Guyer Group
Stratigraphic label: [marc]gy
Map symbol: mAgy

First published: 24 May 2018
Last modified:














Translation of original French

Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
mAgy5 Ultramafic wacke
mAgy4 Komatiite and magnesian basalt
mAgy3 Iron formation and wacke
mAgy2 Felsic and intermediate tuffs
mAgy2b Gneiss derived from felsic and intermediate volcanics
mAgy2a Metasomatic rock of felsic volcanics
mAgy1 Amphibolite derived from basalt, local paragneiss horizons
Author:Goutier et al., 2002
Reference section: 
Type area:Guyer Lake area (NTS sheet 33G11)
Geological province:Superior Province
Geological subdivision:La Grande Subprovince
Lithology:Ultramafic to felsic volcanics, iron formation, ultramafic wacke


The first detailed work on volcanics in the Guyer Lake area was carried out by St. Seymour (1975a, 1975b), Stamatelopoulou-Seymour and Francis (1980a and 1980b), Stamatelopoulou-Seymour (1982), Stamatelopoulou-Seymour et al. (1983) and St. Seymour and Francis (1988). These studies have highlighted the stratigraphy of komatiitic flows and ultramafic sedimentary rocks of Guyer Lake. A geochemical and petrogenetic study of all volcanics in the area was also completed. A final study involved sampling basalts, ultramafic rocks and migmatites from an area slightly to the west to determine their petrogenesis (Liu 1985).

Goutier et al. (2002) later described this sequence in more detail, dividing it into five different units and giving it the name Guyer Group. The majority of the work has focused on improving the definition of the group, including U-Pb isotope datations, and identifying new occurrences of Guyer Group units (Goutier et al., 2001, 2002; Bandyayera et al., 2011, 2013, 2014; Hammouche et al., 2015, 2017; Gigon and Goutier 2017; Goutier et al., in preparation).



Guyer Group 1 (mAgy1): Amphibolite Derived from Basalt, Local Paragneiss Horizons

This unit is the dominant unit of the Guyer Group. They consist of amphibolite derived from basalt, previously defined as Breton and Guyer Metabasalt by St. Seymour and Francis (1988). In general, primary structures are poorly preserved following several phases of deformation and metamorphism. This structural variation allows the rock to split up into centimetric slabs. However, pillow, massive or brecciated lava facies are well preserved. Locally, they alternate on metric thicknesses. The pillows are usually elongated and flattened with preserved hyaloclastic borders. The rock is dark green or black with a brownish patina. Grain size varies from fine to medium with minerals such as hornblende, actinolite and plagioclase. Grain size increases with deformation and metamorphic recrystallization. The minerals present are biotite, garnet, magnetite, epidote, carbonate and quartz (Goutier et al., 2002). Hammouche et al. (2015) observed this amphibolite in sheet 33H11. The rock varies from very fine to fine-grained and from nematoblastic to porphyroblastic (hornblende). Pyrite, pyrrhotite and arsenopyrite were also observed in amphibolite.

Several facies variations are visible on a local scale. Some areas form narrower and more deformed bands. In this case, basalt is completely metamorphosed into amphibolite. The structure is banded, alternating between black to dark green bands (1 to 40 cm) composed mainly of hornblende and plagioclase, grey bands (1 to 5 cm) richer in plagioclase and millimetric white quartzofeldspathic bands. Within these three bands, centimetric lenses and a few millimetric bands of apple-green epidote and diopside are visible (Goutier et al., 2002).

Another facies variation is observed in levels of glomeropphyric basalt with plagioclase (Stamatelopoulou-Seymour, 1982). Plagioclase phenocrystals are yellow white (1 to 35%), rounded and measure from 0.2 to 4 cm. The dark green matrix is very fine-grained. Phenocrystals can be weakly deformed or strongly stretched in foliation (Goutier et al., 2002; Bandyayera et al, 2011).

Hammouche et al. (2015, 2017) added paragneiss derived from wacke horizons to unit mAgy1. These horizons are present in the most easterly band of the unit in sheets 33H10 and 33H11. They vary in thickness from one decimetre to a few metres. Paragneiss is usually light brown or dark grey with rust shades and light grey to brown grey in fresh exposure. It consists of fine grains of feldspar, quartz and green biotite. It also contains minor amounts of hornblende, garnet, carbonates, magnetite and locally pyrite. 


