The major-trace element chemistry of garnet and biotite in metamorphosed hydrothermal alteration zones, Paleoproterozoic Stollberg Zn-Pb-Ag-(Cu-Au) ore field, Bergslagen district, Sweden: Implications for exploration

Abstract 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.