The geological setting of the southeast part of the Lac Montbray property is characterized by felsic (rhyolite and rhyodacite) and andesitic volcanic rocks. These volcanics are related to the Cycle 2 volcanism of the Blake River Group that took place between 2701.8 and 2699.3 Ma. Diorite dykes oriented NE-SW appear to cut mainly rhyolites. The various geological units were emplaced during the collapse of the New Senator Caldera. This collapse phase took place in successive stages, from NW to the SE, and led to the formation of arcuate structures and the migration of volcanic centres parallel to this direction. The many phases of collapse and eruption were interspersed with short latency periods during which chert horizons developed around felsic domes (Pearson and Daigneault, 2009).
The Hunter Creek Fault is one of the major structures in the area. This NE-SW fault represents the northwest edge of the Noranda Caldera, which overprints the New Senator Caldera. To the northwest of the Hunter Creek Fault, the stratigraphic orientations of the volcanic units suggest a domal structure (Duprat Dome). The contacts between the volcanic units trend NW-SE with a dip of 25–65° to the SW or NE. To the southeast of the Hunter Creek Fault, the stratigraphy trends NE-SW with a dip of 80° to the SE.