The region where the Matamec Sakami property is located is in the interior of a large hydrographical basin draining the eastern part of the James Bay territory. The latter occupies the central part of the Superior geological province which includes four subprovinces : these are, from north to south, La Grande, Opinaca, Nemiscau and Opatica. These sub-provinces are constituted of volcano-plutonic groups and metasedimentary groups. The whole unit is cut and structurally deformed by numerous folds and shears, generally E-W to WNW-ESE as well as NE-SW.
More specifically, the La Grande sub-province consists in mafic volcanites with intrusions of diorite, granodiorite and tonalite, often interbedded with sandstone and arenite metasediments. The Opinaca sub-province consists mostly of greywackes, arkoses and quartzites; the units are strongly altered by metamorphism with the consequence that the paragneiss are rather predominating. The base rock, itself, is generally tonalite. According to Gauthier, the iron formations belong to the La Grande sub-province.
The metamorphism varies from the green schists to the amphibolitic facies in La Grande. The Opinaca metasediments are now typically metamorphosed into biotitic paragneiss. Regarding « datation », Gauthier mentions the following measures in his 2000 report : 2618 millions years (Ma) in a Opinaca paragneiss, 2632 Ma in a felsic unit of La Grande and 2788 Ma for a tonalite of the base rock.
The major character of the structural geology of the region surrounding the Matamec property is the contact between the two sub-provinces, La Grande and Opinaca, belonging to the archean province the Superior. The Matamec Sakami property straddles over that contact for a length of 35 km approximately. This contact structure has the shape of a large flattened « S », measuring over 200 km. Starting from a point, close to LG-2 reservoir, the top part of the « S » stretches out easterly for more than 100 km; from the same point, the central part of the « S » makes an abrupt curve southeasterly along a distance of about 35 to 40 km where it curves again westerly, approximately where is located the Sakami La Pointe sector, of the Matamec property. This contact zone is interpreted as a straddling reverse fault along which the eastern and younger Opinaca formations have plunged beneath the western and older formations, with the result that in the surrounding of the fault, the formation of La Grande are straddling over the Opinaca paragneiss.
This type of reverse fault has a certain resemblance with the Larder-Cadillac fault, in the surroundings of which are found the mining camps of Larder and Kirkland Lake, the Noranda, Cadillac, Malartic and Val d’Or mining camps, about 500 km south of the James Bay Sakami region.