First published: 20 January 2020
Last modified:
DISCLAIMER: This English version is translated from the original French. In case of any discrepancy, the French version shall prevail.
| Author(s): | Dimroth, 1972, 1978 |
| Age: | Paleoproterozoic |
| Stratotype: | The type locality is in the NW part of the Bacchus Lake area (NTS sheet 23O07), where the volcanic member of the formation outcrops. On the other hand, sedimentary members are much better exposed between Otelnuk and Coussinets lakes (sheet 24C08). |
| Type area: | Bacchus Lake area (NTS sheet 23O07) |
| Geological province: | Churchill Province |
| Geological subdivision: | New Quebec Orogen (Labrador Trough) |
| Lithology: | Basalt, flysch |
| Category: | Lithostratigraphic |
| Rank: | Formation |
| Status: | Formal |
| Use: | Active |
Background
The name Bacchus Formation was introduced by Dimroth (1972, 1978) to refer to a unit of slate, impure sandstone, quartzitic wacke and basalt located in the central Labrador Trough. The unit takes its name from Bacchus Lake where the volcanic member of the formation outcrops. The rocks of the Bacchus Formation, as well as those later correlated with the Bacchus Formation, have also been described by Baragar (1967), Dimroth (1964, 1965, 1966, 1969, 1970a, 1970b) and Dimroth et al. (1970) in the southern Labrador Trough, and by Fahrig (1955, 1956, 1965), Roscoe (1957), Hashimoto (1964, 1968), Dressler (1973, 1975, 1979), Clark (1977, 1978, 1986), Penrose (1978), Kheang (1984) and Girard (1984a, b, c, 1988) north of the area mapped by Dimroth. Kheang (1984), in particular, assessed the metallogenic significance of rusty zones in the felsic volcanic sequence of the Bacchus Formation located in the La Lande and Douay lakes area (sheets 24F01 and 24F03). The Bacchus Formation was previously assigned to the « Attikamagen Subgroup » (Baragar, 1967; Dimroth, 1970b, 1971; Dimroth et al., 1970; Wardle and Bailey, 1981), later redefined as the Attikamagen Group (Clark and Wares, 2004). According to Dimroth (1978), the Bacchus Formation was partly the base of the Attikamagen Group. He also noted that the Bacchus Formation is interdigitated with the Le Fer Formation westwards. Because of the natural affinities between these formations, Clark and Wares (2004) proposed that the Bacchus Formation be included in the Swampy Bay Group.
A sequence of basaltic, gabbroic and sedimentary rocks, located north of latitude 56°30’N, was assigned to the Mistamisk Formation by Dimroth (1978). Dimroth (1972, 1978) had mentioned lithological similarities between the Bacchus and Mistamisk formations, and had indicated that the latter could either overlie the Bacchus Formation or represent its upper part. He also established the contact between these formations in an arbitrary manner. The name Mistamisk Formation was also used by Dressler (1979), Girard (1984b, c, 1988) and Clark (1986), who also followed Dimroth’s (1972, 1978) interpretation of the stratigraphic position of the Bacchus Formation relative to the Mistamisk Formation. Rocks of the Mistamisk Formation were later incorporated into the Bacchus Formation by Clark and Wares (2004) and the name Mistamisk Formation was abandoned. Clark and Wares (2004) also proposed to abandon the Nachicapau Formation (Dressler, 1979) as this unit is, in their opinion, partly equivalent to the Bacchus Formation.
The following table presents the evolution of the Bacchus Formation, including its informal units, through geological mapping work.
| Actual Unit | Girard (1984b, 1988) | Clark (1986) | Girard (1984c) | Kheang (1984) | Dressler (1979) | Clark (1978) | Dimroth (1978) | Penrose (1978) | Clark (1977) | Hashimoto (1964) |
| pPbc1 | 18a (Nachicapau Formation) | 10e (1/100 000) | ||||||||
| pPbc2 | 3h (Mistamisk Formation) |
8b |
9c 14a (Menihek Formation) 15b (Mistamisk Formation) 18b (Nachicapau Formation) 19f (Murdoch Formation) |
10f (1/100 000) 16a 21b (Mistamisk Formation) |
5c | 5a, 6d, 7a | ||||
| pPbc3 | 3f (Mistamisk Formation) | 15c (Mistamisk Formation) | 10d (1/100 000) | 5a | ||||||
| pPbc4 | 9d 15f (Mistamisk Formation) |
7b | ||||||||
| pPbc5 | 3a (Mistamisk Formation) | 8a 9a (Mistamisk Formation) |
9a 15a (Mistamisk Formation) 19b (Murdoch Formation) |
10g (1/100 000) 16b 21a (Mistamisk Formation) |
5b | |||||
| pPbc6 | 3d (Mistamisk Formation) | 9b (Mistamisk Formation) | 7b (Mistamisk Formation) | 9 (Murdoch Formation) | 15d (Mistamisk Formation) 19a (Murdoch Formation) |
10h (1/100 000) | ||||
| pPbc7 | 16c | |||||||||
| pPbc8 | 9d (Mistamisk Formation) | |||||||||
| pPbc10 | 16e | |||||||||
| pPbc11 | 16d | |||||||||
| pPbc12 | 4a, 4b (Mistamisk Formation) | 6a (Mistamisk Formation) | 5 (Murdoch Formation) | 15g (Mistamisk Formation) 19e (Murdoch Formation) |
7a | 6b | ||||
| pPbc13 | 8a | |||||||||
| pPbc14 | 8b | |||||||||
| pPbc15 | 8a | 12 (Montagnais Group) | ||||||||
| pPbc16 | 3g (Mistamisk Formation) | 7c (Mistamisk Formation) | 15b (Mistamisk Formation) |
