Whitehorse 2016
Technical Program

SS4:  Structure, magmatism and metallogeny of the evolving North American Cordilleran margin 
Organizers / Organisateurs:  Sarah Gleeson, Murray Allan & Craig Hart
Room / Salle:  T1023
Date:  June-02-16
Time:  10:20 AM
Presenter:  Luke P. Beranek

Mid-Paleozoic magmatism and timing of VMS mineralization in the Pelly Mountains, Yukon:  Implications for Cordilleran tectonics and metallogeny

Beranek, L.P., lberanek@mun.ca, Piercey, S.J., Memorial University of Newfoundland, 300 Prince Philip Drive, St. John's, NL  A1B 3X5, and Friedman, R.M., University of British Columbia, 2020-2207 Main Mall, Vancouver, BC  V6T 1Z4

Felsic volcanic and volcaniclastic rocks of the Devonian-Mississippian Seagull group (informal) are recognized hosts to polymetallic (Zn-Pb-Cu-Ag) volcanogenic massive sulphide (VMS) mineralization in the Cassiar terrane, south-central Yukon.  Despite abundant evidence for mid-Paleozoic magmatism and hydrothermal activity along the Cordilleran margin of western Canada, many fundamental questions remain about the precise age of the Seagull group and its genetic relationship with rift-related units of the Yukon-Tanana terrane, Slide Mountain terrane, and ancestral North America.  To investigate these questions, we initiated a project to constrain the timing and tectonic setting of Seagull group magmatism in the central Pelly Mountains (NTS 105F and G).  At one locality near the headwaters of the McConnell River, Upper Devonian(?) black to tuffaceous shale and lithic sandstone units at the base of the Seagull group conformably grade upwards into Lower Mississippian(?) lapilli tuff and rhyolite flows.  Volcanic rocks at this and other localities yield both non-arc and arc-like trace element signatures that together are consistent with high-temperature felsic magmatism in a continental rift setting.  Peralkaline rhyolites at the Wolf VMS deposit in the Finlayson Lake map area (Yukon MINFILE 105F127) are geochemically similar to other non-arc felsic rocks in the Pelly Mountains, including those with Nd-Hf isotope signatures that indicate the sampling of old crustal material during regional magmatism.  Notably, this non-arc geochemical fingerprint is also recognized within gossanous, Tournaisian felsic rocks in the Quiet Lake map area that we recently analyzed by chemical abrasion (CA-TIMS) zircon U-Pb geochronology.  The available geochemical evidence therefore strengthens the idea that some VMS-associated Mississippian rhyolites in the Canadian Cordillera are the result of crustal melting processes.  Our working hypothesis is that Upper Devonian strata of the Seagull group likely comprise part of a turbidite basin that developed during regional extension and initial felsic volcanism, whereas Lower Mississippian strata were deposited in a metalliferous volcanic rift basin that was located behind the Yukon-Tanana continental arc-rift system, along what would become the passive margin side of the Slide Mountain Ocean.