be found, suggesting that much of the movement attributed to it may have occurred by shearing in a broad zone along the "contact" between the two terranes.
Some of the carbonate layers in the lowest part of the Cariboo terrane (or upper part of the Barkerville terrane) are enriched in zinc and lead. Since the 1970's, preliminary exploration on stratiform Zn-Pb targets has been conducted in this area over a strike length of 23 km from the vicinity of the head of the North Arm via Maeford Lake to the Cariboo (Maybe) prospect.
The Cariboo terrane was cut by the Jurassic-Cretaceous Little River stock, a medium-grained granodiorite grading to quartz monzonite. A normal fault along its southwest side (Little River fault) dips east and extends southeasterly to Limestone Point, on the western side of the North Arm of Quesnel Lake. It intersects, and in some literature has been confused with, the Pleasant Valley thrust. It moved chlorite-biotite metamorphic grade strata of the Cariboo terrane eastward to rest against staurolite-kyanite metamorphic grade strata of the Barkerville terrane.
Quesnel Terrane
A small southwestern portion of the Little River area is underlain by the Late Triassic to Early Jurassic, allochthonous Quesnel terrane. It was accreted to the North American continent, in part by subduction and in part by obduction. The Eureka thrust fault marks the boundary between the Quesnel and Barkerville terranes as well as that between the Intermontane and Omineca physiographic belts. The terrane is partly submarine and partly subaerial, consisting of volcanic and volcaniclastic rocks and comagmatic intrusions, with minor carbonate lenses and related sedimentary rocks. Regionally, it hosts many important mineral deposits, mainly of Cu and Cu-Au, such as Highland Valley, Craigmont, Copper Mountain, QR and Mt. Polley. The Bullion Pit, from which 175,700 oz. of placer gold were produced, is near Likely just on the west side of the boundary between the Barkerville and Quesnel terranes.
Slide Mountain Terrane
Rocks of the allochthonous, Devonian to Late Triassic, Slide Mountain terrane underlie a very small part of the Little River area. Portions of these rocks were obducted, while others were subducted during collision of an oceanic plate with the continent. It is exposed east of Wells and Barkerville as the upper plate overlying the generally low-angle Pundata thrust fault. This fault it is nearly vertical where it crosses the southwestern part of the Little River area. Small slices of mainly mafic volcanic rocks and alpine-type ultramafic rocks of the Slide Mountain terrane occur in and parallel to the Eureka thrust. Minor lithologies include chert, meta-siltstone and argillite.
Quesnel Lake Gneiss
The name 'Quesnel Lake Gneiss' (QLG) has been applies to numerous intrusions along the western margin of the Barkerville subterrane between Cariboo Lake and the southern arm of Quesnel Lake. These intrusions have been interpreted to represent the northern extent of Mid-Paleozoic intrusive activity, which is found sporadically along the western margin of the Kootenay terrane. The QLG has been described as a series of sill-like intrusions that are similar to the Mount Pegasus and Boss Mountain gneisses. In general, individual occurrences of the QLG vary in size from 1 to 100 km2 and appear to be intruded along the tectonic boundary between the Barkerville subterrane and the Slide Mountain terrane (Ferri et al., 1999).
Rees (1987) noted a definite spatial variation in composition and texture, by which the suite of intrusions can be separated into an eastern and a western portion. Both portions display a flattened fabric and associated lineation, which can be amplified through the occurrence of very large feldspar phenocrysts. Ferri et al. (1999) noted that feldspar crystals ranging up to 5 cm comprise up to 30% of the rock and characterize the western portion of the QLG. Quartz commonly is recrystallized, but also form ribbons up to several centimetres long. In general, the matrix of the western portion of the QLG is composed of quartz, plagioclase, K-feldspar, muscovite, biotite, and trace garnet. Some metasomatic effects such as chloritization of biotite and sericitization of K-feldspar. The eastern portion of the QLG is typically deficient in feldspar megacrystals, but plagioclase crystals up to 0.5 cm across comprise 5-15% of the rock. This portion of the QLG is more homogeneous than the western portion and contains prominent xenoliths of country rock. Its matrix consists of quartz, plagioclase, microcline, muscovite, biotite, and clinozoisite.
