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 geology in the Mount Nansen area, of the Dawson Range, comprises a metamorphic basement overlain and intruded by mid and Late Cretaceous magmatic rocks. Mid-Cretaceous epithermal deposits are recognized in the Brown-McDade cluster in the south, and Late Cretaceous porphyry and epithermal occurrences in the centrally located Klaza cluster. Here we report eight new U-Pb zircon crystallization ages, two CA-TIMS and six LA-ICPMS, and Pb isotopic data for igneous feldspar from six samples. A 199.06 ± 0.96 Ma crystallization age demonstrates the host rock to the Brown-McDade deposit is the Late Triassic to Early Jurassic Minto suite. We report crystallization ages of 111 ± 1.8 Ma for the Dickson Hill porphyry, 107.96 ± 0.03 and 107.86 ± 0.03 Ma for the central and border phases of the Bow Creek granite, respectively, and 107.0 ± 0.72, 107.0 ± 0.78 and 107.5 ± 0.67 Ma for equigranular and porphyritic phases of the Dawson Range granodiorite. These ages confirm a mid-Cretaceous Whitehorse suite affinity for these rocks with porphyritic textures suggesting high-level emplacement. Feldspar Pb isotopic data of igneous rocks in the Mount Nansen area become more radiogenic with time and are distinct for Late Triassic to Early Jurassic plutonic rocks versus Cretaceous rocks. Cretaceous feldspar isotopic data broadly overlaps Pb isotopic values of galena from deposits throughout the Dawson Range suggesting coeval magmatic rocks are a significant source of metals.

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The Mount M'Clintock map area, northeast of Whitehorse, is dominated by Middle Triassic to Jurassic sedimentary and volcanic strata of Stikinia with small portions of the Cache Creek and Yukon-Tanana Terranes. These assemblages were deformed prior to the mid-Cretaceous intrusion of three plutonic suites and the deposition of two suites of volcanic rocks. Sedimentary rocks previously mapped as undifferentiated Lewes River and Laberge Group strata are separated into their respectve groups and further sub-divided into several members. Lewes River Group rocks form three units that are Carnian and older, Norian, and Upper Norian in age and are represented by siliceous siltstone and calcareous sandstone, conglomerate and limestone respectively. Laberge Group strata are divided according to lithology and dominated by siltstone-sandstone couplets and massive siltstone with lesser conglomerate and volcanogenic sandstone. Volcanic rocks previously mapped as Hutshi Group are divided into the dominantly mafic and submarine, MiddleTriassic Joe Mountain volcanic complex (JMVC) and the dominantly felsic and sub-aerial Mount Byng volcanic complex (BCVC). The volcanic and sedimentary rocks generally increase in age from Middle Triassic to Jurassic from west to east across the map area. Plutons cutting these strata belong to the M'Clintock Lakes (120 Ma), the Whitehorse (115 Ma), and the Mount McIntyre (109 Ma) plutonic suites. The predominantly felsic fragmental rocks of the BCVC are genetically associated with the Byng Creek pluton of the Mount McIntyre plutonic suite. The BCVC is nested into the country rocks and its eastern margin and is preserved as the rim of a tilted caldera. Strata of the Laberge and Lewes River groups are folded throughout. Wavelengths are on the order of approximately 1-2 km, but are much tigter in black siltstone units and adjacent to northwest-trending faults. Faults are ubiquitous throughout the map area and form three sets. North-trending faults are the most common. They are spaced a few kilometres from each other and dictate the physiography and drainage of the region. Northwest-trending faults, in the northeastern corner of the map area control the Teslin River valley and juxtapose Yukon-Tanana rocks with Stikinia. Older faults are dominantly east-trending but are terminated or reactivated by younger faults. Copper (gold-molybdenum-tungsten) skarns and gold-bearing quartz veins are the two mineral deposit types most likely to be discovered in the map area. The source of the placer gold in Sheldon Creek is unknown but may be related to gold veins in the JMVC, BCVC, Sheldon Creek volcanics or the surrounding sedimentary rocks. Hydrothermal activity in the JMVC is characterized by orange weathering alteration, breccias, and carbonate veins. Cache Creek rocks in the southern part of the map area are targets for listwaenite associated gold veins.

A copy of this thesis is available at the EMR library – EMR Library QE195.A66 1998.

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.

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Preliminary geological map (1:50,000 scale) of Mount M'Clintock area, southern Yukon (NTS 105D/16) including geological cross sections and mineral occurrences.

Surficial geological map (1:50,000 scale) of Mount Haldane area, central Yukon (NTS 105M/13) including marginal notes on physiography, glacial history, placer gold potential and terrain hazards.

A preliminary geological map (1:25,000 scale) of Mount Skukum volcanic complex, southern Yukon (NTS 105D/2,3,4,5), including one cross section.