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.

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 White Channel Gravel of the Klondike is an unconsolidated gravel preserved as erosional remnants lying on rock cut benches above younger streams. It contains important concentrations of gold, and eroded parts of the gravel are the likely source of gold in the newer gravels of streams cut through them (e.g., Bonanza, Eldorado and Hunker Creeks). The purpose of this note is to point out that the base of the White Channel Gravel and the bedrock below it are altered and that this alteration coincides with the gold localized in the sediments. Groundwater flowing through the gravel may have precipitated the gold and produced the alteration of gravel and bedrock. Previously, gold in the White channel Gravel has been considered a fossil placer concentration.

A post-depositional hydrothermal alteration product in White Channel sediments and underlying bedrock is divided into 3 zones. These zones, termed the Bleached Zone, the Iron Zone, and the Footwall Zone, are characterized by the development of secondary clay minerals with moderate to high crystallinities. Trace element concentrations of Fe, Mn, As, Sb, Hg, Co, Ba and S are anomalously high in the Iron and Footwall zones. Three types of low temperature, post-metamorphic veins appear to be spatially related to both the distribution and intensity of alteration. Field relationships of altered and unaltered White Channel sediment show zoning patterns which cannot be explained by surface weathering and percolation of meteoric surface fluids. Economic implications of the alteration of White Channel alluvium are that there may be a hydrothermal style of gold mineralization, in addition to gold which was initially deposited in a placer environment. Testing and exploration of altered White Channel alluvium should be done with this in mind, particularly for extremely fine-grained gold which may accompany the alteration product.

Placer deposits in the Mayo area occur in a wide variety of geomorphic settings, including alluvial fans, gulch gravels, valley-bottoms (alluvial plains), and bedrock terrace (bench gravel) settings which have been variably buried and reworked by glaciofluvial processes. Placer gold is also known to occur in glacial till and glaciofluvial gravels especially where these sediment types are close to bedrock. Three major Quaternary glaciations (the pre-Reid, Reid and McConnell, in order of oldest to most recent) and their associated interglacials have modified the drainage and topography of this area, and these events have affected the formation, preservation and proportionate size of the District's placer gold deposits. Duncan Creek, a tributary of the Mayo river which drains Mayo Lake, is one of the most actively-mined drainages in the Mayo District. Placer mining began in the Duncan Creek area in the early 1900's and has continued almost continuously to the present day. Gold production from Duncan Creek in the last 15 years has been nearly 20,000 crude ounces, with historical production estimated to be at least twice that for the last 95 years. Although the McConnell ice limit only reached into the first few kilometres of the Duncan Creek valley at its mouth and its headwaters, associated glaciolacustrine and glaciofluvial sediments have inundated the valley to depths of up to 40 or more metres. Gold-bearing gravels are currently being mined beneath this thick cover of barren overburden. Preliminary sedimentological and stratigraphic data indicate three main lithostratigraphic assemblages:: 1) Crudely stratified, imbricate boulder-cobble gravel and muddy boulder-cobble diamict; 2) Stratified cobble-pebble gravel, stratified sand and laminated silt; and 3) Stratified silt, massive boulder-cobble gravel and silty boulder diamict. Lithostratigraphic assemblage 1 is gold-bearing and is interpreted as Reid-age subglacial or proximal glaciofluvial outwash and Reid-age proximal alpine glacial till. Gold in these sediments is concentrated either as a function of hydraulic interaction with bedrock topography, or as a result of incorporation and dispersion of a pre-existing placer deposit formed during a previous interglacial period. Lithostratigraphic assemblage 2 is interpreted as an interglacial wandering gravel bed river indicated by several fining upward sequences and wood radiocarbon dated at 32 320a. ±1270 B.P. (Beta-86851). Lithostratigraphic assemblage 3 is interpreted to be McConnell age glaciolacustrine silt, glaciofluvial outwash and glacial till. The Keno Hill Silver District (United Keno Hill Mines) lies a few kilometres upstream of the placers and native silver nuggets have been recovered during present and historic placer mining. Lode sources of gold are also known to occur on nearby Mt. Hinton, which indicates the gold in the placers is likely from local bedrock sources. The ubiquitous and extensive nature of facies assemblage 2 combined with the possible existence of other hardrock sources of gold indicates that a significant potential exists for more placer gold reserves in the same drainage.

