Genesis of palaeoproterozoic iron skarns in the Misi region, northern Finland

Tero Niiranen, Irmeli Mänttäri, Matti Poutiainen, Nicholas H. S Oliver, Jodie A Miller

    Forskningsoutput: TidskriftsbidragArtikelVetenskapligPeer review


    Sodic alteration is widespread in Palaeoproterozoic greenstone and schist belts of the northern Fennoscandian shield. In the Misi region that forms the easternmost part of the Perdpohja schist belt, several small magnetite deposits show intimate spatial relationships with intensely albitised gabbros, raising the possibility that regional sodic alteration released iron, which was subsequently accumulated into deposits. Two of these magnetite deposits, Raajrvi and Puro display a typical paragenesis as follows (from oldest to youngest): (1) diopside, (2) actinolite/tremolite-magnetite chlorite, biotite, and (3) serpentine hematite, chlorite. Mass balance calculations suggest that significant amounts of Fe, Ca, Mg, K, Cu, V, and Ba were lost, and Na and Si gained during the albitisation of the gabbro, at near-constant Al, Ga, Ti, and Zr. Significant amounts of Si, Ca, Fe, and Na were enriched in the formation of skarn related to magnetite deposits. Fe and V leached from country rocks deposited during the skarn-alteration and formed the vanadium rich iron deposits while Cu passed through the system without significant precipitation due to low sulphur fugasity. Variations in Na, Ca, Mg, K, and Ba contents reflect the composition of the infiltrating fluid during alteration. Conventional heating-freezing measurements and proton-induced Xray emission (PIXE) analyses of the fluid inclusions related to actinolite/tremolite-magnetite stage alteration indicate that the fluids that caused the alteration and the Fe-mineralisation were complex, oxidised, highly saline H2O +/- CO2 fluids that contained high amounts of Na, Ca, K, Fe, and Ba as well as elevated concentrations of Cu, Zn, and Pb. The oxygen isotope thermometry suggest that temperature during the Fe-mineralisation stage was between 390 and 490 degrees C. Calculated delta(18)O(fluid) values of 6.1-9.8 parts per thousand. SMOW and delta(13)C values of calcites in the ores and skarns were between -7.7 and 10.9 parts per thousand. PDB and most likely reflect admixture of C-13 depleted, possibly magmatic fluids with the marble wall rocks that show delta(13)C(calcite) values of 13 parts per thousand PDB. The SIMS U-Pb data on the zircons in the albitised gabbro next to the Raajarvi and Puro deposits suggest that intrusion of the gabbro took place at 2123 +/- 7 Ma and was accompanied by the formation of diopside skarn. The TIMS data on the metasomatic titanites related to sodic alteration yielded ages of 2062 +/- 3 and 2017 +/- 3 Ma. Iron was probably stripped from the mafic country rocks by sodic alteration between 2123 and 2017 Ma, driven by repeated brine influxes. Subsequently, the metal-rich brine was focused by a fault system and the iron was precipitated from this fluid by a combination of wall rock reaction, fluid mixing, and a drop in the temperature.
    TidskriftMineralium Deposita
    Sidor (från-till)192-217
    Antal sidor26
    StatusPublicerad - 2005
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