Superimposed Late Cretaceous and earliest Eocene gold mineralization and deformation events along the Llewellyn-Tally Ho deformation corridor in northwest British Columbia and southern Yukon

Abstract

A revised geological and metallogenic framework for the Llewellyn-Tally Ho deformation corridor in southern Yukon and northwest British Columbia is proposed. The long-lived fault zone generally demarcates the eastern and western limits of the Nisling (Yukon-Tanana) and Stikine terrane, respectively. Epithermal (e.g. Engineer mine), intrusion-related and 'mesothermal' (e.g. Bennett plateau - Middle Ridge areas and part of the Wheaton district and Wann River area) gold deposits and occurrences are spatially associated with the Llewellyn-Tally Ho deformation corridor. The Llewellyn fault is a southeast-striking, steeply dipping, brittle and dextral strike-slip structure that overprints penetrative fabrics and ductile deformation. The Tally Ho shear zone, Yukon, represents a ductile deformation zone with a penetrative foliation and local mylonite zones. The Tally Ho shear zone is overprinted by late brittle faults, which likely correspond to the northern extension of the Llewellyn fault. Previous work and this study demonstrate that brittle strike-slip deformation along the Llewellyn fault occurred between ca. 56 and 50 Ma. Two unfoliated ca. 75 Ma granodiorite intrusions crosscut the earlier ductile fabrics, which deform a ca. 98 Ma porphyry dyke in the Tally Ho shear zone and a ca. 98 Ma granodiorite pluton along its western limit, indicating that parts of the early ductile fabrics formed between ca. 98 and 75 Ma. At Bennett plateau, the Skarn Zone represents one of the 'early' gold mineralization styles consisting of an amphibole-sulphide skarn with structurally controlled quartz-actinolite-carbonate veins within metasedimentary rocks of the Stuhini Group adjacent to Late (?) Cretaceous intrusions. A massive ca. 76 Ma dyke crosscuts the highly strained and hydrothermally altered host rock and hence provides a minimum age for the mineralization and ductile fabrics. To the south, the Engineer mine is a structurally controlled epithermal vein system related to second-order splays of the Llewellyn fault. A ca. 54 Ma dyke is cut and offset by the main-stage veins and related fractures locally providing a maximum age for mineralization. Vanadian-illite, dated at ca. 50 Ma and intergrown with gold from the Engineer vein constrains the timing of the main-stage veining and mineralization. The ca. 20 Myr time-gap between the early and late deformation events and their respective gold mineralization not only confirms these occurred at different times and at different structural levels, but also suggest they are related to distinct tectonic events. The intrusion-related and 'mesothermal' gold mineralization along the Llewellyn-Tally Ho deformation corridor is associated with Late Cretaceous deformation and magmatism, and as such, is broadly coeval and analogous with some of the mineral occurrences of Dawson Range polymetallic belt in west-central Yukon. The epithermal mineralization and late brittle event are related to the earliest Eocene magmatism and the Llewellyn fault, and match the timing and tectonic framework of the Juneau gold camp in southeastern Alaska.