Referred to as the “Strangway granite” by Longley (1946) and the Rivière Bell Pluton (Abel) in the Ministère’s compilation (MNRF, 2010), Guemache (2020) formally adopted the name Strangway Pluton (based on rapids of the same name on the Bell River) to remove any ambiguity with the Rivière Bell Complex, a stratiform intrusion of mafic to ultramafic composition near the town of Matagami.

Historical work (Longley, 1946), geofiche outcrops and sonic drilling RO-419 indicate that this unit consists of hornblende-biotite granite containing mafic enclaves. According to Longley (1946, page 14): “The rock is medium grained and pink to grey. It is generally massive, but the texture is slightly gneissic in places. In macroscopic specimen, hornblende and biotite are visible, each forming 15-20 [%] of the rock. The first is usually the most abundant. The other main constituents, as they appear in thin sections, are quartz (~25%) and microcline (~30). Plagioclase (oligoclase-andesine) and orthose are present in small amounts. Orthose and plagioclase are slightly altered, but microcline looks fresh and probably secondary. Ordinary accessory minerals are zircon, titanite and apatite. The rock can be classified as a medium-grained hornblendic biotite granite.”

Microscopic analysis of samples collected by sonic drilling RO-419 (Guemache, 2020) reveals that quartz commonly has undulatory extinction, and plagioclase, locally zoned, is commonly sericitized. In places, microcline occurs in large anhedral zones, giving the rock a porphyraceous texture. Myrmekites and microperthites are not uncommon. Biotite (10%), more or less chloritized, hornblende (10%), epidote and titanite tend to form clusters. Apatite, which is scarce, occurs locally as inclusions in biotite, while opaque minerals are sparse. Zircon was reported by Wanless et al. (1978).

Thickness and distribution

The Strangway Pluton corresponds to a relatively small, near-circular intrusion (10 km x 14 km), well visible on aeromagnetic maps (Keating et al., 2010; Keating and d’Amours, 2010; Dion and Lefebvre, 2006).

Dating

K-Ar isotopic dating yielded cooling ages of 2538 ±53 Ma and 2605 ±3 Ma (Wanless et al., 1978).