Project: Role of colloidal transport in the formation of high-grade gold veins at Brucejack,British Columbia
Measured gold concentrations and theoretical gold solubility maxima in hydrothermal fluids are orders of magnitude lower than what is observed at some high-grade epithermal gold deposits. For example, assays from electrum bearing quartz-carbonate veins at Pretivm Resource’s Brucejack deposit in northwestern British Columbia, yield gold grades as high 41,500 g/t Au. Deposition of such high grades would require an extremely large flux of gold in solution through a small area even at maximum theoretical concentrations. The motivation of my project is to study whether gold or electrum can be transported as a nanoparticulate solid in suspension (colloid) to allow for a higher total gold flux in order to deposit such concentrations.
Rapid precipitation of silica from hydrothermal solutions at structural dilation sites may lock gold particles up in amorphous silica phases such as opal. Recrystallization of opal to quartz pushes any impurities to quartz grain boundaries causing gold or electrum to coalesce at interstitial sites. Amorphous silica formed along active faults at Dixie Valley, Nevada, has been documented to contain a few wt.% gold while associated quartz has gold concentrations below detection limits. Any amorphous silica formed at the early Jurassic Brucejack deposit will have recrystallized to quartz. Therefore, it is important to compare textures from this locale to the relatively fresh samples from Dixie Valley.
This project will attempt to find textural evidence for colloidal gold deposition at Brucejack and using samples from Dixie Valley, observe how these textures evolve with recrystallization and metamorphism.A rapid drop in fluid pressure is a likely cause of amorphous silica formation and this can occur at site of structural dilation. If the colloidal deposition of gold can be associated with these sites, this will impact how we explore for gold and potentially any other minerals that may be controlled by this process.