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 Mesozoic 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.

Between the Cretaceous granitic rocks (Hake Batholith on the west; Cassiar Batholith to the east) are three belts of metamorphic rocks, collectively part of Yukon-Tanana terrane. These are remnants of oceanic and continental volcanic arcs, and marginal basin sediments of Early to mid-Paleozoic age. At the head of Borden Creek are thick carbonate and andesitic volcanic rocks correlated with Klinkit Group. The Ram Creek fault and Hidden Lake fault are not exposed but deduced to be steeply dipping brittle structures with northeastward thrust or transpressional offset, based upon more complete exposure to the southeast in 105B/3 map area. The former is likely of Cretaceous age; the latter was active between mid-Permian and Early Jurassic time.

This property, which adjoins the Mt. Nansen property on the north side, is bisected by Discovery Creek, a significant placer gold producer. Two parallel zones of anastomosing quartz veins and porphyry dykes cut Cretaceous intrusive rocks and contain variable amounts of gold and silver over substantial widths. The two mineralized zones lie approximately on trend with the Brown-McDade and Webber-Huestis zones on the Mt. Nansen property to the south, and with gold and silver-bering veins on the Tawa property to the north. The mineralized zones are deeply oxidized, and the property appears to have good potential as a bulk tonnage low-grade oxide gold deposit.

In the eastern Ogilvie Mountains, geological mapping (1:50,000) was undertaken on map sheets 116A/10 and 11, which straddle the boundary between the Foreland and Omineca belts of the Cordilleran Orogen. The area is underlain by a diverse assemblage of epicontinental and miogeoclinal sediementary, and subordinate volcanic and intrusive rocks, ranging in age from Early Proterozoic to Triassic. These rocks represent most of the geological history of the northern Cordilleran miogeocline and its supracrustal basement.

Most epigenetic occurrences of gold and silver in the Pelly Mountains of central Yukon form two adjacent clusters at the headwaters of the Ketza River and Seagull Creek. The origin of these deposits has never been clear because they have been scarcely studied. Many are in Mississippian volcanic and intrusive rocks to which some have been attributed, and none are clearly associated with Mesozoic intrusions. This paper summarizes the characteristics of the deposits in this district which are probably epigenetic, and presents evidence that they are related to a domal uplift here named the Ketza-Seagull Arch, and to one or more buried Cretaceous intrusions.