|Author:||Labbé and Grant, 1998|
|Type area:||Natel Lake area (NTS sheet 33B04)|
|Geological province:||Superior Province|
|Geological subdivision:||La Grande Subprovince|
Table des matières
Early lithological descriptions come from works by Eakins (1961), Carlson (1962), Hashimoto (1962), Eakins et al. (1968), Bourne (1972), Valiquette (1975), Dubé (1978) and Franconi (1978, 1983). The name “Natel Formation” was introduced by Labbé and Grant (1998) to describe a volcanic-dominated unit in the Natel Lake area (sheet 33B04), in the Lower Eastmain River Volcano-Sedimentary Band (LERVB). The Natel Formation was traced by Simard and Gosselin (1999) and Moukhsil and Doucet (1999) to the east of the Natel Lake area. Bandyayera and Caron-Côté (2019) confirmed and detailed the extension of this formation in sheet 32O14, west of Lamothe Lake.
The Natel Formation consists of massive or pillowed flows of amphibolitized basalt, andesite, komatiite and rhyolite, as well as volcaniclastic units (block and lapilli tuff, lapilli tuff and tuff). Volcanic rocks are mainly of tholeiitic affinity (Moukhsil and Doucet, 1999). The Natel Formation was subdivided by Labbé and Grant (1998) into six units: 1) amphibolitized tholeiitic basalt (nAnt1); 2) mudstone and wacke (nAnt2); 3) komatiitic basalt and spinifex komatiite (nAnt3); 4) felsic to intermediate tuff (nAnt4); 5) basalt and andesite (nAnt5); and 6) rhyolite and felsic to intermediate tuff (nAnt6). Unit nAnt1 is located at the base of the Natel Formation and contains interstratified horizons of units nAnt2, nAnt3 and nAnt4. Unit 5 probably forms a more differentiated horizon in the volcanic sequence. Finally, unit nAnt6 forms the summital part. As described by Moukhsil (2000), unit nAnt4 consists of intermediate lapilli and block tuff; therefore, it was separated from the rest of the unit and now forms subunit nAnt4a. Rocks of the Natel Formation have been metamorphosed successively to the greenschist and upper amphibolite facies (Moukhsil and Doucet, 1999). The Natel Formation contains several occurrences of orogenic-type gold mineralization associated with quartz±tourmaline±carbonate veins and sulphide-bearing walls (Moukhsil et al., 2003).
Unit nAnt1 forms the base of the Natel Formation. It consists of generally massive, locally pillowed, amphibolitized tholeiitic basalt (Labbé and Grant, 1998). The rock is fine to medium grained, dark green in altered surface and fresh exposure. It is schistose or banded in some places and has a very apparent lineation highlighted by hornblende (Valiquette, 1975; Franconi, 1983). Pillows are generally decimetric to metric and are partially deformed in some areas (Eakins et al., 1968; Valiquette, 1975). Locally, pillows are not preserved well enough to indicate polarity (Valiquette, 1975). Amygdules are well preserved at the base of some pillows, while others contain cavities that are open or filled with quartz and calcite (Valiquette, 1975).
In thin section, basalt is nematoblastic to granoblastic. Hornblende or acicular and poikilitic actinolite are the dominant minerals and make up 55 to 80% of the rock. Plagioclase (8-40%) is rarely twinned, slightly saussuritized or sericitized and recrystallized. Quartz (15%) forms small straight-contact zones. Garnet can reach up to 5% of the rock locally (Moukhsil and Doucet, 1999). Accessory minerals are apatite, zircon, sphene, epidote, chlorite, sericite, biotite and opaque minerals. Epidote and opaque minerals can account for up to 7% of the rock (Moukhsil and Doucet, 1999).
Gabbroic sills are associated with tholeiitic basalts. Gabbro, diorite and granodiorite intrusions are observed mainly at the edges of intrusions (Labbé and Grant, 1998).
