Although the White Channel gravel (WCG) of the Klondike district, Yukon, contains gold placers which have been exploited for over a century, few sedimentological studies have been undertaken. This study reports a four stage evolution of the WCG, comprising:i. An initial downcutting period which preferentially retained gold particles on the base of the strath.ii. An aggradational stage in which gold concentration occurred within sedimentary features.iii. A lacustrine layer representing a depositional hiatus.iv. A final, more rapidly aggrading fluvial stage.Identification of the lacustrine layer has clarified the evolution of the WCG depositional fluvial systems. Architectural element analysis and detailed sedimentological observations have been synthesized to gain a clearer understanding of the spatial variations within the WCG. Additionally, the identification of plant species from pollen within the lacustrine layer provides irrefutable evidence that the Klondike district was at least 7°C warmer during the Pliocene compared to the present.

The Kluane Ranges, located in southwest Yukon, are underlain by Late Paleozoic to Late Triassic volcanic and sedimentary rocks assigned to Wrangellia. Bedrock mapping completed within the White River area indicates Wrangellian rocks underwent several phases of deformation between Late Triassic and Miocene time. Middle Triassic marine, fine-grained sedimentary rocks are preserved in grabens where they are overlain by basal conglomerates and breccias of the Nikolai formation. The grabens are related to uplift associated with the deposition of Nikolai formation flood basalts and intrusion of ultramafic bodies. Late Jurassic to Early Cretaceous compression resulted in structural stacking of older rocks and northeast- and southwest-verging overturned folds. Latest (?) Cretaceous to Miocene dextral strike-slip along the Denali fault system led to the formation of steeply dipping faults, extensional and compressional basins and refolding of older regional scale folds. Reactivation of Jura-Cretaceous faults also occurred at this time. An enigmatic pre-Middle Triassic deformation event is believed to be preserved locally in rocks of the Hasen Creek Formation.

Geological map (1:50,000 scale) of the West Klondike area, central Yukon (NTS 115O/14,15 and 116B/2,3) including marginal notes on mineral occurrences.

Geological map (1:50,000 scale) of the East Klondike area, central Yukon (NTS 115O/9,10,11,14,15,16 and 116B/2) including marginal notes on geologic history, economic geology and mineral occurrences.

The high level bench gravels of the Klondike region were first described by R.G. McConnell. He divided the gravels into two formations; the older and stratigraphically lower White Channel gravel, and the younger Klondike or high level river gravel. The White Channel gravel occurs on bedrock benches composed of metasedimentary and metavolcanic rocks, and is usually 50 to 100 metres above present day stream courses. McConnell further subdivided the White Channel gravel into interbedded white and yellow gravel units. This suggests contemporaneous deposition, with the white gravel unit containing important concentrations of placer gold. A distinct alteration zone is recognized in White Channel clastic sediments at Dago Hill. Templeman-Kluit suggested that increased gold values at the White Channel gravel and bedrock contact were directly linked to the alteration of the gravel and bedrock. This idea prompted a study by the authors during the summer of 1983 to determine the relationship (if any) of the alteration zones to the deposition of gold.

The White Channel placer deposit in the Klondike area is a Pliocene to early Pleistocene braid plain sequence with tributary alluvial fan and gravity flow deposits. Lithofacies types range from laminated silt and clay to massive and disorganized boulder gravel. A total of 14 lithofacies types have been identified with cryogenic features such as ice wedge casts found in the upper half of the White Channel section. Dominant lithofacies types include distinctly stratified and crudely stratified gravel which is clast-supported and matrix-filled. An alteration product is recognizable in both White Channel sediments and underlying bedrock. Proximal White Channel sedimentation is characterized by interbedded channel sequences deposited under fluctuating, high discharge and flood conditions. Medial to distal positions are dominantly sequences of low relief unit barforms and channel lag deposits. Sorting and stratification improves up-section, and facies trends are not consistent. Deposition of White Channel sediments in valley margin positions is characterized by gravelly mass flows which are interbedded with stratified fluvial sediments. Placer gold is found in all of the White Channel gravelly facies sampled. Concentration of gold in proximal gravelly facies is due to the development of convergent flow in shallow channels during peak discharge intervals. In medial to distal positions, heavy minerals are concentrated in channel sluiceways and aggraded unit bars. Convergent flow in sluiceways between unit bars is not as susceptible to fluctuating discharge levels, and as a result, sorting and concentration mechanisms are more efficient up-section. Gold is also concentrated in gravelly mass flow deposits through shear and suspension sorting. A copy of this thesis is available at the EMR library – TN414.C32 Y87 1985.

