Geology of The Lake Ewart Region, La Grande and Opinaca Subprovinces, East of Radisson, Eeyou Istchee James Bay, Quebec, Canada

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New pillow metabasalt and polygenetic conglomerate strips southeast of Radisson
Following fieldwork in the summer of 2016, two mapping targets were identified…

 

New prospective zones southeast of Radisson
Two new prospective zones in the Lake Ewart region are identified in this bulletin…

 

 

A new geological map of the Lake Ewart Lake (NTS sheets 33G04 and 33G05) was produced at a 1:50,000 scale following a survey conducted in the summer of 2016. The mapping of this area, located in the La Grande and Opinaca subprovinces, revealed a new strip of metabasalt, a polygenetic conglomerate horizon and an anomalous gold content in a paragneiss of the Laguiche Complex. A new beryllium index was found in sheet 33G04, in a banded granitic pegmatite located on the edge of a granitic pluton. Our work has also led to new shear zones, one of which is associated with ultramafic intrusions showing anomalous gold levels.

Methodology

The region was mapped using the established method for surveys in isolated areas with some road access. Geological mapping was conducted by a team of six geologists and six students between June 13 and August 19, 2016. Outcrops along and around the Transtaïga Road, as well as along a road near the transmission line, were revisited by truck. Mapping was also done on NTS sheets 33F02 and 33F07, in addition to spot checks on other sheets.

It should be noted that in 1997, a team from the Ministère had mapped sheets 33G03, 33G04, 33G05 and 33G06 at a scale of 1:250,000 (Simard and Gosselin, 1999). « Geofiches » were added in SIGÉOM, but maps were produced later and were not included in the Simard and Gosselin (1999) report. In 2000, sheets 33G05, 33G06 and 33G11 were mapped to a 1:50,000 scale by a team from the Ministère (Goutier et al., 2002). The major differences between the 1997 and 2000 work and the 2016 work are the use of high-resolution aeromagnetic survey (Goldak Airborne Surveys, 2008; D’Amours, 2011), as well as a new approach to mapping the Laguiche Complex based on assessing the mobilisate percentage. In 2014, a sample was taken near the Transtaiga Road to date the main pluton of the Bezier Suite.

Geological mapping of the study area has produced and updated information presented in the table below:

Previous work

The table below presents a selection of work done in the study area since 1957 that has had an impact on the geological understanding of the area. Interested readers can obtain a complete list of the region’s exploration work from the EXAMINE documentary base.

Lithostratigraphy

The study area is located in the northwestern part of the Opinaca Subprovince and touches part of the La Grande Subprovince. These large units are part of the Superior Province (Card and Ciesielski, 1986). Rocks of the La Grande Subprovince are of Mesoarchean and Neoarchean ages, while rocks of the Opinaca Subprovince are of Neoarchean age. Rocks in these two subprovinces are cut by Neoarchean and Paleoproterozoic intrusions that form sills parallel to the main foliation, egg-shaped pluton or unconformable straight-line dykes.

The relative timing of emplacement of these units is shown in the Ewart Lake time scale and stratigraphic diagram that takes into account crosscutting relationships observed in the field and data from adjacent sheets. This pattern also highlights the higher metamorphism of the units at the base of the Laguiche Complex, located in the southern part of the mapped area. Much of the units in the Opinaca Subprovince are conformable with the main foliation. The various units in the area are described in more detail in the Québec Stratigraphic Lexicon. References associated with unit U-Pb datings can be found in the map legend of Ewart Lake and in the Québec Stratigraphic Lexicon records.

LA GRANDE SUBPROVINCE

Mesoarchean

Guyer Group

Rocks of the Guyer Group (mAgy, 2820-2806 Ma) are only exposed in the NE portion of sheet 33G05 where it forms two strips. The western strip consists of the highly deformed unit mAgy2b. It consists of a gneiss and a banded amphibolite after basalt. Gneiss is interpreted as the result of deformation and metamorphism of intermediate and felsic volcanics. The eastern strip represents the western end of the main strip of the Guyer Group as shown in sheets 33G06, 33G07, 33G10 and 33G11. This strip is important because it bears most of gold mineralization. The units are mAgy1 (amphibolite after basalt), mAgy2 (felsic and intermediate tuffs) and mAgy3 (iron formation). Unit mAgy2 was removed from the 33G05 sheet following work by Cayer (2010, 2011) which indicates that the proportion of deformed felsic intrusions exceeds that of felsic volcanics. Unit mAgy3 trace was modified from the more detailed aeromagnetic map.

