The Stollberg ore field - petrography, lithogeochemistry, mineral chemistry, and ore formation

Sammanfattning: The Paleoproterozoic Fe-Pb-Zn-Mn(-Ag) Stollberg deposit is situated in the Bergslagen region of south-central Sweden. 6.65 Mt of sphalerite, galena and manganiferous skarn magnetite ore occur as disseminations and semi-massive to massive ore bodies hosted by volcanogenic sediments and carbonate rocks. Field relations, mineralogy and geochemistry suggest that the felsic volcanic rocks at Stoll- berg were originally rhyodacitic in composition. The footwall porphyries and the host-rock succes- sion were intensely altered during the mainly exhalative, ore-forming event. Later, the rocks were tightly folded and meta- morphosed in the amphibolite facies (510-560 oC at c. 3 kbar). In the altered footwall lithologies, gedrite, biotite, and muscovite formed from the original alteration minerals. Preserved chemical variation over short distances in minerals suggests that this prograde metamorphism was largely iso- chemical. The metamorphic alteration assemblages of the host rocks consist of biotite-garnet, amphibole-garnet+/-staurolite+/-cordierite+/- biotite, and olivine+/-pyroxene+/-garnet para- geneses. The variations in alteration assemblage mineralogy are related to the varying proportions of carbonate rock to volcanic debris in the pre- metamorphic assemblage, and to the character of hydrothermal alteration. Subsequently, the Stollberg rocks were affected by retrograde metamorphism and deformation at 400-500 oC and less than 3.5 kbar. Gedrite was altered to chlorite and lizardite, biotite to chlorite, and plagioclase to epidote. The mineral chemistries and high-resolution transmission electron micro- scopy observations suggest that the retrograde reactions were metasomatic. The last event to affect the rocks was brittle faulting. The combined geological and geochemical observa- tions at Stollberg suggest that the Stollberg de- posit is a volcanogenic, exhalative magnetite ore and base metal formation.