Guyer Group 2 (mAgy2): Felsic and Intermediate Tuffs

This volcanic unit is 0.3 to 1 km wide and 6.5 km long. It is a lapilli and block tuff with a white, beige and in places yellow patina and a grey colour in fresh exposure. Fine beige and greenish lamines are visible, linked to mylonitic schistosity or foliation. In this case, the tuff becomes a sericite schist. Foliation, as well as strong sericite alteration, make it difficult to observe primary volcanic structures such as the presence of fragments. Quartz porphyritic dykes cut this unit (Goutier et al., 2002).

This felsic tuff is made up of quartz, plagioclase, sericite-muscovite, biotite and chlorite. It is also possible to find sulphides, actinolite, epidote, tourmaline, cordierite, staurotide, sillimanite, carbonates and titanite. Some of these minerals are derived from metamorphism associated with synvolcanic hydrothermal alteration (Goutier et al., 2002). Clasts are mafic (negative relief) to felsic (positive relief) and make up 60% of the rock. These fragments are aligned with foliation and are usually stretched. Locally, white centimetric leucocratic bands with plagioclase alternate with dark bands rich in mafic lapillis. The latter contain 1-3% garnet and 10-20% biotite and hornblende (Bandyayera et al., 2011).


Guyer Group 2a (mAgy2a): Metasomatic Rock of Felsic Volcanics

This subunit was defined during the summer 2011 work in the southwest of sheet 33H12. It is associated with two metasomatic zones of felsic volcanics. Garnet is very abundant (up to 22%) and has a diameter of up to 6 cm. Disseminated sulphides are also present in smaller quantities. Locally, it is possible to observe staurotide and sillimanite (Goutier et al., in preparation).


Guyer Group 2b (mAgy2b): Gneiss Derived from Felsic and Intermediate Volcanics

This unit was introduced in 2018 to define a highly deformed Guyer Group metavolcanic band. It is shown in sheet 33G05 as a WNW-ESE band 8.7 km long and up to 800 m thick. Outcrops are well exposed on the shores of the islands of the La Grande 3 Reservoir. This unit consists of a complex assemblage of gneiss whose protolith is interpreted to be intermediate, felsic and, to a lesser extent, mafic volcanics. The rock has a zebra structure, with centimetric horizons that are either beige with more biotite-rich lamines or black, dark grey with hornblende, pale grey and whitish. These horizons alternate with metric horizons consisting of either dark green banded amphibolite, diopside and epidote, or white centimetric fragments (possibly lapillis). These rocks are also cut by many tonalite, hornblendite and granite dykes of centimetric to metric thickness.


Guyer Group 3 (mAgy3): Iron Formation and Wacke

Iron formation forms several horizons in the Guyer Group. These horizons are present throughout the sequence and have a strong magnetic signature (Goldak Airborne Surveys, 2008). They vary in thickness from one metre to several decimetres. Facies are usually interstratified with the surrounding lithologies, either amphibolite, tuff or wacke (Bandyayera et al., 2011). The dominant facies of iron formation is the oxide facies. It outcrops as magnetite-rich millimetric to centimetric beds. These beds alternate with granoblastic chert beds. In general, magnetite is fine-grained and has been recrystallized by metamorphism. The rock patina is rusted due to the presence of a small amount of sulphide. The silicate-oxide facies is recognizable by the presence of coarse amphibole and garnet. Silicate horizons are composed of grunerite, hornblende, cummingtonite, biotite, garnet, hedenbergite and chlorite. Disseminated magnetite grains and sulphides are visible in silicate laminas. Millimetric to centimetric intervals of disseminated to massive magnetite are also present (Goutier et al., 2002).

Massive sulphide lenses (pyrrhotite-pyrite ± chalcopyrite ± arsenopyrite) are associated with the less common facies of iron formation, the sulphide facies. These lenses are metric and are located at the interface between felsic tuff and amphibolite, or along a gold-bearing horizon in sheet 33G06 (Goutier et al., 2002).

 Unit mAgy3 also includes horizons of wacke and tuff interstratified with thin iron formation (centimetric to metric). This unit has a variable colour ranging from green to pink and in places brown and beige. Wackes are usually dark grey to green in fresh exposure. Foliation takes over bedding in places. The wacke is composed up of quartz, feldspar, hornblende and biotite with spots of epidote and garnet (up to 20%). It is different from the nearby Ekomiak Formation wacke, since its size is finer and it is possible to find disseminated magnetite within the wackes only associated with conglomerates.

