|Author:||Beauchamp and Massei, 2018|
|Type area:||Bohier Island region|
|Geological province:||Superior Province|
|Geological subdivision:||Opatica Subprovince|
|Lithology:||Mafic and ultramafic volcanic and intrusive rocks, chert and iron formation|
Table des matières
The volcanic and volcaniclastic rocks of the Upper Eastmain Greenstone Belt (UEGB) were described in the works of Hocq (1985), Roy (1985, 1988), Couture (1986, 1987a, 1987b, 1987c, 1993) and Couture and Guha (1990). At that time, these authors included all the rocks within the René Group, since no formation was individualized. Following geological mapping in the Bohier Island region during summer of 2017, Beauchamp and Massei (2018) divided the René Group into four formations. The Dolent Formation, located south of the Éponyme lake, was introduced to define the volcanic rocks of the western and northern branches of the UEGB.
Unit nAdln1 usually corresponds to pillow and massive flow very fine or fine-grained basalts. The pillow facies dominates the west and north branch of the UEGB and south of Dolent Lake. The massive facies is most prevalent in the Bohier Island area and north of the eastern branch of the belt. Some outcrops exhibit weakly-deformed pillows, while others were deformed and flattened. Locally observed were peduncles, molar tooth-shaped pillows, quartz chambers, saussuritized pillow cores, micro-pillow (<10 cm) and mega-pillow (>1.5 m wide) areas. Pillow edges are blackish and well defined. They contain biotite, chlorite, amphiboles and sulphides. Basalt is generally non-magnetic. In thin section, unit nAdln1 basalt contains 50-70% green hornblende (locally actinolite), damouritized plagioclase, epidote, sericite and traces of chlorite, calcite, quartz, biotite, garnet, sphene, zircon and opaque minerals. Several veins and veinlets made up of quartz, feldspath, calcite and epidote cut basalt in several directions. In some cases, metamorphism has significantly increased basalt size. Medium-grained rock contains amphiboles up to 1 cm in size. White, locally pegmatitic, coarse-grained massive muscovite-biotite automorphic garnet apatite granite intrudes the mafic lava sequence. These intrusions are similar to the Wahemen Granite. Minor amounts of felsic tuff and gabbro, not visible on the scale of the Ministère’s geological maps, are present in the pile of mafic effusive rocks.
From a geochemical perspective, mafic effusive rocks of the Dolent Formation are subalkaline basalt and andesitic basalt. Some samples are magnesian basalt. These rocks are of tholeitic to transitional affinity characterized by a flat profile on the rare earth spider diagram.
Dolent Formation 1a (nAdln1a): Porphyritic or Glomerophyric Basalt
Unit nAdnt1a represents the facies of porphyric and glomerophyric basalt. As a marker of volcanic stratigraphy, this unit was isolated on the geological map of Beauchamp and Massei (2018). Generally pillowed, unit nAdln1a contains 2-15% plagioclase phenocrystals ranging in size from 0.5 to 1 cm. Glomerophyric structures are also present. Phenocrystal clusters (1 to 5 cm) are rounded to subrounded. Because glomero-porphyries are denser than individual crystals, they tend to accumulate in the lower portion of magmatic bodies. The presence of phenocrystal aggregates makes it possible to locate the base of the stratigraphy.
The Dolent Formation gabbro unit has two mappable features, a gabbroic sill that follows the northern boundary of the western branch of the UEGB, and a smaller area of 1.3 km by 300 m in the heart of the same branch. It is a magnesium-rich (8.5-12% MgO) gabbro of tholeitic affinity. Medium coarse-grained, it is particularly melanoratic and black grey to dark greenish grey. It can be confused with pyroxenite by its general appearance. The majority of the rock is composed of amphibole measuring a few millimetres, and plagioclase feldspath in smaller equigranular crystals. The rock is homogeneous, non-magnetic and massive to weakly foliated. In the northern gabbro band, unit nAdln2 is cut by local shear zones. It is also highly altered with a high presence of epidote (zoisite) and sericite replacing plagioclase grains. Amphiboles belong mainly to the actinolite-tremolite series (outcrops 17-JM-6066 and 17-JF-5074). At the heart of the western branch, the gabbro is less altered, sericitization is very low and the most abundant amphibole is green hornblende.
