From the beginning of geological mapping in Quebec and Ontario, geologists have identified dykes and late diabase sills (e.g., Murray et al., 1897). L’Espérance (1948) and Gill and L’Espérance (1952) noted that these intrusions, present throughout the Canadian Shield, were most likely of Keweenawian (Mesoproterozoic) age and could be classified into three categories: 1) undifferentiated or ordinary diabase; 2) quartz diabase; and 3) olivine diabase. Moore (1929) concluded that olivine diabases were consistently younger than quartz diabases. The Fahrig and West (1986) map, based on compilations and interpretation of aeromagnetic surveys, presents the distribution of diabase dyke swarms in the Canadian Shield and their chronology based on isotopic ages. On this map, NE orientated dykes of western Quebec were associated with the Preissac Swarm. Later, the paleomagnetism and geochronology work of Buchan et al. (1993) divided these dykes into two groups: the Senneterre Dykes and the Biscotasing Dykes.

The Biscotasing diabase Dykes were traced from outcrop distribution and the magnetic field vertical gradient map. These mafic intrusions, oriented NNE to NE extend in a straight line over kilometers.

The thickness of the dykes varies from a few meters to 133 m. Diabase has a brown patina and a green fresh exposure. The contact with host rocks is sharp and generally corresponds to an aphanitic chill margin one to a few centimeters thick. The center of the dykes is medium-grained. The rock is massive, homogeneous and slightly deformed. Fracture networks perpendicular to the dyke orientation are recognized.

The Biscotasing Dykes consist of medium-grained gabbronorite composed of augite, plagioclase and orthopyroxene showing an intergranular to ophitic microstructure. Accessory minerals include magnetite-ilmenite (1 to 3%), quartz (1 to 4%), apatite and traces of biotite. The level of uralization and sericitization varies from low to high. Orthopyroxene is more altered (to chlorite, serpentine, amphibole) and is therefore more difficult to recognize in thin sections. In Ontario, Halls et al. (2005) report that the Hillport Dyke, one of the thickest dykes in this unit, contains plagioclase crystals up to one centimeter in length.