Controls on the genesis of gold mineralisation in the Brucejack epithermal deposit, British Columbia

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Some epithermal deposits are characterised by extraordinarily high metal concentrations of gold and silver, but despite their obvious attraction, they are difficult to explore and develop because zones of hyper-enrichment (bonanza zones) are commonly adjacent to barren zones. The Brucejack deposit in northern British Columbia, where Pretium Resources Inc. initiated mining in 2017, is an example of such a deposit. There, veins containing kilograms of Au/ton are accompanied by veins of the same generation that are devoid of gold (electrum). The reason why this and some other deposits contain these locally hyper-enriched gold-silver veins has not been satisfactorily explained. Indeed, it is difficult to understand how the fluids considered to be responsible for the formation of epithermal ore deposits could fill cm-wide fractures with gold for distances of several metres (the case at Brucejack) by simple precipitation, given that the gold content of these fluids has been shown to be on the order of 10's of ppb. An alternative hypothesis, which is suggested by preliminary high resolution transmitted electron microscope (HRTEM) images of gold-silver nano-particles (3-7 nm diameter) in samples from Brucejack, is that the gold/silver super-saturates as electrum nano-particles which subsequently flocculate, accumulate and then are mechanically transported into selected fractures, perhaps subsidiary to the major fluid conduits. We will test this hypothesis with further HRTEM imaging, EDS-assisted TEM analyses, and experiments designed to produce colloids and determine the controls on their flocculation. The bonanza gold mineralisation at Brucejack was preceded by auriferous arsenian pyrite that is developed locally within a five km-long corridor of phyllic alteration, possibly reflecting a porphyry system at depth, and contributes significantly to the resource and is closely associated with the electrum. The proposed research will develop a comprehensive model for the genesis of the Brucejack deposit that will explain the phyllic alteration, arsenian pyrite and bonanza-type electrum (and the links between them), consider the possibility of a telescoped epithermal-porphyry system, and help guide the ongoing exploration and mine development.