The Obatogamau Formation was established by Cimon (1977) in reference to the Obatogamau lakes located in NTS sheet 32G09. On the northern flank of the Chibougamau Anticline, basalt and andesitic basalt initially assigned to the lower portion of the Gilman Formation (Ludden et al., 1984; Daigneault and Allard, 1990) were grouped by Leclerc et al. (2008) in the David Member. However, by including the overlying Allard Member in the Waconichi Formation, the Gilman Formation was abandoned and the David Member was integrated into the Obatogamau Formation (Leclerc et al., 2011).

The Obatogamau Formation represents the lower portion of the Roy Group’s first volcanic cycle (Cimon, 1977; Daigneault and Allard, 1990). In the Queylus Township area (sheet 32G09), where it was originally recognized, the Obatogamau Formation consists of three informal basalt units (lower, median and upper) distinguishable by the percentage of plagioclase phenocrystals. Percentages range from 1% to 3% in the lower unit, typically reaching 20% in the median unit, and decreasing below 1% in the upper unit, which is generally aphyric. However, these divisions could not be generalized to the entire formation. The summit of the Obatogamau Formation also includes some rhyolite lenses and pyritic rhyolitic tuff. On the northern flank of the Chibougamau Anticline (sheet 32G16), the upper part of the Obatogamau Formation corresponds to the David Member (Leclerc et al., 2011). To the northwest of Chapais (sheets 32G14-200-0201 and 32J03-200-0101), the Obatogamau Formation contains a lens of felsic volcanic rocks attributed to the Pichamobi Member (Charbonneau et al., 1991).

Massive, pillowed and brecciated basalts and andesitic basalts of the Obatogamau Formation are characterized by the common presence of plagioclase phenocrystals associated with the development of a glomerophyric texture. Massive and pillowed facies are the most common. The pillows’ long axis ranges from 10 to 40 cm, but can reach more than one metre in deformation corridors. They may contain millimetric ovoid carbonate-chlorite amygdules. The brecciated facies is scarce and characterized by angular fragments ranging in size from 1 to 5 cm. The rock is uniformly green in fresh exposure with beige phenocrystal clusters (glomerorphyric texture) up to 3 cm in diameter. The rock has a dark green to orange-brown patina in shear zones dominated by chlorite and ankerite alteration, while it is rather light green to beige in areas of abundant silica and epidote. Silification is associated with intense fracturing and the increased presence of millimetric quartz veinlets. In all cases, alteration accentuates the contour of primary structures (pillow margins, flow breccia fragments). In the Kapunapotagen and Gwillim deformation corridors, mafic volcanic rocks are transformed into chlorite-sericite ± ankerite schist. Intense deformation and alteration have partially obliterated primary structures.

Obatogamau Formation 1a (nAob1a): Garnet-Pyroxene Amphibolite

Garnet-clinopyroxene amphibolites (unit Aob1a) located in the Lac Dufresne Deformation Zone, on the western edge of the La Dauversière Pluton, are the metamorphozed equivalent of massive, pillowed and brecciated mafic volcanic rocks of unit nAob1. Primary structures are still recognizable but are affected by deformation and by local migmatitization. Garnet-clinopyroxene amphibolite enclaves within the La Dauversière Pluton are fine to medium grained and banded with alternating bands rich in mafic minerals (hornblende, garnet and clinopyroxene) or in plagioclase (Roy et al., 2005).