In Karelia, sulphuric pyrite deposits are associated with sedimentary volcanogenous formations of the Early Proterozoic era (2500 ± 100 m.y.). Two formations can be isolated, viz. basalt—andesite—dacite—liparite, with the differentiated succession mentioned, and a non-differentiated basalt formation (spilite—diabase). Two main stages have been isolated in the ore origin: (1) a volcanogenous sedimentary stage (a deposit of pyrite-ore beds), and (2) metamorphogenous stage (recrystallization of pyrite ores and their metamorphic regeneration with the formation of pyrrhotite mineralization). The isotopic age of the pyrite ores, obtained by the isochronic lead method, is 2350 ±100 m.y. and that of the pyrrhotite ores 1875 ± 200 m.y.