This map presents a compilation and re-interpretation of the bedrock geology in the Mount Nansen area. The Mount Nansen porphyry district is herein defined as the area of historic placer gold and hard rock gold-silver production centred on the headwaters of the East Fork of Nansen Creek. The majority of MINFILE occurrences in the district are within a 12 x 6 km, northwest elongate area (Hart and Langdon, 1997) shown at 1:20 000 on sheet 2. In the northwestern part of the district, vein and porphyry mineralization was related to Late Cretaceous porphyritic rocks of the Casino and Prospector Mountain suites. Mineralization in the southeastern part of the district appears to have been primarily related to Early Cretaceous porphyritic dikes and plugs. Our mapping is combined with previously undigitized 1:30 000 (published at 1:50 0000) bedrock mapping. Geochronological data are from this study, Lee (2021) and the Yukon geochronology database (YGS, 2020b). The location of approximately 20 samples in YGS (2020b) were modified based on field checks and location notes in the database; geochronological samples which could not be confidently located are omitted.

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.

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.

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Metasedimentary and meta-igneous rocks in ‘Mendocina Creek’ (NTS 105F/5) and eastern Livingstone Creek (NTS 105E/8) areas are part of three distinct stratigraphic sequences: from east to west, the Sheep Creek, Scurvy Creek and Dycer Creek successions. The Sheep Creek succession contains extensive carbonate horizons and is likely part of the Cassiar terrane. To the west, metaclastic rocks of the Scurvy Creek succession are extensively intruded by sills and dykes composed of augen meta-granite of Early Mississippian age; they are correlated with the Snowcap assemblage of Yukon-Tanana terrane. The overlying Dycer Creek succession in the southwest comprises marble, carbonaceous rocks, greenstone and quartzite of Lower Mississippian (and younger?) age that probably correlate with the Finlayson assemblage of Yukon-Tanana terrane. The ‘Mendocina Creek’ area experienced at least four phases of deformation and greenschist- to amphibolite-facies metamorphism. An east-verging thrust locally imbricates the Scurvy Creek succession and the boundary between the Yukon-Tanana and Cassiar terranes corresponds with a west-verging, brittle-ductile thrust fault in the eastern part of the area. Re-interpretation of the geology in western Quiet Lake map-area indicates that this boundary is located 20 km east of the d’Abbadie fault, the previously inferred terrane boundary.

Epithermal vein and porphyry-related gold-silver deposits in the Mount Nansen area are mainly hosted in Paleozoic Yukon-Tanana Terrane metasedimentary rocks and Early Jurassic Big Creek Batholith intrusive rocks. Mineralization is spatially, and probably temporally, related to a northwest-trending belt of mid-Cretaceous hypabyssal felsic intrusions and dykes along the Mount Nansen Trend. The proximal relationship between the veins and mid-Cretaceous intrusive rocks suggests that mineralization may be genetically related to felsic magmatism. The Dickson stock yields a U-Pb zircon age of 108.3 ± 0.7 Ma, and proximal dykes in the Flex, Dickson, Brown-McDade and Weber zones give ages of 107.9 ± 0.9 Ma to 109.0 ± 0.7 Ma, similar to the age of the Mount Nansen Group volcanic rocks. Granodiorite that hosts the Dickson deposit gives a U-Pb titanite age of 191.5 ± 2.9 Ma, and is likely part of the Big Creek Batholith. Previous studies indicated two periods of mineralization in the Dawson Range: mid-Cretaceous and Late Cretaceous. Dating indicates that Mount Nansen mineralization is associated with the mid-Cretaceous emplacement of the high-level felsic intrusions.

Detailed exploration conducted during 2006 in the western part of the Skukum Creek deposit has revealed new structural, mineralogical and geochemical features. The deposit incorporates a number of (at least six or seven) sub- parallel narrow mineralized zones, coincident with andesite-dacite-rhyolite dyke swarms extending for at least 1 km along strike and for hundreds of metres down-dip. Various mineralized zones differ in size, structural setting, intensity and composition of mineralization, and, in total, form a large mineralized package more than 200 m wide, corresponding to a property- to district-scale fault zone extending for over 10 km and traced by a dyke belt. Significant potential exists for the exploration of these structures along strike and down-dip. The diamond drilling intersected numerous high-grade intercepts of gold and silver mineralization corresponding to the low-sulphidation sub-type of epithermal gold-silver deposits. However, strong enrichment in base metals (up to 25% of combined Zn+Pb+Cu) and arsenic suggests essential differences from typical epithermal mineralized systems.

The Hart River area (maps 116 A/10 and 11) of the southeastern Ogilvie Mountains is underlain by two varied and widespread successions which offer the most potential for discovery of sediment-hosted base metal deposits in the northern Canadian Cordillera. A 'shelf sequence', north of the Dawson fault, includes:: Middle Proterozoic Wernecke Supergroup; Middle and (?) Late Proterozoic Fifteenmile group; the Late Proterozoic Windermere Supergroup; Ordovician and Silurian carbonate; Ordovician to Devonian (?) Road River Group; Devonian Ogilvie Formation; and Devono-Mississippian Earn Group. The 'offshelf sequence' includes:: Late Proterozoic Hyland Group; Cambrian volcanic rocks; Lower Cambrian Vampire Formation equivalent; Cambrian Gull Lake Formation equivalent; Cambro-Ordovicain Rabbitkettle Formation equivalent; Ordovician and younger(?) Road River Group; Mississippian Keno Hill quartzite; and Devonian(?) to Jurassic(?) Lower Schist. The most promising exploration targets are:: 1) massive sulphide deposits associated with the Hart River volcanic rocks in the Gillespie Lake Group (upper part of the Wernecke Supergroup); 2) sedimentary copper deposits in the Fifteenmile Group; 3) 'Anvil-type' pyritic massive sulphide deposits in the transition zone between the Gull Lake and Rabbitkettle Formations; 4) 'Howard's Pass-type' zinc-lead deposits in previously unmapped black shales and chert of the Road River Group; 5) sulphide deposits associated with Cambrian, Ordovician(?) and Devonian(?) mafic volcanic and intrusive rocks; 6) covered areas at the base of the Lower Schist which may overlie the Earn Group, with potential for 'Macmillan Pass-type' zinc, lead, silver, barite deposits; 7) volcanogenic massive sulphide deposits associated with felsic volcanic rocks in the Keno Hill quartzite.

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The Mount Nansen camp hosts approximately 30 mineral occurrences of epithermal and porphyry origin. The largest occurrence is the Brown-McDade with approximately 600 000 tonnes of drill-indicated reserves at 6.1 g/t Au and 55.5 g/t Ag. The majority of the northwest-striking, steeply dipping epithermal quartz/sulphide veins are found within a 12 km long by 3 km wide northwest-trending corridor called the Nansen trend. The precious-metal-bearing veins occur in all lithologies within the Nansen trend and contain (in order of decreasing abundance) pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, stibnite and tetrahedrite. Two types of epithermal veins are recognized: an early “cherty” quartz-sulphide vein with fine-grained pyrite and arsenopyrite; and a later coarse-grained quartz-sulphide vein with pyrite, galena and sphalerite and higher precious metal values. Within the trend is the Mount Nansen porphyry complex which hosts two cores of potassic alteration. The character of mineral occurrences varies with increasing distance away from these Central Porphyry Zone and are here recognized as the Peripheral Porphyry, Transitional and Epithermal zones. Together, they comprise a porphyry-to-epithermal transition. Placer mining is predominant on creeks draining from the porphyry complex, implying a porphyry source for the placer gold.