|Author:||Bandyayera and Fliszár, 2007|
|Age:||Precambrian / Archean / Neoarchean|
|Type area:||Eeyou Itschee James Bay region (NTS sheets 33A, 33B, 33C 33F, 33G, 33H, 23D, 23E and 32O)|
|Geological province:||Superior Province|
|Geological subdivision:||Opinaca Subprovince|
|Lithology:||Sedimentary, metamorphic, volcanic rocks and various intrusions|
Table des matières
The definition of this unit was introduced by Eade et al. (1958, 1966) to define the large assemblage of paragneiss, cut by multiple intrusions of white to pink granite and pegmatic granite. Lavoie et al. (1976) wrote in their research that the “Laguiche Unit” corresponds to a gneiss of the Grande Pointe unit and its suite of granitoids. Franconi (1978) grouped migmatized metasedimentary rocks into the “Rossignol Group”. Subsequently, Fouques and Schumaker (1979) re-used the term “Laguiche” by assigning the name “Laguiche Series” to metasedimentary rocks. In order to consolidate all metasedimentary rocks in the northern part of the Opinaca Subprovince, Gauthier et al. (1997) introduced the term “Rossignol-Laguiche Group”. This name was simplified by Simard and Gosselin (1999) and Goutier et al. (2000) by “Laguiche Group”. The Laguiche Complex (nAlgi) was defined in the preliminary report of Bandyayera and Fliszár (2007), which includes all the migmatized paragneiss of the Opinaca Subprovince.
The following table lists the different names and equivalencies of the units of the Laguiche Complex over time and their first mention in a ministry report.
The Laguiche Complex is a metasedimentary unit composed mainly of paragneiss, metatexite and diatexite. This unit consists mainly of biotite paragneiss from the gradual transformation of feldspathic wackes and mudrocks. A few arkosic arenite horizons were also observed in NTS sheet 33F07. Minor components of this unit are felsic ash tuff, iron formation and polymictic conglomerate. The Laguiche Complex contains rare iron formations interstratified with metasediments. Finally, the mobilisate is generally granitic in composition, but may vary from tonalitic to granodioritic (stain analysis is currently not sufficiently representative to estimate the percentage of granitic and tonalitic mobilisate or a possible geographic distribution of mobilisate compositions).
Paragneiss located in the Opinaca Subprovince may be confused with those of the Rivière Salomon Formation. However, paragneiss of the Laguiche Complex may contain orthopyroxene, while this mineral is absent from paragneiss of the Rivière Salomon Formation. The granoblastic structure is, on the one hand, less developed in the Rivière Salomon Formation because of a weaker recrystallization. On the other hand, protoliths of the Laguiche Complex paragneiss are only arkosic, whereas they can be quartz-rich or arkosic in the Rivière Solomon Formation.
Units nAlgi2a, nAlgi3a and nAlgi4a are defined based on the percentage of mobilisate observed on outcrop: 1) between 0 and 10% mobilisate, these are paragneiss derived from wacke (nAlgi2a); 2) 10-50%, paragneiss metatexite (nAlgi3a); and 3) paragneiss diatexite (nAlgi4a) is identified at more than 50% (Gigon and Goutier, 2016). These percentages are valid for the northern part of the Laguiche Complex (sheets 33F, 33G, 33H and 23E), while the southwest part is mainly based on a less than 20% mobilisate interval for units nAlgi2a and nAlgi2b, and 20-50% mobilisate interval for units nAlgi3a and nAlgi3b (Bandyayera and Lacoste, 2009; Bandyayera et al., 2010).
Units nAlgi12 and nAlgi13 were derived from field studies conducted in 1997 by Simard and Gosselin (1999); the knowledge of the region is still the same today. Work is planned in these areas to redefine these units to the 1:50,000 scale.
The following table shows the distribution in area (square kilometres) and in percentage (%) of the different units of the Laguiche Complex. Unit nAlgi12 represents nearly 41% of the complex; however, it is a unit being modified as a result of the implementation of mobilisate percentages (nAlgi2a, nAlgi3a and nAlgi4a). Units nAlgi2a and nAlgi3a alone represent half of the Laguiche Complex.
