The Mount Skukum Volcanic Complex (MSVC) is the northernmost extension of the Sloko volcanic province in western British Columbia, a broad northwest-trending volcanic belt along the northeast margin of the Coast Plutonic Belt. Rocks of the Sloko volcanic province are preserved as downfaulted blocks and as erosional remnants on higher upland surfaces. They comprise an assemblage of intermediate to felsic volcanic rocks and derived sedimentary rocks that lie unconformably on Cretaceous granitic rocks, folded Jurassic rocks and Precambrian (?) metasedimentary rocks. The MSVC is Paleocene-Eocene in age, and elliptical in plan; it covers an area of about 140 km squared. It is a downfaulted volcanic block, deposited on Cretaceous granitic rocks of the Ruby Range Batholith and older metasedimentary rocks of the Yukon Group. The complex is surrounded peripherally by several high-level rhyolite intrusions which have recently been dated at 53 ±1.1 Ma using rubidium-strontium geochronology. The MSVC has a maximum vertical thickness of 850 m. It includes:: 1) a downfaulted part of an andesitic stratovolcano which forms the western and southern parts of the complex and comprises the distal and medial facies assemblage of Formations 1 and 2, and the more proximal facies assemblage of Formation 3 a small felsic cauldron subsidence structure in the northeast corner of the area represented by the felsic cauldron-fill deposits of Formation 4; 3) an andesitic vent facies environment located in a small area in the midwestern part of the complex and comprising the deposits of Formation 5; and 4) a central quartz-feldspar-phyric rhyolite intrusion along the western boundary of the small felsic cauldron. A NNE-trending fracture system is the dominant and perhaps the latest structural feature in the MSVC. Economic epigenetic gold veins are found in three major subparallel NNE-trending fault zones. The veins consist of quartz and calcite and are unusual in that they contain no sulphides and have poorly developed wall rock alteration. As of February 1984, the average grade was 27 g/t gold and 22.63 g/t silver with proven reserves of 149,114 tonnes.

Veins in Main Cirque are hosted in major normal fault zones bounding down-dropped blocks to depletion of a magma chamber below the adjacent eruptive centre. Residual heat within the exhausted magma chamber or perhaps a slight resurgence of intrusive material may have driven the hydrothermal circulation which resulted in regional alteration and deposition of the quartz-carbonate veins. Due to the close relationship of these factors, the identification of further eruptive chambers within the volcanic complex as well as any associated collapse features is critical to future exploration. Identification of additional faults associated with the collapse feature represented by Main Cirque is also important because these faults potentially host additional ore. Gold-silver veins at Mt. Skukum display many of the classic characteristics of a relatively low temperature epithermal system as well as a low-sulphide adularia-sericite system. Vein formation probably occurred within several hundred metres of the paleosurface which may be represented by the intense kaolinitic alteration zone and the crater on the southeastern wall of Main Cirque. The Mt. Skukum deposits likely formed in a near surface environment by circulating meteoric waters in a hydrothermal system driven by a heat source associated with felsic dykes present in the area.

The Eocene Skukum volcanic complex, 60 km south-southwest of Whitehorse, is elliptical in plan, covers an area of about 140 km², and unconformably overlies Cretaceous granitic rocks and Precambrian metasedimentary rocks. The complex is fault-bounded and in places has been intruded by felsic dykes and stocks. A major north-trending fault divides the area into two parts: a western part which includes a lower interlayered sedimentary-volcanic sequence and an upper unit, approximately 500 m thick, characterized by andesite lava flows, pyroclastic flows and sedimentary units; and an eastern part which comprises about 800 m of altered felsic pyroclastic flows and brecciated, flow layered and spherulitic felsic lava flows. Study of the interlayered sedimentary-volcanic formation provides a control on the paleotopography of the Skukum area, and the depositional environment and provenance of the formation..

