Background

Description

Auclair Formation 1 (nAai1): Paragneiss

Unit nAai1 is the main unit of the Auclair Formation. It consists essentially of metamorphic mineral paragneiss subdivided into several facies composed of the following main mineralogical 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 in fresh exposure and rusty brown in altered surface. Its protolith is sedimentary and includes sandstone horizons, clay horizons and feldspathic wacke (Moukhsil and Doucet, 1999). The rock is porphyroblastic to interlobate granoblastic (Moukhsil, 2000). Porphyroblasts are 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 Iron Formation

Unit nAai2 consists mainly of oxide and locally silicate and sulphide facies iron formations (Moukhsil and Doucet, 1999). Oxide iron formations are interstratified in unit nAai1 paragneiss. They form one or more conformable horizons 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). Oxide iron formations have a rusty patina and a banded appearance marked by alternating centimetric chert beds, dark blue in fresh exposure, 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 Agdi3 of the Middle Eastmain Gabbro and Diorite) are associated with these iron formations (Moukhsil and Doucet, 1999).

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

This unit consists of a few horizons of andesite, rhyolite, rhyodacite and intermediate to felsic lapilli and block tuff (Moukhsil and Doucet, 1999; Moukhsil, 2000). Lapilli tuffs form metre-thick horizons that are dark grey in altered patina. They are fine grained, locally porphyritic and contain anhedral plagioclase phenocrystals (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; Moukhsil, 2000; Moukhsil et al., 2001). 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 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 covers a vast area within the Middle and Lower Eastmain Greenstone Belt (MLEGB) (Boily and Moukhsil, 2003; Moukhsil et al., 2003). It extends E-W for ~260 km from the Béryl Lake area (sheet 33B02) to the east coast of James Bay (sheet 33D02). The Auclair Formation was mapped sporadically in the Wapamisk Lake area (sheet 33C08). According to Franconi (1978), the thickness of Auclair Formation paragneiss is not very large and possibly duplicated by folding. Metamorphic mineral paragneiss (unit nAai1) accounts for at least 95% of the area of this unit (Moukhsil et al., 2003). The other units (nAai2, nAai3 and nAai4) cover smaller areas.

Dating

The Auclair Formation corresponds to the earliest period of Archean sedimentary activity of the MLEGB (Boily and Moukhsil, 2003; Moukhsil et al., 2003). It is interpreted by Moukhsil et al. (2003) as part of the second sedimentation period of the MLEGB. A U-Pb dating on zircon was performed on a porphyritic tonalite sample from unit Airn1 of the Rivière au Mouton Intrusions (sheet 33D01) cutting metasedimentary rocks of the Auclair Formation. An average age of 2706 ±1 Ma was obtained. This age would represent the maximum sediment deposition age of the Nemiscau Basin (Moukhsil and Legault, 2002).

Stratigraphic Relationship(s)

The Auclair Formation constitutes the top of the Eastmain Group volcano-sedimentary sequence (Moukhsil and Legault, 2002). The passage from Auclair paragneiss to volcanics 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 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). Rocks of the Auclair Formation are considered to be equivalent to Laguiche Complex metasedimentary rocks (Simard and Gosselin, 1999; Moukhsil and Doucet, 1999). Several post-tectonic and late-tectonic intrusions cut the Auclair Formation (Moukhsil and Doucet, 1999; Moukhsil et al., 2003). Small masses of syntectonic to late-tectonic gabbro, belonging to the “Middle Eastmain Gabbro and Diorite” (Agdi), are also intruding into metasedimentary rocks of the Auclair Formation (Moukhsil, 2000; Moukhsil and Legault, 2002).

Paleontology

Does not apply.

References