The Laguiche Complex can be simplified into four large subdivisions representing more than 90% of the overall area of the complex. The following table shows the number of occurrences and the percentage of outcrops containing the mineral in question in relation to all outcrops associated with the subdivision. Garnet is thus very well represented (between 8 and 28% occurrence), as well as sulphides and K-feldspar.
The nAlgi1 unit consists of banded amphibolite, iron formation, mafic gneiss, as well as locally pillow metabasalt. This unit is present mainly in sheets 33B13 and 23D14. Polygons on the map were mostly drawn by following the magnetic highs.
In the Ewart Lake area, a 2.6 km long by 320 m thick metabasalt strip was drawn following geological mapping of sheet 33G04 in 2016. This strip forms a ridge and has a positive magnetic signature (Goldak Airborne Surveys, 2008). It is composed of a massive pillow metabasalt, metamorphosed to the amphibolite facies. The rock has a green to dark grey patina and a dark green fresh surface. Pillows are usually decametric. A few quartz chambers are visible, suggesting a stratigraphic polarity towards the north. Generally, pillows are weakly to moderately deformed, with an apparent flattening on the surface. In the northern part, more deformed and banded horizons are observed. The base of this sequence is a metric horizon of sulphidic chert, metawacke and pyrrhotite-pyrite metasomatic rock with traces of sphalerite and chalcopyrite (outcrops 2016-RH-7305 and 2016-JG-2709). Diopside, pyrrhotite, pyrite and tourmaline are present in a metawacke sample. These rocks are rusty and stratified. Contact between this unit and paragneiss of the Laguiche Complex was not observed.
These metabasalts often exhibit alteration horizons in the form of centimetric bands, which are either whitish (plagioclase-quartz-hornblende-diopside garnet), cutting or following the edges of pillows, either dark green and partially rusted (hornblende-garnet-sulphides ± biotite ± quartz ± plagioclase) along the pillow edges. Disseminated garnet is also observed in the pillows. A network of late millimetric quartzo-feldspathic veinlets in positive relief affects metabasalts.
Laguiche Complex 2a (nAlgi2a): Biotite ± Garnet Paragneiss Derived from Wacke, Containing <10% Mobilisate and Local Orthopyroxene, Amphibole and Hornblende
The nAlgi2a unit covers most of the Laguiche Complex and includes a biotite wacke paragneiss package with or without garnet. It also forms the least migmatized unit of the Complex, since the rocks contain less than 10% mobilisate (Goutier et al., in preparation). The primary structure, however, has completely disappeared, leaving room for a granoblastic structure, with the exception of local compositional and size banding that could correspond to primary bedding (Bandyayera et al., 2011).
Paragneiss is fine-grained and is derived from an arkosic wacke. The patina is rusty brown and the fresh surface is light grey. It is generally composed of 10-60% quartz, 10-30% plagioclase, 1-2% K-feldspar, 10-35% biotite and 7-15% hypersthene with local garnet, hornblende and pyrite or magnetite-type opaque minerals (Hammouche and Burniaux, in preparation). Accessory minerals such as tourmaline, rutile, zircon, monazite, apatite, allanite, pyrrhotite, ilmenite and diopside are also observed in thin section (Gigon et al., in preparation). The alteration horizon can be seen by sericitization of plagioclase, uralitization of hypersthene and weak serpentinization of hypersthene cracks.
The mobilisate is whitish, granitic in composition and appears either in thin bands (stromatic structure) parallel to foliation, or evasively in diffuse contact with the restite. It is frequently affected by tight or ptygmatic folds. In the presence of medium grain size, biotite and garnet are particularly visible in paragneiss.
Laguiche Complex 2b (nAlgi2b): Nodular Biotite ± Hornblende ± Garnet ± Cordierite ± Sillimanite Paragneiss, Containing <20% Mobilisate
The nAlgi2b unit is a nodular paragneiss containing less than 20% mobilisate. Simard and Gosselin (1999) defined it as the Alu2 unit of the Laguiche Group. This unit is predominantly present in southwestern James Bay (Bandyayera et al., 2010). Occurrences have been observed in the northern part, but are too small to be mapped. The nodular paragneiss forms weakly migmatized kilometric bands. The nAlgi2b unit differs from the nAlgi2a unit because of its garnet concentration; the nodular paragneiss is rich in aluminosilicates and garnet. The nodules correspond to porphyroblasts consisting of cordierite, andalousite or sillimanite. These nodules are in positive relief on outcrop and can be up to 5 cm in diameter. They also highlight banding between bands that are rich and poor in aluminosilicates respectively; these bands can be interpreted as a primary bedding relic.