Gold mineralization at the Mt. Skukum deposit occurs in nearly vertical quartz-carbonate veins which crosscut flat-lying andesites with a NNE trend. The mineralized veins represent the second stage of a two stage hydrothermal system, the first of which resulted in emplacement of thin chalcedonic veinlets. These two stages of veins are probably indicative of an evolving hydrothermal fluid rather than being representative of two separate events. Vein emplacement is one of the latest of a series of events which began with volcanism, producing felsic and andesitic volcanic rocks which overlie basement in this area. Subsequent periods of tectonism produced large faults along which rhyolitic dykes were emplaced. Continued tectonism resulted in reactivation of old faults along which andesitic and dacitic dykes were injected, crosscutting rhyolite dykes in many cases. As volcanic activity waned, the faults remained active, leaving zones of high permeability which acted as conduits for the still active hydrothermal circulation. Veins appear to have been emplaced at low temperature in a circulating hydrothermal system driven by a heat source at depth associated with dykes present in the area. Circulating hydrothermal fluids may have leached gold from the surrounding andesitic volcanics during propylitization. Permeability may have been controlled by faulting, brecciated flow tops and bottoms, and lapilli tuff horizons. Gold was precipitated in highly permeable conduits, such as the Main Fault Zone and breccia bodies.

A preliminary geological map (1:25,000 scale) of Mount Skukum volcanic complex, southern Yukon (NTS 105D/2,3,4,5), including one cross section.

A copy of this thesis is available at the EMR library – QE195 M321. This thesis is available online at https://open.library.ubc.ca/cIRcle/collections/ubctheses/831/items/1.0302650.

Mainly schitose clastic strata of the northern Selwyn basin underlie Lansing map area. These strata form rounded mountains, although jagged ridgelines occur in the thermal metamorphic aureoles surrounding six Cretaceous granitic plutons. Major faults occupy some broad northwest-trending valleys: two of these extend eastward as the Hess and Macmillan faults (Abbott and Turner, 1990) in the Macmillan Pass area; another appears to contine westward as the Robert Service Thrust Fault. Argentiferous galena veins were intermittently mined from the east edge of the map area from 1976 to 1985; whereas the stratiform base metal and disseminated gold potential of these rocks have been investigated during the 1990s.

The Skukum area is located 58 km south-southwest of Whitehorse. It is an elliptical area of volcanic rocks, Tertiary in age, and surrounded by hypabyssal rhyolite intrusives. Field and petrographic evidence, fluorite and tourmaline stockwork, breccia pipes, roof pendants, miarolitic cavities and spherulites in the nine Skukum rhyolites suggest that they were emplaced at a high level. The intrusives vary in composition from rhyolite to dacite. The variation in texture within and between the intrusives can be explained by different rates of crystallization, temperature differences and compositional variability. Chemical data are in accord with the expected trends in a cogenetic suite of igneous rocks. Relatively low CaO and MgO, high SiO2 and anomalously low Sr concentrations indicate that the rhyolites were formed from a highly differentiated magma. Sr and Ba versus Al2O3 plots show that both k-feldspar and plagioclase were important fractionating phases. Rare earth element data further support this conclusion and also suggest that some accessory phase(s), such as monazite, allanite or fluorite help control the rare earth element behaviour. Partial melting of an already depleted source rock with residual plagioclase can also explain the patterns. The Bennett Lake ring and associated dykes are petrographically and chemically similar to the Skukum intrusives. However, Zr and TiO2 are present in higher concentrations in the Bennett Lake complex, indicating that they were derived by a slightly different fractionation process.

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.

The history of an eclogite sample and a mica schist sample from the western Teslin zone are discussed in view of garnet zoning profiles. The preliminary metamorphic results support the contention (de Keijzer et al., in press), based on earlier regional and structural arguments, of a structural contact (the "basal thrust" of de Keijzer et al., in press) between the Last Peak eclogite (part of the Anvil assemblage) and metasedimentary rocks of North American affinity to the west of it. Consequently, the eclogite is considered "exotic" with respect to the metasedimentary rocks. The proposed position of the Last Peak eclogite, a few hundred metres above the interpreted basal thrust within the zone of ductile thrusting, explains why it has experienced pervasive amphibolitization (hydration) since fault zones commonly act as conduits for fluid. It is unclear how much of the amphibolite-to-greenschist facies Anvil rocks surrounding the eclogite have experienced earlier high-pressure metamorphism.