Ultramafic sills were observed at the UEGB north branch (outcrops 17-MQ-8015 and 17-JF-5209) and east branch (outcrop 17-AB-1043). The 90-EM-05 to 90-EM-12 survey series conducted by Kingswood Explorations 1985 in 1990 confirms the extension of the ultramafic band of the eastern branch. Unit nAdln3 consists of two ultramafic rock facies, that is, with or without cumulate structures. Ultramafic rocks with cumulate structures are dark grey to orange brown. The rock is very magnetic and fine to medium-grained. Up to 40-50% of minerals of orange altered colour (Beesley and Goulet, 1992), possibly orthopyroxene or altered olivine, are observed. The matrix consists of tremolite and antigorite. Foliation is mainly marked by serpentine flakes. When the cumulate structure is not present, the rock shows a marked penetrative foliation overprinted by a crenulation schistosity. It is also amphibolitized, biotitized and contains talc, tremolite (locally radial fibrous acicular crystals) and magnetite. Under the microscope, the rock shows serpentine and small post-kinematic hornblende crystals.
Ultramafic volcanics were mapped 2 km southwest of Lépante Lake (Couture 1987) and 2 km south of Dolent Lake (Beauchamp and Massei, 2018). The ultramafic volcanite unit is relatively thin, 200 to 300 m thick (Couture, 1987). In altered patina, ultramafic volcanics are light green and very soft. In fresh exposure, they are blackish to dark greens. Two types of flows were recognized by Couture (1987): typical spinifex flows and flows without a spinifex sommital zone. Typical flows consist of an olivine cumulate basal zone and a spinifex sommital zone. The shape of the spinifex needles suggests that their primary mineral was pyroxene rather than olivine. In thin section, the primary structure of the cumulate zone is relatively well-preserved, despite complete metamorphic recrystallization. Olivine pseudomorphs are replaced by serpentine and magnetite. Long tangled needles recrystallized as a mosaic of monoclinic acicular amphiboles in the tremolite-actinolite series (Couture 1987). Other less frequent minerals including talc, chlorite, epidote, carbonate and mica were observed by Dejou (1987). Spinifex-free flows outcrop on a hill about 2.5 km southwest of Lépante Lake and south of Dolent Lake. These flows are lenticular and are a few metres thick (about 5 to 10 m). Piling of the flows appears to have been rapid since no interbedded sediments are present. Spinifex-free flows consist of three distinct parts, an olivine phenocrystal basal part, an aphyritic median part and a small olivine phenocrystal sommital part. In situ basal and sommital breccias are well developed and allow for the identification of the top of the layers. In some places, the flows form elongated lobes. The edges of these lobes pinch-out (outcrops 17-AB-1106 and 17-AB-1109). According to Dejou (1987), only a few samples of ultramaphic rocks have spinifex structures.
A number of chert and iron formation bands were observed in the northern half of the western branch of the UEGB. Chert bands are recrystallized, laminated and interstratified with centimetric massive sulphide horizons. Outcrops have a rust-out alteration crust. Sulphides are mainly pyrrhotite, pyrite and, in lesser amounts, chalcopyrite. Chalcopyrite is more present in the walls of quartz veins cutting the unit. Main minerals are quartz and sulphides. Accessory minerals are amphibole, epidote, muscovite and, more rarely, plagioclase and carbonates.
The Dolent Formation is located in the western and northern branches, as well as the northern part of the eastern branch of the UEGB.
The stratigraphic position of the Dolent Formation within the René Group is not known at this time. The Bohier Group sedimentary rocks (<2712 ±6 Ma) are younger than the Dolent Formation volcanic rocks. In the western branch of the UEGB, a faulted contact separate the overlying Bohier Group conglomerates (nAbh2) from the underlying Dolent Formation basalts. The Misasque (Namiq) and Cadieux (nAcad) plutons cut volcanic rocks of the Dolent Formation to the north and south.
Does not apply.