The Brown-McDade deposit was the first vein system discovered in the Mount Nansen camp and has produced approximately 34,000 ounces (1058 kg) of gold and 131,000 ounces (4,075 kg) of silver from 225,000 metric tonnes of ore since production began in November, 1996. Production rates have varied since the mill start-up but the carbon-in-leach (CIL) plant is currently operating near capacity at 700 metric tonnes per day. Mining at the Brown-McDade open pit has exposed two separate and distinct deposit types. The first type is gold-silver vein mineralization hosted by a massive feldspar porphyry dyke. These fine-grained quartz-sulphide veins and vein breccia are enclosed by silicified and/or intensely clay-altered brecciated feldspar porphyry. The feldspar porphyry dyke has intruded along an igneous-metamorphic contact that has been mined over a strike length of 50 m in the southern portion of the pit. The second deposit type that occurs at the north end of the pit consists of a siliceous, sulphide-rich breccia in a pipe-like structure hosted by metamorphosed carbonate and clastic rocks of the Nasina Assemblage. The pipe is elongate in plan with a high-grade core approximately 15 m wide and 25 m long surrounded by a low-grade envelope consisting of quartz-sulphide stringers in a silicified breccia. The deposits are separated by a northeast-striking fault which truncates and offsets the main vein-dyke mineralization. The ore is composed of fine-grained quartz and sulphides in narrow veins or as matrix to a breccia of silicified and pyritized wall rock fragments. Unoxidized ore contains dark grey silica and pyrite, arsenopyrite, sphalerite, galena, sulphosalts, bornite, stibnite and chalcopyrite. Gold is genetically related to the pyrite phase of the mineralization and occurs as 5 to 50 micron-sized inclusions in pyrite grains. Oxidation of sulphide minerals extends to depths of up to 70 m and a large portion of the gold grains have been exposed by oxidation of the sulphides and post-depositional cataclastic fractures in the pyrite. The silver mineralogy is not as well understood but appears to be related to the base metal sulphide mineralization.

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The Len porphyry gold prospect is located 47 km north of Mayo, Yukon, in the Tombstone Suite intrusive belt. The area was explored as a Keno Hill-style silver prospect in the 1960s and 1970s. An arsenic-in-soil anomaly first identified in 1980 was followed up by soil geochemistry and excavator trenching in 1996. Multiple sheeted quartz-sulphide veins hosted in a previously unmapped granodiorite stock were discovered during the trenching program. A six-hole program of diamond drilling in 1997 encountered grades ranging up to 2.22 g/t gold across 18.6 m, and showed that gold mineralization is dominantly within, but not restricted to, the intrusive stock.

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 Boulevard gold prospect, located in the Independence Creek area of the Dawson Range, comprises sheeted, auriferous quartz-sulphide-carbonate veins and fault breccia, hosted mainly by mafic schist. The nearby Toni Tiger molybdenum showing is characterized by quartz-molybdenite veins cutting Late Permian meta-aplite and garnet-pyroxene skarn of uncertain age. We present geochronological evidence that gold and molybdenum were deposited at 96-95 Ma, approximately 3 m.y. after intrusion of the Dawson Range batholith and Coffee Creek granite. Fluid inclusions from mineralized quartz veins suggests that gold at Boulevard and molybdenite at Toni Tiger were formed from similar H2O-CO2-NaCl type fluids between 279 and 310°C and >1 kbar. We conclude that both are part of the same mineralizing system, and that structurally-hosted gold at the nearby Coffee deposit and in the Moosehorn Range of western Yukon may be broadly related, post-arc orogenic systems developed during exhumation of the Dawson Range in mid-Cretaceous time.

A copy of this thesis is available at the EMR library – QE390.5 M54.

This report includes a summary of the mineral exploration history of Yukon, an overview of major discoveries and a survey of exploration trends in the territory.

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The Myschka property overlies a large mineralized area within and adjacent to a 1200 m by 600 m Cretaceous Tombstone Suite granodioritic intrusion. Mineralization is controlled by at least four wide east-west-trending lensoid zones of faulting, brecciation and hydrothermal alteration. Just north of the intrusion, the zones were followed for a distance of 1500 m; widths of individual zones vary from 20 to 100 m. The zones tend to coalesce into much wider (up to 200 m) brecciated packages that dip steeply (70-85°) to the south and apparently crosscut the intrusive. A network of northerly dipping fault and alteration zones crosscut these breccias. The breccias include intensive quartz stockwork and thicker quartz-filled shear zones containing disseminated gold-bearing sulphide (pyrite, arsenopyrite) mineralization. Rock samples returned numerous strongly anomalous gold values ranging from 200 ppb to 1.05 g/t throughout the extent of the breccia zones. Larger quartz veins locally exhibit much stronger sulphide enrichment, resulting in higher Ag, Bi, Sb, Pb, Zn and Cu values influencing property-scale geochemical zonation. A distinctive gold and pathfinder element soil anomaly is coincident with the breccia packages. During the 2002 exploration program, the prospect was advanced to drill-ready stage. Proposed drilling will test subsurface continuation of gold-bearing fault/breccia and alteration zones into the intrusive rock.

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