Geology and Genesis of the Mount Skukum Epithermal Gold-Silver Deposits, Southwestern Yukon (NTS 105 D/3, 6)

The Eocene Mt. Skukum gold-silver epithermal deposits are 65 km southwest of Whitehorse in the Yukon Territory. Veins are in nearly flat-lying Eocene andesitic volcanic rocks of the Mt. Skukum Caldera Complex, part of the Sloko Volcanic Province, which unconformably overlies the Mesozois Coast Plutonic Complex and Paleozoic to Precambrian metamorphic rocks. Significant veins are contained in a regional halo of propylitic alteration centered on a graben in the southwestern corner of the Mt. Skukum Caldera Complex. Zones of steeply-dipping quartz-carbonate-sericite veins are associated with major faults and rhyolite dykes which bound blocks in the graben. Electrum and native silver form fine grains which average 15 to 20 microns and locally exceed 1 mm across, in veins containing only trave amounts of sulphides. Fliuid inclusions indicate that vein minerals were deposited from 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 indicate deposition about 470 m below paleosurface. Oxygen and carbon isotope composition of minerals in the deposit and surrounding wall rocks indicate that depositional fluids were meteoric. Large depletions in O18 content of andesitic rocks in the deposit area indicate a minimum water: rock mass ratio of 0.81:1. Precious metals at the Mt. Skukum deposit were emplaced at relatively low temperature, near surface, by a meteoric water dominated hydrothermal system driven by heat from associated rhyolite dykes. Gold and silver were leached from andesitic and rhyolite stocks and volcanic rocks as well as metamorphic and granitic basement, and precipitated with quartz and carbonate in permeable conduits such as fault zones, and breccia bodies.