Background

Description

Sokoman Formation 1 (pPso1): Hematite or Magnetite Oxide Facies Iron Formation, Arenite and Jaspe

Sokoman Formation 2 (pPso2): Silico-Carbonate Facies Iron Formation with Carbonate Porphyroblasts and Chert

Silico-carbonate facies iron formations are abundant in the Roberts Synclinal and in the Brochant River area. They were observed at several sites during Hardy’s work (1976) and during the 2017 mapping work. This unit represents the upper member of the Sokoman Formation in these areas (Hardy, 1976).

The lower contact with the underlying oxide facies unit (pPso1) and the higher contact with the Menihek Formation (pPme) are conformable and generally very sharp (Hardy, 1976). However, some outcrops expose a rather transitional lower contact with oxide facies iron formations, where there are centimetric to decimetric interstratifications of both facies over a few metres (Bilodeau and Caron-Côté, 2018). Locally, silico-carbonate facies iron formations are overlain by a metre-thick sulphide chert. In the Kangirsuk area, silico-carbonate facies iron formations are generally 5 to 50 metres thick. It also occurs as decametric to metric beds interstratified with quartz arenites and oxide or silicate facies iron formations.

Silico-carbonate facies iron formation is brown, orange, rust, white and beige in altered surface and grey, brown and beige in fresh exposure. It is generally fine grained, locally medium grained. It is commonly banded and consists mostly of millimetric to decimetric beds of white or beige quartz arenite and brown or rust iron carbonates. More locally, it contains black beds of ferriferous amphibole. It also usually occurs with 1 to 20% centimetric nodules of red-brown iron carbonates in a recrystallized quartz or white saccharoidal chert matrix. These carbonate nodules are usually stretched and elongated parallel to regional schistosity and have a shallow dip. The nodules are dissolved in several places, giving a porous appearance to the rock. This iron formation is cut in several places by millimetric to centimetric quartz veins.

Silico-carbonate facies iron formation is generally schistose and locally highly fractured, where the rock occurs as centimetric to decimetric fragments. Generally, the white quartz material of the rock, whether in interstratification, matrix or vein, is in positive relief while the carbonate material is in negative relief. More locally, horizons are composed of an iron carbonate matrix with ferriferous amphibole centimetric nodules. In these horizons, the matrix is in negative relief and fine grained, while the amphibole nodules are medium grained and in positive relief.

Under the microscope, quartz is usually granoblastic and fine or medium grained. In banded rocks, quartz ribbons are well defined while carbonate ribbons are less defined. They also have millimetric to centimetric nodules of fine-grained carbonate. Nodular rocks have a quartz matrix that commonly contains 1% very fine-grained disseminated carbonate. The most common minor and accessory mineral phases are magnetite, grunerite and stilpnomelane. These minerals are generally fine to very fine grained.

Some chert horizons overlying silico-carbonate facies iron formations were observed during the 2017 mapping work. Metre-thick, these horizons have a massive structure and a saccharoidal appearance. They have a rust-coloured alteration patina due to the presence of 1-3% fine-grained sulphide disseminated in the rock.

The following descriptions are taken from the Dimroth report (1978). They concern the Labrador Trough section between latitudes 54º30′ and 56º30′, inside sheets 23I, 23J, 23N, 23O, 23P, 24B and 24C.

“Silicate-carbonate ferriferous rocks are more subject to variation than hematite ferriferous rocks. They are grey or greenish in fresh exposure and brown or red brown in altered surface. Colour intensity is a function of iron mineral content. Light brown altered light green cherts contain minnesotaite. Rocks containing large amounts of minnesotaite are dark greenish grey and alter to light red brown. Carbonate cherts are grey and become brown during alteration. The intensity of the brown alteration depends on the carbonate content.” Silicate-carbonate iron formation consists of millimetric to centimetric lamination rocks that can contain millimetres to centimetre-diameter intraclasts. « Concretions are common in intraclastic and oolitic silicate-carbonate facies cherts. They are more or less spherical and vary in diameter from 1 mm to several centimetres. The alteration of carbonate concretions produces cavities.”

