Hébert (1989) discovered wollastonite mineralized zones in the Grandes Pointes Lake area in the Lake Saint-Jean region (sheet 22E04). Following this discovery, the Ministère scheduled mapping projects in this area. SW of sheet 22E04, Gervais (1993) conducted mapping at scale 1:50 000 and identified, among other things, five klippes of metasedimentary rock that he associated with the Grenville Supergroup (Wynne-Edwards, 1972). During geological compilation of sheet 22E04 (in Hébert et al., 2009, off-text map), the author named these metasedimentary rocks the “Saint-Onge Supracrustal Sequence”. Gervais (1993) divided these supracrustal rocks into five informal units, taken up by Hébert et al. (2009). The work of Moukhsil and El Bourki (2020) identified four of these units in the Normandin area (sheets 32A16 and 32A09).

Rocks of the Saint-Onge Supracrustal Sequence consist of quartzofeldspathic paragneiss, marble, calcosilicate rocks and quartzite. According to geochemical data (Moukhsil and El Bourki, 2020), marble varies from calcitic to dolomitic.

Calcosilicate rocks of this unit are grey to greenish grey and contain diopside, quartz, calcite, calcic plagioclase, scapolite and pyrite. Locally, diopside-rich layers (up to 90% of the rock) are present in these calcosilicate layers. Accessory minerals are garnet, K-feldspar, phlogopite, epidote, biotite, hornblende, titanite, tourmaline and zircon .

Marble of this unit is less abundant and contains up to 25% diopside, which gives the rock a grey to blackish colour. Accessory minerals are phlogopite, biotite, hornblende, plagioclase, scapolite, titanite and garnet. A large wollastonite deposit and some small zinc mineralized zones are found in this unit.

Saint-Onge Supracrustal Sequence 2 (mPong2): Calcosilicate Rocks, Marble, Paragneiss and Quartzite

Saint-Onge Supracrustal Sequence 3b (mPong3b): Marble and Calcosilicate Rocks

Unit mPong3b was defined in the SW part of sheet 22E04 (Gervais, 1993), where it occurs as kilometre-long and decametre-wide layers. It also outcrops in sheet 32A16 as folded layers in quartzofeldspathic paragneiss of the same supracrustal sequence (mPong5). In sheet 22E04, the largest marble strips are located north of Grandes Pointes Lake. Near the Rats River in the village of Saint-Eugène-d’Argentenay, the best rock outcrops of this unit are found in two quarries (Bouchard – Grenon and Pères Trappistes).

Marble is mainly calcitic, white to grey, coarse grained and locally banded. It contains calcite with traces of phlogopite and graphite. Some dolomitic and calcosilicate marble layers have also been observed locally. Calcosilicate rocks are greenish, fine to medium grained and composed of diopside, calcite, phlogopite, <1% scapolite and trace pyrite.

Saint-Onge Supracrustal Sequence 4 (mPong4): Quartzite

Quartzite forms thin beds in restricted areas north of Grandes Pointes Lake (Gervais, 1993; sheet 22E04) and in the NNE corner of sheet 32A16 (Moukhsil and El Bourki, 2020). These rocks are very light grey to milky white, very fine grained and composed of >80% quartz. They also contain small subrounded grains of K-feldspar and plagioclase (Hébert et al., 2009).

Saint-Onge Supracrustal Sequence 5 (mPong5): Quartzofeldspathic Paragneiss with Quartzite, Calcosilicate Rocks and Some Marble

Unit mPong5 was mapped by Gervais (1993) at the western boundary of sheet 22E04, SW of Ouest Lake, where it does not exceed 8 km²; it outcrops more in the Normandin area (sheet 32A16). It consists of fine to medium-grained greenish grey quartzofeldspathic paragneiss containing quartz, K-feldspar, biotite, hornblende, augite and enstatite. These rocks are locally interbedded with quartzite and calcosilicate rock layers, and some millimetric marble layers in places. At the contact between unit mPong5 and the Sainte-Hedwidge Intrusive Suite (mPshe2; sheet 32A16), quartzofeldspathic paragneiss is highly cut by pink granite dykes and pegmatitic veins giving a granitic gneiss appearance to the rock.

Thickness and Distribution

Marble strips are generally thin and their extension is difficult to follow. Their thickness can be evaluated in the Bouchard – Grenon and Pères Trappistes quarries. In the former, marble layers are predominant and can reach a thickness of 3 m. In the second quarry, marble layers observed show strips exceeding 5 m in thickness.

Dating

Ider (1997) and Higgins et al. (2002) obtained an age of 1160 ±20 Ma and 1163 ±18 Ma respectively on a wollastonite skarn sample belonging to the Saint-Onge Supracrustal Sequence (mPong1). These ages correspond to the crystallization age of wollastonite formed by contact metamorphism induced by the emplacement of the Du Bras Granite (1148 ±5 Ma; Ider, 1997).