Trace elements in ore minerals as indicators of hydrothermal fluid evolution in the Au-rich porphyry system of Iron Cap, British Columbia, Canada

Trace elements in ore minerals are increasingly utilised to illuminate genetic processes, and improve mineral exploration and processing approaches. However, the ore minerals of porphyry deposits have received limited study, despite the need to better understand the nature of hydrothermal fluid evolution in porphyry systems. In this thesis, the trace element contents of the ore mineral assemblages of successive vein generations are examined to investigate variations in hydrothermal fluid characteristics throughout formation of the Au-rich porphyry deposit of Iron Cap, British Columbia, Canada. Detailed petrographic analyses have led to the classification of eight vein generations at Iron Cap. Trace element analyses by LA-ICP-MS reveal that different vein generations exhibit unique geochemical signatures. Both Se and Ag display consistent ore mineral partitioning trends between vein generations, but their concentrations vary systematically. This suggests that these elements record specific changes in hydrothermal fluid characteristics between vein generations, such as temperature, pH, and/or salinity. A lattice strain model for pyrite has been developed, which shows that Co and Ni are lattice-hosted, and actually exhibit higher partition coefficients for pyrite than Fe does. This means that Co/Ni ratios in pyrite (0.2 to 186.6) directly record Co/Ni ratios in the hydrothermal fluids. Fluctuations in Co/Ni ratios in pyrite growth zones revealed by trace element mapping illustrate that the characteristics of the hydrothermal fluids precipitating the pyrite at Iron Cap are regularly oscillating, which can only be explained by the influx of multiple different fluids during vein formation. This research shows that trace elements in porphyry ore minerals have the potential to elucidate genetic processes. In particular, hydrothermal fluid evolution in porphyry systems is shown to be inherently chaotic and complex on both the local and deposit scale, with specific veins necessarily formed by numerous episodes of fluid pulsing, fluid mixing, and/or vein re-opening.