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.

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 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 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.

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.

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.

Results of a study to determine the origin of alteration in the Plio-Pleistocene White Channel sediments and bedrock using field observations, mineralogical and geochemical analysis. The study was coordinated with a sedimentological study of the White Channel gravel by S.R. Morison in 1985. A copy of this thesis is available at the EMR library – TN414.C32 Y86. This thesis is available online at https://doi.org/10.7939/R32F7JW6S.

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.