Kevin Ng, PhD

An integrated petrographic and microthermometry study focused on the bonanza grade gold-bearing veins at the Brucejack epithermal gold-silver was carried out to investigate the fluid chemistry, fluid source, and physiochemical conditions of the ore-forming processes of 1. primary bonanza grade electrum; and 2. the consequent structural modification. The SEM-CL textures of the bonanza gold veins re-veal that the Phase 1 quartz crystals comprise (1) CL-homogenous cores and (2) oscillatory-zoned over-growths that predate bonanza grade mineralisation. This quartz was followed by CL-unresponsive Phase 2 quartz, which is the immediate host to electrum. It accompanied deformation and the destruction of Type 1 quartz growth textures. Phase 1 calcite is zoned in optical-CL and occur as infills of interstitial space of quartz crystals, massive bands in multi-incremental veins, massive calcite vein. Fluid inclusion studies reveal that the auriferous Type A veins were deposited from relatively saline (7 to 12 eq. wt. % NaCl; Phase 1.1 quartz) magmatic fluids with homogenisation temperatures (Th) ranged from 170 to 200 °C. In Phase 1.2 quartz, a decrease in salinity (3 to 6 eq. wt. % NaCl) and Th (150 to 170 °C) was record-ed. The calcites in Type A and Type C veins were deposited at low temperature of 120 to 150 °C from low salinity to non-saline fluids (0-1.7 eq. wt. % NaCl). A resurgence in temperatures is present during the (re-)crystallisation of Phase 2 quartz with Th between 165 and 190 °C and a wide range of salinities from 0 to 6 eq. wt. % NaCl. Barren, post-mineral tectonic veins (Vn 4) and quartz strain fringes of pyrite in wall rock were deposited from 7 to 12 eq. wt. % NaCl. A disparity in Th was recorded with most of the FIAs clustered between 170 and 190 °C with some ranging from 270 °C to in excess of 300 °C. The FIA with an electrum inclusion bearing fluid inclusion has a salinity of ~12.5 eq. wt. % NaCl and a Th of ~170 °C. Laser ablation ICP-MS analyses of secondary fluid inclusions hosted in quartz crystals detected Au, Ag, As, Sb, Sr, Cu and Pb, indicating that electrum grains were mechanically (re-)mobilised and captured in SLV inclusions during the associated deformation.