Bulletin of the Geological Society of Finland (Dec 1991)
Mineralogy, geochemistry and metamorphism of the early Proterozoic Vähäjoki iron ores, northern Finland
Abstract
The Vähäjoki iron ores occupy the uppermost part of the Karelian (Early Proterozoic) quartzite-dolomite sequence deposited on the late Archaean basement in southern Lapland. The ores occur as magnetite veins brecciating dolomite and as magnetite disseminations in chlorite and mica schists. Their average iron content is 40%, and they contain up to 3.8% P2O5 features which together with the division of the ores into 14 small bodies make them uneconomic. The only main ore mineral is magnetite, with which there occur accessory ilmenite, haematite, pyrite, chalcopyrite, pyrrhotite, arsenopyrite and cobaltite with small gold inclusions. The main gangue minerals classified by microanalysis are dolomite or Fe-dolomite in the dolomites, ferri-tremolite, tremolite, ferri-actinolite and ferrian actinolite, cummingtonite, magnesio-hornblende and tschermakitic hornblende in amphibole-bearing hosts rocks, green Mg-rich biotite, brown Fe-rich biotite and Ba-bearing (BaO 0.18‒7.9%) biotite, and chlorite in chlorite and mica schists. Major and trace elements were analyzed in 18 samples. The dolomites contain 24‒53% CaO and 3‒20% MgO, while the amphibole rocks, amphibole schists, chlorite schists and mica schists are chemically rather similar, containing about 13‒27% Fetot and 7‒21% MgO, the amphibole-bearing varities being a little richer in these elements than the phyllosilicate-bearing ones. The ore samples from magnetite matrix of the breccia type contain 71‒85% Fe2O3 and 0.05‒0.2% P2O5. The garnet-biotite and calcite-dolomite geothermometers and the mineral chemistry of the Ca-amphiboles suggest that the Vähäjoki iron ores were metamorphosed under greenschist ‒ amphibolite facies conditions (T = 465 °C and P = 2‒4 Kbar). Flunctiation in metamorphic grade is reflected in the change in amphibole composition from tremolite-actinolite to tschermakitic hornblende. The Vähäjoki iron ores were remobilized and enriched during early Proterozoic regional metamorphism and deformation, forming an epigenetic iron ore.
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