Geology of the Conviac Lake Area, Opinaca and La Grande Subprovinces, Eeyou Istchee James Bay, Quebec, Canada

 

A new 1:50,000 scale geological map of the Conviac Lake area (NTS sheets 33B02 and 33B07), located ~100 km NE of Nemiscau on the Eastmain River, was produced following a survey in the summer of 2021. Mapping helped clarify the boundary between the Opinaca and La Grande subprovinces in this area, shifting the contact ~30 km to the NE. The Natel Formation, composed of metavolcanic rocks, extends eastwards over some thirty kilometres. Above this formation, the Village Batholith forms granodioritic to tonalitic domes overlying the paragneiss of the Auclair and Prosper formations. The latter corresponds to a new slightly migmatized metasedimentary unit, forming the transition between the Laguiche Complex paragneiss and the La Grande Subprovince rocks. The Bauerman Intrusion crosscuts the previous units. The boundary between the subprovinces is marked by the Prosper Regional Shear Zone (ZCpro). The variably migmatized paragneiss of the Laguiche Complex in the Opinaca Subprovince occupies the NE of the area, cut by the intermediate intrusive rocks of the Féron Suite. To the SE of the study area, the Voirdye Formation paragneiss overlies the La Hutte Complex intrusive rocks.

The area is cut by several shear zones trending more or less E-W, which overprint the ZCpro in places. To the SE, the Nisk Shear Zone separates the Prosper Formation paragneiss from that of the Voirdye Formation. The presence of domes and basins as well as a series of isoclinal folds characterize the regional tectonics.

Methodology

Previous Work

 

The table below provides a list of work completed in the study area since 1966. It also includes the references cited in the report. An exhaustive list can be found in the EXAMINE documentary database.

Previous Work in the Study Area

Author(s)	Type of Work	Contribution
Eade, 1966	Regional geological survey at scale 1:1,000,000	First mapping work in the study area carried out by the Geological Survey of Canada
Franconi, 1983	Geological mapping at scale 1:63,360	Geology of NTS sheets 33B01, 33B02 and 33B03
Lanthier and Ouellette, 1996 Chapdelaine, 1997 Lanthier and St-Cyr, 1997 Chapdelaine and Lachance, 1998	Mining prospection	Discovery of several gold mineralized zones in the Middle Eastmain Belt, west of the project
Labbé and Grant, 1998 Moukhsil and Doucet, 1999	Geological mapping at scale 1:50,000	First description of several important units in the study area, in adjacent sheets (33B04 and 33B03, respectively)
Simard and Gosselin, 1999	Geological mapping at scale 1:250,000	Geology of sheet 33B and the southern part of sheet 33G
Aubin and Girard, 2008	Exploration work	Mapping and sampling in the Conviac Lake area (sheets 33B06 and 33B07)
D’Amours, 2011	Aeromagnetic surveys	Aeromagnetic survey coverage of the region

Lithostratigraphy

The Conviac Lake area is located in the Superior Province, in the central south part of the Opinaca Subprovince, in contact with the La Grande Subprovince. The units in the area can be divided into three packages:

1. units of the La Grande Subprovince;

2. units of the Opinaca Subprovince;

3. later units common to both subprovinces.

A chronology of the emplacement of units is presented in the figure below. This order takes into account the crosscutting relationships observed in the field and available geochronological data. The stratigraphic diagram illustrates the different relationships between the units in the area.

 

 

The present work proposes a major revision of the position of the contact between the La Grande and Opinaca subprovinces, pushing the contact some thirty kilometres to the NE. This change is motivated by the following factors:

• the differences observed between the stratified paragneiss of the Prosper Formation (La Grande Subprovince) and the migmatite of the Laguiche Complex (Opinaca Subprovince);
• the uniformity of the magnetic pattern in the new edition of the La Grande Subprovince in the Conviac Lake area;
• the redefinition of boundaries between the Village Batholith and the Natel volcano-sedimentary Formation, units of the La Grande Subprovince, extending them eastwards;
• the presence of deep crustal margins typical of the La Grande-Opinaca contact (Cleven et al., 2020).

In the Conviac Lake area, the La Grande and Opinaca subprovinces are separated by the ESE-trending Prosper Shear Zone.