Description
The Bacchus Formation is an allochthonous volcano-sedimentary unit consisting mainly of mudstone, locally graphitic and pyritic, and basalt flows, as well as some layers of sandstone, conglomerate and silty or sandy greywacke (Baragar, 1967; Dimroth, 1970b, 1978; Dimroth et al., 1970; Dressler, 1979). The formation also includes dolomite, mafic pyroclastic rocks, schist locally graphitic and pyritic, and minor amounts of paragneiss, marble and chert (Hashimoto, 1964; Dimroth, 1978; Dressler, 1979; Penrose, 1978; Clark, 1978, 1986). Pillow to massive basaltic flows of oceanic tholeiitic affinity are characteristic of the formation and constitute the bulk of volcanic rocks (Baragar, 1967; Wardle and Bailey, 1981). Minor amounts of rhyolite and felsic volcanics are present locally (Dressler, 1979; Kheang, 1984). These felsic volcanics are found in the upper part of the Bacchus Formation (Dimroth, 1978; Clark and Thorpe, 1990; Clark and Wares, 2004). The latter is intruded by a large volume of gabbroic sills of tholeiitic affinity (Wakuach Suite). These are petrographically and chemically similar to lavas and are generally interpreted as contemporaneous and comagmatic (Baragar, 1967; Dimroth, 1978; St. Seymour et al., 1991; Rohon et al., 1993; Skulski et al., 1993; Findlay et al., 1995).
Rocks of the Bacchus Formation are generally metamorphosed to the greenschist facies. In the area NE of Romanet Lake, they reach the amphibolite facies (Dimroth, 1978; Dimroth and Dressler, 1978). Volcanism of the Bacchus Formation was interpreted as the result of rifting along the Archean continent’s margin (Wardle and Bailey, 1981). According to Dimroth (1978), rocks of the Bacchus Formation were deposited in a relatively deep marine basin. The formation hosts abundant silver-lead-zinc vein mineralization and appears to have potential for volcanogenic massive sulphide mineralization (Kheang, 1984; Clark and Wares, 2004).
Bacchus Formation 1 (pPbc1): Polymictic and Monomictic Conglomerate; Locally Silty and Sandy Greywacke
Polymictic and monomictic conglomerates form the base of the Bacchus Formation (Dimroth, 1978; Dressler, 1979). In the Cramolet and Ribero lakes area (sheet 23O13), basal conglomerate consists of alternating beds (0.6-10 m thick) of grey, locally pink or red, coarse-grained granule sandstone, and conglomerate containing pebbles and boulders of dolomite and grey dolomitic sandstone having a brown patina (Dimroth, 1978). According to Dimroth (1978), this basal conglomerate is similar to that of the Romanet Formation. Boulder conglomerate also outcrops on a small island in Ribero Lake. The boulders, up to 30 cm, consist of pink Dunphy-type dolomite in a matrix of green siltstone. Dimroth (1978) points out that this dolomite conglomerate was metamorphosed at the contact of a gabbro sill (Wakuach Suite) on the northern shore of Ribero Lake. The rock is now largely recrystallized as a calcium silicate (actinolite)-calcite-quartz assemblage. In thin sections, the only minerals recognized are actinolite tremolite, diopside, chlorite, quartz and calcite (Baragar, 1967).
In the area SW of Cramolet Lake and NW of Musset Lake, Dimroth (1978) observed local grey granule sandstones between rocks of the Seward Group and the Bacchus Formation. In this area, he could not clearly determine stratigraphic relationships at the base of the Bacchus Formation, as his mapping was not sufficiently detailed. According to Dimroth (1978), lower clastic rocks of the formation are usually overlying coarse-grained clastic rocks of the Seward Group. Their dark grey or dark purple colour, the relative absence of feldspar, loose structure of clastic fragments and a low but characteristic argillaceous fraction distinguish them from Seward Group rocks. According to Baragar (1967), the lower strata of the Bacchus Formation are generally conformable with underlying strata of the Seward Group.
In the Nachicapau Lake area (sheet 24C09), conglomerate is locally interbedded with silty to sandy greywacke which Dressler (1979) associated with turbidites. Conglomerate is poorly to non-stratified and lacks sorting and imbrication. Constituent fragments are angular to rounded, cobble size (64-256 mm) and of varied composition: silty and sandy greywacke, quartzitic sandstone or fine-grained conglomerate. The matrix has a pasty or sandy texture. Conglomerate fragments contain debris (1-2 cm) consisting of arkose, granite and basalt in a fine-grained matrix of angular to subrounded crystals of plagioclase, quartz, and a smaller amount of K-feldspar (Dressler, 1979). Silty and sandy greywacke is dark grey and displays sedimentary structures such as convolute laminations, sorting and crossbedding. In places, Dressler (1979) notes small fragments (1-3 cm) in greywacke. These fragments form 2 to 5% of the rock and consist of silty or sandy greywacke. A few subrounded fragments, locally up to 1 m in diameter, are also reported by Dressler (1979).
