Evolution Model for the Paleoproterozoic Talvivaara Mudstone‐Hosted Ni‐Zn‐Cu‐Co Deposit in Fennoscandia

ABSTRACT The Paleoproterozoic Talvivaara mudstone is a very large sulfidic Ni‐Co deposit that records enhanced organic matter accumulation during the global Shunga Event. Here, rock geochemistry and microscale S and Fe isotope composition of in situ pyrrhotites and pyrites are studied in well‐characterised, well‐preserved drill core samples. δ 34 S values range between −17‰ and +7‰ in response to changes in the depositional environment. Organic matter (C graphite ), clay minerals and iron monosulfides (presently pyrrhotite ± pentlandite), with negative δ 56 Fe values (typically −2.5‰ to −1‰) seem to be important for the primary Ni‐enrichment, whereas syngenetic and diagenetic pyrite precipitation has a diluting effect. We propose a conceptual evolution model at 2.0–1.95 Ga where tectonic activity, basin opening and subaerial weathering of mafic–ultramafic landscape led to fluvially transported Fe‐Mn‐rich nickeliferous muds on an estuarine mudflat, and their rapid redeposition to an adjacent basin with euxinic bottom water.