Cryptic Telescoping in Porphyry-Epithermal Systems: An Integrated Microtextural-Chemical Study of Pyrite from the Brucejack Bonanza-Type Epithermal Au-Ag Deposit

The Brucejack epithermal Au-Ag deposit in northwestern British Columbia, Canada, is host to some of the world’s highest-grade bonanza gold ores. The trace element composition of pyrite in the deposit records a series of abrupt changes in the chemistry of the hydrothermal ore fluids that preceded and ultimately led to ultrahigh-grade gold mineralization. Pyrite cores in preelectrum stage I veins are Co-Ni rich and record the passage of early porphyry-type fluids, which evolved to epithermal-type fluids during telescoping of the system, as evidenced by oscillatory, colloform Au-As growth zones that mantle the Co-Ni cores. The vertical extent of this telescoping, as determined from the deepest pyrite samples that display composite epithermal-porphyry compositions and associated zoning, is ~1,000 to >1,300 m. This hydrothermal activity predated the emplacement of bonanza-grade epithermal stage III to stage V carbonate-quartz-electrum veins that crosscut stage I to II veins throughout the deposit. The time between the emplacement of prebonanza and bonanza vein stages was brief, as demonstrated by the respective 182.7 ± 1.0 and 182.6 ± 1.1 Ma U-Pb zircon ages of syn- to postmineral trachybasaltic dikes and the youngest mineralized host rock, an andesitic crystal tuff unit. Secondary ion mass spectrometry (SIMS) sulfur isotope exchange thermometric analyses show that the Co-Ni pyrite formed in a porphyry environment, whereas bonanza-stage veins formed under epithermal conditions. Our study reveals how pyrite chemistry integrated with geochronology can be used to map the evolution of hydrothermal activity in porphyry-epithermal environments in four dimensions (x-y-z-t), even if early alteration has been overprinted, or obliterated, by the products of telescoping.