Bacchus Formation 2 (pPbc2): Mudstone, Slate, Siltstone, Silty Greywacke and Phyllite, Locally Graphitic and Pyritic; Some Dolomite and Sandstone Locally
Unit pPbc2 consists mainly of mudstone, slate, siltstone, silty greywacke and phyllite. These rocks are grey to dark grey or black, locally graphitic and pyritic, and finely bedded or laminated (Hashimoto, 1964; Dimroth, 1978; Clark, 1978, 1986; Dressler, 1979). Rocks of unit pPbc2 form sedimentary layers (3-30 m thick) generally interstratified with basalt flows (pPbc5) and gabbro sills (Dressler, 1979; Dimroth, 1978; Clark, 1986). At Cramolet and Nachicapau lakes, they are overlying conglomerate and greywacke of unit pPbc1. In the Mistamisk and Romanet lakes area, slate is characterized by slaty cleavage and one or more crenulation cleavages (Clark, 1986). Silty, rarely sandy greywacke is fine to medium grained and locally displays sorting (Dressler, 1979; Clark, 1986). NE of Romanet Lake, Clark (1986) observed interbedded slate and greywacke up to 50 cm thick. At one location, Clark (1986) also noted the presence of sorted beds (2-3 m thick) of grey quartz wacke containing blue quartz grains 0.5-3 mm in diameter. In thin sections, wacke is mainly composed of plagioclase and quartz in a matrix of biotite, sericite, very fine-grained fragments and a minor amount of carbonate. Opaque minerals form the accessory minerals (Dressler, 1979).
Interbedded dolomite and sandstone are locally observed in unit pPbc2 (Dressler, 1979; Dimroth, 1978; Girard, 1984b, Clark, 1977, 1978, 1986). In the Cramolet Lake area, unit pPbc2 is overlain by mudstone and siltstone interbedded with sandstone and impure quartzitic wacke (Dimroth, 1978). In the Cramolet Lake area, Dimroth (1978) also observed mudrock breccia and mudrock-pellet sandstone in the lower part of the Bacchus Formation. Sandstone displays very well-developed sorting and structures indicative of syn-sedimentary deformation (e.g. erosion channels, convolute laminations). The presence of massive sandstone beds at the top of the formation is also mentioned by Dimroth (1978). NE of Otelnuk Lake, this author also observed beds (1-2 m thick) of grey, massive or brecciated dolomite having a brown patina. Clark (1986) also reports some thinly interbedded white dolomite and light grey chert in slate and greywacke NE of Romanet Lake. In the Colombet Lake area, carbonate-quartz veins mineralized in pyrite and chalcopyrite cut sandstone and siltstone (Girard, 1984b).
Bacchus Formation 3 (pPbc3): Impure Sandstone, Greywacke, Quartzitic Sandstone; Local Conglomerate or Mudstone Interstratifications
Unit pPbc3 consists mainly of impure sandstone, greywacke and quartzitic sandstone (Dimroth, 1978; Dressler, 1979; Girard, 1984b, 1988). In the Fort McKenzie area (sheet 24C16), unit pPbc3 is part of what Dressler (1979) has termed the Mistamisk Formation. This formation was integrated into the Bacchus Formation by Clark and Wares (2004). In the Colombet Lake area (sheets 24C15 and 24F02), unit pPbc3 occurs as metric (3-50 m) greywacke beds and some sandstone interbedded in basalts (pPbc5). The thickness of greywacke beds increases westwards. The greywacke is basaltic in composition. It has a good cleavage and medium grain size. The rock is composed of 30% subrounded quartz and 10% altered feldspar in a fine-grained matrix of chlorite, epidote, iron oxide and sphene fragments (Girard, 1984b, 1988). Interbedded conglomerate, arkose or mudstone are locally reported (Dimroth, 1978; Dressler, 1979).
Bacchus Formation 4 (pPbc4): Dolomite
Dolomite is massive, grey in fresh surface and buff or rusty in altered surface, indicating that the rock contains some iron (Hashimoto, 1964; Dressler, 1979). It may occur as beds and contains several thin quartz veins. According to Hashimoto (1964), its composition is very similar to that of ferrodolomite.
Bacchus Formation 5 (pPbc5): Massive and Pillow Basalt; Graphitic Mudstone Interstratifications
According to Dimroth (1978), it is not possible to lithologically distinguish the Bacchus Formation lavas and those of the Menihek, Thompson Lake and Willbob formations. The author also pointed out that there is a gradual – lateral or vertical – transition from fine-grained massive or pillow basalts to medium to coarse-grained gabbros, and that it is commonly impossible to distinguish between the extrusive and intrusive varieties. Lavas occur as alternating massive and pillow tholeiitic basalt flows. These flows range in thickness from 2 m to >20 m and are ~600 m thick at Bacchus Lake and >1500 m thick in the Mistamisk Lake area (Dimroth, 1978). The presence of hyaloclastic material over a thickness of a few centimetres is common at the base or top of the flows (Dimroth, 1978). Interstratified graphititc mudstone (a few centimetres to >1 m) is usually found between flows (Dimroth, 1978; Dressler, 1979; Girard, 1984a).
Basalt is grey in fresh surface and light grey-green or green to dark grey in altered surface. Locally, the alteration patina is slightly rusty brown due to oxidation of sulphides, either disseminated or in fissures. The rock is fine grained to aphanitic, homogeneous and locally vesicular (Dimroth, 1978, Girard, 1984b, 1988; Kheang, 1984). In places, the rock is sheared and has a well-marked foliation (Hashimoto, 1964; Girard, 1984b, 1988). Numerous chlorite-carbonate-quartz veinlets cut basalt (Girard, 1988). Columnar structures are common in massive flows (Dimroth, 1978). Pillows generally have a chilled margin and are rimmed by black hyaloclastic material (Dimroth, 1978; Kheang, 1984). They commonly contain tabular cavities filled with quartz (Dimroth, 1978). In places, Girard (1984b, 1988) points out that it is difficult to determine the polarity of the flows due to the highly deformed nature of pillows. Basalt is composed mainly of plagioclase (albite), actinolite, chlorite, epidote and sphene (Dimroth, 1978; Girard, 1984b, 1984c, 1988).
