Origin of the high-grade Early Jurassic Brucejack epithermal Au-Ag deposits, Sulphurets Mining Camp, northwestern British Columbia

摘要

The high-grade (8.1 million ounces of gold; 15.6 Mt grading 16.1 g/t Au; Pretium, 2016) Brucejack epithermal Au-Ag deposits are located in the Canadian Cordillera of northwestern British Columbia, and formed in association with extensive early Mesozoic island arc magmatism. Porphyry-type Cu-Au-Mo mineralization occurs nearby at the Kerr-Sulphurets-Mitchell (196-190 Ma; Febbo et al., 2015), Bridge Zone (191.7 +/- 0.8 Ma, 191.5 +/- 0.8 Ma), and West Zone (188.9 +/- 0.9 Ma) prospects. Gold-silver vein-type mineralization at Brucejack is hosted by variably altered and deformed Early Jurassic porphyritic latite lava flows, volcaniclastic rocks, and volcanic-derived sandstones, siltstones, and conglomerates. This study focuses on the Valley of the Kings Zone at Brucejack, where host rocks have been dated at 188-184 Ma (U-Pb, zircon). Post-mineralization (but not postalteration) basaltic and trachybasaltic dikes that cut the veins are geochemically similar to the host volcanic sequence and follow the same structural trends of the mineralized vein corridors. We suggest these dikes are broadly coeval with mineralization, and that ore formation at the Valley of the Kings formed during the overall Jurassic tectonomagmatic event.