Rocks of this unit were originally included in an undivided volcano-sedimentary unit called the Doublet Series by Labrador Mining and Exploration geologists (1949, in Frarey and Duffell, 1964), and then the Doublet group by Harrison (1952). Frarey and Duffell (1964) later formalized the name Doublet Group and introduced the name Thompson Lake Formation to refer to the mainly sedimentary sequence overlying the Murdoch Formation. The unit takes its name from Thompson Lake (sheet 23O08) where it is well exposed (Frarey and Duffell, 1964). Rocks of the Thompson Lake Formation have been described by Frarey (1952, 1967), Donaldson (1966), Baragar (1967) and Dimroth (1972, 1978). Wardle (1979, 1982) mapped sedimentary rocks of the Thompson Lake Formation on the Labrador side.

In terms of economic potential, a mafic-ultramafic sill of the Gerido Intrusive Suite intruding along the upper contact of the Thompson Lake Formation contains abundant copper, nickel and PGE mineralization including Lac Retty (Lac Bleu), Lac Retty (Lac Pogo), Lac Retty (Centre) and Lac de la Chance (Clark and Wares, 2004). In addition, a sulphide-rich (pyrite and pyrrhotite) black mudrock probably of syngenetic exhalative origin is found near the top of Thompson Lake (Frarey, 1967; Rohon, 1989).

Unit pPtl1 was mapped locally SE of Irène Lake (sheet 23O10), in the contact zone between the Murdoch and Thompson Lake formations (Baragar, 1967; Dimroth, 1972, 1978). It consists of oxide facies and carbonate facies iron formations associated with chert. Iron formations occur as NW-SE oriented lenses, the largest of which extends for >3.2 km with a maximum thickness of ~200 m. Other smaller lenses are located a few hundred metres north and ~2 km south of the main lens. They are hosted by the Thompson Lake and Murdoch formations respectively. Oxide facies rocks are finely banded with alternating hematite or magnetite-rich bads and white, red, green or brown chert or quartz-carbonate bands. The bed thickness generally varies from 5 mm to 10 cm. Some beds are dominated by iron oxides. Locally, they can be cut by 5 to 12 mm wide, massive hematite veinlets. Quartz veinlets are ubiquitous. Carbonate-facies iron formations are formed by alternating bands, 1 to 20 cm thick, of iron carbonate having a brown altered patina and grey chert (Baragar, 1967).

In thin sections, oxide facies rocks are composed of microcrystalline quartz, 5 to 50% iron oxides (magnetite and hematite) and a minor amount of iron hydroxides (limonite, goethite). Iron carbonate constitutes ~5% of the rock. Variable amounts of riebeckite-arfvedsonite, tremolite-actinolite, acmite and stilpnomelane are also present. Quartz grains vary in size from 0.01 to 0.5 mm, but most are 0.03 to 0.05 mm in diameter. Iron oxide crystals generally range in size from 0.05 to 0.1 mm, while crystal aggregates can be 1 mm or larger. Magnetite is commonly euhedral and appears to be the dominant oxide. Carbonate facies iron formations contain 10 to 50% carbonate. Iron oxides are sparse or absent. Minor amounts of stilpnomelane, goethite or limonite, graphite and quartz are also present. Carbonate grain size generally ranges from 0.03 to 0.3 mm (Baragar, 1967).

Thompson Lake Formation 2 (pPtl2): Mudrock (Locally Pyritic) and Impure Quartzite with Some Greywacke, Siltstone, Chert and Conglomerate

Thickness and distribution

The Thompson Lake Formation belongs to the Retty Lithotectonic Zone as defined by Clark and Wares (2004). The Thompson Lake volcano-sedimentary sequence is restricted to the eastern side of the central Labrador Trough. It extends from the Argent Lake area (sheet 23O14) to the Roymart Lake area (sheet 23I12), a NW-SE distance of ~170 km. The Thompson Lake Formation occurs as narrow strips that are easily eroded and form valleys. These strips are interstratified with mafic to ultramafic sills (Gerido Intrusive Suite) (Baragar, 1967). The thickness of the formation is difficult to estimate due to the lack of outcrops and sill intercalation (Baragar, 1967). It is estimated at 600 m in the Doublet Lake area, 700 m in the Hyland Lake area and 500 m in the Ahr Lake area. The Thompson Lake Formation thins to the north and is ~90 m thick NE of Silver Lake (Baragar, 1967; Frarey, 1967; Dimroth, 1972, 1978).

Dating

None.