The felsic, mafic and ultramafic volcanic rocks of the Upper Eastmain Greenstone Belt were described in the works of Hocq (1985), Roy (1985, 1988), Couture (1986, 1987a, 1987b, 1987c, 1993) and Couture and Guha (1990). In this work, the authors included all of these rocks within the René Group. No formations had been individualized. Following mapping and geochronology work in the Bohier Island and Cadieux Lake areas, Beauchamp et al. (2018) and Beauchamp (2019) divided the René Group into four (4) formations, each corresponding to a volcanic event. The Érasme Formation was introduced to define the rocks belonging to the René Group first cycle (Mesoarchean age).

The Érasme Formation is the René Group’s oldest. It is divided into five (5) informal units. Unit mAerm1 consists mostly of basalt. The porphyritic basalt facies was individualized in the mAerm1a subunit. The basalts are interbedded with several horizons (<50m thick) of felsic and intermediate ash, lapilli and block tuff, and felsic volcanics (mAerm2). Unit mAerm3 includes the gabbros, while the mAerm4 unit represents the ultramafic Sills.

The volcanic sequence of the Érasme Formation is intruded by several types of dykes including porphyritic andesitic dykes, aphanitic felsic dykes, and felsic dykes with quartz eyes and plagioclase phenocrysts. In the Eastmain Mine area, porphyritic granodioritic dykes and sills were interpreted by Eastmain Resources (Frappier-Rivard 2015). These felsic intrusions, which were not observed during the Ministère de l’Énergie et des Ressources naturelles (MERN) mapping work, were intercepted in drilling (Frappier-Rivard, 2015).

The Érasme Formation hosts several gold deposits (Eastmain Mine – Zones A, B and C; Lac Julien; Hillhouse-Lac Julien-SE; Ola River) along the same stratigraphic horizon. These mineralizations are contained in a characteristic lithological package known as the “Mine Series package”. The Mine Series Package consists of a subplanar assemblage of felsic rocks (tuffs and flows), mafic flows, and altered and deformed ultramafic sills that dip 35° to the northeast (Frappier-Rivard 2015). The gold horizon is included in recrystallized and mineralized cherty horizons associated with massive to semi-massive sulphide lenses (pyrrhotite, pyrite, chalcopyrite, sphalerite) and silicified zones in felsic, mafic and ultramafic rocks. The Mine Series Package, which was plotted laterally for 10 km in a NW-SE direction, is a marker level for gold mineralization in the Bohier Island area. 

The mafic effusive rocks, which represent the dominant lithology of the Érasme Formation, have a dark-green colour in fresh exposure and altered patina. The mineralogical composition of these submarine lavas is relatively constant. They contain 50 to 80% green amphibole (hornblende and/or actinolite), 10 to 35% low to strongly damouritised plagioclase, 5 to 10% quartz, 5 to 10% epidote (pistachite, zoisite and clinozoisite) with varying amounts of sphene, biotite, muscovite, chlorite, calcite, zircon, magnetite, ilmenite and opaque minerals (including sulphides). Amphiboles are represented by hornblende, more rarely by actinolite. The more metamorphosed basalts contain hornblende porphyroblasts, commonly poeciloblastic, which are retrograded to epidote and chlorite. Actinolites are twinned and radial. Basalts are commonly cut by variously altered veins made of quartz, calcite, epidote, chlorite, amphibole and/or tourmaline. On the vertical gradient map of the residual total magnetic field, the majority of basalt flows are not magnetic, but some are. These magnetic flows, which form NNW-SSE to NW-SE oriented lineaments, define stratification. Basalts of the Érasme Formation have retained several primary volcanic structures. Pillows are well preserved and very common.