In thin sections, basalt is composed of albite (~50%) and actinolite (15-20%) in a fine-grained matrix formed by intergrown chlorite crystals accounting for 25% of the rock (Girard, 1984c). Albite occurs as slightly sericitized crystals. Albite microlites (up to 0.2 mm) or phenocrysts (2 cm long and 0.5 mm wide) are also observed (Dimroth, 1978). Actinolite is formed by uralitization of pyroxenes and occurs in fibrous aggregates. Pyroxene pseudomorphs may be present. Accessory minerals form ~5 to 10% of the rock and are represented by sphene, epidote, magnetite, pyrrhotite, pyrite and rarely chalcopyrite (Girard, 1984c; 1988). Calcite is also reported in minor amounts (Hashimoto, 1964).
Locally, sulphide mineralization is associated with sodic metasomatism (albitization) (Girard, 1988). Near the Romanet River, basalts (pPbc5) and sedimentary rocks associated with the Bacchus Formation have undergone sodic metasomatism (pPmik3) and have been mineralized in copper and gold (Clark, 1986).
Bacchus Formation 6 (pPbc6): Mafic Pyroclastics: Agglomerate, Tuff, Chloritic Schist
Mafic pyroclastic rocks include agglomerate, tuff, lapilli tuff and breccia layers. Mafic agglomerates occur in the upper part of the Bacchus Formation (Dimroth, 1978). Mafic pyroclastic rocks contain angular (<1-50 cm) fragments and blocks of mafic to intermediate composition in a fine-grained, dark chlorite matrix (Clark, 1978, 1986; Dressler, 1979). Fragments are light green in fresh surface, white to grey in altered patina, elongate to equidimensional, and commonly display an altered rim. Dark grey ovoid vesicular fragments (up to 20 cm wide) are locally observed (Clark, 1986). The volcanic fragments contained in pyroclastic rocks are mostly basaltic in composition (Dressler, 1979; Clark, 1986). In places, Dressler (1979) observed stratification in finer-grained pyroclastic rocks. In thin sections, the fragments and matrix of pyroclastic rocks are composed of actinolite, albite, chlorite, epidote, leucoxene and opaque minerals. Stilpnomelane and biotite are also observed (Dressler, 1979).
Chloritic schists occur in 3 m to 20 m thick layers interstratified with metabasalts of unit pPbc5. These rocks are light to dark green, fine grained, schistose, friable and strongly chloritized. They are very vulnerable to erosion and are easily split into sheets. Schsitosity planes have the same orientation as regional schistosity. Chloritic schists are composed of chlorite (65%), actinolite, epidote (15%), and a few small phenocrysts of albite (5-10%) and sphene (10%) (Girard, 1984a, b, c, 1988). Sulphides are observed locally. They consist of pyrite traces associated with carbonate veinlets, some pyrrhotite and a few chalcopyrite grains (Girard, 1984a, 1988). Chloritic schists probably represent the metamorphosed equivalent of mafic tuff (Hashimoto, 1968; Girard, 1988).
Bacchus Formation 7 (pPbc7): Hyaloclastic Basalt
Unit pPbc7 was observed, among others, south of Bertin Lake (sheet 24B05), where it is represented by a metabasalt flow and a hyaloclastic basalt layer (Dimroth, 1978; Chevé, 1985). Hyaloclastic basalt consists of pillow breccia composed of flat pillows (50 cm long and 10 cm thick), small spherical pillows (up to 5 cm in diameter) and pillow fragments, all in a matrix of partially cemented devitrified glass shards and glass of various textures (Dimroth, 1978). Hyaloclastites commonly contain chlorite and epidote zones, amygdules or veinlets filled with calcite, chlorite or epidote and plagioclase phenocrysts (5 mm long) in a fine-grained matrix, and disseminated fine sphene crystals (Dimroth, 1978).
Bacchus Formation 8 (pPbc8): Marble, Chert
Unit pPbc8 consists of marble and chert. It was mapped by Clark (1986) west of Mistamisk Lake.
Bacchus Formation 10 (pPbc10): Amphibolitized Basalt, Amphibolite; Some Paragneiss and Marble Locally
Unit pPbc10 consists mainly of amphibolitized basalt and amphibolite, and may include lesser amounts of paragneiss and marble. This unit was mapped locally by Dimroth (1972, 1978) in the Villandré Lake area (sheet 24B05). At this location, basaltic flows of unit pPbc5 gradually change to amphibolitized rocks of unit pPbc10, which are interstratified with unit pPbc11 schists (Dimroth, 1964, 1972, 1978). Amphibolitization of basalt begins at the top and base of the flows and near faults. Basalt flows have amphibolitized edge zones and a basalt core (Dimroth, 1964). Amphibolitized basalt is green, dark green or black, fine to very fine grained, homogeneous, massive or schistose (Dimroth, 1978). According to Dimroth (1978), structures are well preserved. In places, irregular veins and clusters of pegmatitic and aplitic material are observed (Dimroth, 1964, 1978). In thin sections, amphibolitized basalt is composed of hornblende or actinolite, albite, epidote and minor amounts of chlorite, calcite, sphene and opaque minerals such as ilmenite or magnetite (Dimroth, 1978).