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.

A poster including a 1:100,000-scale map showing the distribution of White Channel gravel within the Klondike Gold Fields, Yukon, Canada, as well as marginal notes including history, geologic setting, stratigraphy and paleogeographic reconstruction.

The White Channel Gravel (Pliocene) is the most important gold-bearing unit in the world famous Klondike goldfields of west-central Yukon. It is up to 46 m thick and consists of framework-supported, poorly bedded, slightly muddy sandy gravel that was deposited by braided rivers. Historically, this unit is subdivided into a lower 'white gravel' and an upper 'yellow gravel'. The colour of the white gravel is due to an abundance of quartz clasts and an alteration of the sand-mud matrix that has been referred to as 'leaching' or 'bleaching'. Previous researchers concluded that the alteration is hydrothermal in origin, but a review of this research shows that there is no unequivocal evidence supporting hydrothermal alteration. Petrographic examination of in situ samples from both the white and yellow gravel reveals a depositional fabric and an alteration fabric, although the alteration is better developed in the white gravel. The alteration is reinterpreted as the result of weathering, and particularly diagenesis due to groundwater flow.

The Haggart Creek study area contains unconsolidated sediments, which represent six main geomorphic settings: 1) Reid-age glacial and glaciofluvial deposits; 2) McConnell-age proximal braided stream deposits to periglacial alluvial fans; 3) McConnell-age medial to distal braided stream deposits; 4) post-McConnell wandering gravel bed river; 5) Holocene gulch deposits; and 6) Holocene colluvium. Placer gold within the study area is known to originate from at least two main local lode sources: 1) gold in sheeted quartz veins within a Late Cretaceous-age granodiorite stock; and 2) gold in isolated quartz-sulphide fissure veins found in bedrock along Haggart Creek. Placer gold is believed to have formed as a result of reworking and re-concentration of gold during interglacial times, as well as reworking of older auriferous interglacial gravel by McConnell-age periglacial alluvium. The primary setting for placer gold deposits is Late McConnell to Early Holocene alluvium, on or near bedrock. Sub-crystalline to crystalline placer gold grains recovered from sediments found far removed from currently known local bedrock sources suggests that other lode sources may be present within the study area. A copy of this thesis is available at the EMR library – QE195.W47 1999. This thesis is available online at http://hdl.handle.net/1880/25379.

Twelve lithostratigraphic units, including ten autochthonous and two allochthonous units, are recognized, and most of these are divided into 27 subunits. They range from Cambrian to Quaternary in age and include a wide range of sedimentary, igneous and metamorphic rock types. Lower Cambrian siliciclastic and carbonate rocks of the Cassiar Batholith predominate. Three phases of structures are identified. First phase structures include bedding and slaty cleavage, of which the latter is attributed to late stage diagenetic recrystallization. Second phase structures trend northwest and include crenulation cleavage and related folds and lineations. They are attributed to northeast-southwest compression resulting from accretion and obduction of allochthonous rocks during arc-continent collision in Late Jurassic-Early Cretaceous time. Third phase structures are approximately 90° to the second phase structures and trend northeast. They include joints and related folds and lineations and are attributed to dextral transcurrent movement on Tintina, Kechika and Cassiar faults. Precious and base metal mineralization is found mostly within Paleozoic sedimentary rocks and Cretaceous plutonic rocks and forms predominantly veins and replacement lenses. The dominant sulphides include galena, sphalerite, pyrite and chalcopyrite. Arsenopyrite, freibergite, tetrahedrite, pyrrhotite, wolframite, cassiterite, stannite, fluorite and lepidolite are also present. Common gangue minerals include quartz, siderite and iron and manganese oxides. Mineralization appears to be structurally controlled by the northeast-southwest jointing and, to a lesser extent, lithologically controlled by limestone-phyllite contacts. It is attributed to hydrothermal solutions migrating along the joints and was probably deposited approximately 50 Ma ago. The most useful exploration guide to finding additional mineralization is iron and manganese gossans.