Revised lithostratigraphy of the Paleoproterozoic Porkonen banded iron formation and associated sulphidic shales, Northern Finland

The Palaeoproterozoic (~2.0 Ga) Porkonen Formation in the Kittilä greenstone belt (or terrane) in northern Finland is an exceptionally well-preserved succession of banded iron formation, shale-dominated units and submarine lava. The Formation comprises six distinct lithological units that from the base upwards include: a. massive chert b. banded magnetite±siderite-chert (iron ore) c. (Mn)siderite±chlorite-chert d. iron sulphide-rich black shale and breccia (3) tuffaceous shale and tuff and, (4) andesite/rhyolite.

The iron ore has a thickness of approx. 8 meters, laterally discontinuous (<12 km) and an average Fe content of 30.5%. The overlying manganese-rich siderite horizon (3-5m) has an average of 2% Mn, whereas the following 7m thick, sulphide-rich black shale averages at 27% Fe showing a maximum content of 47.9% Fe in one-meter interval. The latter shows elevated contents in Zn, Cu, Ni, Ag, Cd and Au.

All lithological units consist of low-grade metamorphic mineral assemblages, exhibiting primary sedimentary and early diagenetic features, and have contacts typically conformable and gradating.

Single-grain layering of micron-sized magnetite alternating with micritic carbonate, apatite and chert characterize the iron-oxide bands whereas primary colloidal textures, abundant contractional fractures, graphite-stylolites and crackle microbreccia characterize the chert bands. In the shale-dominated units the rhythmic, planar-parallel lamination of authigenic carbonates, diagenetic pyrite, and chert and pyrite nodules, have been contorted by penecontamporenous soft-sediment deformation manifested by slump structures, load casts, and intraformational breccia with angular graphite fragments in a massive iron-sulphide matrix.

The lithostratigraphy may predict a layered -in oxygen ocean with hydrothermally-dominated Fe-Mn-rich deep water mass.