Unconsolidated sediments in the Mount Nansen area can be subdivided into eight clastic facies: 1)clay-rich diamicton; 2) massive/stratified silt/clay; 3) massive/disorganized pebbly sand/sand; 4) stratified pebbly sand/sand; 5) disorganized muddy gravel; 6) massive/stratified sandy gravel; 7) disorganized gravel; and 8) massive to crudely stratified gravel. Diamicton is interpreted as glacial till; other facies are fluvial/glaciofluvial in origin. Sediments have a number of geomorphic setting, including: 1) Holocene colluvium, alluvial fans and stream deposits; 2)Reid periglacial alluvial fans; and 3) pre-Reid glacial and glaciofluvial deposits. Previous workers suggested that significant placer gold concentrations occur only in alluvium that lies either upon bedrock or glacial till. New data from this study suggest that significant amounts of placer gold also occur in the diamicton, primarily at the diamicton/bedrock contact. Gold concentration in the diamicton is likely due to glacial erosion and incorporation of a supergene- enriched bedrock mantle and pre-existing auriferous alluvium.

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Eight lithofacies types have been identified. These include: 1) Gms - massive, matrix support conglomerate; 2) Gm - massive or crudely bedded, clast supported conglomerate; 3) Sr - rippled sandstone; 4) Sp - planar crossbedded sandstone; 5) Sh - massive sandstone; 6) F1 - laminated sandstone, siltstone and mudstone; 7) Fsc - laminated or massive siltstone and mudstone; and 8) L - limestone. All lithofacies are present in the Sixtymile River area, but only lithofacies Sp, Sh and F1 were observed in the Grayling Creek area. The eight types are grouped into four assemblages and are interpreted as: 1) Scott type braided river deposits; 2) Donjek type braided river deposits; 3) South Saskatchewan type braided river deposits; and 4) lacustrine deposits. Lithofacies assemblages are arranged into at least ten fining-upward and thinning-upward megacycles reflecting a history of pulsed and rapid uplift in the source area.

Between the Cretaceous granitic rocks (Hake Batholith on the west; Cassiar Batholith to the east) are three belts of metamorphic rocks, collectively part of Yukon-Tanana terrane. These are remnants of oceanic and continental volcanic arcs, and marginal basin sediments of Early to mid-Paleozoic age. At the head of Borden Creek are thick carbonate and andesitic volcanic rocks correlated with Klinkit Group. The Ram Creek fault and Hidden Lake fault are not exposed but deduced to be steeply dipping brittle structures with northeastward thrust or transpressional offset, based upon more complete exposure to the southeast in 105B/3 map area. The former is likely of Cretaceous age; the latter was active between mid-Permian and Early Jurassic time.

The Mount Nansen and Stoddart Creek map areas (NTS 115 I/3, 6) are in the southern part of the Dawson Range. They contain a number of porphyry and vein related mineral occurrences which have been undergoing extensive re-evaluation for precious metals potential. Basement rocks are part of the Yukon Crystalline Terrane and include metamorphosed and deformed sedimentary, volcanic and plutonic rocks of uncertain age. These are intruded by two suites of foliated plutonic rocks, the Upper Triassic to Jurassic Klotassin Suite, mainly hornblende-biotite granodiorite, and the Jurassic Big Creek Suite, including K-feldspar porphyritic syenite, quartz syenite and monzonite. Latest metamorphism of basement rocks is likely related to emplacement of these suites. Latest lower Cretaceous was marked by intrusion of the Dawson Range Batholith, consisting of the regionally exposed Casino Granodiorite and the more localized Coffee Creek Granite. The Mount Nansen Volcanics, mainly andesite with a lesser felsic component, are possibly cogenetic with these intrusives. Numerous intermediate to felsic porphyry stocks and dikes may be of Mount Nansen age in part, but they are at least in part younger. The Bow Creek Granite is a newly defined, high level, granophyric pluton with related, peripheral quartz-feldspar porphyry dikes. These rocks appear to cut the Mount Nansen volcanics and may be as young as the Carmacks volcanism. The Caribou Creek Conglomerate is a very localized sedimentary sequence which underlies the Carmacks volcanics. The Carmacks Volcanic Suite, uppermost Cretaceous in age, is relatively flat-lying and has been subdivided into three units. The lowermost consists of felsic pyroclastic rocks and associated glassy domes or plugs. The middle unit, which appears to be quite thin in the map area, consists of andesite flows and pyroclastics with minor basalt. The most extensive is the upper unit, which consists mainly of basalt flows. Mineral deposits are of four main types, including porphyries, veins, skarns and placer. Transitional varieties are associated with brecciation and porphyry dike emplacement. The porphyries are low grade copper-molybdenum deposits with local gold enrichment in the upper parts. Breccias with elevated precious metal values occur within the porphyries and also peripherally associated with quartz-feldspar porphyry dikes. Gold and silver-bearing quartz veins occur in dilational fracture systems which are also peripheral to the porphyries. In the presence of calcareous meta-sediments of Yukon Crystalline Terrane, gold-bearing, iron-rich skarns have formed. Base metal-rich veins are rare and distal from the porphyry centres. Mineralization controls are recognised as follows:: 1. Proximity to major regional structures such as the Big Creek Fault and the Minto Linear which extends north-northeasterly through the map area. 2. Local structures, ranging in trend from northwesterly to northeasterly, are important as hydrothermal channelways and vein sites. 3. Presence of a favourable host, including Mount Nansen volcanics, siliceous meta-sediments and Casino Granodiorite. 4. Proximity to porphyry stocks or quartz-feldspar pophyry dikes.

Sedimentological analysis of Quaternary sediments along the Stewart River near Mayo, central Yukon Territory, is used to determine the environmental history of the region. Thirteen sedimentary facies are defined and interpreted. Seven stratigraphic units are interpreted from these facies and they form five informal chronostratigraphic intervals: interglacial, proglacial, glacial, immediate postglacial and Holocene. A copy of this thesis is available at the EMR library – QE696.C376 1993. This thesis is available online at https://doi.org/10.7939/R3222RG09.

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