Abstract: In the Gåsborn area, West Bergslagen, central Sweden, Mn-silicate-magnetite-jacobsite skarns were formed as the result of three successive processes. (1) Deposition of exhalative-sedimentary manganiferous iron-ore-bearing sediments together with cherts and volcanics. (2) Intrusion of a synvolcanic anorogenic granite and some slightly younger gabbros and tonalites: under influence of these intrusives the manganiferous iron-ore-bearing sediments were metamorphosed into (a) rhodonite(I)-magnetite-pyrophanite-garnet assemblages; (b) (manganiferous) hedenbergite(I)-allanite-titanite-garnet-magnetite assemblages; (c) tephroite-jacobsite-pyrophanite-garnet assemblages and (d) magnetite-bearing quartzites. (3) Release of hydrothermal fluids from the granite and subsequent alteration of the above assemblages into metasomatic infiltration skarns, consisting of rhodonite(II), garnet, hedenbergite(II), biotite, actinolite or edenite (with up to 20.11 wt.% MnO), chlorite, bementite, fluorite, helvite, rhodochrosite, hematite, rutile and accessory galena, sphalerite, wittichenite, aikinite, pyrrhotite, chalcopyrite and pyrite.

The maximum temperature (T) and pressure (P) during contact metamorphism are estimated at 550°C and 1.0 kbar respectively. The fluid under influence of which the metasomatic infiltration skarns were formed was relatively rich in Fe, Cl and F and carried little or no Mg and Mn.

During early diagenesis (beginning of stage 2) fo₂ was between the hematite/magnetite (hm/mt) and the hausmannite + hematite = jacobsite buffers (h + m = j). During stage 2, with increasing T, fo₂ changed from above to below hm/mt. Magnetite and jacobsite at some distance from the hydrothermal veins from which the metasomatic skarn-forming fluids were released, were altered during stage 3 into hematite. Magnetite in, and close to the hydrothermal veins was not altered to hematite, implying an increase in fo₂ to above hm/mt with increasing distance from these veins.