Laguiche Complex 3a (nAlgi3a): Biotite ± Garnet Metatexite Derived from Paragneiss, Containing 10-50% Mobilisate and Local Orthopyroxene and Amphibole; Migmatized Paragneiss
The nAlgi3a unit differs from the nAlgi2a unit only by the amount of mobilisate (10-50%) that transforms the paragneiss into metatexite (Hammouche and Burniaux, in preparation). Restite is fine-grained and arkosic in composition, with 20-25% quartz, 10% feldspath, 10-15% biotite, 15% hypersthene, 2-3% garnet and 15% amphibole in places. Biotite usually forms schlierens in contact with the mobilisate. The main accessory minerals are zircon, magnetite and pyrite. Garnet, hypersthene and amphiboles reflect the metamorphic mineralogy. Alteration phenomena remain damouritization of plagioclase, chloritization of biotite and replacement of orthopyroxene with hydrous phases such as biotite, cummingtonite or anthophyllite.
The mobilisate varies from grey-white to pink grey. Its composition is granitic and its structure is stromatic. In addition, it is possible to observe ptygmatic, folded or phlebitic structures. Millimetric banding can also be observed, although it may be late intrusions unrelated to partial melting of the rock.
The presence of coarse-grained garnet-rich leucosome clusters, quartz inclusions in garnet crystals, and melanosome rim around leucosome are evidence of in situ partial melting (Sawyer, 2008). Cordite closely associated with the garnet is also observed on outcrop and in thin section.
Despite intense migmatization, the paragneiss shows compositional bands marked by alternating biotite-rich beds and hornblende-rich beds that may reflect primary bedding (Bandyayera et al., 2010).
Laguiche Complex 3b (nAlgi3b) Nodular Migmatized Paragneiss
The nAlgi3b unit is a nodular miamatized paragneiss package with 20-50% mobilisate (Bandyayera et al., 2010). As with the nAlgi2b unit, some occurrences are observed in the northern part of the complex, but they are only consistent with non-mappable horizons. The nAlgi3b unit is very similar to the nAlgi2b unit, the difference is in the amount of mobilisate. This unit is very geographically restricted and rocks of this family appear only in sheets 33B12 and 33B13. Some horizons show enrichment in aluminosilicates and alternate with biotite ± hornblende ± garnet horizons. Pegmatitic dykes with garnet and tourmaline cut the paragneiss package.
Unit nAlgi4 consists of diatexites derived from paragneiss. It concerns the southwest part of the Laguiche Complex, in the area defined by Bandyayera et al. (2010). These diatexites are composed of more than 50% mobilisate. Units nAlgi4 and nAlgi4a contain similar rocks, but correspond to two different geographic areas. Bandyayera et al. (2010) define the nAlgi4 unit in southwestern James Bay, while Gigon and Goutier (2016) and Hammouche and Burniaux (2015) use the nAlgi4a unit for diatexites with more than 50% mobilisate in northern James Bay.
Laguiche Complex 4a (nAlgi4a): Biotite ± Garnet Diatexite Derived from Paragneiss, Containing >50% Mobilisate and Local Orthopyroxene and Amphibole
The nAlgi4a unit is a heterogeneous and banded diatexite unit composed of biotite, hornblende, garnet and pyroxene (Bandyayera et al,. 2010). They are the most migmatized rocks as they are composed of more than 50% mobilisate and intrusions. The composition of this mobilisate varies from tonalitic to granodioritic. Rock banding is chaotic on outcrop with mobilisate enriched areas and others enriched in stretched enclaves of biotite ± hornblende paragneiss, locally pyroxene ± garnet. Biotite, hornblende, magnetite and garnet crystals are found in high concentrations near leucosomes, while orthopyroxene and clinopyroxene are observed in both the mobilisate and paragneiss enclaves or restites.
