Trapping nanoparticles: A structural approach for concentrating precious metals in vein-hosted ore deposits
Nicolas Harrichhausen, McGill University
Christie Rowe, McGill University
Warwick Board, Pretium Resources Inc.
Charles Greig, Pretium Resources Inc.
Super-saturation of silica is common in fault fluids. This may coincide with pressure changes associated with fracturing, or with transport across temperature gradients in a hydrothermal system. These conditions lead to precipitation of amorphous silica, which will subsequently recrystallize to quartz under typical geologic conditions. Additionally, the changing pressure and temperature conditions may promote the precipitation of precious metals (such as gold), as well as trap suspended pre-existing nanoparticles within amorphous silica phases. We report evidence for silver nanoparticles, and for electrum, an alloy of gold and silver, that is associated with recrystallized amorphous silica at the Brucejack epithermal gold-silver deposit in northwest British Columbia, Canada. We propose a deposit model in which transport of nanoparticles in suspension and as a solution, allows for a higher flux of gold and silver within a hydrothermal system than does transport as a solution alone. This high flux may account for the extraordinarily high gold concentrations, up to 41 582 ppm, that are commonly reported across 0.5 m to 1.5 m intervals at Brucejack. Structural data and maps of electrum-bearing quartz-carbonate stockwork veining are presented, showing that major stockwork systems are related to extension and normal faulting in subaqueous volcanic sequences. Electrum within quartz-carbonate stockwork at Brucejack is extremely localized within veins, with host rock and vein material immediately surrounding the richest mineralization typically averaging much lower grades of gold and silver (<1 ppm Au). We propose that fault rupture concentrates and localizes electrum deposition by trapping silver and gold nanoparticles within rapidly precipitated amorphous silica, at specific structural sites. Repeated rupture events could concentrate electrum even further. Recrystallization of amorphous silica to quartz may cause localized remobilization of electrum and the formation of the ore textures that are observed today.