Geochemistry of wall-rock alteration and of mixed volcanic-exhalative facies at the Proterozoic Stollberg Fe-Pb-Zn-Mn(-Ag)-deposit, Bergslagen, Sweden

Abstract

The Proterozoic Fe-Pb-Zn-Mn(-Ag) ores at Stollberg in Bergslagen ore province, south central Sweden were formed as stratiform, stratabound exhalites in a volcano-sedimentary environment. The surrounding rhyolitic rocks were altered hydrothermally. This alteration predominantly affected the foot-wall rocks. The metamorphic mineral assemblages in the altered rocks were formed during a subsequent episode of regional metamorphism. They comprise gedrite, biotite and muscovite, indicating chlorite and sericite as original hydrothermal alteration products. Structures resembling hydrothermal breccias support an alteration model. More than 2500 samples have been taken from drill-cores and outcrops, and analyzed for major and trace elements. The geochemistries and the densities of the least and the most altered samples have been used to estimate gains and losses during hydrothermal alteration. Fe, Mg, Mn, Ti and K have been added, whereas Si, Ca, and Na have been removed. Plotting elements along the length of a drill-core, cutting the karnaltered exhalitic horizon, shows that the exhalites comprise potassium-rich meta-volcanic material mixed with chemical precipitates. Relative to unaltered volcanites these rocks are richer in Si, Fe, Mg, Ca, Mn, Pb and Zn, and poorer in Ti, Al, Ba, Na and K. The metamorphic mineral assemblages (ortho- and clinoamphiboles, garnet, diopside, epidote, gahnite, cordierite, staurolite, serpentine/olivine, fluorite, calcite, quartz, micas and feldspar) are developed in rocks with a bulk marly composition.