The late Archean Hattu schist belt, located within the Ilomantsi greenstone belt, formed at approximately 2750 Ma and is composed of felsic volcanic and epiclastic deposits and hosts multiple diverse orogenic-style gold deposits. The Korvilansuo prospect is located within the southern portion of the schist belt and is situated between the Kuittila tonalite and the Silvevaara granodiorite intrusions. The deposit hosts hydrothermally-altered gold-bearing quartz-tourmaline veins located near the Korvilansuo shear zone.
Korvilansuo is enriched in important ore minerals and metals such as gold, sulfides, and tellurides. A combination of structural, metamorphic and hydrothermal events occurring within the region contributed to mineralization of the deposit. The metamorphic grade of the deposit reached the upper greenschist to lower amphibolite facies based garnet-biotite geothermometry indicating peak temperatures of 570 ± 30 °C. Pressures of 2.9 – 4.2 kbar occurred at Korvilansuo at peak conditions. Hydrothermal alteration is highly prevalent within tonalite and mica schists. Extensive structural deformation is visible as the deposit lies within the brittle-ductile transition zone.
Gold appears throughout the deposit, occurring in fractures of tourmaline-quartz veins, as well as disseminated throughout the mica schist samples alongside sulfides and as telluride compounds. Sulfides encountered in the Korvilansuo area include: pyrite, pyrrhotite, chalcopyrite, gersdorffite, arsenopyrite, and other minor phases. Tellurides occur primarily as tsumoite and other telluride minerals occur as residual disseminated phases within pore spaces.
Three main fluid types are found within the deposit consisting of high-vapor, high salinity (approximately 27.7 wt. % NaCl), and low-salinity, aqueous-carbonic fluids. Vapor-rich fluids are the oldest, while the NaCl-rich inclusions were determined to be older than aqueous-carbonic fluids and may be responsible for transporting ore. Homogenization temperatures of the late low-salinity, aqueous-carbonic fluids are approximately 323 °C, with hydrothermal chlorites recording mineralization temperatures of up to 450 °C. Temperatures therefore suggest that mineralization occurred after peak metamorphic conditions.
