In the Fen complex, hematite-calcite-dolomite carbonatite ("rødberg") was formed by the action of oxidizing hydrothermal fluids on preexisting ankerite ferrocarbonatite ("rauhaugite II"). Oxygen and strontium isotope data indicate that the fluids were derived from groundwater flowing into the complex from surrounding Precambrian gneisses. During oxidation hematite and other insoluble phases were concentrated by selective dissolution of carbonates. Changes in strontium contents and La Sm ratios suggest that La and Sm were partially removed by the hydrothermal fluid. These results suggest that reactions involving groundwater convecting in hydrothermal cells set up by the intrusives themselves may play a substantial part in the post-magmatic reequilibration of carbonatites, and may lead to important chemical changes in mineral and whole-rock systems.