Bacchus Formation 11 (pPbc11): Biotite-Muscovite Schist, Biotite-Garnet ± Graphite Schist, Biotite Quartzite, Biotite-Amphibole ± Graphite Schist
Unit pPbc11 is the metamorphic equivalent of unit pPbc2. In the area NE of Romanet Lake, Dimroth (1978) distinguishes a biotite schist unit, which comprises biotite-muscovite, biotite-garnet and biotite-amphibole schists, and a biotite quartzite unit. Biotite-muscovite schist is dark grey to black and fine to medium grained. It is composed of biotite, muscovite and quartz with minor amounts of apatite, epidote, zircon and opaque minerals. Biotite-garnet schist is dark grey to black and fine grained. It is characterized by the absence of muscovite and the presence of graphite in smaller amounts. Amphibole-biotite schist is black and also characterized by the absence of muscovite and by the presence of a minor amount of graphite.
In thin sections, biotite is dark brown or reddish brown and forms large, slightly poikiloblastic, anhedral porphyroblasts. It is undeformed and contains graphite inclusions that emphasize the rock’s schistosity. Hornblende is commonly dark blue-green and nematoblastic. Crystals are usually zoned. Hornblende contains little graphite. Quartz occurs in angular, polygonal or elongated grains parallel to schistosity. Feldspars form small grains elongated parallel to schistosity. However, grains are too small to identify the type of feldspar. They contain graphite flakes. Garnet forms more or less rounded poikiloblastic porphyroblasts. The latter contain numerous aligned quartz inclusions that mark earlier schistosity in the rock. Minor amounts of zoisite containing graphite inclusions are also observed. Apatite and zircon are the main accessory minerals (Dimroth, 1978).
Biotite quartzite is a finely recrystallized rock, grey in fresh surface and white in altered patina. It is composed mainly of quartz, biotite and lesser amounts of plagioclase. In thin sections, quartz is present as more or less polygonal grains, elongated parallel to schistosity. Biotite occurs in laminae also oriented parallel to schistosity. It usually contains graphite, which reveals the lamination of the earlier slaty cleavage. Dimroth (1978) notes that the latter may be oblique to the rock’s schistosity.
Bacchus Formation 12 (pPbc12): Rhyolite, Felsic Tuff
The felsic volcanic rocks are sparse within the Bacchus Formation. They consist of rhyolite and locally felsic tuff, interstratified with unit pPbc5 basalts (Hashimoto, 1964; Dressler, 1979; Kheang, 1984; Girard, 1988). In the La Lande Lake area, felsic volcanics are clearly dominated in volume by basalts and represent between 5 and 10% of the volcanic sequence (Kheang, 1984).
Rhyolite forms flows up to 10 m thick, NNW oriented, generally discontinuous (Dressler, 1979). The rock is dark grey, black, beige or greyish, and alters to light beige, locally to rusty brown due to the presence of sulphides. Rhyolite is very hard, fine grained to aphanitic, massive, rarely porphyritic, and has a fluidal texture and conchoidal fracture (Dressler, 1979; Kheang, 1984; Girard, 1984b, 1984c, 1988). Finely bedded (1-3 mm beds), vesicular or brecciated varieties are locally observed (Clark, 1978; Dressler, 1979; Kheang, 1984; Girard, 1984c, 1988). Rhyolite is mainly composed of quartz and microcrystalline feldspar. Albite is the dominant feldspar and alters to epidote. Small deformed vesicles stretched parallel to laminae locally represent up to 1% of the rock. These vesicles are filled with albite, quartz and a minor amount of calcite (Girard, 1988). In thin sections, the rock is composed of a few plagioclase, quartz and microcline phenocrysts in a very fine-grained leucocratic to melanocratic matrix. Minor amounts of chlorite, biotite or stilpnomelane, apatite, sphene and leucoxene are also observed. Opaque minerals consist of magnetite, pyrite and pyrrhotite (Dressler, 1979; Girard, 1984c).
Felsic tuff is aphanitic, locally sheared and brecciated. It is composed of angular to subrounded fragments of pinkish rhyolite, varying in size from 1 mm to 10 cm, in a microcrystalline melanocratic matrix of albite and quartz. Some fragments are elongated and fractured (Girard, 1984c, 1988).
Rhyolite and felsic tuff were locally altered by sodic metasomatism (albitization), which induced a calc-alkaline composition to the rocks of initially tholeiitic composition. Altered rhyolite then has a coarser grain size and the fluid texture is partially to completely obliterated by metasomatic alteration (Girard, 1988). Felsic volcanic rocks display low mineralization in disseminated pyrite and chalcopyrite or in quartz-ankerite-calcite veins (Kheang, 1984; Girard, 1988). Mineralization is associated with sodic metasomatism (albitization) (Girard, 1988).
Bacchus Formation 13 (pPbc13): Massive Basalt
Massive basalt occurs as 30 m thick flows that are interstratified with pillow lavas (pPbc14) and mafic pyroclastics (pPbc6). Unit pPbc5 also includes massive basalt flows (for a detailed description see Dimroth, 1978). Massive lavas are green to dark green and fine grained. They locally change to a medium-grained gabbroic rock (Clark, 1978). In places, Clark (1978) observed that adjacent massive flows are separated by zones of flow-top breccia that are 1 m to 2 m thick.
