The bedrock of the Eskilstuna region immediately to the south of Lake Mälaren (Fig. 1) belongs to a synclinorium within an old Archean range known as the Svecofennides. It comprises two supracrustal series, viz. the basal leptite-hälleflint series, which is mainly volcanic, and the overlying quartzite-graywacke-schist series. These are the oldest Svecofennian rocks encountered, and most of them have changed to gneisses, granite, and granodiorite during the Svecofennian orogenesis. The development of granite and granodiorite did not only involve granitizationsin situbut also implied large-scale homogeneizations and mobilizations often on addition of juvenile matter, mineralizers especially.