Post-Archaean dykes and diabase sills have been known since the beginning of geological mapping of the Canadian Shield of Quebec and Ontario (e.g. Murray et al., 1897). L’Espérance (1948) and Gill and L’Espérance (1952) noted that these dykes were most likely of Keweenawian (Mesoproterozoic) age and could be divided 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 map of Fahrig and West (1986), based on compilation and interpretation of aeromagnetic surveys, illustrates the distribution of diabase dyke swarms in the Canadian Shield, as well as their chronology based on isotopic ages. On this map, NE dykes of western Quebec are 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 S6 dyke of Buchan et al. (1993) is the reference dyke and cuts Archean rocks in the Senneterre region of Quebec. It is outcropping (2016-JG-2859) along Route 113, 300 m south of the entrance to Lafontaine Road (sheet 32C03, UTM NAD 1983, Zone 18, 324308 mE, 5337948 mN). This outcrop corresponds to Site 30 sampled by Buchan et al. (1993) during their paleomagnetic and geochronological study. U-Pb isotopic dating yielded Paleoproterozoic age for these dykes. In 2016, a sample was taken from this site for geochemistry (analysis 2016067640).

The Senneterre diabase Dykes are traced from outcrops and the magnetic field vertical gradient map. They have a straight signature over kilometers with a NE to ENE orientation.

Their contact with surrounding rocks is sharp. Their thickness varies from a few meters to 120 m. Diabase has a brown patina and a green fresh exposure. A centimeter-thick aphanitic chill margin is commonly observed. The dykes’ center is typically medium-grained. The rock is massive, homogeneous and slightly deformed. Fracture networks are typically perpendicular to dyke orientation.

Petrographic description is based on thin section observations from NTS sheets 33F, 33G and 33H. Medium-grained dykes are composed of plagioclase, augite and orthopyroxene, which form an intersertal or intergranular, and rarely equigranular, microstructure. Ophitic and subophitic structures have not been observed since pyroxene crystals are typically smaller or the same size as plagioclase. Pseudomorphs of olivine and orthopyroxene automorphic phenocrysts (0.5 to 1 mm) were observed in thin sections in the chilled margin of some dykes. Magnetite-ilmenite is scarce (1 to 3%, 0.5 to 1 mm). Quartz is generally absent, but traces are observed in thin sections (<1%, <0.5 mm) of some dykes and up to 3% (0.2 to 0.5 mm) in one of the Senneterre Dykes spanning sheets 33H03 and 33H07. Traces of apatite, biotite, usually chloritized (0.5 to 1 mm) and pyrite are also noted. The level of uralization and sericitization varies from medium to high. Orthopyroxene is usually altered (chlorite, amphibole, serpentine) and therefore more difficult to recognize in thin sections. Epidote and carbonate are also observed in several thin sections.