A fergusonite sample from the Berere region in Madagascar was studied in detail with a wide range of analytical methods, including optical and scanning electron microscopy, transmission electron microscopy techniques, electron probe microanalysis and mapping, powder X-ray diffraction, and Raman, photoluminescence and infrared spectroscopy. The specimen contains high concentrations of actinide elements (U up to 6.9 wt%, Th up to 3.0 wt% as oxides) and, as a consequence, it is highly radiation-damaged. The sample has experienced intense, low-T chemical alteration through fluid-driven replacement reactions. The altered areas have sharp boundaries to their neighbouring host and are mainly located adjacent to large fractures. High-resolution element distribution maps show the complexity of the compositional changes. Altered areas are generally enriched in Si and Ca, and depleted in Y; however, most elements show variable trends and heterogeneous distribution patterns pointing to a non-uniform, presumably multi-step alteration history. Most remarkably, the actinides U and Th were comparably immobile; their concentrations having remained almost constant, being only mildly affected by the alteration reaction. This appears to support the potential suitability of the fergusonite-group minerals as host phases for the long-term immobilisation of nuclear waste.