La Grande Subprovince

Units of the La Grande Subprovince in the Conviac Lake area can be divided into two groups: those associated with the Lac des Montagnes Belt (LMB) and those associated with the Middle and Lower Eastmain Greenstone Belt (MLEGB).

The Champion Complex (nAchp; 2889 ±14 Ma, Bynoe, 2014; 2816 ±9 Ma, David, 2019) and the Hutte Complex (nAhue; 2952.2 ±3.8 to 2790.4 ±5.4 Ma; David, 2020a and 2020b respectively) are considered to form the Mesoarchean basement of the LMB (Bandyayera and Caron-Côté, 2019). The Champion Complex is composed of tonalitic gneiss, locally granitic (nAchp1). Foliated tonalite (nAhue2) forms the majority of the Hutte Complex in the study area. The Nasacauso Mafic-Ultramafic Suite, composed of peridotite and pyroxenite (nAnas1), is intrusive in the Hutte Complex. However, no outcrops have been recorded in the Conviac Lake area (Bandyayera and Caron-Côté, 2022). The Voirdye Formation paragneiss (nAvrd2, nAvrd2a) overlies the whole.

In the area, the Eastmain Group of the MLEGB comprises the Natel Formation and the overlying Auclair and Prosper metasedimentary formations. The Natel Formation occurs as bands of generally massive, locally pillowed basalt (nAnt1; 2739 ±5 Ma, Moukhsil, 2000), in places well amphibolitized (nAnt1a). They contain horizons of volcanic and volcaniclastic rocks with a felsic to intermediate composition (nAnt6). The Auclair Formation is present in the form of paragneiss containing cordierite-sillimanite nodules (nAai1) and an oxides iron formation (nAai2).

The Prosper Formation (nAprp; new unit) consists of paragneiss (nAprp1), migmatitized in places (nAprp1a), and oxides and silicate iron formations (nAprp2). These rocks had been assigned to the Laguiche Complex by Simard and Gosselin (1999), but the presence of centimetric to metric stratification defined by a variable content of ferromagnesian minerals is atypical of the Opinaca Subprovince paragneiss. The Prosper Formation has therefore been assigned to the La Grande Subprovince and is thought to be a transitional unit between the less metamorphosed Auclair Formation sedimentary rocks and the Laguiche Complex migmatites. It is thought to be the youngest and most distal unit of the Eastmain Group. The Prosper Formation is separated from the Voirdye Formation to the south by the Nisk Shear Zone.

The Village Batholith (nAvil; 2720 ±2 Ma, Mouhksil, 2000; 2697 +6/-4, David et al., 2010) is probably older than the Auclair and Prosper formations and is thought to be injected above the Natel Formation. It is a large package of homogeneous, foliated granodiorite and biotite tonalite (nAvil1) covering more than 70 km along an E-W trend.

The Bauerman Intrusion (nAbau; new unit) cuts the Prosper Formation, and extends across the entire interface between sheets 33B02 and 33B07. The Bauerman Intrusion is composed mainly of quartz monzodiorite, with tonalite, monzonite, quartz diorite and granodiorite. The rock is homogeneous and only slightly deformed.

The Kauakiekamatsh Intrusive Suite (nAkkm; new unit) represents a series of NE-SW trending post-tectonic pegmatitic granite intrusions. Near the Natel and Auclair formations in particular, these intrusions are particularly rich in tourmaline (nAkkm2), with contents ranging from 1% to 25%.

Opinaca Subprovince

The variably migmatitized paragneiss of the Laguiche Complex (nAlgi2a, nAlgi3a, nAlgi4a) account for most of the Opinaca Subprovince. These metasedimentary rocks have ages ranging from >2710.4 ±2.4 Ma to >2671.6 ±1.8 Ma (Augland et al., 2016; David et al., 2011), corresponding respectively to the age of emplacement of the Féron Suite, which post-dates the Laguiche Complex, and to the first recorded episode of migmatitization. Hectometre-thick bands of amphibolite derived from mafic rocks (nAlgi5) are present throughout the unit. A band of pillowed basalt (nAlgi1) intercalated with ultramafic sills is found in the western part of sheet 33B07.

In the study area, the Féron Suite, consisting of diorite, tonalite and monzodiorite (nAfer2; 2710.4 ±2.4, Augland et al., 2016), takes the form of an ovoidal injection or bands of multi-hectometric thickness throughout the Laguiche Complex paragneiss.

