New stratigraphic interpretations of exposures at Dago, Preido and Paradise hills on lower Hunker Creek suggest a more complex stratigraphic framework for the development of high-level bench gravels in the Klondike than has been previously put forward. A highly-altered gravel has formerly been interpreted to be part of the lower White Channel Gravel sequence and its degree of alteration attributed to enhanced groundwater diagenesis near the bedrock contact. New mining exposures indicate this altered gravel, here informally termed ‘Paradise gravel’, is laterally extensive and the alteration is not restricted to the bedrock interface. Furthermore, a cross-section reconstruction shows the White Channel Gravel is stratigraphically inset into the Paradise gravel. Alteration of the Paradise gravel is possibly a function of pedogenesis, suggesting a period of landscape stability followed its deposition. This pattern of sedimentation and potential stability was repeated within the lower White Channel Gravel and recorded by a zone of clay alteration and iron oxidation. Economically, the Paradise gravel is significant. At each locality investigated, the primary pay streak occurs in the Paradise gravel. Conversely, the White Channel Gravel is only economic where it sufficiently erodes and reworks the Paradise gravel. In nearby drainages, including upper Hunker Creek, the White Channel Gravel more completely reworks Paradise gravel to bedrock and becomes the pay unit. Preservation of the Paradise gravel documented in lower Hunker Creek may be due to its broad valley morphology. Future studies are recommended on the sedimentology of the Paradise gravel to understand gold distribution both vertically and within the pay channel(s). In addition, research into the pedogenic alteration could provide information on past climates during the Neogene.

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.

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.

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 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.

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.

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.

Within the Sixtymile River drainage basin, Quaternary placer deposits include tributary valley bottom gulch gravel, a broad main valley alluvial plain, and high level terrace systems in both tributary and main valley settings. After downcutting, paleo-Miller Creek underwent valley widening during alluvial fan erosion and sedimentation. During this time, the main Sixtymile River valley was also being laterally eroded during braidplain aggradation. Terrace development and incision through both fan and braidplain sediments coincided with significant debris flow sedimentation. The Sixtymile River changed from a braided environment to single channel meandering environment during terrace development. Incision of Miller Creek through the fan surface to present levels probably coincided with downcutting of the Sixtymile River. Economic concentrations of placer gold are found in:: 1) the upper Miller Creek terrace where coarse-grained, massive and angular flood flow gravel forms a crude pay streak above the bedrock surface; 2) the distal Miller Creek terrace where discontinuous concentrations of gold were formed over a broad paleo-surface of lag gravel prior to fan aggradation; 3) laterally discontinuous pockets in the main Sixtymile valley terrace which was originally part of an aggraded braidplain sequence; and 4) lower, but still economic placer concentrations in gravels of the alluvial plain in the main Sixtymile valley.