Alteration of ferromagnesian minerals in felsic Archean rocks apparently was accomplished by diffusion of Gunflint Formation-derived fluids across the Proterozoic–Archean contact. The geochemical evidence documented here is consistent with slow mineral–fluid exchanges under diagenetic or low-grade metamorphic conditions. Chemical changes in mafic minerals include additions of iron, manganese, and water and losses of silica, calcium, and magnesium. The reaction layer extends ~3 m into felsic rocks immediately underlying the Proterozoic–Archean contact. From the evidence presented and the experimental information available it is concluded that these chemical changes occurred as Gunflint Formation-derived, iron-rich fluids diffused into underlying rock over a time frame of 105–107 years. The thermal energy for the reactions was most likely generated by the weight of the overlying sediments and water column.