Gold mineralization at the Mt. Skukum deposit occurs in nearly vertical quartz-carbonate veins which crosscut flat-lying andesites with a NNE trend. The mineralized veins represent the second stage of a two stage hydrothermal system, the first of which resulted in emplacement of thin chalcedonic veinlets. These two stages of veins are probably indicative of an evolving hydrothermal fluid rather than being representative of two separate events. Vein emplacement is one of the latest of a series of events which began with volcanism, producing felsic and andesitic volcanic rocks which overlie basement in this area. Subsequent periods of tectonism produced large faults along which rhyolitic dykes were emplaced. Continued tectonism resulted in reactivation of old faults along which andesitic and dacitic dykes were injected, crosscutting rhyolite dykes in many cases. As volcanic activity waned, the faults remained active, leaving zones of high permeability which acted as conduits for the still active hydrothermal circulation. Veins appear to have been emplaced at low temperature in a circulating hydrothermal system driven by a heat source at depth associated with dykes present in the area. Circulating hydrothermal fluids may have leached gold from the surrounding andesitic volcanics during propylitization. Permeability may have been controlled by faulting, brecciated flow tops and bottoms, and lapilli tuff horizons. Gold was precipitated in highly permeable conduits, such as the Main Fault Zone and breccia bodies.

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Paleozoic rocks of the Pelly Mountains, central Yukon, preserve greater than 150 m.y. of sedimentation, magmatism and base-metal mineralization. To identify secular trends in regional tectonics and metallogeny, a multi-year project on the stratigraphy of the Pelly Mountains in the Quiet Lake (105F) and Finlayson Lake (105G) map areas was initiated. Field studies during summer 2015 focused on two stratigraphic intervals: (1) mafic volcanic, volcaniclastic and clastic rock successions assigned to the Cambrian-Ordovician Cloutier and Groundhog formations (Kechika group); and (2) felsic volcanic, volcaniclastic and clastic rock successions assigned to the Devonian-Mississippian Black Slate and Felsic Volcanic formations (Seagull group). Cambrian-Ordovician strata were deposited in a marine environment characterized by episodic mafic volcanism and extensional tectonism. Devonian-Mississippian strata record the transition from an extensional turbidite basin to a metalliferous volcanic rift basin, and resemble key rock assemblages of the Selwyn basin (Earn Group) and Yukon-Tanana terrane (Grass Lakes and Wolverine Lake groups).

Marble, calc-silicate rock and pelitic layers of the Ram Creek assemblage surrounding the Dan Zn (± Cu-Pb-Ag) occurrence display ample evidence of a monocyclic structural evolution with three main events of progressive deformation (D1-D3). These events developed a tightly folded package of west-northwest-trending tectonites. Primary planar structures (S0) generally lie sub-parallel to two tectonic foliations (S1 and S2), which dip shallowly to steeply southwest. Inter-foliation slip (D3) resulted in a transverse, sub-vertical foliation (S3) that dips generally shallowly to moderately north. Cross-sections based on new mapping and fold analysis indicate that similar folds containing stratabound zinc-sulphide mineralization should be present south of the Dan occurrence, as part of regional north-northeast-verging folds or a thrust-fault-repeated succession.