Within the diatexites, protected enclaves of amphibolite and tonalite are visible. There are also polyphased intrusions of conformable or unconformable mobilisate. Late pegmatitic intrusions (with crystals with a graphic structure up to 70 cm in diameter) cut the whole and, in particular, migmatitic banding.
The nAlgi5 unit is an amphibolite with hornblende, with or without diopside and biotite. The first unit definition is from Simard and Gosselin (1999) describing amphibolite and amphibole gneiss (Alu5 unit) units interbedded in paragneiss sequences. These horizons are generally small and heavily intruded with granitic mobilisate.
The protolith is likely to be a metamorphosed, intermediate, intrusive rock within metasedimentary rocks. Its primary mineralogy is hornblende (40-50%), diopside (1-5%), sulphides, sphene and biotite locally (Gigon et al., in preparation). In outcrop, the rock has a greyish patina and a medium to dark green fresh exposure. It is slightly foliated, fine-grained and granoblastic. Compositional banding is visible between the bands richer in ferromagnesian minerals and those richer in plagioclase.
Unit nAlgi6 is a series of felsic ash tuffs, polymictic conglomerates and iron formations defined by Goutier et al. (2000). It outcrops along the Transtaïga Highway, around kilometre 85 (sheet 33F09). This unit had previously been assigned to the Yasinski Group (Fallara et al., 1999), but stratigraphic continuity between these rocks and sedimentary rocks of the Laguiche Complex have been discovered. This is the upper unit of the Laguiche Complex, located in the centre of a synclinal overfold southward. The maximum preserved thickness is 200 m.
The dominant rock of the Algi6 unit is a felsic ash tuff, of white patina and brown grey fresh exposure. It contains millimetric crystals of plagioclase that float in a finer quartzo-feldspathic matrix. These crystals are evident in altered patina. When the surface of the outcrop is clean and sedimentary structures are attenuated, the rock resembles a fine-grained porphyry intrusion. Quartz phencrystals are not abundant and total not more than 7% of the rock. The felsic ash tuff appears as thin to medium, graded beds with parallel laminations at the top of the beds. When beds are thick and massive, contacts are diffuse. Centimetre to metric horizons of mudstone thin graded beds are interstratified with the ash felsic tuff. Horizons of lapilli pyroclastic rocks are rarer. A few decametric to metric lamprophyre dykes containing tuff and iron formation enclaves cut this sequence. These dykes have sometimes been confused with block tuffs (11144-C1, TRH-96-13; Fallara et al., 1999). They are grey to green, fine-grained, in an amphibole matrix and contain some biotite. The shape of the fragments in these dykes varies from angular to rounded.
One of the features of the nAlgi6 unit is the presence of an gold-bearing oxide-silicate iron formation and a polymictic conglomerate with chert fragments. These two lithologies are generally adjacent. They are located next to the wacke-felsic ash tuff contact in each side of the synclinal. The thickness of the iron formation-conglomerate pacquage does not exceed 5 m. The conglomerate beds are lenticular, whereas the iron formation is rather continuous and shows a decametric lateral extension. The latter consists of millimetric beds of magnetite and chert with a variable proportion of silicate-rich millimetric to centimetric beds (grunerite and hornblende ± garnet ± biotite). The polymictic conglomerate consists mainly of lenticular and subangular pebbles and stones of white feldspathic porphyry, chert and amphibolite. More rarely, fragments of iron formation, amphibole andesite and mudstone are observed. The beds are massive to graded and the fragments are usually joint. The matrix is dark due to the presence of amphiboles.
The nAlgi7 unit consists mainly of metamorphosed biotite wacke. Previously, this unit was part of the Alu1 unit of the Laguiche Group defined by Simard and Gosselin (1999). This unit is present in the centre-east of sheet 33F. Primary sedimentary structures within the metamorphosed wackes distinguish it from paragneiss units. In addition, the presence of chlorite gives a green colour to the rock in fresh exposure, while paragneiss have a greyish fresh exposure. The wacke is fine-grained, foliated and homogeneous. It is more common to find carbonates and epidote in this type of rock (Gigon et al., in preparation).
