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 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.

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.

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.

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.

Surficial geology mapping of the lower Stewart River valley has revealed sets of high level terraces formed when the paleo-Stewart River and the paleo-Yukon River were at higher base levels during the Pleistocene glaciations and during preglacial time. These terraces are composed of bedrock and a variety of gravelly alluvial fill, some of which are glacial in origin, others appear to be nonglacial in origin. The terraces are dominated by pebble and cobble gravel deposits which are typically covered by aeolian sand and silt deposits. The age of these high level terraces was determined by paleosol development, height, and relationship to glacial limits. At best, these are crude correlations which serve to model drainage evolution. For example, inferences can be made about the aggradation of these main valleys. A regional base level was established possibly in late Tertiary time which is identified as a bedrock terrace level above the present flood plain level. The timing of this feature is probably older than White Channel gravel age (Pliocene-Early Pleistocene). Subsequent aggradation of valley fill gravel in the Stewart River drainage followed. The style of deposition is considered nonglacial for this highest alluvial surface. The main rivers incised their valleys due to a lowering of base level; perhaps due to tectonic uplift and/or isostatic readjustment subsequent to a long period of stability. Placer gold distribution on the terraces appears to be widespread. Gold grains are typically flat, smooth and smaller than 1 mm. Gravel sampling of lithofacies was conducted on these high level terraces followed by concentration of heavy minerals by sluicing and panning. Gold is present on many of these terraces. Favourable targets for placer exploration may be in tributary valley terraces which grade to the main trunk stream high level terraces or basal gravel overlying bedrock. Erosion of outwash terraces is believed to contribute gold to modern bars along the Stewart River.

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.

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 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.

Extensive conglomeratic strata in the Late Jurassic to Early Cretaceous Tantalus Formation were deposited in both shallow gravel-bed braided rivers, and deeper meandering gravel bed rivers. Overbank, marsh and swamp deposits, with potential to contain abundant terrestrial organic materials, are restricted to recessive intervals associated with small sandy and gravelly highconstructive river systems. Medium- to high-volatile bituminous and anthracitic coals in these intervals have limited potential as a source of additional gaseous hydrocarbons. Most of the conglomerates have a high fracture density, which would make them good reservoirs for coal-bed methane in settings where the Tantalus Formation lies beneath a seal of younger volcanic strata. Strata of the Late Norian Mandanna member of the Aksala formation near Takhini Hotsprings do not contain fluvial strata: laminated, bioturbated, intraclast-bearing red sandstones were deposited in an intertidal setting, and may have lost most of their organic material prior to burial.