Fault zone architecture, deformation conditions, and kinematics of the Camp Lake and Offset faults of the Lac des Iles Mine, Northwestern Ontario, Canada

Abstract

Faults and their associated damage zones are important geologic structures that serve as permeable pathways through the upper crust. The development of fault cores and damage zones is typically controlled by the strength and composition of the protolith, conditions of deformation, and fluid chemistry. The effect of host lithology on fault core development and damage zone structure is currently poorly constrained. This study uses the Lac des Iles palladium mine in northwestern Ontario, Canada as a natural laboratory to study how faults behave in different lithologies. The Lac des Iles mine is hosted in the 2.689 Ga ± 1.0 Ma Lac des Iles mafic-ultramafic intrusion with current reserves of ~5 Moz of 3E (Pd+Pt+Au) at an average grade of 2.6 g/t. The intrusion hosts numerous mineralized zones, most notably the Roby, Offset, and Camp Lake zones, divided by faults. The Camp Lake and Offset faults are two major structures that offset blocks of the ore body within the Lac des Iles mine, with displacements of ~500 and ~275 m, respectively. The faults crosscut gabbronorite and tonalite, often with these rock units in fault contact with each other. We studied the variation in fracture density surrounding the Camp Lake and Offset faults to quantify how damage zone structure changes with respect to the host lithology. Fracture density decay rates within damage zones (with distance from the fault core) show that fractures in tonalite decay at a faster rate than gabbronorites, irrespective of whether they are in the hanging wall or footwall. The fault cores in gabbronorites are characterized by chlorite-rich gouges whereas tonalite fault cores are composed of silica-rich cataclasites. It is hypothesized that the development of a frictionally weak, chlorite-rich fault core impeded the development of a more fracture-dense damage zone in the gabbronorite. Results from whole-rock geochemistry show variations in major elements that correlate with the largest zones of visible alteration and deformation within the damage zone, and no significant rare earth element variation that can be attributed directly to the faulting. Electron microprobe analysis of chlorite was conducted on fault core and host rock samples to constrain the temperatures of deformation. Three notable clusters in the temperature data were observed, interpreted to represent periods of chlorite formation that were pre-, syn- and post-faulting (290°C, 236°C, and 110 – 175°C, respectively). A combination of structural and geochemical data shows that the Camp Lake and Offset faults have undergone multiple deformation events, with pulses of hydrothermal fluids altering the mineralogy and geochemical signature of the surrounding rocks in the Lac des Iles mine. Palladium mineralization is depleted within the fault core and damage zones. It is hypothesized that hydrothermal fluids associated with faulting are the cause of stripped palladium mineralization. This research highlights the interplay of the geological processes associated with faulting and fluid migration and how they have affected the distribution of palladium within the Lac des Iles mine. Understanding the relationship between faulting and economic mineralization can improve exploration strategies and guide mining efforts for the future.