Garnet and biotite are common minerals in and adjacent to metamorphosed massive sulphide deposits, but their trace element compositions are rarely used to explore for such ores. Both minerals are present in hydrothermal alteration zones metamorphosed to the amphibolite facies spatially related to semi-conformable massive sulphide horizons in the Paleoproterozoic Stollberg Zn-Pb-Ag-(Cu-Au) plus magnetite ore field, Bergslagen district, Sweden. The major-trace element chemistry of garnet in metamorphosed altered rocks, mafic dykes and sulphide mineralisation shows that garnet in garnet-biotite alteration (and high-grade sulphides) is Fe-rich (almandine ratio > 0.5) whereas garnet in skarn and garnet-pyroxene alteration contains significantly higher amounts of Ca and Mn and elevated concentrations of Co, Cr, Ga, Ge, Sc, Ti, V, Y, Zn and the heavy rare earth elements (HREEs). Chondrite-normalized REE patterns of garnet in all rock types are depleted in light REEs and enriched in heavy REEs. Garnet in sulphide-bearing altered rocks, including garnet-biotite and garnet-pyroxene alteration, shows a strong positive Eu anomaly and the highest concentrations of Ga, Ge, Mn, Pb and Zn. Rocks more distal to sulphide mineralisation typically contain garnet that exhibits no or negative Eu anomalies and lower mean concentrations of these elements and higher concentrations of Ti. Biotite shows variable Fe/(Fe+Mg) ratios with most centred around 0.5 and enrichments in Ga, Mn, Sn, Pb and Zn in and adjacent to sulphides. This suggests that garnet and biotite can be used as a vectoring tool to ore in the Stollberg ore field and potentially for metamorphosed massive sulphides elsewhere.

The Paleoproterozoic Stollberg Zn-Pb-Ag plus magnetite ore field contains SVALS-type stratabound, limestone-skarn hosted sulphide deposits within volcanic (bimodal felsic and mafic rocks)/volcaniclastic rocks metamorphosed to the amphibolite facies. The sulphide ores consist of semi-massive to disseminated to vein-network sphalerite-galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite and magnetite). Thermochemical considerations and stabilities of minerals in the systems K-Al-Si-O-H and Fe-S-O and sulphur isotope values for sulphides of δ34SVCDT = +1.12 to +5.71 ‰ suggest that sulphur most likely formed by inorganic reduction of seawater sulphate that was carried in hydrothermally modified seawater fluid under the following approximate physicochemical conditions: T = 250o–350 oC, δ34SΣS = +3 ‰, I = ∼1 m NaCl and a total dissolved S content of ∼0.01 to 0.1 moles/kg H2O. However, a magmatic contribution of sulphur cannot be discounted. Carbon and oxygen isotope compositions of calcite in altered rocks spatially associated with mineralisation show values of δ13CVPDB = −2.3 to −0.8 ‰ and δ18OVSMOW = +9.5 to +11.2 ‰, with one anomalous sample exhibiting values of δ13CVPDB = −0.1 ‰ and δ18OVSMOW = +10.9 ‰. Most carbonates in ore show lighter C and O isotope values than those of Proterozoic (Orosirian) limestones and are likely the result of premetamorphic hydrothermal alteration involving modified seawater followed by decarbonation during regional metamorphism. The isotopically light C and O isotope values are consistent with those for carbonates spatially associated with other SVALS-type deposits in the Bergslagen ore district and suggest that such values may be used for exploration purposes.

Magnetite is a common mineral in the Paleoproterozoic Stollberg Zn–Pb–Ag plus magnetite ore field (~6.6 Mt of production), which occurs in 1.9 Ga metamorphosed felsic and mafic rocks. Mineralisation at Stollberg consists of magnetite bodies and massive to semi-massive sphalerite–galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite and magnetite) hosted by metavolcanic rocks and skarn. Magnetite occurs in sulfides, skarn, amphibolite and altered metamorphosed rhyolitic ash–siltstone that consists of garnet–biotite, quartz–garnet–pyroxene, gedrite–albite, and sericitic rocks. Magnetite probably formed from hydrothermal ore-bearing fluids (~250–400°C) that replaced limestone and rhyolitic ash–siltstone, and subsequently recrystallised during metamorphism. The composition of magnetite from these rock types was measured using electron microprobe analysis and LA–ICP–MS. Utilisation of discrimination plots (Ca+Al+Mn vs. Ti+V, Ni/(Cr+Mn) vs. Ti+V, and trace-element variation diagrams (median concentration of Mg, Al, Ti, V, Co, Mn, Zn and Ga) suggest that the composition of magnetite in sulfides from the Stollberg ore field more closely resembles that from skarns found elsewhere rather than previously published compositions of magnetite in metamorphosed volcanogenic massive sulfide deposits. Although the variation diagrams show that magnetite compositions from various rock types have similar patterns, principal component analyses and element–element variation diagrams indicate that its composition from the same rock type in different sulfide deposits can be distinguished. This suggests that bulk-rock composition also has a strong influence on magnetite composition. Principal component analyses also show that magnetite in sulfides has a distinctive compositional signature which allows it to be a prospective pathfinder mineral for sulfide deposits in the Stollberg ore field.