Background

Description

Théodat Complex 1 (Athe1): Tonalitic Gneiss and Gneissic Tonalite

Théodat Complex 2 (Athe2): Foliated Granodiorite Containing Tonalite Enclaves

Unit Athe2 consists of biotite ± hornblende granodiorite, grey beige to whitish pink in altered surface and beige to medium light pinkish grey in fresh exposure. The rock is fine to medium grained, locally coarse grained, homogeneous, weakly to highly foliated, hematitized and slightly magnetic in some areas. It generally contains 2 to 15% biotite, locally 20 to 25%. Hornblende (≤10%) and magnetite (≤2%) are also observed in places. Ferromagnesian minerals occur in places as very magnetic clusters, while the rest of the rock is weakly or not magnetic. The medium-grained or coarse-grained rock is the least deformed and contains more hornblende than biotite. The fine-grained granodioritic phase is so similar to the foliated tonalite of unit Athe2a that it is locally difficult to differentiate them in outcrop.

Théodat Complex 2a (Athe2a): Biotite ± Hornblende Tonalite, Foliated

Unit Athe2a consists of tonalitic rocks that, in outcrop, resemble the fine to medium-grained granodiorite of unit Athe2. The rock is dark grey to pinkish grey in fresh exposure and beige or whitish grey in altered surface. Tonalite is distinguished from granodiorite by the presence of foliated diorite enclaves that form 1 to 10% of the rock. Subunit Athe2a also contains metric to decametric tonalitic gneiss enclaves. Biotite±hornblende is weakly to highly foliated. It is medium grained, locally fine grained, typically homogeneous. K-feldspar makes up <10% of the rock, while ferromagnesian minerals (biotite, hornblende, epidote, chlorite) represent up to 25%. The rock typically contains 5 to 15% biotite, but 20 to 25% locally. Hornblende (≤5%) and magnetite (1%) are also present. Accessory minerals are sphene and magnetite.

Théodat Complex 3 (Athe3): Porphyritic Granodiorite

The porphyritic hornblende-biotite granodiorite unit (Athe3) occurs as more or less rounded or oblong masses within the Théodat Complex, and generally within units Athe1 or Athe2. It is characterized by a strong, distinct magnetic susceptibility on regional aeromagnetic maps (D’Amours and Intissar, 2012; D’Amours, 2011). The rock is light grey in fresh exposure and whitish to pinkish grey in altered surface. It is massive to foliated, homogeneous, medium to coarse grained and characterized by a porphyraceous texture. It contains K-feldspar phenocrystals, euhedral to subhedral, locally subrounded, disseminated in a medium to coarse-grained matrix. Their content ranges from 10 to 30% and their size from 0.5 to 3 cm, locally >3 cm. In places, the preferential alignment of feldspar phenocrystals highlights a magmatic foliation. K-feldspar phenocrystals commonly contain hornblende or biotite inclusions. They locally feature whitish reaction rim, typical of a rapakivi texture. Quartz is grey, millimetric and represents 25% of the rock. Ferromagnesian minerals include hornblende (8-20%) and biotite (5-10%). The rock generally contains 1 to 3% disseminated magnetite.

Unit Athe3 also contains 1 to 3% highly-deformed porphyraceous monzodiorite enclaves, more or less aligned with foliation planes of the porphyraceous granodiorite host. These enclaves are black with a grey to whitish alteration patina, and are more deformed or foliated than the host rock. The deformation also affects feldspar phenocrystals. These enclaves contain hornblende (30%), biotite (5%), magnetite (1%) and K-feldspar phenocrystals (3-10%).

Théodat Complex 3a (Athe3a): Quartz Monzodiorite and Quartz Monzonite, Locally Porphyraceous

Subunit Athe3a is observed in the SW corner of the Goulde Lake area (sheet 32O11), in spatial association with the porphyritic granodiorite of unit Athe3. It consists of porphyraceous quartz monzodiorite, quartz monzonite and locally granodiorite. These rocks are also characterized by centimetric hornblende clusters and the ubiquitous presence of diorite enclaves. These rocks are medium to coarse grained, weakly to strongly foliated, homogeneous and magnetic. They are pinkish grey to whitish pink in altered surface and medium light grey in fresh exposure. The pinkish colour is locally accentuated by strong hematitization. These rocks are characterized by 5 to 30% centimetric K-feldspar phenocrystals or porphyroclasts, euhedral to subhedral locally, more or less aligned with foliation planes. Ferromagnesian minerals mark foliation and commonly form small elongated centimetric hornblende clusters. The rock contains 10- to 20% hornblende (locally up to 40%), 5 to 25% biotite and 2% magnetite locally. Accessory minerals are abundant and include sphene, epidote, muscovite, chlorite, allanite, apatite and opaque minerals.

