Mining concession number
Ore tonnage mined
Reserves (proven probable)
|The Outokumpu mine (Keretti and Vuonos mines)
0715/2a, 1503/1a (Keretti), 1807/1a (Vuonos)
Keretti 1910, Vuonos 1965
Keretti 1910-1989, Vuonos 1967-1986
Serpentinite rocks (mantle peridotites)
Submarine exhalative, Ni mineralisation by metamorphic remobilisation
Keretti 29.2 Mt, Vuonos 11 Mt
Keretti: Cu 3.3%, Zn 0.84%, Co 0.25%, S 21.3%
Vuonos: Cu 2.14%, Zn 1.31%, S 14.6%, Co 0.14%, Ni 0.17%
Keretti: Cu 956,000 t, Zn 208,000 t, Co 20,000, Au 25,300 kg, Ag 285,200 kg
Keretti new shaft
Vuonos open pit and
plant in 1970's
Keretti old mine
The eldest shaft
and plant at Keretti
|The Keretti (Outokumpu) ore was discovered in 1910 following the discovery, in 1908, of a large glacial erratic boulder 50 km to the SE of the deposit. The Vuonos ore body, 10 km NE of Keretti, had no surface expression. Its discovery in 1965 was a result of geochemical exploration. The Keretti mine at Outokumpu formed the main basis for modern mining and metallurgical industries in Finland.
The main minerals at Keretti are pyrite, pyrrhotite, chalcopyrite and sphalerite, and at Vuonos pyrrhotite, chalcopyrite and sphalerite. The main Co carriers in both of the ore bodies are Co pentlandite and pyrite. Contents of Au, Ag, Pb and Sn in the ore are 0.5, 5, 50 and 100 ppm, respectively.
The deposits are deformed and metamorphosed strata-bound masses. The Cu-occurrences were originally formed in a submarine exhalative environment on peridotitic sea floor at ca. 1950 Ma. Mineralised stockwork in cordierite-antophyllite rock below both of the massive ore bodies represents the upper parts of hydrothermal conduit(s). During early stages of the Svecofennian orogeny, the proto-ores and their hosting sequences were obducted onto Archaean basement. During the orogeny, also the disseminated Ni sulphides were formed through chemical interaction between the obducting peridotites and adjacent black schists: silicate Ni was liberated and formed Ni sulphides. Also, formation of large recumbent isoclinal folds resulted in modification of the originally flat saucer-shaped ore lenses into elongate ruler-shaped masses.
Further modification took place at a later remobilisation stage during the orogeny with much of the pyrrhotitic ore now occupying the thickest parts of the ore bodies in the form of breccia or microbreccia. Despite deformation, many primary features can still be detected in many parts of the ore.
Keretti banded ore
Keretti massive ore
Keretti, underground in 1940
Keretti breccia ore
Vuonos banded ore
Mineral assemblages in the country rocks indicate mid-amphibolite facies metamorphism. During subhorizontal thrusting, the host-rock assemblage was tectonically incorporated within a regionally extensive mica schist sequence. This was related to movement of a thrust nappe with interdigitation of an ophiolite assemblage and flysch deposited during ocean closure associated with Svecokarelian tectonism at ca. 1.90 Ga ago.
ZDROJ: Metalogenetic Survey of Finland