Bacchus Formation 14 (pPbc14): Pillow Basalt
Pillow lavas are interstratified with massive flows (pPbc13) and mafic pyroclastic rocks (pPbc6). Unit pPbc5 also includes pillow basalt flows (for a detailed description, see Dimroth, 1978). Pillows are elongated, very slightly flattened in places, ~1 m long and locally up to 3 m long. Pillows associated with massive basalts are generally not vesicular, whereas pillows in pyroclastic rocks are strongly vesicular (Clark, 1978).
Bacchus Formation 15 (pPbc15): Chloritic, Commonly Pyritic, Schistose Basalt; Local Dolomite Interstratifications
Unit pPbc15 forms a ~1 km to 2 km wide, NW-SW oriented strip near the Caniapiscau River (sheet 24F06). It consists mainly of greenish grey to grey schistose and chloritic basalt, commonly containing a minor amount of disseminated pyrite. The feldspar phenocrysts and small black cherty lenticular fragments are locally observed (Clark, 1977). Volcanic rocks locally include interstratified dolomite (Clark, 1977, 1978).
Bacchus Formation 16 (pPbc16): Mudstone or Graphitic and Pyritic Schist
Unit pPbc16 was described in the Colombet Lake area by Dressler (1979) and Girard (1984b, 1984c, 1988) who assigned it to the Mistamisk Formation (now abandoned). Unit pPbc5 also includes graphitic mudstone layers intercalated between basalt flows (for a description, see Dimroth, 1978). In the Colombet Lake area, unit pPbc16 consists of thin, highly deformed layers of mudstone or graphitic schist, 2 m to 10 m thick, interstratified with basalt (pPbc5). Unit pPbc16 unit is rarely exposed as it is easily eroded. The rock is generally black, glossy, fissile and very fine grained to aphanitic. It is formed from pelitic material and may contain up to 3% disseminated pyrite and a minor amount of iron oxides. Graphitic schist is cut by veins of quartz, carbonate or both, ~0.5 cm to 10 cm thick, locally mineralized in pyrite and rarely in chalcopyrite (Girard, 1984b, 1988). Graphitic schist has well-developed schistosity that follows a general N-NE orientation (Girard, 1984a). Graphite schists represent pelitic sediments, rich in organic matter, which were deposited between two episodes of volcanism (Girard, 1984b).
Thickness and distribution
The Bacchus Formation belongs to the Howse and Romanet (Wheeler) lithotectonic zones, as defined by Clark and Wares (2004). It occupies a large portion of the central and eastern parts of the orogen between latitudes 55°N and 57°30’N. It extends in a NW-SE direction for a distance of ~315 km. The total thickness and width of the formation are difficult to estimate due to the numerous gabbroic sills (Montagnais Supersuite) interspersed in the formation. The basaltic unit (pPbc5), which shows a thickness of ~600 m at Bacchus Lake, constitutes the major part of the formation (Dimroth, 1978). At Tait Lake, the basaltic unit is 300 m thick (Baragar, 1967). SE of Mistamisk Lake, a sequence of basalt, gabbro and sedimentary rock layers originally assigned to the now-abandoned Mistamisk Formation is ~1700 m thick in total (Dimroth, 1978).
Dating
None.
Stratigraphic Relationship(s)
The Bacchus Formation forms the top part of the Swampy Bay Group, as recommended by Clark and Wares (2004). According to Dimroth (1978), the interpretation of this formation is very complex. Relationships at the base of the formation are poorly exposed and poorly understood. In the central part of the orogen, the Bacchus Formation is locally overlying the Chakonipau, Portage and Dunphy formations (Seward Group) and the Lace Lake Formation (Pistolet Group). To explain these stratigraphic relationships, Dimroth (1978) suggested the presence of an erosional unconformity between the Bacchus Formation and the underlying lithostratigraphic units. According to the author, this unconformity is present at least in the area between Romanet Lake and Otelnuc Lake, north of latitude 56°N. However, the nature of the base of the Bacchus Formation is difficult to assess. Dimroth (1978) noted lithological similarities between the Lace Lake Formation and sedimentary rocks at the base of the Bacchus Formation. In addition, in the Mistamisk Lake and Romanet River area, Clark (1986) noted that it is not possible to determine the nature of the Lace Lake-Bacchus contact as it is not exposed. Basalts and interstratified sedimentary rocks of the Bacchus Formation are in fault contact with rocks of the Mistamisk Lake and Romanet River valley (Dimroth, 1978; Chevé, 1985; Clark, 1986; Clark and Wares, 2004).
In the southern part of the Labrador Trough, the Bacchus Formation is interdigitated westwards with the Le Fer Formation. In the Hurst and Chassin lakes area (sheet 23O10), it is overlain conformably by dolomite of the Denault Formation (Dimroth, 1978). Thus, Dimroth (1978) proposed that most of the Bacchus Formation is the temporal equivalent of the Le Fer Formation and possibly parts of the Otelnuc, Du Chambon and Romanet formations. Slightly different interpretations have been presented by Le Gallais and Lavoie (1982) and Clark (1986). Essentially, these authors proposed that the sequence containing the Bacchus Formation was deposited east of the platform rocks of the Pistolet Group. Also, that the Bacchus Formation is the temporal equivalent of these platform rocks, and was thrust over the Seward and Pistolet Groups on major Ferrum River and Argencourt thrust faults. The schist unit (pPbc11) is equivalent to the biotite-muscovite ± garnet paraschist subunit of the Freneuse Suite (pPfru1a) described further east by Charette et al. (2016).
