The Late Devonian to Early Mississippian Sicker Group of Vancouver Island, British Columbia, Canada, is host to the Myra Falls polymetallic, volcanic-hosted massive sulfide (VHMS) deposits (production has exceeded 30 Mt at average grades of 5.5% Zn, 1.6% Cu, 0.6% Pb, 2.0 g/t Au, and 54.0 g/t Ag), along with numerous historic VHMS workings and occurrences. Two cycles of bimodal volcanism and marine sedimentation at Myra Falls are prospective for VHMS deposits. These cycles are defined as the lower and upper subdivisions of the Myra Formation. The lower Myra Formation consists of the H-W member, Hanging-Wall Andesite, and Lower Mixed Volcanics units. Rhyolitic rocks of the H-W member host the HW, Battle, Trumpeter Zone, Extension Zone, Gap, Ridge, and Marshall Zone orebodies. The upper Myra Formation includes the L-M-P member, Upper Mixed Volcanics and the Upper Mafic units. Rhyolitic rocks of the L-M-P member host the Lynx, Myra, and Price orebodies. Prior to this study, geological observations suggested that the H-W and L-M-P VHMS mineral deposits were likely coeval based on similar volcanic host rocks, base metal sulfide mineralization zoning, and only 200 vertical meters of stratigraphic separation. Previous attempts to date the duration of felsic volcanism at Myra Falls have resulted in relatively imprecise crystallization ages and were unable to resolve the temporal relationship between the L-M-P and H-W members and the VHMS deposits they host. New chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon geochronology yielded crystallization and maximum deposition ages of volcanic rock units from Myra Falls. This new data temporally constrains the felsic volcanic history at Myra Falls by a precise CA-ID-TIMS age of 362 +/- 1 Ma for coherent rhyolite of the H-W member and a LA-ICPMS maximum depositional age of 356 +/- 3 Ma for volcaniclastic rhyolite of the L-M-P member. This new geochronological data suggests there is a 3.5-11 myr time interval between the H-W and L-M-P members and therefore, a previously unrecognized unconformity exists between the lower and upper subdivisions of the Myra Formation and that the VHMS deposits of the H-W and L-M-P members were not coeval, but likely formed from temporally, distinct hydrothermal events. The results of this study, together with published age constraints from across Vancouver Island, provide a revised geochronological and lithological framework for volcanism and VHMS deposit formation within the Sicker Group. At least three Paleozoic felsic volcanic events (similar to 362 Ma; similar to 355 Ma; and 310-300 Ma) host VHMS mineralization on Vancouver Island. Recognizing the specific chronostratigraphic intervals, and the lithostratigraphic setting, of VHMS deposits is important for genetic and mineral exploration models, and will aid explorers in identifying key stratigraphic units that are most prospective for VHMS mineralization.
