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Geology and genesis of the Mount Skukum tertiary epithermal gold-silver vein deposit, southwestern Yukon Territory (NTS 105D SW) McDonald, Bruce Walter Robert


The Tertiary Mt. Skukum gold - silver epithermal vein deposit occurs 65 km southwest of Whitehorse in the Yukon Territory. Veins are hosted by a sequence of nearly flat-lying Eocene Skukum Group andesitic volcanic rocks of the Mt. Skukum Volcanic Complex, part of the Sloko Volcanic Province which unconformably overlies these intrusive complexes as well as metamorphic rocks of the Yukon Group. Major known mineralized zones occur within a regional halo of propylitic alteration centered on a fault-bounded graben within Main Cirque in the southwestern corner of the Mt. Skukum Volcanic Complex. Each zone consists of steeply-dipping quartz-carbonate-sericite veins associated with major faults and rhyolite dykes which bound blocks of the graben. Precious metals occur as electrum and native silver as fine grains averaging 15 to 20 microns and locaIly exceeding 1 mm across, in veins containing only trace amounts of sulphides. Fluid inclusions indicate that vein minerals were deposited from hydrothermal fluids averaging 313°C with an average salinity of 0.7 weight percent NaCl equivalent. Primary inclusions show that depositional fluids existed under two pressure regimes; one close to hydrostatic, the other approaching lithostatic. Both reflect depths of deposition of about 470 m below paleosurface. Variable fluid pressures reflecting similar depths of deposition combined with variable liquid to vapour ratios in primary inclusions as well as abundant textural evidence of hydrothermal brecciation indicate that boiling was common during mineralization. Oxygen and carbon isotope composition of minerals in the deposit and surrounding wall rocks indicate that depositional fluids were meteoric in origin with no contribution from magmatic sources. Large depletions in 0¹⁸ content of andesitic rocks in the deposit area indicate a minimum water rock ratio over the life of the deposit of 0.81:1. Precious metals at the Mt. Skukum deposit were emplaced at relatively low temperature in a near surface environment by a circulating, meteoric water dominated, hydrothermal system driven by a heat source associated with the rhyolite dykes. Gold, leached from andesitic volcanic rocks and metamorphic and granitic rocks was precipitated with quartz and carbonate in permeable conduits such as fault zones, and breccia bodies.

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