Background

Description

Auclair Formation 1 (nAai1): Paragneiss

Unit nAai1 represents the main unit of the Auclair Formation. It consists essentially of a paragneiss with the following metamorphic mineral assemblages: (a) biotite-sillimanite-cordierite; (b) biotite-garnet-staurotide-kyanite-fibrous sillimanite; (c) biotite-garnet-staurotide-kyanite; (d) biotite-andalusite-garnet. However, these facies are very difficult to map as distinct lithological units (Moukhsil and Doucet, 1999; Moukhsil, 2000). Paragneiss is fine to medium grained, dark grey to bluish on a fresh surface and rusty brown on an altered surface. Its protolith is sedimentary and includes sandstone beds, argillaceous layers and feldspathic wacke (Moukhsil and Doucet, 1999). The rock has a porphyroblastic to interlobate granoblastic structure (Moukhsil, 2000). Porphyroblasts correspond to aluminosilicate nodules composed of cordierite, andalusite, staurotide or sillimanite. These nodules are in positive relief in outcrop and can be a few centimetres in diameter (Franconi, 1983; Moukhsil and Doucet, 1999). They highlight strong banding defined by alternating centimetric bands (20-30 cm) rich in aluminosilicates and bands lacking them (Franconi, 1983).

Auclair Formation 2 (nAai2): Oxide Facies Iron Formation

Unit nAai2 consists mainly of oxide and locally silicate and sulphide facies iron formations (Moukhsil and Doucet, 1999). The oxide iron formation is interstratified in unit nAai1 paragneiss. It forms one or more conformable layers ranging from a few metres to ~40 m thick, which are systematically associated with andalusite-garnet pelitic paragneiss (Franconi, 1983; Moukhsil and Doucet, 1999). Lateral changes in facies and thickness (primary) are noted (Chapdelaine, 1997). The oxide facies iron formation has a rusty patina and a banded appearance that is marked by alternating centimetric chert beds, dark blue on a fresh surface, and beds (≤10 cm) of fine-grained magnetite (Franconi, 1983; Moukhsil and Doucet, 1999). South of Béryl Lake, iron formations locally contain garnet, which may be an alteration product (metasomatism; Moukhsil and Doucet, 1999). Gabbro sills (unit nAgdi3 of the Moyenne Eastmain Gabbro and Diorite) are associated with these iron formations (Moukhsil and Doucet, 1999).

In thin section, the oxide facies iron formation consists of 25-50% iron oxides, 30-40% quartz, 20-30% feldspar and 20-30% biotite (Côté-Roberge et al., 2021). These proportions vary greatly from one bed to another within the same sample.

Auclair Formation 3 (nAai3): Andesite, Rhyolite, Rhyodacite and Lapilli and Block Tuff

This unit consists of a few layers of andesite, rhyolite, rhyodacite and intermediate to felsic lapilli and block tuffs (Moukhsil and Doucet, 1999; Moukhsil, 2000). Lapilli tuffs form metre-thick layers with a dark grey alteration patina. They are fine grained, locally porphyritic and contain anhedral plagioclase phenocrysts (5-7%, 1-3 mm in diameter). Tuffs also contain up to 50% of small grains of sericitized plagioclase in the matrix (Moukhsil and Doucet, 1999). In thin section, the main minerals observed are quartz (35%), biotite (5%) and actinolite (8%). Quartz is polygonal. Brown biotite is anhedral and moderately to highly chloritized. Actinolite occurs as coarse subhedral to anhedral poikilitic grains. Accessory minerals are epidote (1%) and small angular grains of opaque minerals (1%). Tuffs are deformed and the schistosity is highlighted by the alignment of biotite and actinolite (Moukhsil and Doucet, 1999).

Auclair Formation 4 (nAai4): Polymictic Conglomerate

This unit is not very widespread and is usually located along volcanic packages (Franconi, 1983). The stratigraphic position of this unit appears ambiguous. Moukhsil and Doucet (1999) mention that the conglomerate occupies the summital part of the Auclair Formation. Nevertheless, they note that the Auclair Formation stratigraphically overlies the Natel Formation, which is supported by polarities towards the NE observed in the field. Conglomerates are located at the interface of the Natel and Auclair formations, suggesting that they would instead correspond to basal conglomerates.