Sokoman Formation 3 (pPso3): Carbonate Facies Iron Formation

The iron formations’ carbonate facies forms the summit of the Sokoman Formation. Its total thickness is estimated to be more than 60 m northwest of the Aigneau River (sheets 24E08, 24F05 and 24F06) (Clark, 1977). The carbonate member shows alternating beds about 1 to 20 cm thick consisting of: 1) ferriferous carbonate rock with orange, reddish-brown or rusty brown patina, typically rich in quartz veins cutting beds at a sharp angle; 2) cherty ferriferous carbonate rock with reddish-brown or purple-brown patina, fine-grained, locally fissile, containing in places brown-altered sericite rhomboid crystals (1 to 2 mm) in a chloritized matrix, and ferriferous amphibole crystals in the form of needles and rosettes; 3) grey or greenish grey chert with red-brown to grey patina, usually brecciated, with disseminated, brown-altered carbonates; 4) laminated grey ferruginous chert; 5) reddish-brown sandstone; and 6) jaspe (Ciesielski, 1975; Clark, 1977, 1978, 1979, 1984).

Well-rounded pebbles of iron formations are interstratified in the unit exposed to Gossen Hill (sheet 24F12) (Clark, 1979). Sandstones are rather lenticular, while cherts are only stratiform. Ferriferous carbonates are stratiform, lenticular and nodular in appearance or form clusters of up to 1 cm in diameter (Ciesielski, 1975; Clark, 1977). Crossbedding is sometimes observed in ferriferous cherty carbonate rocks and sandstone (Ciesielski, 1975; Clark, 1978). South of Forbes Lake, Clark (1977) indicates that cherty rocks are silicified and strongly recrystallized. Contact between the carbonate facies and the underlying oxide facies (pPso1) is gradual. Jaspe, reddish-brown sandstone beds or carbonate lenses 30 to 60 cm long in rocks of the oxide facies mark the transition (Ciesielski, 1975; Clark, 1977, 1984). 

Sokoman Formation 4 (pPso4): Black Chert

Black chert is observed in the Goethite Lake and Otelnuk Lake areas (sheet 24C01) where it typically forms a bedding several metres thick in the silico-carbonate facies (pPso2). It is dark green or black grey in fresh exposure and brown or grey in altered surface. The unit consists of intraclasts, oolids and pisolites cemented by a matrix consisting mainly of cherty quartz with minor amounts of minnesotaite, greenalite, siderite and magnetite. Intraclast diameter ranges from 0.5 mm to >1 cm. Siderite concretions of 1 to 60 mm in diameter or siderite lenses up to 30 cm thick and 1.83 m long are observed locally. The intraclastic facies often contains jaspe fragments up to 10 cm in diameter and up to 2 cm thick. Chert is recrystallized and intraclastic structures are generally visible, albeit poorly preserved (Dimroth, 1969, 1972, 1978).

Sokoman Formation 5 (pPso5): Silicate Facies Iron Formation and Micaceous Schist

Silicate facies iron formations were observed very locally in the Roberts Synclinal, but are very common in the Brochant River area. Hardy (1976) defined this unit as the lower member of the Sokoman Formation within the Roberts Synclinal. However, Bilodeau and Caron-Côté (2018) observed in some locations that it also occupies other stratigraphic levels, especially in the Brochant River area. At these locations, it is interstratified with carbonate or oxide facies iron formations and has conformable and sharp or gradual contacts. At the locations where it represents the formation’s lower member, silicate facies iron formations overlie the Wishart Formation with sharp and conformable contact, and underlie the oxide facies iron formations with conformable, sharp and locally transitional contact. Locally, a micaceous schist is interbedded between the Wishart Formation and the iron formation. This schist has sharp lower contact with the Wishart Formation, while its upper contact with the silicate facies iron formation is gradual (Hardy, 1976).

