Fluid inclusion and stable isotope constraints on the origin of Wernecke Breccia and associated iron oxide-copper-gold mineralization, Yukon

Iron oxide - Cu +/- Au +/- U +/- Co (IOCG) mineralization is associated with numerous Proterozoic breccia bodies, collectively known as Wernecke Breccia, in Yukon Territory, Canada. Multiphase breccia zones occur in areas underlain by Paleoproterozoic Wernecke Supergroup metasedimentary rocks and are associated with widespread sodic, potassic, and carbonate alteration assemblages. Fluid inclusion data indicate syn-breccia fluids were hot (185-350 degrees C) saline (24-42 wt.% NaCl equivalent) NaCl-CaCl2-H2O brines. Estimates of fluid pressure vary from 0.4 to 2.4 kbar (1 kbar = 100 MPa). Carbon and oxygen isotopic compositions of breccia-related carbonates range from similar to-11 parts per thousand to +1.5 parts per thousand (Pee Dee belemnite (PDB)) and -2 parts per thousand to 20 parts per thousand (Vienna standard mean ocean water (V-SMOW); delta O-18(water) similar to-8 parts per thousand to +15 parts per thousand), respectively. delta C-18 and delta O-18 values for host Wernecke Supergroup limestone/dolostone vary from similar to-2 parts per thousand to 1.6 parts per thousand and 12 parts per thousand to 25 parts per thousand, respectively. Sulfur isotopic compositions of hydrothermal sulfides and sulfate vary from similar to-12 parts per thousand to +13 parts per thousand and +8 parts per thousand to +17 parts per thousand (Canon Diablo Troilite (CDT)), respectively. Syn-breccia biotite, muscovite, and actinolite have delta D and delta O-18 values of similar to-141 parts per thousand to -18 parts per thousand and +7 parts per thousand to +12 parts per thousand (V-SMOW; delta O-18(water) similar to 7 parts per thousand to 11 parts per thousand), respectively. The Wernecke Breccias and the associated IOCG mineralization appear to have formed from largely nonmagmatic fluids - based on isotopic, fluid inclusion, and geological data. The emerging hypothesis is that periodic overpressuring of dominantly formational/metamorphic water led to repeated brecciation and mineral precipitation. The weight of overlying sedimentary rocks led to elevated fluid temperatures and pressures; fluid flow may have been driven by tectonics and (or) gravity with metals scavenged from host strata.