The works on the Waconichi Formation resulted in a distinction between the Lemoine and Queylus facies (Daigneault and Allard, 1990; p. 113): “We define the Lemoine-type Waconichi, which essentially consists of rhyolitic rocks on the edge of the Chibougamau Pluton, and the Queylus-type Waconichi, which consists of essentially pyroclastic rocks at the interface of the Obatogamau and Gilman Formations.” The name Lemoine Member is associated with the Lemoine Township and volcanogenic massive sulphide (VMS) mine of the same name. A lithostratigraphic synthesis of the Lemoine Mine area based on detailed mapping and multiparameter study of drillings (Lafrance et al., 2006; Roy et al., 2007; Boulerice et al., 2014; Mercier-Langevin et al., 2014; Ross et al., 2013; Ross et al., 2014) leads to an informal division of the Lemoine Member into lower and upper parts and allows for the the definition of up to ten informal subunits.

A detailed description of the Lemoine Member’s units can be found in the Daigneault and Allard (1990) report on page 117: “Rocks of this unit generally contain 2-10% quartz and plagioclase phenocrystals. Quartz is more abundant, making up on average more than 70% of phenocrystals. Their diameter is smaller than that which characterizes rhyolitic porphyries, varying around 1 mm. The interpretation of these rocks as flows is difficult to demonstrate with certainty. The unit is characterized by the presence of flow structures such as flow lines, which are well developed and locally closely resemble primary bedding. There are sericite-rich layers characterized by a high concentration of subspheric cavities interpreted as vesicles. Layers are thin (4-8 cm) and contain about 30% vesicles with an average diameter of 1 cm but locally a few centimetres. The edge zone is characterized by a cream colour likely due to high sericite content. These layers almost mimic stratification in tuffs. However, the irregular and closed geometry defined by margins contest this interpretation. Lobbed structures can reach 10 m in size; some are less than one metre. The larger ones have an irregular shape and the smaller ones are usually elliptical. It is possible that an emerging rhyolitic dome will produce this type of structure by budding of the warm rhyolitic mass upon contact with water. The high viscosity of rhyolitic fluid could be responsible for the complex geometries that these structures describe and which resemble lobes.”

The Lemoine Member is divided into two informal units based on the change in geochemical affinity of volcanic rocks on either side of the sulphide cluster represented by the Lemoine Mine mineralized deposit (Ross et al., 2014). The lower unit of the Lemoine Member (nAlem1) consists of six basal subunits, while the four top subunits are grouped in the upper unit (nAlem2).

Lemoine Member 1a (nAlem1a): Quartz-Plagioclase Porphyritic Dacite

The base of the Lemoine Member’s lower unit corresponds to subunit nAlem1a. This is a horizon of dacite with bluish quartz (2-10%, 2-6 mm in diameter) and plagioclase phenocrystals (3-10%, 1-5 mm in diameter) similar to subunit nAlem1e, but with significantly higher Zr content (500-800 ppm Zr; see Figure 5 of Lafrance et al., 2006). Quartz phenocrystals are typically smaller and less abundant than plagioclase crystals (Boulerice et al., 2013). Hyaloclastites, lobe edges and laminar flow structures recognized in drilling and outcrop indicate that this unit is of extrusive origin (Riverin and Boily, 2003; Ross et al., 2014). The unit shows locally intense chlorite-sericite hydrothermal alteration and is therefore an interesting exploration target for sulphide clusters comparable to that of the Lemoine Mine (Riverin and Boily, 2003; Boulerice et al., 2014).

Subunit nAlem1c consists of a package of aphyritic dacite and rhyolite associated with volcaniclastic rocks of the same composition (Lafrance et al., 2006). In the eastern portion of the Lemoine Member’s lower unit, it overlies subunit nAlem1b.

Subunit nAlem1d consists of aphyritic basalt and andesite flows and derived volcaniclastic rocks up to 100 m thick (Lafrance et al., 2006; Ross et al., 2014; Mercier-Langevin et al., 2014). It is overlying subunits nAlem1b and nAlem1c.

In the sommital portion of the Lemoine Member’s lower unit, subunit nAlem1e consists of quartz (7-8%, 1-2 mm in diameter) and plagioclase (3-7%, 1-3 mm) porphyritic dacite, invariably of massive appearance in drilling, suggesting an intrusive origin.

Subunit nAlem1f occurs as andesitic to dacitic hypovolcanic domes and dacitic sills characterized by bluish quartz (2-10%, 2-6 mm in diameter) and plagioclase (3-10%, 1-5 mm in diameter) phenocrystals. Unconformable contacts, as well as weak hydrothermal alteration, suggest an intrusive origin (Ross et al., 2014). Two dacite dykes <5 m thick and particularly rich in quartz and plagioclase phenocrystals have also been recognized in the Lemoine Mine area.