Turbidite structures in thin to medium graded beds are observed in many places in the metamorphosed wacke. The base of some beds is composed of polymictic granules as observed in the Sakami Lake area (sheet 33F07, Goutier et al., 2000). The mudrock is interstratified with wacke and more abundant in the area. It has a brown or green patina and commonly exhibits parallel laminations. Fine biotite is widely developed, while garnet and amphibole are more rarely observed. Some horizons of polymictic conglomerate with quartz, arenite, biotite shale, amphibolite or felsic rock granules are interbedded with the wacke.
Unit nAlgi8 is an assemblage of paragneiss derived from wacke, magnetite paragneiss, altered and sheared paragneiss horizons, and graphite and sulphide schist. A geographic type area describes this unit, located in the SE corner of sheet 33F08 and measuring 3 km long by 200 m wide (Gigon et al., in preparation).
Paragneiss are usually foliated and some levels are sheared. The mobilisate is poorly represented in this unit. The peculiarity of this is the high concentration of pyrrhotite, graphite and amphibole in the rock. Schist horizons derived from graphite and sulphide mudstone show a darker and finer matrix than other paragneiss lithologies. They vary in thickness from 25 to 75 cm. There are centimetric to metric intrusions of granite containing biotite, tourmaline and muscovite.
Centimetric to decametric, conformable and unconformable horizons of alteration are also present in this unit. They contain diopside, quartz, plagioclase, microcline, amphibole and sulphides.
Unit nAlgi9 corresponds to iron formations, located in sheet 33F09, which were previously part of the Alu4 unit of the Laguiche Group established by Simard and Gosselin (1999). These levels of iron formations are a few metres thick and have little lateral development. Iron formation is banded by alternating bands of centimetric to decametric magnetite and laminas of recrystallized and whitish chert.
The nAlgi10 unit consists of a few horizons of polymictic joint granule and pebble conglomerate interbedded with wacke. The most important lens is located southeast of Bonfait Lake (sheet 33F09). It consists of metric beds of polymictic conglomerate with rounded pebbles of various lithologies, including basalt, gabbro, tonalite, quartz, arenite, biotite, amphibolite and felsic rock (Goutier et al., 2001). A second lens is located in the northeast portion of sheet 33G05 along the Transtaïga Highway. This unit marks contact between the La Grande and Opinaca subprovinces. It has been deformed, as shown by L/S tectonic, foliation and shear evidence. The majority of the matrix consists of biotite (10-20%), amphibole (5-20%), and traces of pyrite and chlorite.
Other non-mappable occurrences were identified during the work conducted in 2015 and 2016. The occurrences of conglomerates belonging to the Laguiche Complex that have been identified are in the northern part of the Opinaca Subprovince. They are generally concentrated within 10 km of the border of this subprovince.
The gneiss of the Prosper Gorge are located in sheets 33B01 and 33B02 and correspond to unit Alu3 by Simard and Gosselin (1999), which was replaced by unit nAlgi11 during this compilation. They were named and described by Franconi (1983). According to Franconi (1983), the typical sequence is banded, in places with stratiform flow, and consists of alternating mesocratic gneiss, finely banded gneiss (repetition of leucocratic and melanoratic centimetric horizons) and amphibole-rich melanoratic gneiss.
Mesocratic gneiss is the most common facies. Macroscopically, they are homogeneous rocks, fine to medium-grained, with a brownish to greenish grey patina and a greyish fresh surface. They contain between 20 and 35% ferromagnesian minerals (amphibole and biotite) in small stocky crystals distributed homogeneously, giving them a pepper and salt appearance. They do not show increased foliation; some facies rich in black minerals show at most a vague schistosity. The finely banded gneiss, which is fairly common, consists of a repetitive alternation of leucocratic, medium-grained bands, composed of a combination of plagioclase and quartz crystals, and thiner biotitic bands (2 to 4 cm). Leucocratic bands contain between 8 and 10% biotite and have an indured aspect. In addition, the patina and fresh surface have a very clear greenish tint. Finally, the less common melanocratic amphibole gneiss is strongly banded and contains lighter bands of the same composition as those that characterize the finely banded gneiss. It is homogeneous and not banded in places.