Units Common to Subprovinces

The units in the area are cut by several families of diabase dykes of Neoarchean to Palaeoproterozoic age. The Mistassini Dyke Swarm (nAmib), the only swarm of Neoarchaean age (2515 ±3 to 2503 ±2 Ma; Hamilton, 2009; Davis et al., 2018), has a NNW orientation and an aphyric diabase composition. The Lac Esprit Dykes (pPesp; 2069 ±1; Hamilton et al., 2001) trend NW in the study area and have a gabbroic to gabbronorite composition. The Matachewan Dyke Swarm (pPmaw; 2473 +16/-9 to 2446 ±3 Ma; Heaman, 1997) is N-S trending and gabbroic to gabbronoritic in composition. Finally, the Senneterre Dykes (pPsen; 2214 ±12.4 Ma, Buchan et al., 1993; 2216 +8/-4, Mortensen in Buchan et al., 1996; 2221 ±4, Davis et al., 2018) trend NE to ENE and have a gabbronorite composition.

Lithogeochemistry

The lithogeochemistry of units in the Conviac Lake area is presented separately in tabular form.

Structural Geology

 

Chronology of Deformation Phases

The extensive recrystallization of the rocks in the area implies a scarcity of preserved primary structures. In the amphibolitized basalt, a few remnants of pillows indicate the polarity of their respective band sequence. The Prosper Formation paragneiss commonly preserves its primary bedding, which takes the form of a centimetric to decimetric band.

The main episode of D_n deformation affecting the area corresponds to a N-S shortening. The main fabrics are characterized by the presence of penetrative foliations marked in particular by the preferential orientation of ferromagnesian minerals such as biotite and hornblende, as well as by the flattening of quartz crystals. Metatexites and diatexites commonly have a stromatic structure exhibiting migmatitic banding that corresponds to this main fabric. The main lineations appear in the form of mineralogical alignments or elongations of hornblende or cordierite crystals. The area is characterized by a dome and basin structure, with a series of isoclinal folds.

 

Structural Domains and Shear Zones

The study area has been subdivided into five structural domains based on their lithological, structural and geophysical characteristics. From south to north, these are the La Sicotière, Beryl, Bauerman, Conviac and Achiyaskunapiskuch domains.

In the study area, the La Sicotière Structural Domain (DSsic) comprises the rocks of the Hutte Complex and the Voirdye Formation. Although its attitude is more or less variable throughout the domain, its S_n foliation is mainly oriented ENE-WSW, dipping steeply to the NNW or SSE. A second phase of deformation affects this domain in the form of a steeply dipping, NE-SW trending crenulation schistosity, which is thought to have been affected by a P_n+1 folding episode.  

The DSsic is separated from the Beryl Structural Domain to the north by the Nisk Shear Zone (NSCZ). In the study area, its impact has not been observed on the surface, so its position has been inferred from geophysics. It is an ENE-WSW structure, with apparent thrusting movement and a dextral component. Its lineations plunge moderately to the SW.

The Béryl Structural Domain (DSber) comprises rocks from the Natel, Auclair and Prosper formations, as well as the Village Batholith and Bauerman Intrusion. It is affected by a WSW-ENE trending foliation, dipping mainly to the NNW, with lineations generally plunging 31° westwards. The domain comprises large elongated units in the same direction as the fabric, visible with geophysics in the form of long bands noticeably less affected by other regional fabric than by N-S shortening. Numerous E-W Pn folds affect the stratigraphy, particularly in the western part of the domain. The presence of the Auclair and Prosper metasedimentary formations around bands of the Natel Formation also suggests the presence of a major P_n-1 folding episode in this area, which folded the sequence on itself, forming an isoclinal anticline.

The Bauerman Structural Domain (DSbau), separated from the DSber by an E-W to ENE-WSW shear zone, contains most of the rocks of the Bauerman Intrusion, as well as rocks of the Village Batholith and the Prosper and Auclair formations. The area is affected by an E-W trending foliation with a predominantly northerly moderate to steep dip. Due to the rather intrusive nature of the domain’s components, this foliation is not well marked.