Unit nAnt2 is interstratified with unit nAnt1. It occurs as lenticular horizons of black argillite and grey wacke, some of which are large enough to be mapped (Labbé and Grant, 1998).
Unit nAnt3 is observed locally within unit nAnt1 (Moukhsil and Doucet, 1999). It consists of komatiitic basalt and komatiite. The latter are undeformed and display spinifex textures and brecciated horizons (Labbé and Grant, 1998). Komatiitic lava is highly altered. Its composition is similar to that of komatiites typical of the Abitibi Subprovince and of those observed in the Upper Eastmain Volcano-Sedimentary Band (sheet 33A) (Boily, 2000; Boily and Moukhsil, 2003; Moukhsil et al., 2003). In thin section, the rock is fine grained, even grained and has no foliation. Actinolite, the dominant mineral (45-94%), is anhedral to subhedral. Plagioclase (2%) is sparse. Other minerals include epidote (3%) and opaque minerals (1%), which form small subangular and disseminated grains (Moukhsil and Doucet, 1999).
According to Labbé and Grant (1998): “the main unit consists of fine-grained tuff observed in the Fed Lake area and near Aupapiskach Lake. Tuff facies are fine grained, usually siliceous and finely laminated, and locally contain quartz crystals. Unit nAnt4 does not appear to be a marker horizon, two horizons being within unit nAnt1 and the other associated with volcanic rocks of unit nAnt5.” Subunit nAnt4a consists of block and lapilli tuffs described further east by Moukhsil and Doucet (1999).
Natel Formation 4a (nAnt4a): Felsic to Intermediate Lapilli and Block Tuff
Horizons of lapilli tuff and block and lapilli tuff, a few decimetres to >50 m thick, were described by Moukhsil and Doucet (1999). A few fragment-rich outcrops show normal sorting consistent with the polarity indicated by unit nAnt1 pillow basalts (Moukhsil and Doucet, 1999).
Unit nAnt5 consists of basalt and andesite with a few horizons of felsic to intermediate tuff and gabbro. According to Labbé and Grant (1998), this unit probably represents a more differentiated horizon of the Natel Formation volcanic sequence. Rocks mapped in the Aupapiskach Lake area (sheet 33B04) are generally schistose, hence primary structures are obliterated and it is difficult to differentiate between lithologies (Labbé and Grant, 1998). To the southeast of Village Lakes (sheet 33B03), the unit consists of aphanitic and equigranular andesitic lava horizons (Moukhsil and Doucet, 1999). Andesite has a tholeiitic affinity (magnesian tholeiite and ferrotholeiite; Boily and Moukhsil, 2003). These rocks are composed of subhedral actinolite (50%), polygonal quartz (25%), slightly to non-sericitized plagioclase (20%) and opaque minerals (5%). The rock has a foliation marked by the alignment of actinolite crystals (Moukhsil and Doucet, 1999).
Unit nAnt6 is the summital part of the Natel Formation. It consists mainly of rhyolite and felsic to intermediate tuffs as massive volcanic horizons of varying thickness and extent. South of Aupapiskach Lake (sheet 33B04), it is represented by a thick and continuous horizon of felsic ash and lapilli tuff within unit nAnt5. These tuffs are similar to those observed in unit nAnt5. In the Brendan Lake area (sheet 33B04), the unit consists of tuff and a massive and very siliceous rhyolite. This rhyolite forms a small hill ~3 km west of Brendan Lake (Labbé and Grant, 1998). Rhyolite and tuff are variously chloritized, sericitized and silicified (Moukhsil, 2000). In the Béryl Lake area, thin felsic tuff horizons were described by Franconi (1983). These rocks are light grey to bluish grey in fresh exposure and fine to very fine grained. They are composed of light grey to dark grey centimetric fragments, stretched and flattened parallel to schistosity, in a light coloured, flattened and finely banded matrix.
