Background

Description

Clément Formation 1 (nAcln1): Amphibolitized Pillow and Massive Flow Basalt and Andesite, Minor Amounts of Gabbro

Clément Formation 2 (nAcln2): Felsic and Intermediate Tuff, Rhyolite, Felsic Volcanics

Unit nAcln2 is not very common in the Clément Formation. Tuffs and flows are beige in altered surfaces and light grey in fresh exposure. These rocks have felsic to intermediate compositions and are of calc-alkaline affinity. Tuffs contain mafic or felsic lapillis. They frequently contain 1-5 % quartz crystals, and weakly to moderately damouritised plagioclase phenocrystals. Felsic to intermediate volcaniclastic rocks contain quartz, feldspath, biotite, chlorite, white mica and epidote. Some contain amphiboles, garnet and sillimanite and pinitized cordierite porphyroblasts. Massive felsic flows with quartz eyes and plagioclase phenocrystals are also present.

Clément Formation 3 (nAcln3): Gabbro

Gabbro is dark green in altered patina and slightly lighter in fresh exposure. It is mesocratic to melanoratic. Generally, this unit contains porphyroblastic amphiboles (0.3 to 1 cm tall) with pyroxene pseudomorphs and plagioclase. Sericite and epidote, which form clusters, moderately to strongly replace plagioclase. There are minor amounts of sphene, rutile and opaque minerals. It is a solid, weakly foliated rock that varies in size from medium to coarse-grained. Some quartz, albite, epidote and hematite veins are present.

Clément Formation 4 (nAcln4): Ultramafic Sill and Flow

Roy (1988) had identified an ultramafic intrusion in the Clément Lake area over a distance of 10 km. The maximum thickness of this intrusion was 200 m. Over short distances, its composition changed from ultramafic (olivine cumulate) to mafic (hornblende gabbro).

As a result of the Ministère’s 2017 mapping campaign, several additional horizons of ultramafic sill or ultramafic volcanics were observed or interpreted using magnetic anomaly maps (Beauchamp and Massei, 2018). Ultramafic rocks of the Clément Formation form bands conformable with the belt stratigraphy and do not show volcanic structure in general. So this is ultramafic sills. However, some outcrops may consist of extrusive rocks. Outcrop 17-JM-6132 shows what could be contact between flows or cooling fractures. There are also very long lobes that close. Ultramafic intrusions have a brownish-to-greenish altered patina and are darker in fresh exposure. Tremolite, talc, magnetite and locally serpentinized olivine cumulates are the main minerals visible under the microscope. Magnetism is medium to strong and granulometry is fine apart from olivine phenocrystals that measure up to 1 cm in the cumulate-structure facies. In thin section, the rocks are made up of amphibole acicular crystals (mainly tremolite and actinolite, locally hornblende). Olivines, when present, are completely replaced by serpentine. The rock contains varying amounts of magnesian chlorite and minor amounts of talc, epidote, calcite, sericite and opaque minerals (including magnetite and sulphides).

Ultramafic sills and flows of the Clément Formation are characterized by contents in SiO₂ between 41 and 46%, MgO between 22 and 33%, TiO₂ between 0.1 and 0.3%, Zr between 6 and 18 ppm and Zr/Y ratio between 1.2 and 2.2. Their rare earth profiles are relatively flat as shown by their ratios (La/Yb)_CN = 0.8 and 1.1 and (Gd/Yb)_CN = 0.8 and 1. These ultramafic rocks plot in the field of non-aluminum-depleted komatiites (Sproule et al., 2002).

Clément Formation 5 (nAcln5): Iron Formation and Garnet Amphibolite

Iron formations within the Clément Formation represent a very small proportion of all rocks. Two main horizons were identified through “geofiche” and compilation outcrops. The iron formations are thin strips and their extension northward and southward is interpreted by magnetic continuities on the residual total magnetic field map. They have a rusted (gossan) altered patina. They contain 32-43 % SiO₂, 28-40 % Fe₂O₃, 5-6 % MgO, 72-112 ppm Zr and a Zr/Y ratio of 4.5-6. They are enriched with light rare earth elements and have anomalies in Nb, Ta and Ti on the spider diagram.

Iron formation at the oxide facies has a banded structure. It consists of an alternation of quartzite-recrystallized white chert centimetric bands and dark grey oxide bands. Sulphide-rich bands contain 30-60 % pyrrhotite and pyrite. The minimum thickness of iron formation is 5 m.

Mixed iron formation shows both silicate and oxide facies. It is made up of quartzite-recrystallized chert bands, and amphibole-rich (hornblende and grunerite) bands containing some magnetite. Primary banding and primary foliation are parallel to subparallel. Quartzite bands are locally boudinaged, and locally a light foliation is visible in the amphibole-rich melanocratic bands. Disseminated pyrrhotite (trace to 5%) is present in amphibole-bearing bands. Laterally, chert bands become thicker (5 to 15 cm) and only thin lode of magnetite and amphibole are present.

Garnet amphibolite horizons, which could be in the continuity of silicate-facies iron formations (MgO between 3 and 5% and Fe2O3T between 11 and 11.5%) were identified on the same magnetic markers as banded iron formations. They are foliated but not banded. They have a dark greenish patina in altered surface and medium green in fresh exposure. They are medium-grained and have a nematoblastic structure. Their paragenesis consists mainly of 40-60% green amphiboles (hornblende, grunerite), 20-30% pyroxenes, 20-30% poecility porphyroblasts of subautomorph almandine garnet, 10-20% opaque minerals (oxides and sulphides) and 5-30% quartz, with minor and variable amounts of biotite, sericitized plagioclase, epidote, calcite and zircon. Garnets are flattened and retrograded to chlorite.

Thickness and Distribution

The Clément Formation is only present in the south branch of the UEGB. At present, lithological and geochronological data do not allow this formation to be extended to other branches of the UEGB. The Clément Formation has a true thickness ranging from 0.85 to 4.2 km and is continuous for approximately 30 km. 

Dating

None.

Stratigraphic Relationship(s)

The Clément Formation is part of the René Group. It corresponds to the third volcanic event of the UEGB. The Clément Formation is younger than the Roman Formation, dated 2770 ±3 Ma (Davis et Sutcliffe, 2018b) and 2770 ±6 Ma (Davis et Sutcliffe, 2018a), and the Érasme Formation, dated 2800 ±6 Ma (Davis et Sutcliffe, 2018b). Its stratigraphic position with respect to the Dolent Formation (2751 ±5 Ma; Davis, personal communication, 2019), which has been mapped in the west and north branches of the UEGB, is not known at this time. The western end of the Clément Formation is in faulted contact with aluminosilicate paraschists (nAbh1a) and conglomerates (nAbh2) of the Bohier Group. The Bohier Group rocks, whose maximum age of sedimentation is 2712 ±6 Ma (Davis et Sutcliffe, 2018b), are younger than the René Group volcanic rocks. Rocks of the René Group overthrusted rocks of the Bohier Group; they are therefore overlying sedimentary rocks in the southern branch. Contact between the Clément Formation and paragneiss of the Opinaca subprovince was observed on outcrop 17-AB-1083. It is marked by a shallow dip thrust (about 25°). Contact between the Clément and Roman formations was not observed. In the south, the Clément Formation disappears from the discordance that borders the Otish Basin. 

Paleontology

Does not apply.

References