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Kauko Laajoki ¹

Affiliations
1 University of Oulu, Department of Geology, Linnanmaa, 90570 Oulu, FI

Abstract

Precambrian banded (cherty) Iron-Formations are encountered in Finland in four main rock associations: 1) Late Archaean (c.2800 Ma) occurrences hosted either by greenstones or turbiditic schists in eastern and northern Finland. 2) Early Proterozoic (Lapponi) (>2200 Ma) iron-formations associated closely with metavolcanics in Central Lapland. 3) Early Proterozoic (Karelia) (c. 2050 Ma) metasedlment-hosted occurrences in Kainuu. 4) Early Proterozoic (Svecofennia) (c. 1900 Ma) occurrences hosted either by metavolcanics or metasediments in southern and central Finland.

The occurrences are mostly relatively small and of low grade (Fe_tot mostly below 30%). With the exception of central Lapland, the occurrences are relatively highly metamorphosed. representing the medium grade as revealed by the fact that grunerite or other iron-rich amphiboles and garnet are commonly present; pyroxene is encountered in only one locality. The low grade Kittila occurrences in Lapland are distinguished by stilpnomelane, biotite, Fe-chlorite and carbonate with minor minnesotaite.

Chemically, the Finnish occurrences are like the iron-formations described from the other Precambrian shields. The Kainuu occurrences and at least a part of the Lapponi occurrences are. however, unusually rich in phosphorus; the average P₂0₅, content being c. 2.5% at Puolaoka. Phosphorus occurs as discrete phosphorite (apatite) bands some millimetres thick. Manganese-bearing types are encountered in Kittila and Kainuu.

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Kauko Laajoki ¹, T. C. Devaraju ²

Affiliations
1 Department of Geology, University of Oulu, Linnamaa, 90570 Oulu, FI
2 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN

Abstract

Electron probe microanalytical data is given for Fe-chlorite (31% FeO) and grunerite (31.4% FeO) occurring in the mixed oxide-silicate facies iron-formation and for ankerite (av. 17.5% FeO), siderite (av. 44,2% FeO) and stilpnomelane (31.6% FeO, 5.9% Al₂O_3, 2% K₂O) in the carbonate iron-formation of Chiknayakanhalli Schist belt. While Fe-rich chlorite, ankerite and siderite appear to represent recrystallized diagenetic and primary sedimentary phases of the same minerals, grunerite has formed from metamorphic alteration of minnesotaite and stilpnomelane from a montmorillonite (nontronite?)-like precursor. The minerals identified in the iron-formation fit into the overall greenschist to low amphibolite facies metamorphism witnessed by the schists.

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