In thin section, dark fragments are composed of saussuritized plagioclase, quartz and minor amounts of partially chloritized biotite, sphene, tourmaline and opaque minerals. Light-coloured fragments are composed of plagioclase and quartz and are difficult to distinguish from the matrix. The latter is composed of quartz and recrystallized plagioclase and, in smaller amounts, microcline and tourmaline.
Rhyolites of the Natel Formation are of calc-alkaline affinity (Labbé and Grant, 1998; Moukhsil and Doucet, 1999). According to Moukhsil and Doucet (1999), rhyolite horizons probably represent repetitive flows within basaltic flows in an underwater environment.
Sparse outcrops of oxide iron formations are observed north of Brendan Lake. West of the same lake, there are also some thin wacke and conglomerate horizons (Labbé and Grant, 1998). According to Eakins et al. (1968), rhyolite horizons are locally accompanied by graphitic schist as well as thin layers of garnet-bearing chlorite schist.
The Natel Formation is located mainly in the eastern part of the LERVB. Two major strips are observed: the first strip runs along much of the south shore of the Eastmain River and extends laterally beyond 50 km along an ENE-WSW axis. There, the maximum thickness of the formation is 2 km (Franconi, 1983; Moukhsil and Doucet, 1999). In the vicinity of Lamothe Lake (sheet 32O14), this strip is oriented along an N-S axis and is affected by a series of antiforms and synforms. A second major strip is located on the north shore of the Eastmain River and extends over 30 km NW of Village Lakes. Klippes of unit nAnt1 were also mapped during the work of Bandyayera and Caron-Côté (2019). They are usually kilometre long and hectometre thick. These klippes were related to the Natel Formation because of their lithological nature and their proximity to the Middle and Lower Eastmain Greenstone Belt (MLEGB).
A rhyolite sample from unit nAnt6 yielded an age of 2739 ±5 Ma (Moukhsil, 2000). This rhyolite crystallization age places the Natel Formation in the second volcanic cycle of the LEGB, aged between 2739 and 2720 Ma (Moukhsil et al., 2003). According to Moukhsil (2000), the age of felsic volcanism in the Middle Eastmain area (unit nAnt6 rhyolite) coincides with the end of the first volcanic cycle (2730-2740 Ma) recognized by Gauthier and Larocque (1998) in the La Grande River area.
|Unit||Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Reference(s)|
The Natel Formation constitutes the base of the Eastmain Group (Moukhsil and Doucet, 1999; Boily, 2000). In the Béryl and Village lakes region, it is conformably overlain by paragneiss of the Auclair Formation (Franconi, 1983; Moukhsil and Doucet, 1999). To the east, the Natel Formation is bordered by migmatized paragneiss of the Laguiche Complex (Bandyayera and Fliszár, 2007). Although the contact between the Natel and Anaconda formations was not observed by Labbé and Grant (1998), these authors suggest that a conformable contact exists. Based on the work of Moukhsil and Doucet (1999), Boily (2000) proposed that the Anaconda Formation should stratigraphically overlie the Natel Formation. In the Natel Lake area, basalts of unit nAnt1 are in fault contact with the Clarkie Formation (Labbé and Grant, 1998). West of Clarkie Lake, the contact between the Natel and Clarkie formations was not observed. According to Labbé and Grant (1998), it is possible that the contact is faulted, but it is also possible that the Clarkie Formation unconformably overlies the Natel Formation. The relationship between these two formations is not clearly established in the Village Lakes area (Moukhsil and Doucet, 1999). Between the Montagnes and Arques lakes, nAnt1 klippes are hosted in the Champion Complex (Bandyayera and Caron-Côté, 2019). Numerous small syntectonic to late-tectonic masses of gabbro (unit Agdi1) and diorite (unit Agdi2) of the Middle Eastmain Gabbro and Diorite are observed in basalts of the Natel Formation (nAnt1; Moukhsil and Doucet 1999). Several syntectonic and late-tectonic intrusions cut the Natel Formation (Labbé and Grant, 1998; Moukhsil and Doucet, 1999).
Does not apply.
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