This paper deals with the formation of magnetite in connection with sulphurization and serpentinization of skarn iron ores in Early Proterozoic (1.9 Ga) metasediments and mafic metavolcanics in northern Sweden. The ores contain Ca-Mg and Mg silicates which are characterized by a high Mg/(Mg + Fe) ratio, being mainly higher than 0.8. This feature is possibly due to an original high content of sulphur in the chemically precipitated ore material. During regional metamorphism involving heat transfer and recrystallization there occurred internal reactions. Sulphur reacting with primary, more Fe-rich silicates, gave rise to more Mg-rich silicates and a simultaneous formation of iron sulphides and magnetite. Calculations made on the Stora Sahavaara ore show, however, that the sulphide-silicate reactions could only have attributed to relatively restricted amounts of magnetite. The magnetite of the ores often contains small amounts of magnesium, at maximum 6 weight% Mg. The magnesium-bearing magnetite is mostly associated with serpentine which is an alteration product of olivine, tremolite and diopside. The occurrence of the magnesium-bearing magnetite is attributed to serpentinization which released iron from the silicates, and their high magnesium/iron ratio could in part be related to this process. However, the formation of magnetite in connection with serpentinization is only known in some deposits, and it is therefore plausible that serpentinization, in similarity with the sulphide-silicate reactions, has given rise to restricted amounts of magnetite in comparison to the amount of magnetite (or its precursor) which was deposited as a primary chemical precipitate.