The nAlgi12 unit is made up of migmatized paragneiss and biotite paragneiss. Originally, Simard and Gosselin (1999) defined the Alu1 biotite paragneiss unit. This unit was also defined by Goutier et al. (2001). This paragneiss is fine-grained and is composed of quartz, plagioclase and biotite. Accessory minerals observed are garnet, hornblende, cummingtonite, tourmaline, cordierite, chlorite and magnetite. The structure is generally granoblastic and foliation is visible by the orientation of biotite grains. The degree of migmatization is highly variable and is not a criterion for the definition of this unit. This unit reflects work done in the region between 1997 and 2000 where migmatization percentage intervals had not yet been defined.
The nAlgi13 unit is a magnetic paragneiss unit, formerly defined as the Alu4 unit of the Laguiche Group by Simard and Gosselin (1999). It is easily recognizable on aeromagnetic maps due to the concentration of magnetite grains in the rock. Otherwise, this unit is similar to the nAlgi12 unit described above.
The nAlgi14 unit was introduced by Talla Takam et al. (in preparation) to group anatectic granites within the Laguiche Complex. These are, therefore, granitic rocks that have completely melted before recrystallizing, generally with a flow structure that differentiates them from the relatively massive granites of the region. This unit was observed only very locally. The rocks are pinkish white to grey-white and coarse-grained. It has a paragenesis typical of felsic rocks: quartz-K-feldspar-plagioclase with biotite schlierens and very locally orthopyroxene or garnet. Zircon was also observed in thin section.
The Laguiche Complex is the dominant unit in the Opinaca Subprovince. It occupies a very large area, more than 22,450 km², in the Eeyou Itschee James Bay region. This unit covers sheets 33A, 33B, 33C 33F, 33G, 33H, 23D, 23E and 32O.
Early dating (Machado, personal communication, 1999) of a sample of polymictic pebble conglomerate from the Laguiche Complex collected on outcrop 1998-JG-2274 (sheet 33F09) revealed a variety of ages, some as young as 2648 ±50 Ma and others as old as 3284 ±75 Ma. Subsequently, many laser dating have helped to determine the age of the various sources, as well as the maximum age of sedimentation.
Several dated intrusions cutting rocks of the Laguiche Complex are the Bezier Suite (2712.3 ±5.8 Ma; David, personal communication, 2017), the Fregate Pluton (2710.4 ±2.4 Ma, Augland et al., 2016), the Pichi Pluton (2693.2 ±5.4 Ma, Davis et al., 2018), the Marjoulet Gneiss (2689 ±4 Ma; David, personal communication, 2017) and the Vieux Comptoir Granite Suite (nAvcr2, 2640 Ma to 2613 Ma; David, personal communication, 2017; Goutier et al., 2000; Bogatu and Huot, 2016). These ages give a minimum age of sedimentation. Isotope analyses of paragneiss detrital zircons give the maximum age of sedimentation, while mobilisate dates indicate the age of different episodes of metamorphism. All of these data establish that the sediments that caused the Laguiche Complex began depositing before 2712 Ma (sample 2014-JG-2088A; Augland et al. 2016), and continued depositing after 2697 Ma (sample 2015-JG-2605A; David, personal communication 2017). Subsequently, these rocks were metamorphosed and deformed during several episodes that are dated to 2672 Ma, 2660 Ma, 2647 to 2677 Ma (samples 2008-PR-6040, 2009-SM-6116 and 2008-PR-6055; David et al. 2011; Morfin et al. 2013).
Most of the Laguiche Complex is separated from other units by shear zones, with the exception of the Bonfait Lake area (sheet 33F09) where the feldspathic wacke is in normal contact with intermediate pyroclastitic rocks of the Yasinski Group. In the Bonfait Lake area, these rocks lie in normal contact with those in the La Grande Subprovince. In sheet 33G07, it is separated from the La Grande rocks by a major shear zone, characterized by mylonitization of host rocks and transposition of intermediate to felsic dykes. In sheets 33F02 and 33F07, feldspathic wacke and biotite paragneiss are separated from mafic volcanics by faults.
Does not apply.