The aim of the present work is to verify along the East Coast of South America the model which links tensional tectonics related to passive margin formation with the emplacement of some ore deposits and the occurrence of diagenetic processes in nearby sedimentary basins. With this in view, two hundred clay samples were collected, prepared and analyzed by XR diffraction to select those useful for dating, from Patagonia to northem Brazil and in the Paranâ Basin. The K/Ar and Rb/Sr ages obtained show: - a diagenetic event dated back to neariy 200 Ma and coeval with the first Gondwana fragmentation; - a second high temperature diagenetic episode, the age of which is equal to that of the Paranâ basalts, from 138 to 129 Ma; - lower temperature diagenetic mechanisms altering the basalts, and F and U vein deposits in the range 120 to 50 Ma. These two later processes enter the whole history of tensional tectonics related to the South Atlantic opening. ln contrast, three other mineralized occurrences find their origin in various mechanisms and their age is different from that of South Atlantic tectonics. The diagenetic alteration of the Paranâ basalts is regarded as responsible for the whole rock K/Ar conventional ages scattering. Ali results lead to the conclusion that the processes related to tensional tectonics may be considered, along the South Atlantic too, as accounting for some vein ore deposits and for diagenetic alteration in large nearby sedimentary basins.
