An X-ray diffraction and electron microscopy study of very-low-grade metamorphic clay mineral assemblages of Upper Carboniferous (Stephanian) mudrocks from the coal-bearing Ciñera-Matallana basin of NW Spain reveals a complex mineralization history related to localized igneous and hydrothermal activity associated with strike-slip-faulting. This thermally active, pull-apart basin experienced peak temperatures (up to ca. 296 C) that reached anchizonal grades in areas of high heat and fluid flow, with hot hydrothermal growth of well crystallized 2M1 illite–muscovite, chlorite, and pyrophyllite occurring pre- and synchronous with Late Carboniferous/Early Permian folding and faulting of the sedimentary sequence. No burial pattern could be recognized in the clay mineral reactions with stratigraphic depth due to the complexities of hydrothermal alteration. Clay mineral growth was controlled by the location of igneous activity and the circulation of CH4-bearing fluids released during the maturation of coal seams and dispersed organic matter. Extensive retrograde reaction to illite-smectite (1Md polytype) with abundant R1 rectorite (50–50) in altered rocks adjacent to igneous intrusions and along faults is attributed to the circulation of cooler (<140 °C) hydrothermal fluids. These and younger fluids were also probably responsible for the extensive crystallization of kaolinite in pore spaces within the mudrocks and related quartz-carbonate veins.