Background

Description

Lac des Montagnes Group 1 (nAmo1): Amphibolitized Basalt; Ultramafic Volcanic Rock Layers

Lac des Montagnes Group 2 (nAmo2): Intermediate Volcanic Rock

The intermediate volcanic rock unit is the least known of the Lac des Montagnes Group. These rocks are generally described in the field as amphibolitized basalt. They have been renamed based on lithogeochemical analysis results. Dark grey on an altered surface, medium grey to greenish grey on a fresh surface, these fine- to medium-grained rocks are homogeneous, foliated and locally magnetic. Locally, there are structures interpreted as isolated, stretched and flattened pillows, amphibolitized flow margins, or pillow breccias. The rock contains 60% hornblende (0.5 to 5 mm) and 1% to 2% magnetite (0.2 to 0.5 mm). In places, silicified and garnet-rich (up to 30%) layers are observed. Geochemically, these are andesites of calc-alkaline affinity and richer in SiO2, TiO2 and light rare earth elements than the basalt of unit nAmo1.

Lac des Montagnes Group 3 (nAmo3): Felsic Volcaniclastic Rock, Locally Intermediate

The felsic volcanic rock unit, in places intermediate, accounts for nearly 10% of the Lac des Montagnes Group. Unit nAmo3 consists of felsic volcaniclastic rocks in the form of decametric to hectometric layers, interstratified in the basaltic sequence of unit nAmo1. The best preserved sequences consist of bedded felsic tuffs characterized by alternating centimetric to metric beds of lapilli tuff, fine-grained massive ash tuff and conchoidal-fractured fine-grained tuff. The rock is whitish on an altered surface and greenish grey on a fresh surface. The matrix is sericitized and chloritized.

Locally, the felsic volcaniclastic rocks contain centimetric to metric beds of intermediate volcaniclastic rocks with mafic blocks and lapilli.

In thin section, the felsic tuffs are granoblastic and mainly composed of quartz and plagioclase grains in a fine-grained recrystallized matrix composed of chlorite, sericite, epidote, quartz and plagioclase. Apatite, sphene and sulphides are present as accessory minerals. Lithogeochemical analyses show that the felsic tuff of unit nAmo3 is of rhyodacitic, locally rhyolitic, composition and calc-alkaline affinity.

Lac des Montagnes Group 4 (nAmo4): Oxide Facies Iron Formation and Silicate Facies Iron Formation

Unit nAmo4 consists of several layers of iron formation <150 m thick, and up to 8 km in length, interstratified in the basalt unit (nAmo1). In places, they are located at the top of the basalts of unit nAmo1, and mark the transition between the volcanic rocks of the Lac des Montagnes Group and the sedimentary rocks of the Voirdye Formation. These highly magnetic layers are clearly visible on aeromagnetic maps.

The silicate and oxide facies iron formations are generally banded. Compositional variations between these two types (oxides or silicates) are observed in places at the outcrop scale. The oxide facies iron formations are characterized by massive magnetite bands, alternating with granoblastic chert bands. The massive magnetite layers are black to bluish grey and reach a metric thickness in places. The silicate facies iron formations consist of magnetite bands and laminae alternating with hornblende-garnet-biotite-chlorite rich magnetic layers and magnetite paragneiss layers. The amphibole-rich layers are greenish grey and very limited in extent. The beige-grey magnetite paragneiss represent about one third of the total thickness of the silicate facies iron formation. The entire iron formation unit is affected by numerous tight isoclinal folds that have axial planes parallel to the regional schistosity.

Layers of silicate facies iron formation have also been observed in volcano-sedimentary rock remnants of the Champion Complex. These have been incorporated into the Lac des Montagnes Group. In thin section, the rock contains up to 75% cummingtonite and 15% garnet poikiloblasts.

