This study describes the structural evolution of the Klondike Schist and the structural setting of mineralized mesothermal veins from which over 500 tonnes of placer gold has been derived. The Klondike Schist was emplaced as a series of thrust slices on top of a structural stack that includes at least three additional thrust slices. A distinctive set of mesoscopic structures, particularly a set of recumbent folds, formed during thrust emplacement. These folds have a general southeast trend, and deform thrust-emplaced ultramafic rocks. Underlying thrust panels contain folds that resemble these thrust-related structures, but have a consistent northeast trend. The Klondike Schist may have been rotated about a vertical axis during the latter stages of thrusting along phacoidal cleavage zones. Extensional sites in post-thrust kink folds and faults host mesothermal gold veins. Hence, gold mineralization postdated thrust stacking. Normal faults offset mesothermal veins, and host late-stage hydrothermal alteration zones.

A copy of this thesis is available at the EMR library – QE446.Y8 S83.

Kalzas, in central Yukon, is a porphyry-style stockwork and sheeted-vein wolframite deposit. Alteration includes a potassic core, a quartz-tourmaline-sericite zone and an outer quartz-sericite-pyrite zone, the latter in excess of 2 km in diameter. Wolframite is confined to the inner two zones, in an oval area 1500 m by 800 m. The wolframite is disseminated within the quartz-tourmaline stockwork and also occurs as coarse crystals in sheeted veins. Mineralization occurs within Neoproterozoic to Early Cambrian Hyland Group quartzites and phyllites, which are likely intruded at depth by a pluton, possibly of the Cretaceous Tombstone Suite. From 1981 to 1984, Union Carbide carried out mapping, soil and rock geochemistry, an airborne magnetometer survey, road building, trenching and drilling of two diamond drill holes. Results from Copper Ridge's 2001 sample program range from 0.3% WO3 to 0.5% WO3 over widths up to 70 m. They demonstrate the potential to define a signifi cant resource of surface-mineable tungsten mineralization at a grade of 0.4% WO3 or better. Drilling is required to confirm grade continuity at depth and along strike.

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.

Geological map (1:50,000 scale) of the West Klondike area, central Yukon (NTS 115O/14,15 and 116B/2,3) including marginal notes on mineral occurrences.

Several centres of continental volcanism are situated in southern Yukon. Two of these, the Skukum Volcanic Complex and the Bennett Lake Cauldron Complex BLCC are closely associated in space and gross geological characteristics, including their important role as hosts for epithermal precious metal mineralization. The BLCC was studied extensively by Lambert and the Skukum complex is currently under investigation by the author. The Skukum complex was previously thought to represent a similar structural and volcanic setting to the BLCC. Study of the Skukum complex began in the summer of 1982 with the idea that it represented a cauldron subsidence feature and that an improved understanding of its geology would assist in developing potential targets for mineral exploration. The geology in the area is complex due to the discontinuous nature and incomplete exposure of volcanic units and is further complicated by severe faulting. The author found that the Skukum complex differs from the BLCC in three respects:: 1) the presence of a bimodal suite of extrusive rocks (which suggests the tapping of two magma chambers), 2) its structural complexity and the absence of major cauldron subsidence and 3) the variability of depositional environments within the complex. Contrary to past speculation, the two complexes, which have been considered to be closely related in time and space, each represent a distinctive structural and volcanic setting.

Carbonate is an important component of gold-bearing quartz veins in the Klondike district, and also makes up an under-recognized proportion of the Klondike schist host rocks. The predominantly metavolcanic Klondike schist contains carbonate as disseminated porphyroblasts and as coarse quartz-carbonate segregations, and contains rare layers of marble. Chemical staining and LA-ICP-MS analyses reveal that, irrespective of paragenesis, carbonate is dominated by Mg-Fe-Mn calcite. Laser spectroscopic analyses of C and O isotopes reveal that marble is a 13C-enriched isotopic reservoir compared to carbonate in micaceous schist. Carbonate in gold-stage veins has a similar isotopic signature to carbonate in metamorphic segregations and porphyroblasts in the host rocks. We tentatively interpret these results to indicate that the CO2 component of vein carbonate has been remobilized from local sources during brittle deformation. The results of this study may bear on interpreting the scale of rock-fluid interaction during orogenic gold mineralization in the area.

One of the challenges in exploring for skarn mineralization is that barren skarn may not give any indication that mineralization is close by. The purpose of the present study is to evaluate titanite as an indicator mineral for tungsten-skarn mineralization. Titanite from samples of scheelite mineralization from the Risby, Ray Gulch and Mactung deposits was analysed by electron microprobe. The titanite is fluorine-rich and some grains contain anomalous tin, but in most grains metal concentrations are at or below the detection limits of the microprobe (~20-50 ppm). Future work is planned to analyze titanite by LA ICP-MS to determine, in particular, the W-Mo-Sn contents to further evaluate the use of titanite as an indicator mineral for W skarn exploration.

Surficial geological mapping in the central Kluane Ranges was undertaken to better understand the distribution and character of surficial materials. Upland surficial materials in the area are dominated by near-source bedrock derivatives. Valley bottom settings, including Shakwak trench and Duke River valley, are characterized by broad open valleys with thick deposits of Quaternary sediment. Permafrost is discontinuous in the study area and its character is affected by slope and aspect, topography and material texture. Mass wasting processes in the study area include rock fall and avalanches, debris flows and avalanches and active layer detachment slides. The distribution and character of surficial materials has significance for soil geochemical sampling programs and infrastructure and implications for mineral exploration and developments are addressed.

Detailed exploration conducted during 2006 in the western part of the Skukum Creek deposit has revealed new structural, mineralogical and geochemical features. The deposit incorporates a number of (at least six or seven) sub- parallel narrow mineralized zones, coincident with andesite-dacite-rhyolite dyke swarms extending for at least 1 km along strike and for hundreds of metres down-dip. Various mineralized zones differ in size, structural setting, intensity and composition of mineralization, and, in total, form a large mineralized package more than 200 m wide, corresponding to a property- to district-scale fault zone extending for over 10 km and traced by a dyke belt. Significant potential exists for the exploration of these structures along strike and down-dip. The diamond drilling intersected numerous high-grade intercepts of gold and silver mineralization corresponding to the low-sulphidation sub-type of epithermal gold-silver deposits. However, strong enrichment in base metals (up to 25% of combined Zn+Pb+Cu) and arsenic suggests essential differences from typical epithermal mineralized systems.