Three allochthonous terranes, igneous rocks of the Cretaceous and Tertiary Coast Plutonic Complex and associated volcanic complexes comprise the geology between Whitehorse and the Yukon-BC border. The Paleozoic and older(?) Nisling Terrane is composed of the quartz-rich Nisling assemblage, the carbonaceous Nasina assemblage and orthogneiss assumed to be Devono-Mississippian in age. The Nakina sub-terrane of the northern Cache Creek Terrane is composed of Mississippian to Permian spilitized basalt, ultramafite, chert and limestone characterized by Tethyan faunal assemblages and dramatic facies variations. The northern Stikine Terrane is composed of the Upper Triassic Lewes River arc and its plutonic roots. Upper Triassic to Middle Jurassic sedimentary rocks of the Lewes River and Laberge groups collectively comprise the Whitehorse Trough overlap assemblage. Basal augite-phyric volcanic rocks of the Lewes River Group (Povoas Fm) are unconformably overlain by sedimentary debris derived from the eroding arc (Aksala Fm). These rocks are disconformably overlain by Laberge Group coarse clastics (Takwahoni Fm) deposited in laterally discontinuous submarine fans and conformably overlain by distal fine-grained equivalents (Inklin Fm). Siliciclastic rocks of the Tantalus Formation were deposited in a variety of marine environments in a successor basin to the Whitehorse Trough during Upper Jurassic and Late Cretaceous(?) times. Plutonic rocks cover approximately 40% of the project area and comprise over 30 individual bodies with a wide range of compositions. Plutonic rocks have been divided into six chrono-lithologically distinct suites: Late Triassic batholiths of the Klotassin plutonic suite; Early Jurassic syn-tectonic(?) foliated diorite; mid-Cretaceous granodiorite of the Whitehorse plutonic suite; mid-Cretaceous granophyric quartz monzonite of the Mount McIntyre plutonic suite; a poorly defined suite of Late Cretaceous intrusions with associated volcanism; and quartz-rich high-level Late Paleocene/Early Eocene intrusions of the Nisling Range Plutonic Suite which form the plutonic roots to Skukum Group volcanism. The Klotassin suite is assumed to form the plutonic roots to the Lewes River arc, but are isotopically dissimilar. The Bennett Granite provides a Late Triassic link between Nisling Terrane and Lewes River arc. Cretaceous and Tertiary intrusions are peraluminous and comprise the Coast Plutonic Complex. Isolated but widespread accumulations of intermediate mid- and Late Cretaceous volcanic rocks form the Mt. Nansen and Carmacks groups. Four Late Paleocene/Early Eocene volcanic complexes of the Skukum Group are aligned in a northerly trend and represent deeper levels of erosion to the north. Skukum Group activity is also represented by numerous northeast-trending rhyolite dyke swarms representative of a transtensional regime. Strata of the Whitehorse Trough were deformed into a set of open to tight, northwest-trending folds during Middle and(?) Latest Jurassic/Early Cretaceous compressional event(s). In northern Cache Creek Terrane, much of the deformation took place during Middle Jurassic terrane accretion, but Late Cretaceous southerly verging thrust faults are documented. Nisling Terrane metasedimentary rocks were metamorphosed prior to the intrusion of the Late Triassic Bennett Granite. The Tally Ho shear zone is a narrow domain of penetrative, ductile deformation with evidence of Late Triassic, sinistral strike-slip displacement. It strikes northwest and dips steeply and marks the easternmost limit of Nisling Terrane exposures. It may represent a terrane boundary between the Lewes River arc and the Nisling Terrane; alternatively it may be a transpressional structure within the arc itself.