The Prosper Shear Zone (PSZ) is a major north-dipping structure that separates the structural domains of the La Grande Subprovince from those of the Opinaca Subprovince. Several E-W shear zones and folds locally overprint this major shear zone. No kinematic indicators were found to indicate the direction of movement of the structure. However, we can assume a polyphase history during which the ZCpro initially acted as a normal fault, allowing the burial of the Laguiche Complex paragneiss. Subsequently, the N-S Dn shortening episode would have caused the reverse movement that exhumed these rocks, explaining the difference in metamorphic grade on either side of the ZCpro. The Laguiche Complex is more metamorphosed than the Prosper Formation despite its younger estimated age.

The Conviac (DScon) and Achiyaskunapiskuch (DSach) structural domains contain the metasedimentary rocks of the Laguiche Complex, as well as the intrusions of the Féron Suite. The DScon has an ENE-WSW penetrative foliation, corresponding to an axial plane foliation, dipping steeply to the NNW and SSE. The lineation plunges slightly eastwards, which would be consistent with a N-S shortening with a lateral component. The area is affected by a series of E-W plane folds, consistent with the Dn deformation event. The dome and basin structure of the DScon is particularly evident in the stratigraphy and geophysics, including large ovoidal units at the surface. The DSach lies further north and is separated from the DScon by an E-W to ENE-WSW shear zone. It has a similar foliation. The major difference between the DSach and DScon is the elongated and folded nature of the units in the former, as corroborated by geophysics.

 

Metamorphism

The metamorphic peak facies is estimated from mineralogical assemblages, the level of migmatization and the recrystallization modes of quartz and feldspar, which are illustrated in the figure opposite. The metamorphic gradient increases from the upper greenschist facies in the extreme SW of the area, to the granulite facies in the northern part of the map. The study area straddles the contact between the La Grande and Opinaca subprovinces, each of which has distinct metamorphic characteristics. The Prosper Shear Zone (ZCpro), which separates the two subprovinces, marks a transition in the intensity of the metamorphism.

 

La Grande Subprovince

 

In the DSber, there is a transition from the upper greenschist metamorphic facies, in the extreme west of sheet 33B02, to the upper amphibolite facies in the direction of the ZCpro. This transition is marked by the disappearance of the actinote-chlorite-epidote assemblage in the metamorphosed basalts of the Natel Formation, at the western end of the domain, in favour of a hornblende-plagiocase-epidote assemblage characteristic of the greater part of the DSber, DSsic and DSbau. Two types of hornblende can be distinguished from petrographic observations. In the western part of the domain, hornblende crystals are generally greenish, strongly pleochroic and have second-order interference colours in the amphibolite. In the eastern part of the domain, they are more brownish, have weaker pleochroism and late first-order interference colours. These variations reflect the transition to a higher metamorphic facies towards the east, from the lower to the upper amphibolite facies (Winter, 2010). The same SE-NW gradient is observed in the metasedimentary rocks of the Prosper Formation in the DSber and Dsbau, where a mineral assemblage characteristic of the amphibolite metamorphic facies is found, with the presence of garnet and cordierite, followed by the gradual appearance towards the NW of an onset of migmatitization, attesting to the transition to the upper amphibolites. The presence of orthopyroxene in the paragneiss is anecdotal, unlike in the Laguiche Complex where this mineral is very common. In addition, a sillimanite-garnet-cordierite mineral assemblage is commonly observed in the metasedimentary rocks of the Auclair Formation. Observation in thin sections of rounded grain boundaries, and locally of amoeboid patterns and extinction chessboards in the quartz and plagioclase of the Village Batholith granodiorite, indicates dynamic recrystallization at high temperature (Passchier and Trouw, 2005). This suggests that the latter is also metamorphosed to the upper amphibolite facies. The same type of structure is rarer in a large part of the Bauerman Intrusion, which indicates an amphibolite facies or a lower facies, although locally, outcrops of this unit located near the ZCpro show an onset of migmatization.

Opinaca Subprovince

In the Opinaca Subprovince, to the north of the ZCpro, the metamorphic facies transitions from the upper amphibolite facies near the contact with the La Grande Subprovince (DScon), to a very marked granulite facies in the northern part of the study area (DSach). In both of these structural domains, the metasedimentary rocks of the Laguiche Complex are mainly composed of paragneiss affected by varying degrees of migmatitization. Indicator minerals for metamorphism observed in the metasedimentary rocks are K-feldspar, garnet, cordierite and orthopyroxene. The latter is observed in places directly to the north of the ZCpro, in the DScon, and then becomes ubiquitous in the DSach. The few klippes of mafic volcanic and intrusive rocks (nAlgi1 and nAlgi5) are highly amphibolitized, reveal evidence of partial melting around hornblende grains in thin sections and their assemblage is commonly completed by clinopyroxene.

