AGE AND ORIGIN OF THE BRUCEJACK EPITHERMAL AU-AG DEPOSIT, NORTHWESTERN BRITISH COLUMBIA

• Author / Creator

  Tombe, Sean P

• Brucejack is an epithermal Au-Ag deposit located in northwestern British Columbia. The deposit is one of many world-class economic deposits that formed in association with extensive volcanic arc-related magmatism in Late Triassic–Early Jurassic time in the Canadian Cordillera. Brucejack mineralization is hosted by island arc-related Early Jurassic porphyritic latite flows, volcanic ash–block-sized fragmental rocks, and volcanic sandstones, siltstones and conglomerates. The variably altered and mineralized volcanic host rocks yield U-Pb zircon dates ranging between 196.4 ± 0.7 Ma and approximately 184 Ma. Molybednite Re-Os age estimates for mineralization at Brucejack range between 191.7 ± 0.8 Ma (Bridge Zone) and 188.9 ± 0.8 Ma (West Zone). Individual mineralizing events spanning ~3 m.y. are unlikely, and multiple pulses of ore formation are more probable, whereby earlier intermediate-sulfidation-style epithermal Au-Ag mineralization (Bridge Zone and Valley of the Kings), which was perhaps distally related to porphyry-type hydrothermal activity, was followed by a younger low-sulfidation epithermal Ag-Au event (West Zone). Brucejack is crosscut by late stage andesite–basaltic andesite amygdaloidal dykes, which truncate all mineralized veins, and which are crosscut by late stage (post-mineralization) quartz-calcite veins. A U-Pb zircon age of 182.7 ± 1.0 Ma has been determined for one of these dykes, providing a minimum age for mineralization. With the exception of West Zone, the deposit shows features typical of intermediate-sulfidation epithermal deposits, characterized by the presence of crustiform and cockade vein textures, minor open space filling, the presence of various sulfosalt minerals, including tetrahedrite-tennantite, chalcopyrite, and FeS-poor sphalerite, scarce arsenopyrite, absence of pyrrhotite, and the vertical extent of mineralization is >1000 m. Wallrocks at Brucejack are moderately phyllic-altered, with variable amounts of pervasive and texturally destructive sericite, pyrite, quartz, and carbonate. Early sericitization and pyritization is recognized, as well as hydrothermal sericitization contemporaneous with gold mineralization. Five vein stages have been recognized at the Valley of the Kings: (I) highly deformed and discontinuous pyrite stringer veins containing carbonate and quartz commonly with chlorite and sericite-altered vein margins, containing no electrum; (II) weakly deformed quartz-carbonate stockwork veins, breccias veins, and subvertical stringer quartz vein networks, also hosting electrum; (III) Zn-Pb-(Cu) sulfide veins containing Ag-sulfosalts and electrum; (IV) highly deformed carbonate ± quartz veins containing abundant orange-coloured, manganoan calcite and electrum; (V) late stage quartz-carbonate shear veins with asymmetrical sericite, chlorite, and pyrite vein banding, often with associated subhorizontal tension gash veins. Fluid inclusions from vein generations II, III, and IV have relatively low homogenization temperatures (means of 166 ± 11˚C, n = 25; 154 ± 18˚C, n = 89; 156 ± 9˚C, n = 34, respectively) and a narrow range in salinity (0.5 to 7.4 wt % NaCl equiv.; n = 54), with the exception of stage III veins which have a range of 0.5 to 15.5 wt % NaCl equiv. (n = 89). Evidence for boiling is recognized in stages II and IV, and therefore homogenization temperatures can be taken as approximating the actual trapping temperature. Boiling is thought to be responsible for the deposition of high-grade gold mineralization for these vein stages in the VOK, where gold precipitated from condensed magmatic liquids during near-surface depressurization. Stage III fluid inclusion assemblages indicate fluid mixing between cooler, more saline meteoric fluids and warmer, dilute magmatic fluids resulting in Pb-Zn-Ag-Au deposition. Oxygen isotopic compositions of vein quartz and calcite (calculated 18Ofluid values ranging from -9.8 to -1.5‰ and -5.8 to +1.4‰ respectively) indicate a progressive dilution from modified magmatic fluids mixing with metal-depleted meteoric surface waters. Carbon and sulfur isotopic compositions also suggest a magmatic source for these components in electrum-bearing veins; hydrothermal calcites have mean 13CCO2 value of -6.4 ± 1.3 ‰ (n = 25), and sulfur isotopic compositions of pyrite, sphalerite, and galena average -0.7 ± 0.3 ‰ (n = 16). With consideration to fluid inclusion and stable isotope analysis, a magmatic fluid is the most likely source for bonanza gold mineralization in the Valley of the Kings at Brucejack.

• Subjects / Keywords

  • Brucejack
  • Au-Ag Epithermal
  • Hazelton Group Formation
  • Sulphurets District
  • Iskut
  • Intermediate Sulfidation

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3QJ7884C

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.