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The Southwest McQuesten-northern Carmacks area is primarily underlain by rocks of the Yukon-Tanana terrane which is divided into two distinct belts separated by the Willow Creek fault: 1) a central belt of polydeformed, upper greenschist-amphibolite facies metasedimentary and metaplutonic rocks of Permian and older ages; and 2) a northeastern belt of generally undeformed and unmetamorphosed volcano-plutonic rocks of the Early Mississippian Reid Lakes complex. The southern part of the area is underlain mainly by rocks of Quesnellia and Stikinia, including: 1) Paleozoic retrogressed metamorphic rocks of the Boswell assemblage; 2) Upper Triassic augite-phyric volcanic rocks; and 3) Early Jurassic granitoids of the Aishihik plutonic suite. These rocks are dissected by a series of dextral strike-slip faults, probably related to the Teslin fault system. Post-accretion rocks include: 1) mid-Cretaceous biotite monzogranite plutons; 2) dacite and minor basalt of the Upper Cretaceous Carmacks Group; and 3) Quaternary basalt of the Selkirk volcanics. The southwest McQuesten-northern Carmacks area is under-explored, but shares many geological attributes with nearby, highly prospective districts such as the Dawson Range mineral belt, the recently discovered White Gold area and the producing Minto Mine.

The map area spans the western edge of the old North American continent and the terranes that accreted to it in Mesozoic time, which together form a generally west-dipping structural stack, separated by layer-parallel thrust faults. Three major elements are represented: 1) tilted strata of the ancient North American margin (Cassiar Terrane), 2) overthrust rocks that originated as volcanic arcs and off-shore sediments (Yukon-Tanana Terrane), and 3) granitic bodes injected into this thickened crust after the Cordilleran orogeny.

The Irvine Lake and Gravel lake map-areas (NTS 105B/10,11) lie within the northern Omineca Belt, west of the Tintina-Northern Rocky Mountain Trench (NRMT) fault. The eastern part of the area is underlain by Proterozoic to early Paleozoic metasedimentary rocks of Cassiar terrane, a fragment of the North American miogeocline which has been displaced northward on the Tintina-NRTM fault. The western part of the area is underlain by basaltic metavolcanics, serpentinized ultramafic rocks, metagabbro, and cherty and calcareous metasediments of the Slide Mountain terrane. Unfoliated to weakly foliated granitic intrusives (Marker Lake and Cassiar batholiths and Cabin Creek and Gravel Creek stocks) occur throughout the area intruding both the Cassiar and Slide Mountain terranes. Slide Mountain and Cassiar terranes are juxtaposed by an east-verging thrust referred to in this area as the Zak fault. Southwest of Irvine Lake, the thrust places serpentine, basaltic meta-volcanics, and an undeformed dioritic intrusion onto a footwall consisting of the Proterozoic Tsaydiz Formation and older units. Northwest of Irvine Lake, near Shootamook Creek, the thrust places cherty metasediments of the allochthon onto marble and quartzite inferred to be lower Cambrian Rosella and Boya Formations, respectively. The northern end of the Cassiar batholith extends into the southwestern corner of Irvine Lake map area. Its northeastern contact with rocks of Slide Mountain terrane is a subvertical, northwest-southeast trending mylonite zone several tens of metres wide. Mesoscopic structures including S-C fabrics and shear bands prove dextral displacement parallel to a variably plunging, but commonly sub-horizontal stretching lineation. The mylonite zone lies along a pronounced topographic lineamnet which extends from the trace of the Cassiar fault south of the Alaska Highway northwestwardly into the Irvine Lake map-area rather than veering to the west as previously mapped. Mineral occurrences in this area are primarily near the contact of granitic intrusions and carbonate rocks. Carbonate rocks hosting the deposits belong to the upper Proterozoic Ingenika Group (Swannell, Tsaydiz and Espee formations) rather than the Lower Cambrian Atan Group as has been inferred for nearby deposits in the Rancheria district. Other, non-carbonate-hosted mineral occurrences include a porphyry Mo prospect and Ag, Pb, Zn veins.

Includes a geological cross section, mineral occurrences and isotopic age dates.

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