The Upper Eastmain Greenstone Belt (UEGB) was first mapped by geologists of the Geological Survey of Canada. The geological survey work of Eade (1966), covering a large area of the Eeyou Istchee James Bay region, identified a strip of greenstones north of the Otish Mountains, but did not really define its extensions. The UEGB was better circumscribed in the 1980s following the work of Hocq (1985), Roy (1985, 1988), Couture (1986, 1987a, 1987b, 1987c, 1993) and Couture and Guha (1990). Couture (1986) divided rocks of the belt into two lithostratigraphic groups: the Bohier Group, which consists mainly of sedimentary rocks, and the René Group, which comprises mostly volcanic rocks (Couture 1987a; Roy 1988). Beauchamp et al. (2018) and Beauchamp (2019) subdivided René Group volcanic rocks into four formations based on geochemical and geochronological data from their geological mapping work in 2017 (Bohier Island project) and 2018 (Cadieux Lake project), respectively. The unit name is from the René Lake located in sheet 33A08.

The René Group comprises volcanic rocks belonging to the UEGB (Opatica Subprovince). The stratigraphy of the René Group units is based primarily on geochronological data available and on the study of the best-preserved outcrops from the belt’s south and west branches (Talla Takam et al., in preparation; Beauchamp et al., 2018; Beauchamp, 2019).

The René Group includes at least three volcanic events. The first event, mostly mafic, is Mesoarchean; it corresponds to the Érasme Formation (2800 ±6 Ma; Davis and Sutcliffe, 2018b). The Roman Formation represents the second volcanic event. In the southern branch, this unit consists of felsic and intermediate volcaniclastics and volcanic rocks dated 2770 ±3 Ma (Davis and Sutcliffe, 2018b). Lithologies of the Roman Formation are more varied in the east branch of the belt than in the south branch. In the former, the formation includes mafic volcanic rocks (2770 ±6 Ma; Davis and Sutcliffe, 2018a) and minor amounts of iron formation and sedimentary rocks. The 30 Ma time gap between the emplacement of the Érasme and Roman formations could be explained by the presence of a hiatus or a major fault that obliterates a portion of the stratigraphy. Finally, in the south branch of the UEGB, the last event corresponds to the Clément Formation, which consists essentially of mafic volcanic rocks, sills, ultramafic flows, iron formation and garnet amphibolite. This last unit has not been dated, but several stratigraphic polarity indicators (pillow peduncles facing south and SSW) indicate that the Clément Formation is at the top of the volcanic pile of the south branch. By relative chronology, the Clément Formation is younger than 2770 Ma. Since the stratigraphy of the René Group is overturned, the younger Clément Formation is structurally underlying older rocks of the Roman and Érasme formations.

The Dolent Formation, located in the west and north branch of the UEGB, consists mostly of basalt and minimally of mafic-ultramafic sills, komatiite and thin beds of chert, iron formation and felsic tuff. Dated 2751 ±5 Ma (Davis, personal communication, 2019), the Dolent Formation was deposited after the Érasme and Roman formations. Its stratigraphic position in relation to the Clément Formation is not known. It could be contemporary (stratigraphic equivalent) or not with the Clément Formation.

Rocks of the René Group are generally metamorphosed to the amphibolite facies (Couture, 1987a; Roy, 1988; Beauchamp et al., 2018; Beauchamp, 2019) and have undergone local retrogressive metamorphism to the greenschist facies (Dejou, 1987; Couture, 1993; Beauchamp et al., 2018). Within the René Group, the intensity of regional metamorphism is not uniform. In general, it is higher in the central part of the south branch, in the René Lake area (sheet 33A08; Couture, 1993; Beauchamp et al., 2018).