Guyer Group 4 (mAgy4): Komatiite and Magnesian Basalt

Unit mAgy4 is composed of magnesium basalt and komatiite which are distributed according to several distinct bands. St. Seymour (1975b) and Stamatelopoulou-Seymour (1982) defined a band north of the Transtaïga Highway in sheet 33G06, while St. Seymour and Francis (1988) listed a second band south of the Transtaïga Highway in the same sheet. Goutier et al. (2002) defined a third band in sheet 33G11, while Bandyayera et al. (2011) identified two other bands west and northeast of the De Pontois River in sheets 33G10 and 33G07.

The patina varies from brownish grey (peridotite facies) to shiny bottle-green (pyroxenite facies); it is easy to scratch. Pillow flows and massive flows are the most prevalent in the region and spinifex and polygonal joints are visible in places. The typical arrangement of komatiite flows, Pyke Hill type (Pyke et al., 1973), does not apply. Fine-grained rock consists mostly of tremolite, serpentine, chlorite and talc. It is not an amphibolite, as there are no black fine needles of amphibole present. Magnetite is common in the peridotite facies and rare in the pyroxenite facies. Millimetric porphyroblasts of brown olivine developed in several places in the rock (Stamatelopoulou-Seymour and Francis, 1980b; Goutier et al., 2002). Orthopyroxene porphyroblasts have also been identified (Bandyayera et al., 2011) and may account for 5-15% of the rock. Felsic intrusions are observed locally in this unit.

The lateral extension of this unit ranges from 1 to 6.5 km per 100 to 600 m wide. Unit mAgy4 is usually associated with iron formations. In contact areas, komatiite is heavily deformed and locally schistose.


Guyer Group 5 (mAgy5): Ultramafic Wacke

The ultramafic wacke unit was identified in sheet 33G06 by Stamatelopoulou-Seymour and Francis (1980) and was exposed for 1.5 km in the northern portion of the sheet. These are very well-preserved ultramafic turbidites. They conformably lie on komatiites and are covered by pillow basalts and pyroxenite. The rock is pale brown and consists of cyclic layers of thin to medium beds with grading structures and parallel and oblique laminations. The mineralogical assemblage consists of fine grains of tremolite, chlorite, talc, serpentine, carbonates, magnetite and chromite. The spot appearance of the rock comes from the presence of olivine porphyroblasts. These olivines are metamorphic in origin (Goutier et al., 2002). 

Thickness and Distribution

The Guyer Group is a lateral extended unit in the La Grande Subprovince. It is present in 12 NTS sheets (33G05, 33G06, 33G07, 33G08, 33G09, 33G10, 33G11, 33G12, 33H05, 33H10, 33H11 and 33H12), representing an area of nearly 720 km². 


The three U-Pb isotope dates of felsic volcanics (unit mAgy2a) of the Guyer Group indicate that this unit is Mesoarchean (2820 Ma to 2806 Ma). 

Isotopic SystemMineralCrystallization Age (Ma)(+)(-)Reference(s)
U-PbZircon2806.12.32.3David et al., 2011
U-PbZircon2815.93.13.1David et al., 2012
U-PbZircon2820.30.80.8Goutier et al., 2002

Stratigraphic Relationship(s)

The amphibolite after basalt (mAgy1) and the gneiss after intermediate and felsic volcanics (mAgy2b) of the Guyer Group are heavily deformed in areas of contact with other units such as the Langelier Complex, the Poste Le Moyne Pluton and the Bezier Suite. These rocks are folded, sheared and mylonitized.

The Marbot Formation lies in part on the Guyer Group. Their different age of emplacement, from 2820 to 2806 Ma for the Guyer Group and <2702 ±2 Ma for the Marbot Formation (David et al., 2011), suggest a discordance between these units (Bandyayera et al., 2011).


Does not apply.


Author(s)TitleYear of PublicationHyperlink (EXAMINE or Other)
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DAVID, J. – MCNICOLL, V. – SIMARD, M. – BANDYAYERA, D. – HAMMOUCHE, H. – GOUTIER, J. – PILOTE, P. – RHÉAUME, P. – LECLERC, F. – DION, C.Datations U-Pb effectuées dans les provinces du Supérieur et de Churchill en 2009-2010. Ministère des Ressources naturelles et de la Faune, Québec; RP 2011-02, 37 pages.2011RP 2011-02


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Levé aéromagnétique sur le territoire de la Baie-James – Opinaca, sud de LG-3 et sud de LG-4. Ministère des Ressources naturelles et de la Faune, Québec; DP 2008-01, 48 pages, 58 plans.

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