Théodat Complex 3b (Athe3b): Foliated Diorite and Quartz Diorite

Unit Athe3b is observed north of the Lucky Strike Creek area (sheet 32J12), which is within porphyritic granodiorite of unit Athe3 (Bandyayera and Caron-Côté, 2021). It consists of foliated diorite and quartz diorite. The rock is medium grained, granoblastic, homogeneous, foliated and moderately magnetic. It is greyish in fresh exposure and greyish beige in altered surface. Locally, the rock contains 5% K-feldspar phenocrystals and corresponds to a monzodiorite. Diorite is composed of saussuritized plagioclase (60%), hornblende (25-35%), biotite (3-5%), epidote (3-5%) and magnetite (1-3%). Accessory minerals include sphene, muscovite, chlorite, allanite, apatite and opaque minerals.

Théodat Complex 4 (Athe4): Biotite Granite

Unit Athe4 occurs as decametric to kilometric intrusions in the Théodat Complex. It consists of pink biotite±magnetite granite. The rock is pinkish beige in fresh exposure and light pink to pinkish grey in altered surface. It is medium to coarse grained, locally pegmatitic, massive, slightly to highly magnetic. Unit Athe4 contains decametric enclaves of foliated to gneissic tonalitic rocks. Unit Athe4 is very poor in ferromagnesian minerals (<5%) and contains no garnet. The main rock minerals are quartz, plagioclase, partially chloritized biotite (<5%) and magnetite (1-5%). Accessory apatite and zircon are observed in thin section.

Théodat Complex 5 (Athe5): Granitic Pegmatite

Unit Athe5 consists of granitic pegmatite that is typically light pink, usually massive, and displaying a widespread graphic texture. The rock is magnetic, heterogranular, commonly pegmatitic or coarse grained, locally medium grained and weakly to highly hematitized. Hematitization gives the rock a reddish colour in some places. Granitic pegmatite is also distinguished by low ferromagnesian mineral contents representing <5% of the rock. It contains 1 to 3% biotite (5% locally) and 1 to 3% disseminated millimetric magnetite grains, locally in clusters or large centimetric crystals. Unlike white granitic pegmatite associated with metasedimentary sequences, unit Athe5 pegmatite does not contain garnet, muscovite or beryl, and tourmaline is very scarce. Unit Athe5 contains enclaves of all the other units of the Théodat Complex (foliated to gneissic tonalite, granodiorite, monzodiorite and diorite).

Thickness and Distribution

The Théodat Complex occupies a very large portion of the Eeyou Istchee James Bay Territory (sheets 32J, 32K, 32N and 32O). It extends for 220 km from east to west between the James Bay Road and the Troilus Mine, and for 50 to 85 km from north to south between the village of Nemiscau and the Frotet-Evans Belt. Outcrop distribution indicates that the Goulde Lake area (sheet 32O11) contains a 30 km N-S section along which all units of the Théodat Complex outcrop.

Dating

U-Pb dating on zircons from a tonalitic gneiss sample (outcrop 2018-DB-1118) returned a crystallization age of 2833.5 ±3 Ma (David, 2020). This age is comparable to that of the Lac Rodayer Pluton interpreted as a tonalitic basement emplaced between 2830 Ma and 2820 Ma (Davis et al., 1994, 1995).

Stratigraphic Relationship(s)

Tonalitic to dioritic gneiss units of the Théodat Complex are the extension of the Rodayer Pluton eastward. The latter is interpreted as an old bedrock established between 2830 Ma and 2816 Ma (Sawyer and Ben, 1992, 1993; Davis et al., 1994, 1995; Bandyayera and Daoudene, 2017). This old crust is intruded by several generations of mafic, intermediate and felsic intrusive rocks, which are cut by late-tectonic granodiorite or granite intrusions. Volcanic rocks of the Frotet-Evans Belt to the south and Lac des Montagnes Group to the north are younger than the Théodat Complex tonalitic basement. The contact between the latter (Opatica Subprovince) and the Lac des Montagnes Belt (La Grande Subprovince) is delineated by a regional shear zone assigned to the Poste Albanel Shear Zone. Pegmatitic granite or granitic pegmatite intrusions, generally enriched in magnetite (2-10%), are observed on outcrops throughout the Théodat Complex. These intrusions are pinkish in altered surface and light pink in fresh exposure, and are usually magnetic and similar to granitic pegmatite masses assigned to unit Athe5. Porphyritic granodiorite (Athe3), quartz monzodiorite and porphyritic quartz monzonite (Athe3a) units are similar to those of the La Sicotière Suite mapped in the La Sicotière Lake area (sheet 32N08; Bandyayera and Daoudene, 2018). These units may be part of the same intrusive suite.

Paleontology

Does not apply.

References

Publications Available Through SIGÉOM Examine

Other Publications

Suggested Citation

Contributors