Paleozoic platformal and basinal strata of Cassiar Terrane are separated from rocks of Yukon-Tanana Terrane to the west by an unexposed fault in southeast Glenlyon map area. Quartzite, marble, phyllite, and amphibolite are grouped in Cassiar Terrane, and no rocks of Slide Mountain Terrane are recognized. The mid-Cretaceous Glenlyon batholith contains pendants of Cassiar Terrane and is intruded by at least five andesite dykes. West of the fault, the Yukon-Tanana Terrane includes: (1) mafic volcaniclastic rocks with preserved primary textures; (2) coarse-grained quartz-feldspar grit; and (3) metasiltstone and semi-pelitic schist. The grit is intruded by foliated hornblende granodiorite, likely of early Mississippian age. Small outcrops of tectonized serpentinite were tectonically emplaced into Yukon-Tanana Terrane, and a positive magnetic anomaly parallel to the fault suggests an unexposed extension to the southwest. Two mylonite localities and evidence of brittle cataclasis up to 1 km on either side of the presumed buried fault suggest a complex structural history along this terrane boundary.

Geological mapping in Wolverine Lake area has outlined new Yukon-Tanana Terrane stratigraphy, constrained the stratigraphic position of the Wolverine Lake volcanogenic massive sulphide (VMS) deposit, and clarified the relationship of Yukon-Tanana Terrane to the Campbell Range belt. Yukon-Tanana Terrane comprises two stratigraphic successions separated by an angular unconformity. Beneath the unconformity are polydeformed felsic and mafic meta-volcanic rocks, carbonaceous meta-clastic rocks, marble and granitic orthogneiss. The Kudz Ze Kayah VMS deposit occurs in felsic meta-volcanic rocks of this sequence. Yukon-Tanana Terrane rocks above the unconformity are deformed by only one phase of deformation and consist primarily of carbonaceous meta-clastic rocks and quartz- and feldspar-phyric felsic meta-volcanic rocks. The Wolverine VMS deposit occurs in this succession, associated with siliceous exhalite and baritic magnetite iron formation. Meta-basalt of the Campbell Range belt, included previously in Slide Mountain Terrane, overlies the upper succession of Yukon-Tanana Terrane with sharp contact. This contact has been observed at several localities and it appears depositional. There is no evidence that it is a terrane boundary fault.

Penetratively deformed rocks of the Yukon-Tanana terrane and Semenof block, and unfoliated Jurassic-Cretaceous intrusions are exposed in the southern Big Salmon Range of south-central Yukon. A newly mapped area, centred on the Boswell River, is divided into three structural panels by two regional-scale discontinuities, the Little Bear fault and the Sidney Creek fault. The panel on the northeastern side of the Little Bear fault includes units dominated by metasedimentary rocks (Slate Mountain succession), mafic volcanic and volcaniclastic rocks (Wiley succession), and a varied metasedimentary/metavolcaniclastic unit (Livingstone Creek succession). Each of these is crosscut by intermediate-mafic intrusions of the Simpson Range suite and is therefore Early Mississippian or older. Fossiliferous limestone is interpreted to unconformably overlie phyllite of the Slate Mountain succession. Other units that crop out in this region are Permian augen schist and Cretaceous plutons, including Late Cretaceous quartz monzonite that hosts the Red Mountain Mo porphyry deposit. The central part of the area, between the Little Bear and Sidney Creek faults, is dominated by mafic volcanic and plutonic rocks of the Sawtooth succession. These are along strike from, and provisionally correlated with the Moose formation of the Boswell assemblage. The Sawtooth succession is overlain by a carbonate and chert-bearing metasedimentary unit (Rosy succession), which hosts a small foliated metagranodiorite intrusion. A younger, as yet undated, mafic metavolcanic unit (Gunsight succession) is locally preserved above the Rosy succession. The metagranodiorite in the Rosy succession formed ca. 336 Ma and underwent metamorphic zircon growth in the Early Jurassic (~195 Ma). This zircon growth is interpreted to have accompanied regional, epidote-amphibolite to amphibolite-facies metamorphism and deformation. Mississippian rocks of the Simpson Range suite dominate much of the southwestern domain. These intrusions crosscut quartzose schist, graphitic phyllite and metabasaltic rocks of the Flat Creek succession, which also hosts Middle Triassic metagabbro. All pre-Jurassic rocks in the area are deformed, while post-tectonic intrusions include members of the Early Jurassic Lokken suite, the mid-Cretaceous Quiet Lake batholith and Iron Creek stock, and the aforementioned Red Mountain suite. Most Early Jurassic intrusions are undeformed, but deformation/hydration of some minor phases suggests they were intruded during the waning stages of deformation. The Sidney Creek fault is cut by the Early Jurassic Sawtooth stock while the Cretaceous Iron Creek stock intruded across the trace of the Little Bear fault. The Little Bear and Sidney Creek faults are interpreted to define tectonic boundaries between Yukon-Tanana terrane and rocks of the Semenof block and may form part of a regional detachment between these units.