In thin sections, the intrusive rocks of the Féron Suite (nAfer2) display recrystallization textures along with undulatory and chessboard extinction. These microstructures of quartzofeldspathic material are also characteristic of high metamorphic conditions (Passchier and Trouw, 2005). Some outcrops of the nAfer2 unit also show an onset of migmatitization.

The Lake Conviac area has undergone at least two episodes of metamorphism. The first, M_n-1, predates the emplacement of the vast majority of the rocks in the area. It was dated 2804.6 +/-5.1 Ma (David, 2020a) in a tonalitic gneiss of the Hutte Complex, to the SW of the study area (sheet 32N09). The main metamorphic episode, M_n, affects all the rocks in the area, in both the La Grande and Opinaca subprovinces. Several microtextures observed in thin sections of paragneiss suggest that the metamorphic peak of the M_n episode is synchronous with or slightly late in the D_n deformation phase. This is evidenced by the preferential orientation of biotite sheets and hornblende grains, as well as the rotation in some garnet porphyroblasts. Some garnets with a skeletal structure suggesting peritectic growth are syntectonic to late-tectonic. The high prevalence of stromatic structures in the migmatites, along with a banding pattern consistent with the orientation of the main foliation, indicates that the episode of metamorphism under anatectic conditions probably took place synchronously with the NNW-SSE shortening episode (Sawyer, 2008). The migmatitization of the metasedimentary rocks of the Laguiche Complex is dated 2671.6 ±1.8 Ma (sheet 33B13, David et al., 2011) and could correspond approximately to the one observed in the study area.

Economic Geology

The Conviac Lake area contains prospective zones for four types of mineralization:

• base-metal massive sulphide mineralization associated with volcanic rocks (VMS);
• magmatic-hydrothermal mineralization associated with mafic-ultramafic intrusive rocks;
• stratiform gold mineralization hosted in iron formations;
• mineralization associated with granitic pegmatites.

The table of mineralized zones below presents the analytical results for the only known mineralized zone in the area.

 

The table of lithogeochemical analyses of metals of economic interest provides the location, description and analytical results for 36 selected samples in order to assess the economic potential of the area.

 

 

Eastward Extension of the Middle Eastmain Volcano-Sedimentary Belt: New Prospective Zones for Base Metal Massive Sulphides Associated with Volcanic Rocks (VMS)

 

This project has extended the Natel Formation, belonging to the Middle and Lower Eastmain Greenstone Belt (MLEGB), towards the east (Natel-est prospective zone). This extension takes the form of subkilometre-thick bands of felsic to mafic volcanic rocks metamorphosed to the amphibolite and upper amphibolite facies. These rocks are locally rusty and contain up to 4% pyrite. They are associated with local hydrothermal alteration characterized by sericitization, chloritization and phlogopite alteration. Evidence suggests that they are conducive to the development of base metal mineralization representative of VMS. Values reaching 968 ppm Cu and 92 ppb Au have been observed (outcrops 21-MY-1182-A1 and 21-WM-2068-A2, respectively). The Clearwater-sud mineralized zone (~75 km further west, sheet 33B04) is also hosted in MLEGB basalt and may represent a comparable mineralization system. Chlorite and sericite alteration are also observed locally.

 

Potential for Magmatic-Hydrothermal Mineralization Associated with Mafic-Ultramafic Intrusive Rocks

 

The Ni-Cu-PGE mineral potential of an amphibolite band in the Opinaca Subprovince has been highlighted in the Conviac prospective zone. Significant and anomalous Ni-Cr ± Co contents were obtained in selected amphibolite samples derived from pillow basalt and metapyroxenite containing centimetric orthopyroxene porphyroblasts. Sulphides are very fine grained and hardly visible to the naked eye or with a magnifying glass. A selected sample from a decametric layer of slightly rusty massive ultramafic rock contains up to 1% disseminated pyrite; it yielded significant Ni-Cu-Cr ± Co contents (21-MK-3060-C1: 524 ppm Ni, 231 ppm Cu and 1270 ppm Cr). All zones with significant contents correspond to positive magnetic anomalies. The best nickel and chromium values were obtained from a selected metapyroxenite sample containing orthopyroxene porphyroblasts and up to 10% finely disseminated chromite (21-MY-1047-A1: 745 ppm Ni and 2540 ppm Cr). These metal values and the nearby presence of sulphide-bearing metasedimentary rocks (21-MY-1045-A1: 3.4% S) highlight the potential for Ni-Cu-PGE mineralization in magmatic-hydrothermal type deposits in mafic-ultramafic intrusions.