Lac des Montagnes Group 5 (nAmo5): Amphibolite Derived from Komatiitic Basalt

Unit nAmo5 represents a small proportion of the Lac des Montagnes Group and consists of amphibolite layers derived from komatiitic basalt. This unit occurs as decametre to hectometre-thick layers interstratified in the basaltic sequence of unit nAmo1. It appears to be spatially associated with a felsic volcanic rock layer of unit nAmo3 located north of Voirdye Lake. Amphibolites derived from komatiitic basalt are fine to medium grained and foliated. They are dark green both on a fresh surface and on an altered patina. They are mainly composed of actinolite (>90%) and contain accessory serpentine. On some outcrops, these rocks contain 1 to 5% tourmaline as millimetre-sized crystals.

Thickness and Distribution

The Lac des Montagnes Group extends discontinuously for ~175 km along a NE-SW axis (sheets 32N07 to 32O15). The unit generally ranges in width from 200 m to 1000 m. In sheet 32N, it occupies mainly the NW portion of the Lac des Montagnes Belt, whereas in sheet 32O, it occurs on both sides of the belt, on its NW and SE flanks.

Unit nAmo2 forms several small lenses that are sparsely distributed, notably west of Hutte Lake and south of Indien and Caumont lakes. Unit nAmo3 occurs mainly in the Voirdye Lake area as well as south of Indien Lake and NW of La Sicotière Lake. Iron formation unit nAmo4 was mapped very locally by Bandyayera and Daoudene (2018a and 2018b). It is located primarily south of Caumont Lake and east of Kanamekuskasich Lake (32N08 and 32N09). Unit nAmo5 is mainly observed between the Voirdye and Chlorite lake areas (Bandyayera and Caron-Côté, 2019). It has also been recognized in sheets 32N08 and 32N09, but does not appear on the geological map due to its small size.

Dating

The dating of felsic volcaniclastic rocks collected from the nAmo3 unit at various locations in the Lac des Montagnes Belt has been performed. In the SW of the belt, two samples collected from sheets 32N08 and 32N09 yielded ages of 2703.6 ±4.9 Ma for a sample 2.8 km east of Caumont Lake (outcrop 2017-DB-1037) and 2720.5 ±2.5 Ma for a sample collected 1 km SE of Kapapineyapiskach Lake (outcrop 2017-RP-6096) (David, 2020a). Further NE, NW, and SW of Voirdye Lake (sheet 32O12), two samples from outcrops 2018-EC-2155 and 2018-DB-1120 yielded crystallization ages of 2723.2 ±3.9 Ma and 2706.4 ±6.7 Ma, respectively (David, 2020b). The geographic distribution of these ages in the Lac des Montagnes Belt suggests a rejuvenation of volcanism from NE to SW.

Stratigraphic Relationship(s)

The Lac des Montagnes Group, formerly part of the Nemiscau Subprovince, has been reassigned to the La Grande Subprovince (Bandyayera et al., 2022). It is in structural contact with the plutonic and gneissic rocks of the La Grande and Opatica subprovinces. To the north, it is separated from the Champion Complex by the Nisk Shear Zone, and to the south, from the Théodat Complex by the Poste Albanel Shear Zone. These volcanic rocks are stratigraphically overlain by the sedimentary Voirdye Formation, which occupies essentially the central part of the Lac des Montagnes Belt. The contact between these two units is concordant, although locally sheared. This concordance is better exposed in the area south of Nemiscau Lake (sheet 32N07). The volcanic belt located south of the Rivière Nemiscau Shear Zone has also been included in the Lac des Montagnes Group, since it shares the same characteristics and lithological associations. 

The Caumont Mafic-Ultramafic Suite is mostly intrusive in the Lac des Montagnes Group. White granitic pegmatites of the Spodumène Suite are also found very locally cutting the Lac des Montagnes Group, west of Spodumène Lake and east of Andalousite Lake. White granitic pegmatites of the Senay Suite also cut rocks of the Lac des Montagnes Group, mainly in sheet 32O12. 

Paleontology

Does not apply.

References

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