Includes geological cross sections, mineral occurrences, isotopic age dates and fossil localities.

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.

Two contrasting lower Paleozoic units underlie the region southwest of Big Salmon Lake in south-central Yukon. The lower unit comprises dolomitic quartzite, quartzite, dolostone, dolomitic shale, siltstone and sandstone, and their metamorphosed equivalents. Two-holed crinoid ossicles indicate an Early–Middle Devonian age for the dolostone. These dolomitic rocks are overlain by largely carbonate-free, dark, fine-grained and siliceous strata. Rock types include graphitic phyllite, siltstone, metachert and porphyroblastic metapelitic schist. The two units are correlated with the Askin and Earn groups, respectively. An interval of metabasaltic schist locally marks the boundary between the Askin and Earn groups. Mafic and ultramafic rocks are intermittently exposed beneath the Askin Group in parts of the region. The lower Paleozoic metasedimentary units are crosscut by deformed Devonian–Mississippian two-mica augen gneiss and by largely undeformed mid-Cretaceous megacrystic biotite granite to monzogranite. Deformation prior to the mid-Cretaceous produced close to tight folds that trend northwest. The associated axial-planar cleavage/schistosity dips northeast at moderate to steep angles, away from the crest of a major antiformal structure.

Preliminary geological map (1:50,000 scale) of Little Kalzas Lake area, central Yukon (NTS 105L/13) including geological cross sections, mineral occurrences, fossil localities and stratigraphic relations.

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Geological mapping of Yukon-Tanana Terrane in Little Salmon Range has outlined a coherent stratigraphic succession in rocks that were previously described as strongly foliated and lineated mylonitic tectonites. The widespread occurrence of primary sedimentary and volcanic textures and the lateral continuity of the units are incompatible with the previous interpretation of the area. A laterally continuous volcanic arc sequence occupies the core of a broad synclinorium and rests unconformably on disparate clastic units to the east and west. The volcanic sequence is structurally overlain by an allochthonous sheet of distal turbidites. The occurrence of massive sulphide and exhalite within the volcanic sequence attests to the high mineral potential of this largely unexplored region.

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

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