 

Potential for Gold Mineralization in the Iron Formations of the Prosper Formation

 

Airborne geophysical surveys (D’Amours, 2011a) reveal numerous highly magnetic bands in the south-central and eastern parts of sheet 33B02. These bands correspond to oxide and silicate iron formations associated with unit nAprp3 of the Prosper Formation, which are mostly hosted in paragneiss and amphibolite layers of the same unit. The Béryl-est prospective zone and the Prosper prospective zone are related to this iron formation subunit. These are mainly characterized by E-W bands of centimetric to metric thickness that are continuous over 5 km to 10 km. In the Béryl-est prospective zone, they form banded layers rich in magnetite. In the Prosper prospective zone, they are accompanied by beds rich in garnet and grunerite and cut by granitic injections also rich in garnet. Although sulphides are present in the iron formations, geochemical analyses indicate only one significant gold value (64 ppb Au, outcrop 21-MY-1070), which is located in the Prosper prospective zone.

In the northern part of sheet 33B02, there are bands of iron oxide formation hosted in the aluminosilicate metasedimentary rocks of the Auclair Formation (Bauerman prospective zone). This sector displays significant structural complexity, which could be conducive to the enrichment of certain zones by the presence of structural traps.  

Numerous gold mineralized zones associated with oxide and silicate iron formations are present in the Middle Eastmain Belt area, directly to the west of the study area. These mineralizations are subconcordant and disseminated in the form of pyrite, pyrrhotite and arsenopyrite in iron formations. These lithologies are continuous and typically occur at the interface between volcanic and metasedimentary units (Lanthier and St-Cyr, 1997). This is particularly true of the Béryl-est prospective zone, in the mapped area, where bands of iron formation occur at the interface between the Natel Formation volcanic rocks and the Prosper Formation metasedimentary rocks. Gold is generally associated with sulphides and garnet in the various mineralized zones in the area (Arianne, Rock n’Hammer, Golden Butterfly, Ti-Beu, Kog-1, Frank, Enterprise and Latour). Gold associated with sulphides may also be found in quartz veins. Several of the mineralizations are associated with biotite-hornblende alteration and Ca-Fe-Mg silicates. Mouhksil et al. (2003) mention a strong potential for mineralization in the area where these iron formations are associated with the hectometric to kilometric P₂ fold hinges. These characteristics are reminiscent of the context of major deposits such as Homestake, Lupin and Musslewhite (Caddey et al., 1991; Moukhsil et al., 2003), all hosted in lithologies metamorphosed to the amphibolite facies.

 

 

The Kauakiekamatsh Intrusive Suite: A New Tourmaline-Rich S-Type Granitic Pegmatite Unit

 

The SW sector of the study area is interspersed with numerous muscovite-garnet-tourmaline S-type pegmatitic granite intrusions likely to contain Li, Be, Ta and Nb mineralization. The intrusive rocks of the Kauakiekamatsh Intrusive Suite (Kauakiekamatsh prospective zone) are notable for their significant tourmaline content, a mineral commonly enriched in incompatible elements. This unit contains on average >1% centimetric tourmaline, and up to 25% (outcrop 21-WM-2067). Beryl has also been observed locally (Esnault and Corriveau, 2020). No economic value has been associated with these intrusions, but significant values of 14 ppm Be and 5.25 ppm Ta have been recorded in a paragneiss host rock (outcrop 21-SG-4227). The Kauakiekamatsh Intrusive Suite is part of a swarm of NE-SW trending dykes hosted in the Prosper Formation paragneiss (nAprp1) and in the volcanic rocks of the eastern part of the Natel Formation (nAnt), within the Middle and Lower Eastmain Greenstone Belt (MLEGB). The geological context of this suite is similar to that of several LCT pegmatite deposits in the La Grande Subprovince, such as the Cyr-Lithium deposit also located in the MLEGB, west of the study area.

