The Brucejack Intermediate Sulfidation Epithermal Au-Ag Deposit, Northwestern British Columbia

English Abstract:

The Brucejack intermediate-sulfidation epithermal Au-Ag deposit is one of many world-class porphyry and epithermal deposits that formed in association with extensive volcanic arc-related magmatism in Late Triassic–Early Jurassic time in the Canadian Cordillera. Mineralization at Brucejack is hosted by Early Jurassic porphyritic latite flows, block and ash flows, and volcanic sandstones, siltstones and conglomerates. Oldest estimates for these variably altered and mineralized porphyritic flows are 196.4 ± 0.7 Ma (U-Pb zircon). Age estimates for mineralization at Brucejack range between 191.7 ± 0.8 Ma and 188.9 ± 0.8 Ma (Re-Os molybdenite). Brucejack is crosscut by late stage trachybasaltic dykes dated at 182.7 ± 1.0 Ma (U-Pb zircon), which truncate all mineralized veins, and are crosscut by barren (post-mineralization) quartz-dominated veins. Six vein stages have been recognized at Brucejack, with high-grade Au mineralization occurring in stages II, III, and IV. Fluid inclusions within these auriferous high-grade veins have relatively low homogenization temperatures (~160˚C) and broad ranges in salinity (0.5 to 15.5 wt. % NaCl equiv.), suggesting fluid mixing. Evidence for boiling is recognized in all mineralized veins, and is thought to be the principal mechanism for Au mineralization deposition. Temperature-corrected oxygen isotopic compositions of Au-bearing vein quartz and vein calcite indicate d¹⁸O_fluid values of -1.5 to -8.0 ‰ and 1.4 to -10.2 ‰ respectively, ranging from modified magmatic to meteoric water values, and suggesting mixing between these fluid sources. Carbon (vein calcite: d¹³C_CO2 = -8.4 ± 0.2 ‰, n= 22) and sulfur (vein pyrite, sphalerite, and galena: d³⁴S= -0.8 ± 0.3 ‰, n= 10) isotopic compositions also suggest a magmatic source for these elements, possibly with a minor crustal component. Gold is inferred to have been sourced from these magmatic fluids.