A sample from the Frank Creek area yielded an age of 357.2±1.0 Ma (U-Pb in zircon). Based on the upper intercepts to two chords that extend from the magmatic age of the QLG, this sample also has an inherited component of age 2.4 to 2.7 Ga. This new U-Pb age date agrees with previous U-Pb geochronology that bracketed the emplacement of the QLG between 335 and 375 Ma (Mortensen et al., 1987). A Rb/Sr age date of 351±70 Ma was obtained from phyllosilicates from the eastern portion of the QLG (Rees, 1987). The agreement in ages from the two portions of the QLG suggests that they are genetically related (Ferri et al., 1999).
Volcanogenic Massive Sulphide Deposits in Similar Terranes
Conformable, semi-massive to massive sulphide deposits of the Besshi type occur in the Kootenay terrane (Goldstream deposit), in the Yukon-Tanana and Nisling terranes, and in the Klondike schist. Recent government work (Hoy and Ferri, 1998) suggests that these may all be part of an elongate terrane that also includes the Barkerville subterrane. Host rocks are deformed complexly and metamorphosed to micaceous quartzite, phyllite, and schist, commonly graphitic. Marble and meta-volcanic rock lenses are common. Besshi-type deposits contain pyrite, pyrrhotite, magnetite; and chalcopyrite, locally sphalerite, and rarely galena. Host rocks are mainly sedimentary, commonly siltstone, quartzite, and carbonaceous schist associated with amphibolite (metamorphosed mafic volcanic rocks).
The regional geology was described by L.C. Struik (1988). The Barkerville terrane is considered to be the northwest extension of the Kootenay terrane, which to the southeast overlies the Monashee metamorphic core complex, a large uplifted mass of high-grade paragneiss, quartzite and marble. The properties are on the flank of the northern, unexposed portion of this core complex. Northwest from the North Arm of Quesnel Lake the characteristic metamorphic minerals change from sillimanite through staurolite-kyanite, almandine garnet and biotite to chlorite northwest of the Ace claims. The garnet isograd runs northerly across the east-central part of the Ace group, while that of biotite is 30 km further northwest. Historic mines near Wells and Barkerville are in rocks of the greenschist facies. The age of both deformation and metamorphism is regarded as Mid-Jurassic, which is interpreted as the time of collision of the North American plate to the east with a group of island arcs to the west. In the Little River area, four geological terranes are represented, most of which are dominated by marine sedimentary or metasedimentary rocks.
Glaciation and Glacial Deposits
The last glacial stage that affected the Quesnel Highland, the Fraser glaciation, began 30,000 years ago. Much of this ice had melted by 10,000 years ago, but small remnants are preserved high in the alpine areas of the Cariboo Mountains. At lower elevations, glaciers of this age scoured the debris left by preceding ice advances, almost completely destroying them, leaving a chaotic assemblage of unsorted till, moraine and drift, with lenses of gravel and sand that had been roughly sorted by meltwater and rivers, leaving behind beds of silt and clay that were stratified by settlement in ice-dammed lakes. In the Cariboo area, the debris covers bedrock in valleys below 1700 m, leaving typical glacial features such as U-shaped valleys, ice-sculpted drumlins, moraine terraces and glacier and river benches. On the Barker Minerals properties, glacial deposits range from one to a few tens of metres thick. Some glacial till deposits are overlain by well-bedded glaciolacustrine clay and silt deposits up to a few tens of metres thick.
In much of the Cariboo district, a layer of distinctive, hard, compact, semi-rigid blue clay sits either on or slightly above bedrock and acts as "false" bedrock. It was formed from glacial drift left behind by the last ice advance prior to the Fraser glaciation and was compacted by the weight of the Fraser stage ice. In the placer-gold areas of the Cariboo, large amounts of gold were recovered from gravel resting on this clay. In places the clay layer was penetrated by the placer miners to reach richer "pay streaks" on true bedrock below.
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