The Dunphy Formation constitutes the upper unit of the Seward Group as defined by Clark and Wares (2004). It is a sedimentary unit that was deposited during the marine transgression that followed deposition of the Chakonipau Formation in the central part of a shallow basin (Dimroth, 1970b, 1972, 1978; Wardle and Bailey, 1981; Clark and Wares, 2004). It is equivalent to the Portage and Milamar formations in the western and eastern parts of the Labrador Trough, respectively (Dimroth, 1972, 1978). The type locality of the Dunphy Formation (additional reference section) is located between Ronsin and Romanet lakes (Table 1; Dimroth, 1972, 1978; Chevé, 1985).

Table 1: Characteristics of the Type Locality

West of Romanet Lake (additional reference section), the Dunphy Formation was divided into four subunits by Chevé (1985). The lower member consists, at the base, of some graphitic black slate overlain by laminated grey to dark grey mudstone and siltstone with lenses (30 cm thick) of grey dolomite having a brown alteration patina. Towards the top, it consists of laminated mudstone and siltstone having a green alteration patina interbedded with centimetric grey dolomite having a brown alteration patina. The upper member of the Dunphy Formation consists mainly of dolomite. At the base, it is represented by massive pink, salmon, beige, grey or white dolomite having a grey or brownish alteration patina, locally sandy. Dolomite contains interbedded purple, green or black mudstone and chert. At the top, it is characterized by massive to laminated pink or salmon stromatolitic dolomite having a white patina, with locally intebedded purple mudstone. The pink colour of dolomite is due to the presence of very finely disseminated hematite in the rock (Dimroth, 1972, 1978; Chevé et al., 1985). The Dunphy dolomite has a characteristic translucent lustre (Dimroth, 1972, 1978). In this area, layers of the Dunphy Formation have been isoclinally folded and refolded.

The rocks of the Dunphy Formation were generally metamorphosed to the greenschist facies (Dimroth and Dressler, 1978; Fraser et al., 1978). Locally, dolomite was ankeritized, particularly in areas where it is highly deformed. The ankeritized rock is yellow in fresh surface and brown in altered patina (Dimroth, 1972, 1978). In places, the Dunphy dolomite has undergone more intense metamorphism. It is represented by a white marble or talc-calcite schist near gabbroic sills of the Wakuach Suite (Montagnais Supersuite) (Dimroth, 1966, 1972, 1978).

The Dunphy Formation was deposited in a coastal or shallow marine shelf environment (Dimroth, 1972, 1978; Chevé et al., 1985). According to Dimroth (1972, 1978), sediments forming the rocks at the eastern and western edges of the formation are thought to have originated in part from continental springs to the east and west of the Trough. The presence of sand grains and hematite dust in dolomite indicates that at least some of the sediments were transported by wind (Dimroth et al., 1970).

The economic potential of the Dunphy Formation lies in lode silver-lead-zinc mineralization, stratiform copper mineralization in sedimentary settings and lode copper mineralization (Clark and Wares, 2004). Laminated to massive dolomites of the Dunphy Formation, formed during the early marine transgression, are of major metallogenic importance for stratiform copper mineralization. The black slate located at the base of the formation, west of Romanet Lake, contains anomalous copper and gold contents (Chevé, 1985; Kish, 1994).

Unit pPdy2 forms the upper member of the Dunphy Formation in the Ronsin Lake area (sheet 24B05) (Dimroth, 1972, 1978; Chevé, 1985). It generally consists of pink, salmon, beige, grey or white dolomite, locally slightly sandy, and pink or salmon stromatolitic dolomite having a white alteration patina. These rocks occur in massive to finely laminated centimetric to metric layers (Dimroth, 1978; Chevé, 1985; Clark, 1986). The Dunphy dolomite has a characteristic translucent lustre (Dimroth, 1972, 1978). The pink or salmon colour of the rock is due to the presence of finely disseminated hematite (Dimroth, 1972, 1978; Clark, 1986). In the Romanet Lake area, Chevé et al. (1985) attribute this pink colour to the oxidation of white dolomite by fluid flow along late fractures. Dolomite contains fine-grained sand near the western margin of the Trough as well as on an island NW of Romanet Lake (Dimroth, 1972, 1978).

Unit pPdy2 also contains interbedded centimetric pink, grey or white chert and interbedded (2-30 cm thick) purple, green or black mudstone locally (Dimroth, 1972, 1978; Clark, 1986). It differs from unit pPdy7 in particular by the absence of sandstone and dolomitic sandstone beds.

In the Canichico Lake area (sheet 24C10) (Dressler, 1979), the Dunphy dolomite has a spotted and banded appearance marked by alternating discontinuous, translucent, white bands (1-10 mm thick) and brownish red bands. Dolomite contains minor amounts of siltstone and quartzitic arenite in the Canichico Lake area and south of latitude 56°N.

Dolomite is locally ankeritized, especially in highly deformed areas south of Ronsin Lake and NE of Otelnuk Lake (Dimroth, 1972, 1978). The rock is honey-yellow in fresh surface, dark brown in altered patina and very coarse grained in places. North of Effiat Lake (sheet 24B04), dolomite is recrystallized to white calcite-actinolite marble at the contact of a gabbro sill (Wakuach Suite). The rock is white to grey, medium grained and highly sheared. It contains stocky actinolite prisms up to 2 cm long (Dimroth, 1966, 1978).

Dunphy Formation 5 (pPdy5): White Quartzitic Arenite with Interbedded Laminated Green to Grey Siltstone