Superimposed auriferous structural events along the Llewellyn-Tally Ho deformation corridor in southern Yukon and northwest British Columbia

Abstract

The Llewellyn-Tally Ho deformation zone in northwest British Columbia and southern Yukon demarcates the eastern limit of the Nisling terrane and the western limit of the Stikine terrane and has spatially related epithermal, mesothermal, and intrusion-related gold deposits and occurrences. The Llewellyn fault is a southeast-striking, steeply dipping brittle dextral strike-slip structure that overprints 'early' penetrative fabrics and ductile deformation. The Tally Ho shear zone, in the Yukon, represents an early ductile deformation zone, comprising one penetrative foliation (Smain) and mylonite zones that are overprinted by late brittle faulting, analogous to 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 granodiorite intrusions (ca. 75 Ma) crosscut the early ductile deformation fabrics. In the Tally Ho shear zone, Smain crosscuts a granodiorite pluton and porphyry dyke, which have yielded preliminary U-Pb zircon dates of ca. 98 and 95 Ma, respectively. As such, we infer that the early ductile fabrics formed before ca. 75 Ma and after ca. 95 Ma. This study further demonstrates that the early ductile and late brittle deformation are separated by at least ca. 20 Ma, indicating the various styles of gold mineralization developed during temporally distinct tectonic events: an 'early' late Cretaceous ductile event typical of the Tintina gold belt; and a 'later' Eocene brittle event matching the timing and structural framework of the Juneau gold camp.