AbstractManganese mineralization occurs throughout the greywackes and pelites of the Harlech Grits Group (Lower/Middle Cambrian) of the Harlech area, North Wales. It is most intense at the base of the Hafotty Formation where it forms a discrete ore bed, and in the upper half of the Gamlan Formation where it forms coticules (quartz–spessartine metapelites). Low‐grade mineralization also occurs in sediments of equivalent age at St. Tudwal's Peninsula. The mineralization is syngenetic/syndiagenetic. Manganese was probably supplied to the basin by submarine exhalative hydrothermal solutions, and may have precipitated directly as an oxide/carbonate sediment, or by replacement of pre‐existing carbonate. Early diagenetic remobilization of manganese oxides, precipitation of manganese carbonates, and replacement of clastics and volcaniclastics has played an important role in manganese concentration in the sediments. In the Harlech ore, many original depositional textures are preserved in spite of lower greenschist facies metamorphic recrystallization of the diagenetic mineral assemb