| Summary: | Abstract. The present study aims to characterize the Petäjäskoski formation in the Paleoproterozoic Peräpohja belt, and to identify the evaporite sequence through petrographic and electron microprobe analysis. The Peräpohja belt in northwestern Finland is known to host for example atypical orogenic Gold (Au)-Cobalt (Co)-Uranium(U) deposits of Rompas-Rajapalot, where evaporites have shown to play a major role in formation of these deposits. Evaporites have also important part in the genesis of various deposits worldwide, including Iron Oxide Copper Deposit (IOCG) and Sediment hosted stratiform (SSC). In order to comprehend the source of the salt rich solutions and rock types, drill core samples from Petäjäskoski were examined through petrographic and electron microprobe studies. Petrographic studies shows the major lithologies as a mudstone-sandstone-dolomite unit with collapsed breccias. Mudstones contains fine and dark layers with quartz, feldspar, muscovite, and sericite. Sandstone is recrystallized as well as well-rounded sub-arkose units with quartz, plagioclase, orthoclase, microcline, biotite, muscovite, and trace zircon and hematite. Dolomite is recrystallized and talc rich while conglomerate is monomictic with mudstone clast and quartz, feldspars, and talc as major minerals. Microprobe analysis of selected samples showed in accessory halite, anhydrite, sylvite and bischofite which are supporting the evaporitic origin the Petäjäskoski formation. Stromatolitic dolomite, flaser and lenticular bedded mudstone, brecciated unit, anhydrite pseudomorphs, and fractures filled with calcite pseudomorphs after evaporite minerals suggests shallow ramp tidal flat/lagoonal depositional condition, where conditions were suitable for evaporite sedimentation. Further support is indicated by collapse breccias within the formation making these rocks oldest evaporitic rocks in the Fennoscandian shield.
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