MARINE AND SUPERGENE ALTERATION PROCESSES IN A CHLORIT IZED

that it had been subjected to two alteration episodes of different origin. Structural and petrographic evidence showed that the alteration processes operated per descensum from the bedrock surface down to shallow depth, thus eliminating any hydrothermal episode due to fluid ascension through deep fractures. The rock was first altered into interstratified chlorite-vermiculite minerals with potassic interlayers. These potassic lay minerals occur in an originally K-deficient rock and seem to have been generated by a marine alteration episode; this episode appears to be contemporaneous with the Cenomanian transgression which covered the amphibole-schist wi h sandy-clayey deposits. The potassic lay minerals were replaced uring a later meteoric alteration episode by kaolinites, iron oxides, and smectites with Mg,Ca interlayer cations. The major difficulty when dealing with rock alteration is to separate several alteration processes of various origins (e.g. hydrothermal, marine or supergene) from a group of observed alteration patterns which may overlap in the same rock. Each of these episodes generates pecific clay minerals, which are easily recognizable when isolated, but more difficult to distinguish when overlapping or replacing one another in the same rock sample