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Brucejack Geology - Summary

The Brucejack Au-Ag deposit, part of the Sulphurets Au-Ag-Cu Camp. The camp contains a well-mineralized gossanous trend, associated with the Early Jurassic Mitchell Intrusions, which runs for at least 25 km from Brucejack in the south to Treaty Creek in the north. Deformed Lower Jurassic island arc rocks of the Stikine Terrane, exposed in the core of the Cretaceous McTagg anticlinorium, host mineralization at Brucejack.

The Brucejack deposit is interpreted as being a deformed porphyry-related transitional to intermediate sulfidation Au-Ag epithermal deposit. High-grade Au-Ag mineralization occurs as coarse aggregates of electrum and silver sulfosalts in steeply dipping and predominantly E- to NW- trending quartz-carbonate vein stockwork and breccia zones within a broader halo of clay alteration and low-grade mineralization. High-grade gold deposition appears to have resulted from a com¬plex series of interactions between various physicochemical parameters.

Progressive island arc development during the Late Triassic-Early Jurassic was associated with active tectonism and multiple stages of magmatism, lasting until c.180 Ma. Porphyry-style Cu-Au(-Mo) mineralization in the Sulphurets Camp is associated with calc-alkaline magmatic bodies formed between 195-191 Ma. Pervasive sericitic alteration and associated low-grade stockwork mineralization at Brucejack was formed at between 191-185 Ma. High-grade Au-Ag epithermal mineralization was superimposed upon the earlier porphyry-associated alteration and mineralization between 185-183 Ma. The age discrepancy between the known porphyry bodies and the Brucejack epithermal system indicates that, if it was porphyry-driven, the source porphyry has not yet been discovered. Mineralized veins and vein stockwork zones appear to have utilized lithological contacts and pre-existing structures. Epithermal veins are largely discordant to the penetrative foliation of the altered host rock, suggesting that mineralization post-dated a fabric-forming event. Mineralized zones were affected by tilting, thrusting, and shearing during Cretaceous compressional deformation.

Detailed surface and underground exploration of the Brucejack Au-Ag vein stockwork, focusing on the West Zone, was conducted during the 1980s and 1990s. The high-grade Valley of the Kings Zone, discovered in 2009, has been the focus of detailed exploration by Pretium Resources Inc. since acquiring the project in late 2010.