Peduncles, which are located at the base of the pillows, indicate a reverse stratigraphic polarity that is systematically oriented towards the SW or SSW in this unit. Pillows are commonly joint, flattened, and the edges are altered in carbonate, sericite and epidote. A few radial joints and quartz chambers filled with quartz are present in the pillows. Other volcanic structures, less common, are preserved in the mAerm1a unit: pillow breccias, massive flows and isolated pillows. Pillow breccias contain hyaloclastic material between fragments of pillows (Roy 1988). Massive flows have no volcanic structure and are difficult to distinguish from some gabbro outcrops. Some facies depicting unit mAerm1a basalt were not isolated as a geological area on the map. These are the amygdaloidal facies, the porphyritic facies and the brecciated facies that is fractured by hydrothermalism. The amygdaloidal facies is characterized by subrounded cavities, 0.1 to 0.5 cm, filled with alteration minerals. These amygdules, observed at the macroscopic and microscopic scale, are filled with epidote, calcite and sericite. The porphyritic facies is distinguished by the presence of 3-10% low to moderate damouritised plagioclase phenocrystals in a finer matrix. The brecciated facies fractured by hydrothermalism is characterized by the presence of 10 to 30% filled fractures. Some dark-green fractures are composed of hornblende that is coarser than the matrix. Other light fractures, white to beige, are filled with quartz ± epidote ± calcite.

Basalts of the Érasme Formation, especially those near and in the Mine Series Package, exhibit various hydrothermal alterations. The most common are silicification, chloritization, carbonatization, sericitization and biotitization. The 17-JM-6008 outcrop, which corresponds to the Susanna showing (Eastmain Resources), exhibits an area of intense and penetrative alteration within the mAerm1 unit. The outcrop shows a bleached pillow basalt, completely altered in silica, sericite, biotite and fuschite. There is also 25% low pinitised poeciloblastic cordierite in the edges and in the hearts of the pillows. This subalcaline basalt is characterized by an alteration rich in Si-Al-K-Cr which is typical of that observed near several metamorphosed massive volcanogenic sulphide gold deposits.

From a geochemical perspective, the diagram of Winchester and Floyd (1977) shows that the samples of mafic effusive rocks are located in the fields of andesitic basalt and subalkaline basalt (3-10 wt. % MgO). Some basalts are magnesian (10-12.5 wt. % MgO). They contain 47-58 wt. % SiO₂, 3-12.5 wt. % MgO, 0.6-1.9 wt. % TiO2 and 38-136 ppm Zr. They are of tholeitic to transitional affinity. Tholeitic basalts are characterized by flat rare earth profiles (La/Yb = 0.7 to 2.2; gd/Yb = 1 to 1.3; normalised to Chrondrite CI; McDonnough and Sun, 1995) and Zr/Y ratios of 2.2 to 3.3. Transitional basalts have fractionated profiles showing light rare earths enrichment (La/Yb = 2.2 and 5.3; gd/Yb = 1.2 and 1.6) and have Zr/Y ratios between 4.1 and 5. Transitional basalts were only identified near the Tourigny Deformation Zone (1989) and Eastmain Mine mineralized corridor (Zones A, B, C). This subunit is not mappable in size at our work scale, but it seems to follow stratification and provides a reference horizon that is located within the structural roof of the Eastmain Mine mineralization. Discrimination diagrams of geotectonic environments indicate a formation environment similar to that of N-MORB and island-arc tholeites for tholeitic basalts. An E-MORB source is proposed for transitional basalts. The Pearce diagram (2008) and Nb-Ta anomalies on the spider diagram suggest that basalts of the Érasme Formation have undergone crustal contamination.

Érasme Formation 1a (mAerm1a): Porphyritic Basalt

A porphyritic basalt unit was identified by Eastmain Resources as a result of their surface mapping and drilling work (Frappier-Rivard 2015). This unit, which is a few centimetres to a few metres, was not observed during MERN mapping work in 2017. The porphyritic basalt is continuous for 4.5 km and extends from Hillhouse hill to Zone C of the Eastmain deposit. This unit, which contains about 10% tabular plagioclase, is considered a marker level. The porphyritic basalt is located in the structural wall of the “Mine Series Package”. Drilling sections conducted by Eastmain Resources show that the porphyritic basalt horizon dips 35° towards the NE, while the entire stratigraphy dips towards the NE at 45°. Frappier-Rivard (2015) suggests that the porphyritic basalt unit represents a late synvolcanic dyke, as it cuts volcanic stratigraphy, and occurs as xenoliths in felsic dykes.