Metal zonation in alteration assemblages at the volcanogenic Stollberg Fe–Pb–Zn–Mn(–Ag) skarn deposit, Bergslagen, Sweden

Abstract

The Palaeoproterozoic Stollberg Fe–Pb–Zn–Mn(–Ag) skarn deposit in the Bergslagen mining district in Sweden is hosted by volcanogenic sandstones to siltstones and carbonate rocks. The host rocks and the underlying porphyritic rhyolites were intensely altered during the ore-forming event. Skarn formation and precipitation from hydrothermal fluids of ore minerals were concentrated at boundaries and in transitional zones between the volcanogenic sediments and carbonate rocks. Subsequent medium-grade metamorphism formed the present mineral assemblages: the altered footwall rhyolites are characterised by gedrite- and biotite-rich zones, the footwall volcanogenic sediments by biotite and biotite-garnet assemblages and the transitional zones by biotite–garnet, amphibole-garnet ± staurolite ± cordierite ± biotite and olivine ± pyroxene ± garnet ± talc assemblages. The latter three alteration assemblages are associated with pyrrhotite ± sphalerite, pyrrhotite ± sphalerite ± galena ± magnetite and sphalerite-galena mineralisations, respectively, and form a metal and mineral zonation. The change in mineral compositions is interpreted as a reflection of bulk rock composition. Compared with the footwall biotite-rich volcanic rocks, the skarn is enriched in Si, Fe, Mg, Ca, P, As, Cl, Cu, Mo, Ni, Pb, Sr, V, W, Y and Zn, and depleted in Ba, Na, K and Rb. Skarn considered to be proximal in relation to the ore-forming, hydrothermal system is richer in As, Co, S, Y and Zn than distal skarn. Distal skarn is richer in Mg, Mn, Cu and Sr. Chemical zonations in terms of higher proximal Zn/Pb and distal Mn/Fe ratios are comparable to sulphide-rich rocks, skarn deposits and iron ore formations elsewhere. These features suggest that the Stollberg mineralisations are volcanogenic, exhalative, combined magnetite ore and base metal formations. However, alteration of the immediate hangingwall suggests that post-depositional, subsurface processes acted as well.