 

The new position of the contact between the La Grande and Opinaca subprovinces significantly changes our understanding of the region. Taking into account the interpretations from the Lake Conviac project, the trace of this contact will be redefined to the east of the study area (in sheets 33B01 and 33A04). This new delineation will eventually tie in with the mapping work by Beauchamp et al. (2018; sheets 33A01, 33A08, 23D04 and 23D05) and Beauchamp (2020; sheets 33A02, 33A03 and 33A07). A synthesis of the relationships between the various subprovinces in the area will then be relevant to a more homogenous understanding of the region. The extension of the MLEGB eastwards also raises questions about its relationship with the Upper Eastmain Greenstone Belt (UEGB). Based on current observations and interpretation of magnetic maps. The two belts seem to meet in sheet 33B01.

The contact will also be observed during future MRNF mapping work planned to the NW (sheets 33B06 and 33B11). These may provide a better view of the contact and ZCpro in outcrop and identify the kinematics of the shear zone. However, contrary to what has been observed to the north (Goutier et al., 2021), the contact could prove to be gradual in the southern part of the Opinaca Subprovince.

Bandyayera and Caron-Côté (2019) suggest that the Lac des Montagnes Group could correspond to the second volcanic episode of the Eastmain Group, specifically the Natel Formation episode as defined by Moukhsil et al. (2003). They highlight the similarity between the Voirdye Formation paragneiss and that of the Auclair and, now, Prosper formations. This episode (2739 ±5 Ma; Moukhsil, 2000; Moukhsil et al., 2003) is older than the Lac des Montagnes Group episode (2723.2 ±3.9 Ma to 2707 ±10 Ma; David, 2020b and 2019, respectively), however volcanic rejuvenation from the NE (towards the Eastmain Group) to the SW has been noted. The mapping work presented here, in conjunction with work by Bandyayera and Caron-Côté (2022), does not confirm the hypothesis which connects these units. Further work will be carried out to the east of the study area, which may provide a cartographic link between the two units. New dating in the area is also underway.

Authors	Myriam Côté-Roberge, P.Geo., M.Sc. myriam.cote-roberge@mern.gouv.qc.ca William Chartier-Montreuil, GIT william.chartier-montreuil@mern.gouv.qc.ca Maxym-Karl Hamel-Hébert, GIT, M.Sc. maxym-karl.hamel-hebert@mern.gouv.qc.ca Daniel Bandyayera, P.Geo., Ph.D. daniel.bandyayera@mern.gouv.qc.ca
Geochemistry	Fabien Solgadi, P.Geo., Ph.D.
Geophysics	Siham Benahmed, P.Geo., M.Sc. Rachid Intissar, P.Geo., M.Sc.
Potential assessment	Virginie Daubois, P.Geo., M.Sc.
Logistics	Marie Dussault, coordinator
Geomatics	Julie Sauvageau Kathleen O’Brien
Template and content compliance	François Leclerc, P.Geo., Ph.D.
Coaching/mentoring and critical review	Abdelali Moukhsil, P.Geo., Ph.D.
Organism	General Direction of Géologie Québec, Ministère de l’Énergie et des Ressources naturelles, Governement of Québec

Acknowledgements:

This Geological Bulletin was made possible through the collaboration of many people who have been actively involved in the various stages of the project. We thank geologists-in-training Sarah Galloway and Charles St-Hilaire, as well as students Erick Guertin, Gabrielle Chaput-Vorobief, Andréanne Blais, Alec Vaillancourt, Anne-Marie Proulx, Aube Gourdeau, Antoine Nadeau, Jeanne Carabin, Stéphanie Simard, Thomas Côté and Michaël Noiseux. We salute the excellent work of cooks Micheline Gagné and Rémi Otis, camp staff Olivier Dumas and Daniel Gosselin, and nurse Marie-Ève Lavoie. Transportation on the field was provided by Héli-Inter and pilots Christophe Zarragoza, Jean-François Ostiguy and Anthony Rolland, along with mechanics Gaël Lachambre and Frédérick Côté. We also thank Dany Trudel for his support in the installation and dismantling of the camp. Finally, we would like to thank geologist Emmanuel Caron-Côté for his help in organizing the camp and for sharing his geological expertise. Discussions on geology with Yannick Daoudene were also particularly beneficial.

References

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