he bedrock geology in the Mount Decoeli area of southwest Yukon is characterized by Paleozoic to Triassic stratigraphy of the Alexander terrane, Wrangellia and the Bear Creek assemblage, overlain and intruded by Jurassic and younger rocks. Alexander terrane rocks comprise mainly Devonian mafic to intermediate volcanic and volcaniclastic rocks overlain by thick successions of carbonate and phyllite. Wrangellia is characterized by Mississippian to Permian volcanic and siliciclastic rocks of the Station Creek and Hasen Creek formations, overlain by Upper Triassic basalts and calcareous rocks of the Nikolai and McCarthy formations respectively. The Alexander terrane and Wrangellia are separated by the Duke River fault, a mainly Late Cretaceous northeast-verging thrust fault. To the northeast, Wrangellia is separated from the Bear Creek assemblage by the Denali fault, a strike-slip fault with as much as 400 km of right-lateral motion. The Bear Creek assemblage comprises metamorphosed and deformed siltstone, mudstone and sandstone interlayered with mafic to intermediate volcanic and volcaniclastic rocks. Preliminary ages suggest the Bear Creek assemblage is Late Triassic (ca. 204 Ma). Regional correlation of the Bear Creek is unclear, but similarities between the assemblage and rocks of the Taku and Alexander terranes suggest possible linkages. Correlation with Upper Triassic rocks of Wrangellia is less favourable.

The study area is underlain by four stratigraphic successions ranging in age from Middle Proterozoic to Early Paleozoic. From oldest to youngest, they are: Middle Proterozoic Wernecke Supergroup; Middle to Upper Proterozoic Pinguicula Group; Upper Proterozoic Windermere Supergroup; and Uppermost Proterozoic to Lower Paleozoic sandstone and carbonate. Together, they represent about a billion years of intermittent sedimentation punctuated by processes such as deformation, uplift, erosion, magmatism and mineralization. Rocks in the study area record eight phases of contractional and extensional deformation, some of which may be related to strike-slip faulting. Two phases of southwest-verging folds and thrust faults may be related to dextral transpression on the Snake River Fault. Mineral enrichments occur in two general forms:: breccia-related (Middle Proterozoic), and veins (Mesozoic to Tertiary). The breccia-related occurrences have enrichments of Cu ± U, Co, Au and Ag, as dissemminations and veinlets in and near intrusive breccia zones (Wernecke breccia). The vein occurrences comprise Zn-Pb-Ag ± Cu and Au, in veins and related lenses and irregular replacements of carbonate.

The tectonic evolution of the Whitehorse trough in central Yukon is largely preserved by the Early to Middle Jurassic Laberge Group, an ~3000-m thick succession of synorogenic clastic strata that unconformably overlies arc and arc marginal rocks of the Lewes River Group. A two-year project was initiated to test a Sinemurian to Toarcian transgression of basal Laberge Group strata westward across the Whitehorse trough and examine the regional relationships between the timing of Jurassic exhumation, sedimentation, and terrane accretion in the northern Canadian Cordillera. Field studies in 2017 targeted basal Laberge Group strata at seven locations in central Yukon. At each field locality, basal Laberge Group strata are known or inferred to unconformably overlie the Povoas formation and multiple units of the Aksala formation. Pre-Early Jurassic unconformities may indicate variable basin topography due to the complex internal stratigraphy of the Lewes River Group, or that regional exhumation and erosion affected the Whitehorse trough prior to Laberge Group sedimentation.

Three events of Paleoproterozoic deformation are recognized in schist of the Fairchild Lake Group (Wernecke Supergroup) in the Wernecke Mountains. The first event produced a chloritoid ± garnet and opaque porphyroblastic, chloritoid-chlorite-muscovite-quartz schist. Pressure-temperature conditions have been estimated to lie between 3-6 kbar and 450-550°C. The second event produced a crenulation, and the third generated kink bands. All of these features are crosscut by 1.60 Ga Wernecke Breccia. These events are regarded as products of the Racklan Orogeny, a Paleoproterozoic interval of orogenesis, which favourably correlates with the Fifteenmile Orogeny in the Ogilvie Mountains of western Yukon and the Forward Orogeny in the Northwest Territories.

Geological map (1:50,000 scale) of Slats Creek area, Wernecke Mountains, central Yukon (NTS 106D/16) including geological cross sections, mineral occurrences and preliminary isotopic age determinations.

Clear Creek map area of central Yukon is underlain by deformed and metamorphosed Proterozoic and Paleozoic rocks of western Selwyn Basin (Hyland Group, unnamed carbonate unit, Road River and Earn groups) and mid-Cretaceous felsic intrusions. Metasedimentary rocks are disposed in a warped northwest to northeast-dipping structural panel with younger, structurally shallow rocks in the north and older, structurally deeper rocks in the south. Younger rocks in the north are deformed into Lost Horses syncline, a southwest-overturned tight to isoclinal syncline with an axial surface trace extending across the northern part of the area. Older rocks at deeper structural levels in the south are deformed by a suite of fabric elements that probably post-date Lost Horses syncline, indicate a top-to-the-northwest sense of tectonic transport, and are probably related to Early Cretaceous displacement on the Tombstone thrust. Subsequent deformation warped the panel into its current orientation. All structures are intruded by mid-Cretaceous felsic intrusions including hornblende-biotite (rare muscovite) granite, quartz monzonite, granodiorite, syenite, and quartz syenite. Mineral occurrences are mainly precious metal, tin, and/or tungsten bearing vein, skarn, and breccias associated with felsic intrusions. Bedded barite occurs in Earn Group strata. New analyses of mineralized samples confirm earlier data reporting significant gold values in veins cutting felsic intrusions and nearby country rock, and correlation of anomalous gold with anomalous bismuth in some veins.

The Upper Triassic to Middle Jurassic Whitehorse Trough is a 500-km-long forearc basin developed on the eastern margin of the Stikine Terrane. Basin fill in the Fish Lake syncline consists of Early to Middle Jurassic strata of the Laberge Group. Re-examined fossil collections record all ammonite zones from the Upper Pliensbachian to lowest Bajocian (except for the recessive or volcanic Aalenian Stage), suggesting that the Laberge Group is a conformable sequence. Biostratigraphic dating of oldest conglomerates indicates that source area uplift occurred first in the north in the Early Pliensbachian, progressing southwards through the Late Pliensbachian. Periodic, basin-wide anoxia was prevalent during the Middle and Late Toarcian, indicated by abundant Bositra, a low-oxygen-tolerant bivalve. The final stage of basin-fill, Early Bajocian chert-pebble conglomerates, provides a maximum age constraint on the amalgamation of the Cache Creek and Stikine terranes.

Sprague Creek map area is underlain by multiply deformed Upper Proterozoic to mid-Paleozoic metasedimentary rocks and undeformed Cretaceous felsic intrusions. The oldest and structurally deepest rocks underlie most of the southern two-thirds of the map area. Three phases of regional deformation and a younger suite of faults and fractures are represented in Sprague Creek map area. Mineral occurrences in the area comprise veins, breccias, and skarns spatially and probably genetically related to Cretaceous intrusions, and gossanous breccias along NNW-trending lineaments.

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Geological map (1:50,000 scale) of Slats Creek area, Wernecke Mountains, central Yukon (NTS 106D/16) including geological cross sections and mineral occurrences.