Numerous gabbroic sills of the Wakuach Suite (Montagnais Supersuite) are intercalated into the Bacchus Formation volcano-sedimentary pile (Dimroth, 1978; Wardle and Bailey, 1981). A rhyolite dyke cutting the top of the Bacchus Formation at Colombet Lake has given a U-Pb age on zircons of 2142 +4/-2 Ma (T. Krogh and B. Dressler, unpublished data cited in Clark, 1984, page 4).
Paleontology
Does not apply.
References
Publications Available Through SIGÉOM Examine
CHARETTE, B., LAFRANCE, I., MATHIEU, G., 2016. Géologie de la région du lac Jeannin, Québec, Canada. MERN; BG 2015-01, 1 plan.
CHEVE, S R. 1985. LES INDICES MINERALISES DU LAC ROMANET, FOSSE DU LABRADOR. I N R S-GEORESSOURCES. ET 83-13, 62 pages and 2 plans.
CLARK, T. 1977. GEOLOGY OF THE FORBES LAKE AREA (NOUVEAU-QUEBEC). MRN. DPV 452, 19 pages and 1 plan.
CLARK, T. 1978. REGION DU LAC HERODIER (NOUVEAU-QUEBEC) – RAPPORT PRELIMINAIRE. MRN. DPV 568, 48 pages and 2 plans.
CLARK, T. 1984. GEOLOGIE DE LA REGION DU LAC CAMBRIEN – TERRITOIRE DU NOUVEAU-QUEBEC. MRN. ET 83-02, 77 pages and 1 plan.
CLARK, T. 1986. GEOLOGIE ET MINERALISATIONS DE LA REGION DU LAC MISTAMISK ET DE LA RIVIERE ROMANET. MRN. ET 83-22, 56 pages and 1 plan.
CLARK, T., WARES, R. 2004. SYNTHESE LITHOTECTONIQUE ET METALLOGENIQUE DE L’OROGENE DU NOUVEAU-QUEBEC (FOSSE DU LABRADOR). MRNFP. MM 2004-01, 182 pages and 1 plan.
DIMROTH, E. 1964. GEOLOGIE DE LA REGION DU LAC ROMANET, NOUVEAU-QUEBEC. MRN. RP 523, 20 pages and 1 plan.
DIMROTH, E. 1965. GEOLOGIE DE LA REGION DU LAC OTELNUK, TERRITOIRE DU NOUVEAU-QUEBEC. MRN. RP 532, 30 pages and 1 plan.
DIMROTH, E. 1966. GEOLOGIE DE LA REGION DU LAC DUNPHY, TERRITOIRE DU NOUVEAU-QUEBEC. MRN. RP 557, 24 pages and 1 plan.
DIMROTH, E. 1969. GEOLOGIE DE LA REGION DU LAC CASTIGNON, TERRITOIRE DU NOUVEAU-QUEBEC. MRN. RP 571, 62 pages and 7 plans.
DIMROTH, E. 1970. CARTES GEOLOGIQUES DU LAC ROMANET ET DU LAC CRAMOLET (FOSSE DU LABRADOR). MRN. DP 068, 3 plans.
DIMROTH, E. 1972. STRATIGRAPHY OF PART OF THE CENTRAL LABRADOR TROUGH. MRN. DP 154, 304 pages and 6 plans.
DIMROTH, E. 1978. REGION DE LA FOSSE DU LABRADOR ENTRE LES LATITUDES 54° 30′ ET 56° 30′. MRN. RG 193, 417 pages and 16 plans.
DRESSLER, B. 1973. GEOLOGIE DE LA REGION DU LAC PATU, TERRITOIRE DU NOUVEAU-QUEBEC. MRN. RP 603, 26 pages and 1 plan.
DRESSLER, B. 1975. GEOLOGIE DE FORT MCKENZIE, CHUTE AUX SCHISTES (1/2E), LAC MORAINE (1/2E), NOUVEAU-QUEBEC. MRN. RP 608, 32 pages and 1 plan.
DRESSLER, B., CIESIELSKI, A. 1979. REGION DE LA FOSSE DU LABRADOR. MRN. RG 195, 136 pages and 14 plans.
GIRARD, A. 1984a. REGION DU LAC COLOMBET (WAPANIKSKAN) – FOSSE DU LABRADOR. MRN. DP-84-19, 1 plan.
GIRARD, A. 1984b. GEOLOGIE DE LA REGION DU LAC COLOMBET, FOSSE DU LABRADOR. MRN. DP-85-09, 1 plan.
GIRARD, A. 1984c. LES INDICES MINERALISES DU LAC COLOMBET (FOSSE DU LABRADOR – NOUVEAU-QUEBEC). I N R S. MB 84-06, 67 pages and 1 plan.
GIRARD, A. 1988. GEOLOGIE ET METALLOGENIE DES INDICES CUPRIFERES ET URANIFERES DE LA REGION DU LAC COLOMBET (WAPANIKSKAN) – FOSSE DU LABRADOR. I N R S-GEORESSOURCES. MB 88-20, 85 pages and 2 plans.
HASHIMOTO, T. 1964. GEOLOGIE DE LA REGION DU LAC JOGUES, TERRITOIRE DU NOUVEAU-QUEBEC. MRN. RP 524, 13 pages and 1 plan.
HASHIMOTO, T. 1968. GEOLOGY OF JOGUES LAKE AREA, NEW QUEBEC. MRN. DP 179, 37 pages and 1 plan.
