Mount Hinton: Geologic SettingThe Mount Hinton gold vein property is located in the Mayo mining district (NTS 105 M) of central Yukon, approximately 10 km southeast of the world-famous Keno Hill silver mining camp (Fig. 1). The Keno Hill and Mount Hinton mining camps are situated in the central part of the 550 km long Tombstone Gold Belt, a region of silver and gold mineralization in Alaska and Yukon (Fig. 1, 2) related to Cretaceous intrusions called the Tombstone Plutonic Suite (TPS). The Roop Lakes stock dated at 92 Ma that lies about 10 km east of the main area of vein mineralization on the Mount Hinton property (Fig. 3, 4) is believed to be responsible for a hydrothermal system that deposited silver and gold-rich veins.
Mount Hinton is located within the Selwyn Basin (Fig. 2), Neoproterozoic (760- 730 Ma) to Paleozoic elongate rift-controlled epicratonic sedimentary basin between the Mackenzie foreland belt (platform) and the accreted terranes & displaced elements of the ancient North American continental margin. Rocks from Neoproterozoic to Cretaceous are found within the present geographic boundaries of the Selwyn Basin that existed in its tectonic entity from Late Properozoic through Devono-Mississippian. Gritty quartz sandstone, shale and carbonate rocks of the Hyland Group underlie northwestern Selwyn Basin (Fig. 3). They are believed to be Late Neoproterozoic to Early Cambrian age (based on primitive trace fossils) and are interpreted as slope to basin facies that correlate with the upper part of the Windemere Group on the MacKenzie platform. On the Mount Hinton property, the Hyland Group is exposed south of the Robert Service thrust fault (Fig. 3, 4) and is locally known as the “Upper Schist”. From Cambrian to Early Devonian relatively thin successions of carbonates, shale and chert of Middle Cambrian-Silurian Road River Group (Gull Lake, Rabbitkettle, Duo Lake and Steel Formations) (Fig. 3) and Late Silurian-Early Devonian Nogold Unit were deposited indicative of shallow marine, periodically anoxic conditions that suggest an isolation of the basin from an open ocean. Episodes of rifting were reflected in unconformities and accompanied by volcanism during Cambrian, Early-Middle Ordovician and Devonian. Upper Camprian-Silurian rocks are not exposed on the Mount Hinton Property. The final and most widespread rifting event in the Selwyn Basin occurred during Early-Devonian to Early Mississippian and resulted in subsidence of the entire miogeocline, local uplift and formation of secondary extensional basins. Slide Mountain Ocean was opened to the west in a backarc setting inboard to a Devono- Mississippian arc (Yukon-Tanana terrane, Fig. 2). Devono-Mississippian rocks in the Selwyn Basin comprise the Earn Group (Fig. 3, 4). Earn Group in the Mayo area (NTS 105M) is subdivided into three units: DMv- felsic metavolcanic unit, DMp- metasedimentary unit of siliceous slate, carbonaceous schist, metachert and metaconglomerate and unit DMe composed of siltstone, shale, chert, sandstone, conglomerate and grit that is only found in the southern part of the Mayo area. Sedimentary rocks of the Earn Group are interpreted as deep-marine deposits of a turbidite basin with coarse clastic input such as submarine canyon. Carbonaceous phyllites of the Earn Group in the Mount Hinton area are locally known as the “Lower Schist” (Fig. 4). Early Carboniferous Keno Hill Quartzite (Fig. 3, 4) overlies the Earn Group and is believed to be at least partly stratigraphically interlayered with it. It is composed of massive to well-foliated quartzite with lesser phyllitic quartzite, chloritic and carbonaceous phyllite and minor limestone. The unit (locally called the “Central Quartzite”) has major economic significance because its fractures host silver and gold-rich veins at the Keno Hill and Mount Hinton camps. The Keno Hill Quartzite is interpreted as deposited in shallow coastal well-aerated environment based on the discovery of plant remains and gastropods. Early Carboniferous conodont fossils were recovered from the Keno Hill Quartzite in the Dawson area (NTS 116 B/C). Zircons from Yukon Gold Marg VMS deposit (Fig. 1, 4) provide broader Late Devonian-Early Carboniferous age. The youngest strata in the Mayo and adjacent areas (do not occur at Mount Hinton) is Triassic-Jurassic Jones Lake Formation composed of slate, shale, sandstone and limestone indicative of shallow-marine environment. Triassic tabular bodies of diorite and gabbro intrude rocks of the Earn Group and the Keno Hill Quartzite at the Mount Hinton and adjacent areas and are locally called the “Greenstone”. The U/Pb isotopic date of 232 Ma was determined for zircons and baddelyite from southern Ogilvie Mountains in Dawson map area (NTS 116 B/7, B/8). Major change in the tectonic evolution of the Selwyn Basin occurred when an orogeny commenced in Jurassic with the exotic elements of the composite Yukon-Tanana terrane (Fig. 2) colliding with the ancient continental margin. Convergence initiated folding, metamorphism and the development of thin-skinned, N-directed thrust sheets: Dawson, Tombstone and Robert Service (Fig. 3, 4). Timing of fold-and thrust-related deformation is constrained to be Late Jurassic to late Early Cretaceous. Main thrust faults are more than 200 km long and are rooted in a single detachment in the Yusezyu Formation (lower part of the Hyland Group) with faults cutting progressively deeper from north to south. The Tombstone and Robert Service thrusts are nearly parallel to each other and the earlier Robert Service Thrust gets deformed within highly strained ductile shear zone in the hanging wall of the Tombstone Thrust called the Tombstone High-Strain Zone or THSZ (Fig. 3). The Dawson Thrust fault is a linear Mesozoic fault that juxtaposes rocks of the Selwyn Basin on its south side to shallow water sequence of the MacKenzie platform on its north side. The Dawson Fault represents moderate amount of displacement compared to other faults with an offset that could be as little as 2-4 km. The Dawson Fault is interpreted to be an ancient underlying basement structure, which repeatedly influenced patterns of sedimentation, igneous activity and faulting since later Proterozoic time. Both the Robert Service and Tombstone faults are folded over regionally-extensive McQuesten and Mayo Lake antiforms (Fig. 4) in the Mayo area. Regional metamorphic grade reaches lower to middle greenschist facies in the lower part of Robert Service and the underlying Tombstone thrust sheets. Collision-related deformation ceased by 105-100 ma and was followed by the Middle and Late Cretaceous emplacement of predominantly felsic to intermediate intrusions of the Tombstone Plutonic Suite (96-90 ma) (Fig. 3) and McQuesten Suite (64-70 ma). The Tombstone gold deposits in the western Selwyn Basin include Brewery Creek (13.3 Mt at 1.44 g/t Au) and Dublin Gulch (99 Mt at 1.1 g/t Au), as well as properties that are currently in different stages of exploration such as Scheelite Dome, Clear Creek and others (Fig. 3). Roop Lakes stock belongs to the Mayo group of TPS intrusions predominantly composed of quartz monzonite. During late Cretaceous-Tertiary newly-assembled continental margin was displaced about ~425 km along the Tintina fault (Fig. 1, 2, 3). The onset of the dextral transcurrent regime is believed to result from the initiation of the Kula plate at ca. 85 Ma, and its subsequent northward movement relative to North America. |
