In the Kopparberg county, small sulphide ores containing sphalerite, galena, pyrite, pyrrhotite as well as a gangue of calcite, baryte and fluorite are known around Boda and on the island Sollerön in Lake Siljan. The ore has developed around tectonized zones in Ordovician limestones as disseminations or as hydrothermal veinfillings with alternating layers of ore minerals and gangue. A replacement of matrix by sulphides has also occurred locally in the Obolus conglomerate, the lowest member of the sedimentary formation. The sphalerite shows colloform banding and the colour of the different zones varies from black or dark brown to light yellow. As a rule the dark zones are narrow and have a much higher iron content than the light coloured zones. Important exceptions are some dark black-brown, iron poor narrow zones which have a comparatively high copper content. The formation of the colloform banded sphalerite is discussed and it is assumed that the zinc sulphide has been precipitated as a gel or as cryptocrystalline particles from a neutral or slightly alkaline solution. Because of variations in the iron content of the hydrothermal solutions, the precipitated zinc sulphide may contain more or less iron. Varying Fe/Zn ratio in the solutions may also have changed the original composition of the surface layer of the precipitated zinc sulphide. Etched polished sections show that the zinc sulphide has crystallized as sphalerite. The formation of the ore is connected with the large tectonic deformations of the Paleozoic sedimentary rocks in the Siljan area.