Neoarchean

Langelier Complex

The Langelier Complex (Algl, 3452-2788 Ma) is a widespread unit in the La Grande Subprovince. In several places, this unit represents the basement on which the volcano-sedimentary units have deposited. In the Ewart Lake area, rocks of this complex are present in the northern portion of sheet 33G05. They belong to unit Algl3 composed mainly of biotite or biotite-hornblend tonalite, foliated and locally gneissic. In detail, tonalite is locally gneissic and a few horizons of diorite are present. This unit’s contact with rocks of the Guyer Group is tectonic in nature. However, an unpublished date of the Langelier Complex from further west (sheet 33F08) indicates a Neoarchea age (2796.5 ±5.3 Ma), suggesting that this unit was intrusive into the rocks of the Guyer Group.

Duncan Intrusions

A hornblende-biotite diorite, in intrusive contact with rocks of the upper part of the Guyer Group, was associated with the Duncan Intrusion unit nAdcn2 (nAdcn, 2716-2709 Ma) by Goutier et al. (2002). This diorite is weakly deformed. Its main feature is that it is the host unit of gold mineralization. Cayer’s work (2009, 2010) revealed the presence of several gold anomalous levels (>500 ppb) and gold showings within this intrusion and along its extension to the NE where the percentage of tonalite increases.

OPINACA SUBPROVINCE

Laguiche Complex

The Laguiche Complex (nAlgi, >2712 to 2640 Ma) is the main unit of the Opinaca Subprovince and covers 85% of the area mapped in the Lake Ewart region. As illustrated in the stratigraphic diagram, there is an increase in metamorphism and mobilisate from north to south, resulting in the appearance and increasing importance of metatexite and diatxite units (nAlgi3a and nAlgi4a). The boundary between paragneiss and migmatized facies is gradual. For example, on the same outcrop of the southern part of the region, the proportion of mobilisate can vary significantly from the metric to the decametric scale. The same is true for the number of centimetric intrusions of granodioritic to granitic composition.

One of the major contributions of the summer 2016 work was the discovery of a horizon of amphibolite after a mostly pillow basalt (unit nAlgi1) in the NW corner of the 33G04 sheet. It was traced 2.57 km long and approximately 300 m thick from outcrops and the vertical magnetic gradient map. The pillow facies is particularly well preserved and dominant. Garnet is present in the edges of the pillows, suggesting metamorphosed volcanogenic alteration. Contact with adjacent paragneiss appears stratigraphic.

Similarly, the 2016 work identified a strip (3.2 km by 160 m) of polygenetic conglomerate in the northern portion of sheet 33G05. This highly deformed conglomerate was associated with unit nAlgi10, which is a very minor part of the Laguiche Complex. Contact with monzodiorites of the Bezier Suite is tectonic in nature, but some monzodiorite dykes cut the conglomerate, suggesting that the Bezier Suite intruded into the sedimentary rocks and that the two units were subsequently deformed. This unit also includes minor horizons of paragneiss, iron formation and amphibolite. It could be thrust slices from the La Grande Subprovince.

Intrusive Rocks

Marjoulet Gneiss

The Marjoulet gneiss (nAgma) is a unit composed of biotite ± hornblende ± magnetite tonalitic gneiss, foliated and banded. It forms two kilometric horizons in sheet 33G04, inside rocks of the Laguiche Complex. This unit is interpreted as syntectonic intrusions emplaced parallel to the main foliation and which have been progressively deformed into gneiss. An unpublished age indicates intrusion around 2689 ±4 Ma.

Féron Suite

The Féron Suite (nAfer1) includes intrusions of hornblende-biotite-magnetite quartz monzodiorite, porphyritic with K-feldspar phenocrystals. These rocks intrude rocks of the Laguiche Complex. They range in shape from egg-shaped plutons to sills parallel to the main foliation. They have a clear signature on aeromagnetic maps due to their high magnetic susceptibility. U-Pb dating dates intrusion of these rocks to 2710.4 ±2.4 Ma.

Bezier Suite

The main intrusion of the Bezier Suite (nAbez) occupies a significant portion of the northern portion of sheet 33G05. This intrusion is composed of quartz monzodiorite and granodiorite, with a K-feldspar porphyritic texture. These rocks intrude rocks of the La Grande and Opinaca subprovinces. Locally, the contact is sheared off. The unpublished dating of 2712.3 ±5.8 Ma from a sample taken in 2014 (2014-JG-2088A) in sheet 33G05 yielded an older age than previously proposed by Goutier et al. (2002) at 2674 ±12 Ma.

