Biostratigraphy and Sr-87/Sr-86 ratios indicate that phosphatisation of the Middle Eocene reef limestones occurred in the Early-Middle Miocene (similar to 16-23 Ma) on Carter and Hirondelle Seamounts and in the Late Miocene (similar to 6 Ma) on Lighthill Seamount. Upwelling near the palaeo-equator probably played a vital role in diagenesis by promoting high biological productivity and a large flux of phosphorus to pore waters in the seamount limestones. Phosphorites are probably widespread on other sediment-capped seamounts and fracture-zone ridges in the Equatorial Atlantic.