Quantitative mineralogy, U-Pb geochronology of zircon and monazite, 40Ar/39Ar geochronology of muscovite and sericite, and Pb isotopes from galena in veins and feldspar in plutons provide insight into the age of metamorphism, mineralization, intrusion emplacement and the sources of metals at the Plateau South (MINFILE 105N 034, 035, 036) occurrences in central Yukon. Orogenic mineralization and metamorphism is ca. 110 Ma to 100 Ma, and possibly as old as ca. 130 Ma. Following deformation and regional metamorphism, two biotite-muscovite plutons, the Russell stock and Armstrong pluton, were emplaced at 95.39 ± 0.03 Ma and 95.51 ± 0.03 Ma, respectively. These plutons are here reassigned to the Tungsten suite based on mineralogy, chemistry and age. Coeval with these plutons are contact metamorphism and possibly intrusion-related mineralization. Lead isotopic data from galena cluster into two groups: Group 1 is enriched in thorogenic Pb with 206Pb/204Pb values between 18.31 and 18.14, 207Pb/204Pb between 15.62 and 15.55 and 208Pb/204Pb between 38.77 and 38.30. Group 2 is isotopically evolved with 206Pb/204Pb values between 19.13 nd 18.91, 207Pb/204Pb between 15.78 and 15.63 and 208Pb/204Pb between 39.24 and 39.07. We suggest that late Early Cretaceous mineralization is related to large-scale orogenic fluids that tapped primitive (deep?) metal sources and early Late Cretaceous mineralization, coeval with local intrusions, sourced isotopically distinct metals from the intrusions. Alternatively, all mineralization could relate to Early Cretaceous orogenic fluids but with heterogeneous, locally derived metal sources and thermal resetting of Ar ages near the intrusions.

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We present new field and U-Pb analytical data from the Sonora Gulch Project that demonstrate a protracted history of polymetallic mineralization (Au-Ag-Cu-Zn ± Mo) associated with several pulses of Cretaceous magmatism. Recent exploration on the Sonora Gulch Project has highlighted the presence of two important mineralized zones: the Nightmusic zone, a mesothermal Au-enriched base metal skarn, and the Amadeus zone, an epithermal Au-Ag system. Four U-Pb age dates determined from each of two feldspar porphyry dykes (ca. 74 Ma), a weakly mineralized quartz porphyry stock (ca. 75 Ma) within the Nightmusic zone and the Au-Ag mineralized Amadeus stock (ca. 75 Ma), demonstrate the widespread occurrence of Late Cretaceous magmatism. The age determinations indicate that mineralization occurring within the Sonora Gulch project area are temporally equivalent to the Casino Cu-Au-Mo deposit, located roughly 40 km to the west-northwest. These new data extend the currently known eastern limit of Late Cretaceous magmatism and associated mineralization.

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 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 Blende zinc-lead-silver deposit, 60 km northeast of Keno City, Yukon Territory, is spatially associated with a mid-Proterozoic fault zone that crosscuts stromatolitic dolostones of the Middle Proterozoic, upper Gillespie Lake Group. It is the largest known strata-bound, carbonate-hosted, zinc-lead deposit in the Yukon Territory. Mineralization, largely epigenetic, consists of sphalerite and galena, with lesser pyrite and chalcopyrite, and trace friebergite in quartz-carbonate veins and veinlets. Veining is zoned from copper- and silver-rich mineralization at the base of the deposit, through lead- and zinc-rich, to zinc-dominated at the top. Detailed cross-section mapping indicates that controls on the deposit are both stratigraphic and structural. Many of the stromatolite beds are mineralized and are especially rich in lead and zinc close to normal faults. Fluid inclusion studies indicate that mineralizing fluids were about 285 degrees C during main-stage mineralization. Sulfur isotope analyses of unmineralized whole-rock samples cluster tightly around an average delta 34 S value of 23.6 + or - 0.4 per mil. The source of sulfur is apparently seawater sulfate, probably from minor anhydrite in the host dolostone. Sulfide sulfur isotope ratios define three populations. Local blebs of pyrite associated with stromatolites have markedly negative ratios (- 15.1%) that are probably bacteriogenic in origin. Vein sulfides in tight dololutites have ratios between 19.0 and 26.3 per mil, with an average delta 34 S value of 23.2 PM 0.9 per mil. This is virtually identical to the whole-rock sulfur isotope ratio. Veins that crosscut stromatolitic beds have sulfur isotope ratios between 18.3 and 7.4 per mil with a mean of 15.0 + or - 1.2 per mil. Intermediate sulfur isotope ratios apparently reflect a mixed sulfur source of both whole-rock sulfate and biogenic sulfide. Carbon and oxygen isotopes in host rocks and ore-stage dolomite spar are related genetically; both are related to ancient seawater. The calculated isotopes for mineralizing fluids are characteristic of basinal brines...

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Gold mineralization has been identified on the foliated margins of Paleozoic gabbroic intrusions, where a protracted series of structural preparation events has enhanced rock permeability. Rheological contrasts between these mafic rocks and amphibolite facies metasedimentary hosts have resulted in variable foliation development, especially at margins of mafic bodies. Initial foliation development centred on granitoid material in magmatic intrusion breccia, and was enhanced by syn-metamorphic biotite formation. Early mineralized quartz vein development occurred under greenschist facies conditions during and/or after regional Mesozoic thrust stacking of kilometre scale rock slabs. Initial vein emplacement was largely controlled by the foliation in mafic gneiss, and minor chloritic alteration and later hydrothermal generations cut across all structures. Gold mineralization involved only minor introduction of As, Sb, and S, and the hosting quartzite has higher As, Sb, and S content than most mineralized veins. Au is accompanied by elevated Bi, Mo, and Te in quartz veins.

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