The Talvivaara deposits are located within the Proterozoic Kainuu Schist Belt, which consists mainly of quartzites, mica schists and black schists resting unconformably on the Archaean basement gneiss complex.
The Ni-Cu-Co-Zn mineralisations at Kuusilampi and Kolmisoppi are hosted almost entirely by high grade metamorphosed and intensively folded black schists. The main mineral assemblage in the black schists is quartz, biotite, graphite and sulphides. Sulphide content in the ore is typically from 15 to 25% and the main sulphides are pyrrhotite, pyrite, pentlandite, sphalerite and chalcopyrite. Roughly about 90 % of the ore is hosted by black schist and the rest 10% by metacarbonate rocks, phyllites, mica schists, quartzites, graywackes and quartz wackes.
Pentlandite contains between 75-88% of the contained nickel and pyrrhotite is the second most important mineral in terms of nickel content. Pyrite contains the main share (between 67-90%) of contained cobalt while chalcopyrite is carrying copper and sphalerite is carrying zinc. The Paleoproterozoic black schists in Finland typically exhibit low concentrations of uranium. Also the black schist hosted ore at Talvivaara contains uranium in concentrations ranging from 0.0001% to 0.004% U. Uranium occurs as thucholite which is a mixture of hydrocarbons, uraninite and sulphides. Also curite type lead uranium hydroxide has been encountered.
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