Placer deposits occur in unusual geological settings in the Mayo Mining District, where gold is mined from Reid-age glacial till and glaciofluvial gravel and from more recent (McConnell and post-McConnell-age) alluvial fans and fan-deltas. In other districts these types of deposits are not generally explored or prospected for placers. In the Mayo area, placer deposits are best preserved near the maximum limit of glacial ice where ice-scouring is minimal and depositional processes dominate. Pre-existing alluvial gold deposits were likely buried in this region where the ice limits of the Reid and McConnell glaciations exist in close proximity. Initial studies of the geomorphology of the region's known placer deposits show that they occur in three main types of landforms of different ages. Alluvial fans and fan-deltas contain placer deposits that are McConnell and younger in age while valley-bottomplacers are likely interglacial, glacial or glaciofluvial deposits of Reid age or older. A number of drainages have not been extensively explored or prospected; however, given similarities with known placer occurrences in the area, the potential for discovery of new placer deposits is good for many sites in the Mayo map area.

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The AurMac property, located 35 km north of Mayo in central Yukon, includes two metasedimentary rock-hosted gold deposits: the 6158 koz Au Powerline deposit and the 845 koz Au Airstrip deposit. Mineralization at the Powerline and Airstrip deposits is characterized by gold in sheeted quartz veins and mineralized skarn horizons, respectively. The AurMac deposits straddle the Robert Service thrust fault whereby the Powerline deposit is hosted in the Late Proterozoic to Cambrian Hyland Group hanging wall, and the Airstrip deposit is hosted in the Mississippian Sourdough Hill Member of the Keno Hill Quartzite footwall. Host rocks comprise siliciclastic metasedimentary rocks, variably calc-silicate–altered calcareous metasedimentary rocks and magmatic rocks. Magmatic rocks in the Powerline zone consist of foliated mafic horizons that are geochemically similar to Cambro-Ordovician magmatic rocks found in Hyland Group metasedimentary rocks in the McQuesten, Mayo, Clark Lakes and Hart River map areas. In the Airstrip zone, magmatic rocks include a steeply south-dipping, unfoliated, aplite dike. Evidence for intrusion-related gold mineralization at AurMac includes sheeted vein and skarn mineralization similar to the intrusionhosted, intrusion-related gold deposits at Dublin Gulch, as well as the presence of metamorphic porphyroblast assemblages that suggest contact metamorphism. These findings suggest potential for further discovery of mineralized intrusion-hosted zones on the AurMac property and sedimenthosted, intrusion-related gold deposits elsewhere in the region.

The Minto and Williams Creek copper (-gold) deposits in western Yukon are hosted by variably deformed Early Jurassic (198-197 Ma; U-Pb) plutonic rocks and to a lesser extent strongly metamorphosed supracrustal rocks. These rocks are pendants and schlieren within slightly younger (197 Ma; U-Pb), intermediate-composition intrusive phases of the Granite Batholith. Chalcopyrite and bornite are disseminated and also occur as stringers in these rocks. Alteration muscovite associated with late quartz-feldspar-epidote veins gives a 182 Ma Ar-Ar age. Geobarometry on postmineral intrusive phases in the area indicate that they were emplaced at a depth of >9 km. Hornblende geochemical studies of plutonic and meta-plutonic host rocks at Minto and Williams Creek indicate that they formed in a continental magmatic arc setting. Cu/Au ratios and field observations indicate that supergene mobility of copper was more extensive at Williams Creek than at Minto. Our results indicate that the two deposits represent variations on typical copper (-gold) porphyry deposits.

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

Despite many years of exploration and relatively limited success, the Rusty Springs prospect retains considerable potential for a large-tonnage deposit. The property lies within the east-vergent Taiga-Nahoni fold belt, occurring in the core of a structural culmination exposing host dolostones of the Lower and Middle Devonian Ogilvie Formation. Mineralization occurs in stratabound and discordant zones along the contact with the overlying Devono-Mississippian unnamed shale. Various deposit models, ranging from Mississippi Valley-type to epithermal vein-type have been employed. Poor exposure and relatively deep weathering, resulting from the lack of Pleistocene glaciation, account for the lack of consensus with regard to genesis. Evidence points to the potential for a high-temperature, carbonate-hosted massive sulphide deposit (manto-chimney complex). The great extent of mineralized and altered rocks, together with their stratabound nature, significant thickness, local high grades, and potential for supergene enrichment, suggest that Rusty Springs remains an attractive drill-oriented exploration target.

The Cynthia property overlies a large (greater than 2x2 km) area of gold mineralization related to a Cretaceous Tombstone Suite quartz monzonite intrusive body. The mineralization is controlled by two district-scale fault zones and is especially intensive in the area of their intersection, located above and adjacent to the intrusive body. These larger structures host abundant gold-bearing massive and drusy quartz and chalcedony veins, zones of intense stockwork and strong brecciation, as well as numerous mineralized felsic dykes. The gold grades within the mineralized structures are commonly in the range of 200 ppb to 2.0-3.0 g/t Au, with higher (up to 16 g/t Au) values attributed to the fault intersection area. Multi-staged gold mineralization found in the quartz veins, stockwork and altered dykes is associated with sulphide minerals (mainly pyrite and arsenopyrite) and elevated As, Bi and Ag values. A later mineralizing episode produced sulphide mineral-bearing chalcedony and drusy quartz veins, with gold concentrations accompanied by elevated Sb, Hg, Ag and Pb values, indicating the affi nity of epithermal style gold mineralization. The property is considered to represent a bulk-tonnage exploration target, with potential of the structures to host a major gold deposit. During the 2002 exploration program, the prospect has been advanced to a drill-ready stage.

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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A new widespread, structurally controlled gold mineralizing system has been identified during the 2010 exploration drilling program at the Coffee Project, west-central Yukon. The Coffee Creek area is underlain by a sequence of shallowly to moderately south to southwest-dipping Paleozoic metamorphic rocks that are considered to be part of the Yukon-Tanana terrane and are intruded by the Cretaceous Coffee Creek granite along a west to northwest-trending contact. During the 2010m drilling program, structurally controlled gold mineralization was discovered in all major lithological units underlying the Coffee property. Importantly, these mineralized zones correspond to a number of discrete structural corridors. The gold zones are steeply dipping and characterized by extensive silicification in addition to sericite and clay alteration accompanied by variable As-Ag-Sb-Ba-Mo enrichment. Polyphase breccias of both hydrothermal and tectonic origin, in addition to andesitedacite dykes, are common within the gold-bearing structural corridors. The dominant sulphide is pyrite, although trace arsenopyrite, chalcopyrite and stibnite are observed locally. The similarity of breccia textures and alteration/sulphide mineralogy between all gold zones currently defined on the Coffee property implies a common mineralizing event.