Kevin M.H. Ng, PhD
K.M.H. Ng1, A.E. Williams-Jones1, J.R. Clark1, J.J. Hanley2, D.F. McLeish1
1Department of Earth and Planetary Sciences, McGill University, Montreal, QC, Canada
2Department of Earth Science, Saint Mary's University, Halifax, NS, Canada
An integrated, in situ microanalytical study was conducted on bonanza-grade gold-bearing veins at the Brucejack epithermal gold–silver deposit, British Columbia, Canada, to constrain fluid chemistry, fluid sources, and the physicochemical conditions of ore formation and any potential remobilisation. Analytical methods included SEM, cathodoluminescence imaging, fluid inclusion microthermometry, LA-ICP-MS, SIMS, microdrill-based gas-source mass spectrometry, and MC-ICP-MS.
The primary auriferous veins formed through mixing between a moderately saline magmatic-hydrothermal fluid (7–12 wt.% NaCl equiv.) with homogenization temperatures of 170–200°C and a low-salinity fluid (0–1.7 wt.% NaCl equiv.) at lower temperatures (120–150°C). In-situ SIMS transects across epithermal quartz yielded δ18Oquartz values of 4.75 to 13.96 ‰ (n = 79). The δ18Oquartz values decrease from the cores into the mantles of the crystals and increase towards the rims. The higher values in the cores correspond to a higher contribution of magmatic-hydrothermal fluid, the low values in the mantle reflect an influx of meteoric water and the high values at the rims record either cooling of the hydrothermal system or renewed input of magmatic-hydrothermal fluid.
The radiogenic Sr87/Sr86 isotope compositions of the calcite indicate that the earliest bonanza-grade quartz–calcite veins precipitated from fluids with a strongly depleted mantle signature, with ratios of 0.7040–0.7055. Carbon and oxygen isotope data for the calcite are consistent with this interpretation and confirm that meteoric water constituted a significant component of the fluids responsible for most of the bonanza-grade gold at Brucejack. Late-stage calcite veins formed from surficial waters that equilibrated with the Stuhini Limestone during the waning stages of epithermal mineralization.
A thermal resurgence associated with post-mineral deformation is recorded by the recrystallization of Phase 2 quartz, and the trapping of fluid inclusions with Th values of 165–190°C. LA-ICP-MS analyses of secondary fluid inclusions yielded elevated concentrations of As, Sb, Sr, Cu and Pb. Isolated Au and Ag spikes record the presence of nanoparticles in the fluid inclusions, indicating mechanical remobilization of electrum during deformation.
This study demonstrates that mixing between colder, likely steam-heated meteoric water and auriferous magmatic fluids was the dominant process driving gold deposition at Brucejack. This induced rapid cooling and pH increase, destabilizing gold colloids and promoting flocculation. Subsequent formation of weakly mineralized, massive calcite veins reflects the addition of surficial water that equilibrated with the Stuhini limestone. These results highlight the importance of multiple hydrothermal events and deformation in generating/remobilizing bonanza-grade gold mineralization in older, structurally complex terranes.