This unit consists of garnet-rich polymictic conglomerate composed of >90% pebbles and <10% highly chloritized matrix. Fragments are very stretched and include different lithologies: quartzite and/or rhyolite, basalt and amphibolite, tonalite to granodiorite, and biotite paragneiss. Quartzite and/or rhyolite fragments are up to 20 cm in diameter and form up to 20% of the rock. Basalt and amphibolite fragments (2-15 cm in diameter) are highly deformed and account for 15% of the rock. Tonalitic to granodioritic fragments are 10 to 25 cm in diameter and form 40% of the rock. They could come from one or more synvolcanic plutons. Biotite paragneiss fragments are 2 to 4 cm in diameter and form 25% of the rock. Some channel sedimentary structures are recognized. However, the intensity of deformation precludes establishing polarity (Moukhsil and Doucet, 1999).

Thickness and Distribution

The Auclair Formation extends discontinuously over ~150 km in the MLEGB. The largest mass belonging to this formation is found in the Village lakes area (sheet 33B03; Moukhsil and Doucet, 1999) and to a lesser extent in adjacent sheets 33B04, 32O14 and 33B02. In this area, the Auclair Formation forms a strongly arcuate strip, the convexity of which points to the ENE. This strip reaches a length of 35 km by an average width of 6 km. A solitary ovoid mass 4 km long, with a particularly complex structure, is present in sheet 33B02. It is distinguished by its strong magnetic contrast due to the presence of unit nAai2.

The Auclair Formation also outcrops in sheets 33C01, 33C02, 33C06, 33C07 and 33C08. In these sheets, it generally appears as a lens 1 to 7 km long and 0.2 to 1.5 km wide. The paragneiss with metamorphic minerals (nAai1) represents the most important unit. The other units (nAai2, nAai3 and nAai4) are located only in the Village lakes area (sheets 33B03 and 32O14). They cover smaller areas and are not as thick.

According to Franconi (1978), the thickness of Auclair Formation paragneiss is not very large and possibly duplicated by folding.

Dating

None.

Stratigraphic Relationship(s)

The Auclair Formation corresponds to the youngest period of Archean sedimentary activity in the MLEGB (Boily and Moukhsil, 2003; Moukhsil et al., 2003). It is interpreted by Moukhsil et al. (2003) as part of the second period of sedimentation of the MLEGB. It is possible that the Auclair Formation is the same age as the uppermost part of the Wabamisk Formation and the Anaconda and Clarkie formations, corresponding to the first period of sedimentation of the MLEGB.

The Auclair Formation constitutes the top of the Eastmain Group volcano-sedimentary sequence (Moukhsil and Legault, 2002). The transition from paragneisses of the Auclair Formation to volcanic rocks of the La Grande Subprovince is marked by a stratigraphic contact (Goutier et al., 1999; Moukhsil, 2000; Moukhsil et al., 2001; Moukhsil and Legault, 2002). In the Village lakes area (sheet 33B03), the Auclair Formation overlies conformably the Natel Formation (Moukhsil and Doucet, 1999). To the west, it is stratigraphically overlying the Wabamisk or Komo formations (Moukhsil, 2000; Moukhsil et al., 2001) and separated from the Jolicoeur Complex by the Causabiscau Shear Zone (Bandyayera et al., 2022). To the east, the Auclair Formation overlaps the Village Batholith (2720 to 2700 ±2 Ma; Moukhsil, 2000) and is in inferred contact with the probably younger Prosper Formation (Côté-Roberge et al., 2021).

Several post-tectonic to late-tectonic intrusions, such as the Pacific Intrusive Suite, cut the Auclair Formation (Moukhsil and Doucet, 1999; Moukhsil et al., 2003; Côté-Roberge et al., 2021). Small masses of syntectonic to late-tectonic gabbro, belonging to the Moyenne Eastmain Gabbro and Diorite unit, are also intruding into the metasedimentary rocks of the Auclair Formation (Moukhsil, 2000; Moukhsil and Legault, 2002).

Paleontology

Does not apply.

References

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