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.

High-level terraces in the Indian River valley, between the confluences of Ruby Creek and Dominion Creek with Indian River, are underlain by a sand-dominated fill. The fill formed when meltwater torrents from the margin of a Late Pliocene ice sheet drained into the Indian River valley from the divide with the Stewart River basin. A lake or lakes existed in the Indian River valley at that time. Mechanisms for ponding of the lake(s) include regional glacial damming of the ancestral Yukon drainage (Glacial Lake Yukon), or local damming by alluvial fans or landslides. Sufficient evidence does not exist to effectively eliminate any of these hypotheses. Placer gravel may exist below the sandy fill in a buried segment of the pre-glacial Indian River valley near the confluence of Montana Creek.

Significant economic concentrations of placer gold were first recognized in an intermediate level terrace near Dawson City in the late 1980's Regional surficial mapping has shown the distribution of many high level terraces of pre-Reid, Reid and McConnell age in central Yukon, but the relationship between economic gold concentration and terraces is not well understood. Sedimentological study of an intermediate level terrace near Dawson City suggests two river types have been dominant:: the first, a 'wandering gravel bed river' is characterized by moderate sinuosity, later accretion deposits, limited sand facies, and generally fine gravel; the second, 'proximal braided river' is characterized by multiple channels, very thick and crudely imbricate gravel, low bed relief, and a maximum particle size greater than the underlying wandering gravel bed river deposits. The gold-bearing 'wandering gravel bed river' assemblage is typical of present-day conditions with river processes dominated by lateral migration and high gravel transport rates through the system, conducive to heavy mineral concentration during an interglacial period. The 'proximal braided river' is characteristic of nearby glacial ice and rapid sedimentation resulting in poor heavy mineral concentration. The transition from a wandering gravel bed river to a proximal braided river is suggested to mark the onset of a pre-Reid glaciation in the Southern Ogilvie Mountains. The sedimentology of the intermediate terrace gravels suggests a geomorphic model which may be used for exploration of terrace placer deposits in central Yukon with a similar pattern of regional glaciation influencing terrace formation.

The layered rocks in this area originated as continental shelf sediments overlain by volcanic arc successions. Now called Yukon-Tanana terrane, they tectonically over-rode the western edge of ancient North America beginning in Middle Jurassic time. Three elements are present in the map area. The west half comprises the Big Salmon Complex; the east half is a separate, in part contemporaneous succession composed of the Dorsey Complex and Swift River Group. Unconformably overlying both these elements are less metamorphosed Klinkit Group and Triassic sediments that are here interpreted as overlap assemblages. The unexposed contact between Big Salmon Complex and Swift River Group is inferred to be an east-side-down normal fault.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

The upper Sixtymile River area is located approximately 128 km west of Dawson City, Yukon. Lithology in this area consists of Precambrian to Paleozoic metamorphic rocks, Paleozoic ultramafic rocks, Middle Jurassic pegmatitic and aplitic dikes, Upper Cretaceous porphyritic dikes and volcanic rocks with intercalated sedimentary rocks, Quaternary alkaline basaltic dikes and Quaternary alluvial sediments. Precious metal occurrences in these volcanic rocks are divided into two types, based on differences in local distribution, petrology and wall rock alteration: a gold-bearing pyrite-arsenopyrite type and a silver-bearing galena-sphalerite type. Both types are characterized by four stages of mineralization.

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The study area provides a clear record of the Proterozoic geological evolution of northern and central Yukon Territory. The area lies in the Wernecke Mountains of east-central Yukon, approximately 150 km north-northeast of the town of Mayo, and 20 km west of the Yukon-Northwest Territories border. The rocks record events of sedimentation, magmatism and deformation ranging in age from Early Proterozoic to Tertiary. Rocks of Early Proterozoic age predominate, but strata of Middle Proterozoic, Late Proterozoic, and Early Paleozoic ages are also abundant.

Includes a geological cross section, mineral occurrences and isotopic age dates.

Much of the Yukon-Tanma Terrane escaped Pleistocene glaciation with the result that there is <1% outcrop across much of the terrane and weathered rock commonely extends to depths of >75m. Weathering has in many cases removed all obvious signs of mineralization and has resulted in the dispersion of soluble metals in the near surface. For these reasons, exploration based on traditional prospecting methods and soil geochemical surveys has met with limited success. Geological mapping is hindered by the lack of exposure and by the complex geology. To address these problems a detailed airborne geological sruvery, combining gamma ray spectrometric, magnetic and VLF sesors, was flown in the Dawson Range, central Yukon Tanana Terrane. Spectrometric data were used to determine the average surface concentration of potassium (K), uranium (U), and thorium (Th). Lithological units are characterized by relatively consistent geochemical signatures permitting improved geological mapping based on the distribution of spectrometric domains. Alteration halves, commonly characterized by the addition of K and magnetite, are identifiable as areas of low Th'K rations and as total field magnetic highs.