Cordierite-mica-quartz schists in a Proterozoic volcanic iron ore-bearing terrain, Riddarhyttan area, Bergslagen, Sweden

Abstract

The Riddarhyttan Proterozoic predominantly felsic, alkali-enriched metavolcanic rocks have been affected by extensive synvolcanic hydrothermal Mg-alteration. After amphibolite facies metamorphism and deformation this is reflected in the occurrence of large elongated zones of tourmaline-bearing cordierite-micaquartz schists, spatially related to numerous volcanogenic-exhalative iron formations. Geochemical evidence suggests that seawater-based fluids caused substantial mobilization of both major and trace elements, resulting in the formation of Fe-depleted and Fe-enriched Mg-schists. The latter is commonly gradational to cordierite-anthophyllite wall-rocks. The geochemical pattern is compatible with subseafloor hydrothermal circulation leaching iron and minor amounts of base metals from the felsic volcanic rocks, the emerging fluids precipitating these elements close to exhalative vents.