Abstract
A Palaeoproterozoic magnesite deposit in the Bergslagen ore province in Sweden is characterized in terms of petrography and major element, trace element and stable isotope compositions. The magnesite horizons are situated at the contact between a metamorphosed gabbro intrusion and felsic metavolcanic rocks, in association with iron-oxide mineralizations and intense Mg-metasomatism. The alteration of the surrounding rocks is used to explain the formation of the magnesite, and chemical dating of hydrothermal monazite constrains the timing of Mg-alteration to a pre-metamorphic age of 1911 ± 16 Ma (2σ). High Y contents in hydrothermal monazite in both metavolcanic and magnesite samples indicate a temperature of>400°C for the hydrothermal alteration. C and O isotopic composition of the magnesite points to interaction with seawater at 200–250°C. We conclude that the magnesite was formed by replacement of a carbonate precursor through hydrothermal alteration, induced by the emplacement of the gabbro intrusion.
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