Lablois Suite

The Lablois Suite (nAslb, 2620 Ma) consists of a series of ultramafic intrusions (~0:32 km²) located near large shear zones that pass through the Laguiche Complex in the southern part of sheet 33G04. The dominant lithology of this unit is an olivine ± phlogopite metawebsterite, usually porphyroblastic with orthopyroxene. The low degree of deformation compared to the host rocks suggests intrusive contact. The interest in these intrusions lies in their proximity to shear zones and gold mineralization, as well as their anomalous gold content.

Vieux Comptoir Granitic Suite

Rocks of the Vieux Comptoir Granitic Suite (nAvcr) come in two forms, kilometric egg-shaped plutons and metric to kilometric pegmatitic dykes or intrusions. In the Lake Ewart area, these granite intrusions cut all the rocks in the Opinaca Subprovince, with the exception of diabase dykes.

Large plutons (nAvcr1 unit) consist of biotite-magnetite ± hornblende ± epidote granite, massive to slightly foliated. Pegmatitic intrusions (nAvcr2 unit) are composed mainly of K-feldspar-biotite ± muscovite ± garnet ± tourmaline granite. Published and unpublished datings indicate that the nAvcr1 unit is older (2683-2656 Ma) than unit nAvcr2 (2640-2613 Ma).

Neoarchean and Paleoproterozoic Diabase Dykes

Archaean rocks in the region are cut by diabase dykes, brown in surafce and green grey in fresh exposure. These diabases are gabbronoritic, medium grained and undeformed to fractured. These dykes are grouped according to their orientation and geochemical composition. Here, the NE-SW diabase dykes belong to the Senneterre dykes (pPsen). NW-SE oriented dykes may belong to the Mistassini Dyke Swarm (nAmib1) or the Lac Esprit Dykes (pPesp). Dykes of unitnAmib1 are richer in TiO₂, K₂O and P₂O₅ than diabase of the Lac Esprit Dykes. The distinction between pPesp1 and pPesp2 dykes is based on geochemistry and U-Pb isotopic dating. The pPesp2 dykes have a flat profile of incompatible elements (see Buchan et al., 2007, Figure 11) and are older (~2090 Ma), compared to the pPesp1 dykes that have a slope profile and are younger (2069 ±1 Ma).

Neoarchean Lithological Units

Biotite-MagnetiteTonalite and Granodiorite

Three intrusions that vary in composition from tonalite to granodiorite cut rocks of the Laguiche Complex (Neoarchean, I1Da). They contain biotite and magnetite, and are massive to locally gneissic. They mainly follow structural grain and have a high magnetic susceptibility, which allows them to be recognized on aeromagnetic maps. At present, these intrusions have not been associated with a particular lithodemic unit.

Mafic Intrusion

A lithological unit representing metamorphosed mafic intrusion was plotted from the work of Laflèche et al. (2004). It is located just north of the centre of sheet 33G04. There is insufficient data to associate this intrusion with a known lithodemic unit.

 Biotite Hornblendite

The lithological unit of biotite hornblendite is located in the NE corner of the 33G05 sheet and continues in the 33G12 sheet. It was plotted and described during the work of Goutier et al. (2002). It is weakly deformed. The exploration work of Cayer (2010) revealed several showings of Mo-Ag-Cu-Re, an unusual metallic association in an ultramafic intrusion.

Structural Geology

Few primary structures in volcanic and sedimentary rocks were observed as the rocks in the Lake Ewart region were heavily deformed and at least two episodes of high regional metamorphism were observed. Grouped planar elements include: primary foliation defined by elongation of minerals such as biotite and hornblende; gneissossity and migmatitic banding defined by alternating  leucocratic neosome bands and messocratic paleosome bands richer in biotite and intrusions. Stereograms of lineations are constructed with tectonometamorphic mineral lineations, magmatic lineations and megascopic fold axes.

Archaean shear zones in the region form foliated, banded or mylonitic zones that are several metres thick. In some cases, in the north, they correspond to gneissic horizons several hundred metres thick, developed in metamorphosed volcanic rocks. They were drawn from the outcrops, structural style changes and vertical gradient map of the total residual high-resolution magnetic field. Traces of the shear zones in the adjacent west sheets (33F01 and 33F08) were taken into account. East sheets (33G03 and 33G06) will be mapped in 2018.

