Stevenson et al. (1969) noted the presence of two thin strips of volcanic rocks inside the predominantly granitic rocks of southern Nantais Lake, north of Klotz Lake (sheets 35A and 35B). Taylor (1981) subsequently mapped a small unit of amphibolite and metasedimentary rocks on the shoreline of the NW end of Nantais Lake (sheet 35G01). As a result of their reconnaissance work, Percival et al. (1997a) defined sequences of metavolcanic supracrustal rocks (amphibolite and mafic gneiss) and paraschists in the Nantais Lakes area that were included in the Pélican-Nantais belt. (Percival et al., 1997b). Subsequently, Madore et al. (2001, 2002b) introduced the term “Pélican-Nantais Complex” to group rocks of the Pélican-Nantais belt and some tonalitic intrusions between Klotz Lake and the Pingualuit Crater. They divided the complex into three informal units: paragneiss (Apna1), metavolcanic rocks (Apna2), and tonalite (Apna3). The Pélican-Nantais Complex was extended southward in the Pélican Lake area (sheet 34P) (Cadieux et al., 2002, 2004). The Nantais belt referred to rocks of the Pelican-Nantais Complex in the Nantais and Klotz lakes area (sheets 35A and 35B; Madore et al., 2001, 2002b), while the Pélican belt represented rocks of the same complex in the Pélican Lake area (sheet 34P; Cadieux et al., 2002, 2004).

Simard et al. (2008) divided the Pélican-Nantais Complex into two separate complexes because of the different ages obtained in the Nantais (2775 Ma) and Pelican (2740-2733 Ma) belts: the Nantais and Pélican complexes, respectively. These authors assigned the Caumartin and Peltier belts (metavolcanics) and Headwind Belt (metamorphosed volcano-sedimentary rocks) as defined by Madore et al. (2002a, 2004) to the Nantais Complex based on the resemblance of lithological assemblages (Madore, personal communication). Finally, they reassigned tonalites of the Pélican-Nantais Complex (unit Apna3) defined by Madore et al. (2001, 2002b) to the Kapijuq Suite. These authors also mentioned that tonalites of the Pélican-Nantais complex and tonalites of the Kapijuq suite could be equivalent. As a result of these adjustments, the term “Pélican-Nantais Complex”, as well as the Caumartin, Peltier, Headwind and Nantais belts were abandoned.

The Nantais Complex comprises several volcanic belts metamorphosed to the amphibolite and granulite facies (Percival et al., 1997a, b; Madore et al., 2001, 2002b; Cadieux et al., 2002, 2004; Simard et al., 2008; Simard, 2008; Lamothe and Simard, 2010, 2011). Metavolcanics are mainly composed of metabasalt and mafic gneiss of tholeiitic affinity. On outcrops, metabasalt has penetrative foliation, while mafic gneiss displays centimetric compositional banding. The most deformed rocks, especially on the edges of klippes, have a mylonitic or straight gneiss fabric. Metabasalt and mafic gneiss contain 5 to 20% felsic mobilisate (granitic and tonalitic material) injected as discontinuous veins, millimetre to centimetre-thick, parallel to gneissosity. They are locally interstratified in various tuff layers of intermediate to felsic composition, with conglomerates and, in places, 1-10 m thick layers of massive to foliated ultramafic rocks (volcanics or sills) or very migmatized paragneiss (>20% granitic mobilisate).

In thin sections, metabasalt and mafic gneiss are fine to medium grained and have a well-developed granoblastic texture. The major mineral phases are green hornblende (60-80%) and plagioclase (10-20%). Clinopyroxene, orthopyroxene, olivine, and garnet and biotite porphyroblasts are observed in places. There are also small amounts of sphene and disseminated opaque minerals (magnetite or pyrite, 1-5%), as well as secondary minerals associated with hydrothermal alteration such as quartz, carbonates and epidote. In a few places, traces of chalcopyrite and native copper are observed. Three volcanogenic massive sulphide-like gold showings were found in the Nantais Lake area (Nantais 1, Nantais 2 and Cabane), suggesting a prospective environment for this type of mineralization (Labbé and Lacoste, 2001).

Thickness and distribution

The Nantais Complex groups four volcano-sedimentary belts elongated along an N-S axis. The latter outcrops between the Vergons Lake area to the north (sheet 34H04) and Couture Lake area to the south (sheet 35B03) as thin klippes a few kilometres thick, up to nearly 40 km long. The belt known as the Headwind Belt (Cadieux et al., 2002, 2004) forms pockmarks (tight synforms) between Nantais Lake and Headwind Lake (sheets 35G01 to 35B10).

Dating

U-Pb dating on zircons of a felsic volcaniclastic rock of the Nantais belt (sheet 35A13) yielded an age of 2775 ±5 Ma, interpreted as the age of emplacement (David et al., 2009). Within 1 km, a staurolite schist at the edge of the klippe yielded a metamorphic age of 2703 ±1 Ma (Percival and Skulski, 2000).