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 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.

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 central Yukon Territory has a number of favourable placer deposit settings due to its unique history of multiple glaciations, active stream sedimentation in association with proglacial outwash settings and terrain which has remained unglaciated. Placer gold was found along the Stewart River on point bars in 1884 prior to the discovery of gold in the Klondike area. This was the first indication that the Yukon Territory contained important economic concentrations of placer gold. This study is concerned with the late Tertiary and Quaternary geology in the Lower Stewart River and adjacent Yukon River above Dawson. Previous systematic surficial geological mapping and testing for placer gold on the high-level terraces along these rivers has been limited. This report describes the sedimentology and stratigraphy of key gravelly exposures in this area because similar high-level terraces in the Fortymile River drainage in Alaska had been mined for gold for many years. Work of this type also provides information on the physical characteristics of gravelly deposits (e.g., grain size distribution) which may assist regulatory decisions on placer mining in the lower Stewart and Yukon drainages. Accompanying this report are two 1:50 000-scale surficial geology maps including marginal notes (Garner Creek, NTS 115O/13 and Matson Creek and Ogilvie NTS 115N/9 (east half) and 115O/12), as well as one 1:250 000-scale topographic map (Stewart River - NTS 115N/O) including field study site locations, heavy mineral sample sites and hardrock mineral occurrences.

The Stewart River map area (115 O&N) is the most important historic and current placer gold producing region in the Yukon. Unfortunately, the historic placer-gold deposits are becoming depleted, and more efficient mining of existing deposits and exploration for new deposits must be encouraged. Although placer deposits in the Klondike district are well described and their origin is quite well understood, placer deposits in the remaining part of the Stewart River map area have not been so well documented. The purpose of the Stewart River placer project is to describe and document the geology of known placer deposits, to interpret the formation of the placer deposits, and to relate the geology of the placer deposits to the regional surficial and bedrock geology. The objectives of the project are to aid in the exploration and mining of placer deposits by providing a comprehensive and up-to-date placer geoscience database. The utility of the placer database is that it can be used to construct placer deposit models (general summaries of given placer settings). These models then serve as predictors for future placer exploration and mining. Fieldwork for the project began in 1998 and will be completed in 2001; results of the project will be published in a final report and a resource appraisal map for placer gold.

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.

Placer gold deposits are widespread throughout the largely unglaciated Stewart River and southern part of the Dawson map areas. These deposits include the world famous Klondike goldfields, the historic Fortymile and Sixty Mile goldfields, and well known placers along Black Hills, Scroggie, Thistle and Kirkman creeks. Although the deposits have been mined for over 100 years and have produced an estimate 311 tonnes of gold, they still account for about 85% of Yukon's annual placer gold production. The placer deposits are classified into three levels of gravel with four main units: high-level gravel, which usually forms prominent, continuous high-level terraces and is subdivided into the White Channel Gravel (which is locally subdivided into a lower White Gravel and an upper Yellow Gravel unit) and Klondike Gravel; intermediate-level gravel, which mostly forms relatively small, irregularly distributed intermediate to low-level terraces; and low-level gravel, which represents alluvium along present day creeks, gulches and rivers. The White Channel Gravel, is up to 46 m thick and characterized by a predominance of quartz clasts (which are generally more abundant in the White Gravel than in the Yellow Gravel). It is considered Early Pliocene to earliest Late Pliocene in age (~5 to 3 Ma). The Klondike Gravel, not considered an economical placer, is up to 53 m thick and is distinguished by chert clasts derived from the Ogilvie Mountains, located northeast of the map areas. It was deposited as glaciofluvial outwash during the end of the initial and most widespread of the pre-Reid glaciations, and is probably latest Early Pliocene to earliest Late Pliocene (~3 Ma). The intermediate-level gravel, the least important economically, is up to 9 m thick. The low-level gravel, historically the most important gold-bearing unit, is 5 m thick in creeks and up to 20 m thick in rivers. The intermediate-level and low-level gravel have similar amounts of quartz, igneous and metamorphic rock particles, although locally, the low-level gravel contains sedimentary rock particles. The intermediate-level gravel is thought to be Late Pliocene to Early Pleistocene (~3 Ma to 750 Ka) in age and the low-level gravel is considered Late Pleistocene to Holocene in age. Practically all of the placers are fluvial in origin and were deposited primarily in braided streams that flowed parallel to the present day streams along which the deposits occur. Gold recovered from the various levels of gravel is detrital in origin and was mainly derived from early Mesozoic auriferous quartz veins. The concentration of gold in the gravel is related to a hierarchy of physical scales: at the lithofacies scale (metres), bed roughness determined sites of gold deposition; at the element scale (tens of metres), gravel bars were preferentially enriched in gold; at the reach scale (hundreds of metres), stream gradient was an important factor; at the system scale (hundreds of kilometres), braided river environments transported large amounts of gold; and at the sequence scale (thousands of kilometres), economic placers formed initially in the high-level White Channel Gravel and later in the intermediate- and low-level gravel.

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.