Yukon-Tanana Terrane in Little Kalzas Lake area consists of a lower quartzite package and an upper metavolcanic package. The lower quartzite package includes a discontinuous metavolcaniclastic and mafic metavolcanic unit. The upper metavolcanic package consists predominantly of intermediate to felsic metavolcanic rocks in the northern part of the map area. These rocks pass southward into a clastic-dominated metavolcanic assemblage. A conspicuous crinoidal marble occurs in the middle of the upper metavolcanic package and can be traced between the northern and southern domains. The layered metamorphic rocks are intruded by the multi-phase Little Kalzas orthogneiss complex in the northeastern part of the map area along the southwestern side of Tintina Trench. The Little Kalzas orthogneiss complex, of uncertain age, comprises granodioritic to granitic gneiss and contains abundant xenoliths of country rock. Younger (Jurassic?), post-kinematic quartz monzonite (in the north) to quartz-diorite (in the south) plutons also intrude the area. The youngest intrusive rocks are small plugs of Tertiary quartz-feldspar porphyry. A pervasive transposition foliation and mineral lineation are developed throughout the area, except in local low-strain domains where primary textures are preserved. The transposition foliation is axial-planar to tight south southwest-vergent folds whose axial surfaces become progressively upright to south southwest-dipping toward the northeast. These structures are deformed by younger crenulation cleavages and associated open folds.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

Upper Devonian and Lower Mississippian metavolcanic rocks of Yukon-Tanana Terrane in southern Finlayson Lake and Frances Lake map areas occur in three thrust sheets, locally modified by a Cretaceous normal fault. The lower thrust sheet, the Big Campbell sheet, comprises the Upper Devonian to Lower Mississippian metavolcanic stratigraphy that hosts the main volcanichosted massive sulphide (VHMS) deposits of the district. Metavolcanic rocks in the middle thrust sheet, the Money Creek sheet, include the Upper Devonian Waters Creek and Early Mississippian Tuchitua River formations. The former comprises primarily felsic metavolcanic rocks and carbonaceous phyllite and is extensively intruded by sheets of comagmatic porphyry. The latter comprises primarily intermediate metavolcanic, volcaniclastic and epiclastic rocks. The upper thrust sheet, the Cleaver Lake sheet, is in part made up of Late Devonian calc-alkaline basalt and rhyolite, the Cleaver Lake formation, and comagmatic felsic to ultramafic plutonic rocks. Of these, the Waters Creek formation and the formations in the Big Campbell sheet have the highest potential to host VHMS deposits.

The Yukon-Tanana Terrane (YTT) is a terrane of pericratonic affinity which occupies an intermediate position between continental margin rocks of Ancestral North America (Cassiar Terrane, Selwyn Basin) to the east and arc and oceanic terranes accreted in Mesozoic time to the west (Quesnellia, Stikinia and Cache Creek). It consists of polydeformed and metamorphosed Paleozoic metasedimentary and meta-igneous rocks which have isotopic and provenance ties to Archean and Proterozoic cratonic source regions, comparable to those of sedimentary strata from northwestern Canada, but whose paleogeographic evolution with respect to the Laurentian craton is enigmatic. The YTT is host to significant base metal occurrences, including the Wolverine and Kudz Ze Kayah deposits in the Finlayson Lake district, in the part of the terrane which lies northeast of Tintina Fault.

Now in its second year, the Ancient Pacific Margin National Mapping (NATMAP) project continues to make progress in elucidating the stratigraphic framework of Yukon-Tanana Terrane of northern British Columbia and Yukon. Updated composite stratigraphic sections for the Finlayson Lake district, Glenlyon, Wolf Lake and Jennings River areas, and from previous studies in the Aishihik Lake area and northern Dawson Range, show that Yukon-Tanana Terrane originated as a dynamic mid- to late Paleozoic (pericratonic) arc system. The evolution of this arc system was punctuated by arc-building events, arc rifting and development of back-arc basins, and episodes of contractional deformation between Late Devonian and early Pennsylvanian. Its development terminated in late Pennsylvanian time with the opening of a marginal basin and the subsequent mid-Permian calc-alkaline volcanism, high-pressure metamorphism and regional deformation of the terrane.

Yukon-Tanana Terrane in Grass Lakes map area south of Finlayson Lake consists of highly deformed though regionally mappable metasedimentary and metavolcanic rocks. Four newly revised map units and various subunits have been traced throughout the area. These include unit 1, a quartz-rich metaclastic unit with felsic metavolcanic rocks at its lowest exposed level and a middle calcareous marker; unit 2, a mafic metavolcanic unit with lesser carbonaceous metasedimentary rocks; unit 3, a carbonaceous phyllite and quartzite and felsic metavolcanic unit, and unit 4, an upper unit of carbonaceous phyllite, mafic metavolcanic rocks and coarse-grained quartzofeldspathic metaclastic rocks. Units 2 and 4 have been redefined from a previous report based on new observations and a consideration of the provenance of coarse-grained quartzofeldspathic metaconglomerates now included in unit 4. In the current interpretation, the metaconglomerate-bearing strata north of and overlying the felsic meta-volcanic rocks hosting Kudz Ze Kayah are considered to unconformably overlie them. Three of these map units are associated with mineral occurrences or deposits. Rusty, locally malachite-stained muscovite-quartz schist at Arcturus Resources' First Base claims and Expatriate Resources' Blue Line and Winger claims belong to the felsic metavolcanic part of unit 1. Columbia Gold's Fyre Lake deposit is hosted in unit 2 mafic metavolcanic rocks, possibly associated with a synvolcanic fault. Cominco's Kudz Ze Kayah deposit occurs in unit 3 felsic metavolcanic rocks. Gossans on Cominco's Cobb claims, Expatriate's Overtime and NHL claims, and Atna/Westmin's Pack claims and anomalous copper in soils at Arcturus' Bas claims are also spatially associated with this unit.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.