Silicate facies iron formations range in thickness from 1 to 10 m and very locally up to 80 m. The rock varies from grey, brown, beige to green, depending on mineralogy. It is fine to coarse grained, locally heterogranular. It is commonly stratified, banded or massive. The thickness of the bands varies from millimetres to decimetres. The amphiboles’ orientation locally marks a nematoblastic texture. In the Brochant River area, the rock usually shows a granoblastic texture and contains amphibole porphyroblasts in several places. These porphyroblasts are usually acicular and locally radial. In a few places, there are iron oxide nodules or bands of oxides or iron carbonates.

Silicate facies iron formation is usually composed of quartz and ferriferous amphibole such as grunerite. Under the microscope, minor and accessory mineral phases are usually hornblende, garnet, magnetite, carbonate and more locally actinolite and tremolite. Garnet is usually porphyroblastic and poikilitic, containing quartz and amphibole inclusions. Banded horizons typically have bands of varying proportions of quartz, amphibole, magnetite and carbonate. In the Brochant River area, magnetite is locally euhedral and appears to be late with respect to crystallization of amphibole. This disseminated magnetite appears to cut amphibole forming the matrix.

Some horizons of micaceous schist were observed locally in the Roberts Synclinal. The schist is interbedded between the Wishart Formation and the oxide facies iron formation. It is metres-thick, greenish-black in altered surface and a black grey in fresh exposure. It is fine grained and usually composed of granoblastic quartz, biotite and garnet. It contains magnetite and grunerite more locally as minor and accessory mineral phases. Grunerite occurs as coarse-grained porphyroblast and contains quartz inclusions. Garnet is porphyroblastic and medium or coarse grained. It is commonly helicitic and contains quartz and magnetite inclusions. The biotite’s orientation usually marks a lepidoblastic texture.

Sokoman Formation 6 (pPso6): Iron Formation, Micaceous schist and Mica-Garnet Schist

The Sokoman Formation 6 is a minor unit that is only exposed in sheets 24N05 and 24N12. It consists of two lenses, one oriented north-south for 15 km west of False Bay, and another oriented east-west for 1 km west of Hopes Advance Bay. It consists of undifferentiated iron formation, micaceous schist and mica-garnet schist. Very little information appears to be available on this unit.

Thickness and Distribution

The Sokoman Formation extends along the length of the Labrador Trough. Within the Roberts Synclinal and in the Brochant River area, its thickness generally ranges from 20 to 100 m.

Dating

In the Dyke Lake area (sheets 32J08 and 32J09), a U-Pb age of 1877.8 ±1.3 Ma was obtained for a syenite clast contained in a conglomerate interstratified with the Sokoman Formation. This dating proposes a maximum sedimentation age for the Sokoman Formation in this area (Findlay et al., 1996).

Stratigraphic Relationship(s)

East of Cambrien Lake (sheets 24C07 and 24C10), a 1880 ±2 Ma U-Pb age was obtained for a carbonatite dyke of the Castignon Complex (Chevé and Machado, 1988). This dyke is contemporaneous of the Sokoman Formation and thus gives this formation’s age (Chevé and Machado, 1988; Clark and Wares, 2004). Dressler (1975) dated a lamprophyre associated with carbonatites of the Castignon Complex and intruded into the Ruth Formation (underlying the Sokoman Formation) and obtained a K/Ar age of 1873 ±53 Ma. In the Knob Lake area (sheet 23J15), Fryer (1972) obtained a Rb-Sr age of 1870 ±50 Ma for the Sokoman Formation based on 19 samples from the Menihek Formation and Attikamagen Group, which are the units respectively overlying and underlying the Sokoman Formation.

In the vicinity of Kangirsuk, in the Roberts Synclinal’s flanks and in the Brochant River area, the Sokoman Formation is commonly overlying the Wishart Formation. Because the Wishart Foramtion is absent locally, the Sokoman Foramtion is thus mapped in these places as being in contact with Archean rocks. This contact between the Sokoman Formation and Archean rocks was not observed in the field. In the absence of the Wishart Formation, the Sokoman Foramtion and Archean rocks are separated by an overburden segment. In these areas, the Sokoman Formation is overlain by the Menihek Formation. Contacts with the Wishart and Menihek formations are conformable and sharp. 

Paleontology

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