The name “Lac Caste Group” was originally attributed to these lithologies by Brett et al. (1976). It comes from a small lake in lots 42 to 49 of range IX and X in Cléricy Township, NTS sheet 32D07. James and Mawdsley (1927), Cooke et al. (1931), Norman (1944), Sharpe (1961) and Dawson (1966) had long identified these sedimentary units and their vast expanse. Dimroth et al. (1973) and Dimroth and Rocheleau (1979) also used this name. Goutier (1997) subsequently proposed the name “Caste Formation”, which is part of the Kewagama Group.

Biotite schist is usually well-foliated and is mainly composed of fine-grained equigranular quartz surrounding large flakes of biotite and sodic plagioclase. Grains have an average size of 0.3 mm. Musovite, chlorite, epidote, apatite, sericite, clinozoisite, zircon and pyrite are accessory minerals (Brett et al., 1976). Chlorite commonly replaces biotite. Biotite schist is medium to fine grained. In altered surface, it is usually grey with iron oxide stains; in fresh exposure, different shades of grey are observed. The general alignment of biotite flakes in the quartz-feldspar matrix defines schistosity. Banding is well developed locally, with bands ranging in width from 1 to 10 cm. Some bands contain mainly quartz, accompanied by biotite.

Schist foliation has an approximate but fairly constant E-W direction, but it locally follows the La Motte and La Corne batholith edges in a N-S direction. Locally, schist can be intensively folded and shows well-developed drag folds, thin ptygmatitic quartz veinlets and quartz veins and clusters. Quartz veins are commonly parallel to or cut schistosity at a sharp angle. Several veins are boudinaged and folded in complex ways. Generally E-W-orientated bedding can be seen in a few places in sheet 32D08. This bedding has the same dip as schistosity but is locally more abrupt. Stratification and bedding commonly dip 30 to 60° northward. Summits generally appear to be facing southwest (Brett et al., 1976).

Partial assimilation of sedimentary rocks by batholith masses is commonly observed. Such examples of partial assimilation, manifested by migmatization, were observed in sheet 32D08 and in the northern portion of sheet 32C04. Biotite schist then occurs as elongated fine-grained enclaves composed of biotite, quartz and feldspar phenocrystals with fuzzy edges.

There does not appear to be any significant change in the degree of metamorphism of schists near batholiths. Grade varies from amphibolite to upper greenschist away from batholiths. Considering that none of these sedimentary units are more than 1600 m away from batholith contact, it can be assumed that all have undergone approximately the same degree of metamorphism (Brett et al., 1976). Enclaved biotite schists are similar to schists found outside batholiths. A well-defined chilled margin between biotite monzogranite of the La Motte massif and this formation is not observed, except for the presence in several places of staurotide and garnet in these schists.