|Author:||Moukhsil and Doucet, 1999|
|Type area:||Village Lakes area (NTS sheet 33B03)|
|Geological province:||Superior Province|
|Geological subdivision:||Nemiscau, La Grande and Opinaca subprovinces|
Table des matières
The first descriptions of these lithologies come from the works of Carlson (1962), Hashimoto (1962), Eakins et al. (1968), Bourne (1972), Remick and Ahmedali (1974), Valiquette (1975), Dubé (1978), and Franconi (1976, 1978, 1983). Simard and Gosselin (1999) later grouped rocks of this unit and metasedimentary rocks of the Opinaca Subprovince under the name “Laguiche Group”. The name Auclair Formation was introduced by Moukhsil and Doucet (1999) to describe a metasedimentary-dominated unit in the Village Lakes area (sheet 33B03), which is part of the Lower Eastmain River Volcano-Sedimentary Band (LERVB). The Auclair Formation is named after a former Hydro-Québec camp (Auclair Camp). It was then mapped by Moukhsil (2000), Moukhsil et al. (2001) and Moukhsil and Legault (2002) west of the Village Lakes area. In the work of Moukhsil et al. (2003) on the geology of the Middle and Lower Eastmain, the Auclair Formation encompassed both paragneiss of the Opinaca and Nemiscau metasedimentary basins, as well as some of the Eastmain Group sedimentary formations in the La Grande Subprovince. All of the migmatized paragneiss in the Opinaca Subprovince have been grouped under the name “Laguiche Complex” (Bandyayera and Fliszár, 2007). All of the migmatized paragneiss in the Sub-province of Nemiscau have been grouped under the name “Rupert’s Complex” (Bandyayera and Daoudene, 2018), while paragneiss in the Lac des Montagnes Belt have been assigned to the “Voirdye Formation” (Bandyayera and Caron-Côté, 2019). As a result, the Auclair Formation now encompasses only a portion of Eastmain Group sedimentary rocks.
The Auclair Formation essentially consists of paragneiss. It was subdivided by Moukhsil and Doucet (1999) into four units: 1) paragneiss (nAai1); 2) oxide iron formation (nAai2); 3) andesite, rhyolite, rhyodacite and lapilli and block tuffs (nAai3); and 4) polymictic conglomerate (nAai4). Metamorphism varies from the greenschist facies to the upper amphibolite facies (Moukhsil and Doucet, 1999; Moukhsil and Legault, 2002). The granulite facies has been reached locally in the Auclair paragneiss (Moukhsil and Doucet, 1999; Moukhsil and Legault, 2002). The Auclair Formation contains several gold mineralization associated with oxide or silicate facies Algoma-type iron formations (Chapdelaine, 1997; Lanthier and Ouellette, 1997; Moukhsil and Doucet, 1999; Moukhsil et al., 2003).
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).
Some beds are strictly composed of cordierite, while others contain only staurotide. Cordierite porphyroblasts are subcircular (Franconi, 1978, 1983), while staurotide ones are light brown and have undefined but angular forms (Eakins et al., 1968). Sillimanite is fibrous or in elongated whitish millimetric clusters. Fibrous sillimanite and cordierite are observed in places (Moukhsil, 2000). Andalusite forms quadrangular grey-pink porphyroblasts with edges reaching several centimetres (4-7 cm) (Franconi, 1983; Moukhsil and Doucet, 1999). Garnet forms euhedral crystals, locally the size of andalusite (Moukhsil and Doucet, 1999). The biotite-andalusite-garnet facies appear to be a marker bed for iron formation (nAai2) (Franconi, 1983; Moukhsil and Doucet, 1999). Outcrops of this facies can stretch for several kilometres. This facies is composed of 45% quartz, 30% biotite and 10-25% andalusite porphyroblasts. Garnet can form up to 10% of the rock. In some places, this facies has a very strong schistose flow, marked by the alignment of andalusite (Moukhsil and Doucet, 1999).
In thin section, unit nAai1 is typically composed of 28-34% quartz, 21-25% biotite, 18-22% plagioclase and 1-2% garnet. Porphyroblastic phases can have 14 to 18% andalusite and 10 to 15% staurotide (Moukhsil, 2000). Andalusite porphyroblasts are poikilitic with abundant quartz inclusions, small biotite lamellae and some sillimanite crystals. These porphyroblasts are rimmed with biotite and sit in a fine-grained matrix of quartz, plagioclase and biotite crystals. Sillimanite occurs in clusters of long fibres with biotite or in isolated nodules, ovoids, consisting of fine needle imbrication (Franconi, 1983). Staurotide porphyroblasts have a sieve texture and occur as euhedral crystals, stretched anhedral masses or large crystals with cruciform twinning (Eakins et al., 1968; Franconi, 1978). Cordierite forms pinitized anhedral masses (Franconi, 1978). Garnet occurs as euhedral crystals or as pink poikiloblastic porphyroblasts (Eakins et al., 1968; Franconi, 1983). Accessory minerals are opaque minerals, epidote, muscovite, graphite, apatite, tourmaline, chlorite and pyrite (Eakins et al., 1968; Franconi, 1983; Moukhsil, 2000).
Paragneiss mapped west of the Anatacau Lake area (sheet 33C02) are increasingly migmatized from north to south and contain 10 to 15% quartzofeldspathic mobilisate. Their grain size also increases southward. Granitic dyke intrusions cut these paragneiss (Franconi, 1978; Moukhsil et al., 2001; Moukhsil and Legault, 2002).
Orthopyroxene is observed in thin sections, especially in the southern portion of sheet 33D01, in paragneiss of the Auclair Formation (Moukhsil and Legault, 2002). In the Village Lakes area, a few biotite paragneiss enclaves also show orthopyroxene. These rocks may have been removed from the Laguiche basin during intrusion of the Village Batholith (Moukhsil and Doucet, 1999).
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).
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).
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).
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.
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).
|Unit||Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Inherited Age (Ma)||(+)||(-)||Metamorphic Age (Ma)||(+)||(-)||Reference(s)|
|Airn1||U-Pb||Zircon||2706||1||1||2743||2||2||2668||2||2||Moukhsil and Legault, 2002|
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).
Does not apply.