Bulletin of the Geological Society of Finland (Dec 2003)
General geology, alteration, and iron deposits in the Palaeoproterozoic Misi region, northern Finland
Abstract
The Paleoproterozoic Misi region forms the northeastern part of the Peräpohja Schist Belt in northern Finland. The area comprises mafic volcanic land sedimentary rocks, differentiated gabbros, and late-orogenic granitoids. Three geochemically different mafic volcanic units were recognised: LREE-depleted amygdaloidal lavas, slightly LREE-enriched lavas, and mafic tuffs that have a flat REE pattern. Sedimentary rocks include arkosites, mica gneisses, dolomitic marbles, quartzites, tuffites, mica schists, calc-silicate rocks and graphite-bearing schists. Two types of gabbros were identified: one with a LREE-enriched pattern and another with flat REE pattern. The age of the former is according to Perttunen and Vaasjoki (2001) 2117 ±4 Ma, whereas there is no age determination for the latter. The granitoid intrusions belong to the ca. 1800 Ma late-orogenic group of the Central Lapland Granitoid Complex. The geochemistry and the stable isotope data on mafic lavas and dolomitic marbles show similarities with the mafic volcanic rocks and marbles of the lower plart of the Kivalo group in the western part of Peräpohja Schist Belt. Peak metamorphic conditions in the region vary from upper-greenschist to upper-amphibolite facies. Three major stages of deformation were distinguished: N-S compressional D1 with ductile deformation, NE-SW compressional D2 with ductile to brittle-ductile deformation, and E-W compressional D3 with brittle deformation. Several magnetite occurrences are known in the region and four of those have been mined for iron. The ores are mainly composed of magnetite with minor haematite pyrite, chalcopyrite and bornite. Besides iron the ores contain small amounts of P, S and V as well as trace amounts of Cu, Co, Te and Au. The magnetite bodies are hosted by skarnoids within the ca. 2220-2120 Ma dolomitic marble-quartzite sequence, and highly differentiated, intensely albitised, LREE-enriched gabbro. Multistage and -type alteration is characteristic for the entire region. The styles of alteration in the region are: scapolitisation, regional and local abitisation, sericitisation and silicification associated with a major shear zone, and late carbonatisation and carbonate veining associated with brecciation of the ores and their wall rocks. Local intense albitisation and formation of skarnoids and magnetite ores took place during the pre-D1 or D1 faulting or shearing which postdate the intrusion of 2120 Ma gabbros. The iron was mobilized from the mafic to intermediate country rocks and/or marble sequence which possibly contained sedimentary iron formation. Regional alteration with the local intense albitisation and ore formation show similar features to the iron oxide-copper-gold type deposits, although the known deposits in the Misi region only contain trace amounts of gold and copper.
Keywords