KHEANG, L. 1984. ALTERATION DES RHYOLITES ET DES BASALTES DANS LA REGION DES LACS LA LANDE ET DOUAY. MRN. DP-84-33, 20 pages.
PENROSE, B. 1978. GEOLOGIE DE LA REGION DU LAC HORSESHOE (NOUVEAU-QUEBEC). MRN. DPV 573, 39 pages and 1 plan.
Other Publications
BARAGAR, W R A. 1967. Wakuach Lake map-area, Quebec-Labrador (23O). Geological Survey of Canada; Memoir 344, 174 pages. http://doi.org/10.4095/123960
CLARK, T., THORPE, R.I. 1990. Model lead ages from the Labrador Trough and their stratigraphie implications. In The Early Proterozoic Trans-Hudson Orogen of North America: Lithotectonic Correlations and Evolution (J.F. Lewry, J.F. and M.R. Stauffer, editors). Geological Association of Canada; Special Paper 37, pages 413-432.
DIMROTH, E. 1970. Evolution of the Labrador Geosyncline. Geological Society of America Bulletin; volume 81, pages 2717-2742. http://doi.org/10.1130/0016-7606(1970)81[2717:EOTLG]2.0.CO;2
DIMROTH, E. 1971. The Attikamagen-Ferriman transition in part of the central Labrador Trough. Canadian Journal of Earth Sciences; volume 8, pages 1432-1454. http://doi.org/10.1139/e71-132
DIMROTH, E., BARAGAR, W. R. A., BERGERON, R., JACKSON, G.D. 1970. The filling of the Circum-Ungava geosyncline. In Symposium on Basins and Geosynclines of the Canadian Shield (A.J. Baer, editor). Geological Survey of Canada; Paper 70-40, pages 45-142. http://doi.org/10.4095/124922
DIMROTH, E., DRESSLER, B.O. 1978. Metamorphism of the Labrador Trough. In Metamorphism in the Canadian Shield. Geological Survey of Canada; Study 78-10, pages 215-236. http://doi.org/10.4095/104534
FAHRIG, W.F. 1955. Lac Herodier map-area, New Quebec. Geological Survey of Canada; Paper 55-1, 15 pages. http://doi.org/10.4095/101302
FAHRIG, W.F. 1956. Cambrien Lake (west half), New Quebec. Geological Survey of Canada; Paper 55-42. http://doi.org/10.4095/108312
FAHRIG, W.F. 1965. Géologie Lac Hérodier, Québec. Commission géologique du Canada; Carte 1146A. http://doi.org/10.4095/107520
FINDLAY, J. M., PARRISH, R. R., BIRKETT, T. C., WATANABE, D. H. 1995. U-Pb ages from the Nimish Formation and Montagnais glomeroporphyritic gabbro of the central New Québec Orogen, Canada. Canadian Journal of Earth Sciences; Volume 32, pages 1208-1220. http://doi.org/10.1139/e95-099
LE GALLAIS, C. J., LAVOIE, J. 1982. Basin evolution of the Lower Proterozoic Kaniapiskau Supergroup, central Labrador Miogeocline (Trough), Quebec. Bulletin of Canadian Petroleum Geology; volume 30, pages 150-166. http://bcpg.geoscienceworld.org/content/30/2/150
ROHON, M.-L., VIALETTE, Y., CLARK, T., ROGER, G., OHNENSTETTER, D., VIDAL, PH. 1993. Aphebian mafic-ultramafic magmatism in the Labrador Trough (New Quebec): its age and the nature of its mantle source. Canadian Journal of Earth Sciences; volume 30, pages 1582-1593. http://doi.org/10.1139/e93-136
ROSCOE, S M. 1957. Cambrian Lake, (east half), Quebec. Geological Survey of Canada; Paper 57-6, 16 pages. http://doi.org/10.4095/101318
SKULSKI, T., WARES, R. P., SMITH, A. D. 1993. Early Proterozoic (1.88-1.87) tholeiitic magmatism in the New Québec Orogen. Canadian Journal of Earth Sciences; volume 30, pages 1505-1520. http://doi.org/10.1139/e93-129
ST. SEYMOUR, K., KIDDIE, A., WARES, R. 1991. Basalts and gabbros of the Labrador Trough: remnants of a Proterozoic failed ocean? Neues Jahrbuch fuer Mineralogie, Monatshefte; Hefte 6, pages 271-280.
WARDLE, R J BAILEY, D G. 1981. Early Proterozoic sequences in Labrador. In Proterozoic Basins in Canada (F.H.A. Campbell, editor). Geological Survey of Canada; Study 81-10, pages 331-358. http://doi.org/10.4095/124192
Suggested Citation
Ministère de l’Énergie et des Ressources naturelles (MERN). Bacchus Formation. Quebec Stratigraphic Lexicon. https://gq.mines.gouv.qc.ca/lexique-stratigraphique/province-de-churchill/formation-de-bacchus_en [accessed on Day Month Year].
Contributors
|
First publication |
Charles St-Hilaire, GIT, M.Sc. charles.st-hilaire@mern.gouv.qc.ca (redaction) Mehdi A. Guemache, P. Geo., Ph.D. (redaction and coordination); Thomas Clark, P. Geo., Ph.D. (critical review); Simon Auclair, P. Geo., M.Sc. (editing); Céline Dupuis, P. Geo., Ph.D. (English version); Nathalie Bouchard (HTML editing). |