Major folds were drawn from variations in the attitude of foliation and gneissossity, as well as the vertical gradient map, aerial photos and portions of satellite images. Folding and gneissossity are associated with a first phase affecting the rocks of the Laguiche Complex, while regional folds are associated with a subsequent phase.

Structural Arrangement and Structural Domains

The region was divided into seven structural domains based on the attitude of the measured structures (structural map). There are changes in structural attitudes on stereograms and on the structural map. In the north, the first domain, La Grande, corresponds in large part to the southern flank of a synform belonging to an ovoid structure affecting Mesoarchean and Neoarchean rocks in the La Grande Subprovince. This structure dips moderately towards the ENE. The antiform portion, located to the north in sheet 33G11, dips similarly (065°/32°). This is a new interpretation from Goutier et al. (2002). The plan form is clearly visible by the image of the magnetic angle tilt in SIGÉOM.

The second domain is the main pluton of the Bezier Suite, intruded in the contact zone between the La Grande and Opinaca subprovinces and in the ovoid structure of the La Grande domain. There is a northward dipping foliation and northward dipping lineations (magmatic and tectonic) indicating that rocks of the Opinaca Subprovince extend beneath rocks of the La Grande Subprovince.

The following domains are all in the Opinaca Subprovince. They reflect an increase in deformation and metamorphism from north to south. In outcrop, the gneissossity becomes more pronounced southward. Regional folds are more abundant and denser in the southernmost area and show some variability in orientation and dip. These variations could be attributed to a later deformation phase.

Interpretation and Structural Model

With U-Pb isotopic dating, it is possible to establish a chronology of the structural and metamorphic events that have affected the Lake Ewart region.

First Episode

Rocks of the Guyer Group were deformed by a first compression event that arrived after their formation (<2806 Ma) and before or during intrusion of a tonalite of the Langelier Complex nAlgl3 unit, newly dated at 2796.5 ±5.3 Ma. This event is important as the rocks at the base of the Guyer Group have been transformed into mylonite and gneiss zones.

Second Episode

The second event is related to an N-S compression carrying the rocks of the La Grande Subprovince over the rocks of the Opinaca Subprovince. The age of this event is constrained by the age of the Bezier main pluton, present in the Lake Ewart area and dated 2712.3 ±5.8 Ma. This pluton intruded at the interface between the two subprovinces. It is less deformed, intrusive into adjacent rocks and contains magmatic mineral lineations.

Third Episode

The most important event is associated with another N-S compression and regional metamorphism that has reached the granulite facies just south of the region. It could have started around 2698 Ma with formation of the first migmatites (Jean David, personal communication, 2016) and ended around 2600 Ma since the youngest phases of the Vieux Comptoir Granitic Suite are locally affected by mylonite zones (e.g., Orfée gold showing; Bogatu and Huot, 2016). This compression event was followed or ended with uplift or extension, exposing the more metamorphosed rocks to the south. This is supported by observations of normal movement in some mylonite zones between the two subprovinces.

Fourth Episode

This episode is associated with a phase perpendicular to the third episode. This causes undulation of large regional folds and changes in the orientation of the folding axes.

Late Deformation

Subsequent deformation phases are brittle in nature and related to the intrusion of different Neoarchean and Paleoproterozoic diabase swarms (see Goutier et al., 2002).

Metamorphism

The region has been affected by at least two episodes of metamorphism to the amphibolite facies. The first episode arrived at around 2800 Ma with significant deformation affecting metavolcanics of the Guyer Group mAgy2b unit in sheet 33G05, transforming them into gneiss, while the adjacent Langelier tonalite (nAlgl3) is not as deformed. This relationship is observed on the shores of the La Grande 3 Reservoir. The second episode (2689-2606 Ma) is the one that affects the entire region, particularly rocks of the Opinaca Subprovince at the amphibolite facies. In detail, there is an important peak between 2683 and 2670 Ma, and a peak around 2645 Ma. There is a growth in metamorphism from north to south, characterized by an increase in the percentage of mobilisate in paragneiss units. The metamorphism reached the granulite facies in sheets 33B located just south with the appearance of orthopyroxene in paragneiss and migmatites (Sawyer, 1998; Morfin et al., 2013).

Economic Geology

(E): selected sample; (R): channel; (F): drillhole

The table of lithogeochemical analyses of metals of economic interest provides the location, description and results of the 37 samples selected to assess the economic potential of the region in 2016. Five samples were collected in 1997 and twelve in 2000 by the teams at Géologie Québec. It should be noted that the sectors targeted by Lamothe’s potential assessment studies (2008, 2009) have been heavily prospected by industry.

New pillow metabasalt and polygenetic conglomerate strips southeast of Radisson

Following fieldwork in the summer of 2016, two mapping targets were identified. The first, identified in sheet 33G04, is associated with a pillow metabasalt horizon, partially altered, which may represent a prospective zone for base and precious metal mineralization. The second, located in sheet 33G05, is a strip of paragneiss containing anomalous gold content and a deformed polygenetic conglomerate lens. This target represents a prospective zone for orogenic-type gold mineralization.

New prospective zones southeast of Radisson

Two new prospective zones in the Lake Ewart area are identified in this bulletin. The first prospective zone is a 2 to 3 km thick aureole located on the edge of an intrusion of the Vieux Comptoir Granitic Suite. This area is characterized by the presence of banded pegmatitic intrusions enriched with beryllium (Blacksmith showing) cutting paragneiss of the Laguiche Complex. The Sinclair model (1996, Figure 21-4) suggests that pegmatites are zoned with a gradual enrichment of the following elements as they move away from the pluton: Be; Be, Nb, Ta; Li, Be, Ta, Nb; and Li, cs, Be, Ta, Nb.

The Lac Ewart-2 prospective zone is a grouping of a point area of gold high-favorability (Lamothe, 2008, 2009; Allard et al., 2015) and ultramafic intrusions near a shear zone. Gold anomalous levels (~300 ppb) were obtained during exploration in this sector (Laflèche et al., 2004, 2006; Bussières and Richard, 2010).

Lamothe (2008, 2009) and Allard et al. (2015) identified several high-favorability zones in their potential assessment for orogenic-type gold mineralization and porphyry-type copper-gold ± molybdenite mineralization. These zones are concentrated in the northern portion of sheet 33G05.

MINERALIZATION ASSOCIATED WITH GRANITIC PEGMATITES

Granitic intrusions are very abundant in rocks of the Laguiche Complex. In the Ewart Lake area, the presence of a beryllium showing in an alkali feldspar banded granite on the edge of a granitic egg-shaped pluton suggests potential for rare earth metal mineralization.

GOLD MINERALIZATION

Gold mineralization contains the largest number of showings in the Ewart Lake region. They are found in the form of orogenic quartz and quartz-tourmaline veins, gold-bearing disseminated sulphides (pyrite and pyrrhotite) in amphibolites after basalt and gold-bearing sulphides in iron formations. These showings are mainly concentrated in the Guyer Group volcanic band in the northern part of sheet 33G05.

The presence of a polygenetic conglomerate unit, a disseminated mineralization (<1% PY, 666 ppb Au) in a paragneiss and anomalous gold levels in an ultramafic intrusion show that rocks of the Laguiche Complex also have some potential for gold mineralization.

MINERALIZATION ASSOCIATED WITH PORPHYRITIC INTRUSIONS

Updating the showings on sheet 33G05 indicates the presence of a large number of molybdenite mineralization on or around a biotite hornblendite intrusion. This ultramafic intrusion is most likely not the source of metals, but the intrusion associated with this mineralization has not been identified. In addition to the Mo content, it is interesting to note the showing levels of silver, copper, gold and rhenium. This type of mineralization is found in the adjacent sheets 33G12 and 33G11. Work is underway (R. Hanes, Ph.D. currently underway at Laval University) in the adjacent sheets to better understand this type of mineralization and its link to the orogenic-type vein systems.

The presence of this type of ultramafic mineralized intrusion, near a major deformation zone, at the boundary between the La Grande and Opinaca subprovinces, increases the mineral potential of the Lake Ewart region. From a more regional perspective, the association of these mineralized intrusions with tectonic areas represents an important metallotect for the exploration of both base and precious metals.

UNKNOWN TYPE DEPOSITS

Three showings were classified as undetermined deposits due to the lack of information in the statutory work reports (GM). The first two are gold mineralization, while the third may be related to mineralization associated with porphyry intrusions.

This bulletin was made possible through the cooperation of many people who have been actively involved in the various stages of the project.

References

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