The off-shore Gabon basin contains large hydrocarbon reserves in Lower Cretaceous argillaceous sandstones deposited in the rifting stage of the opening of the South Atlantic. The study area ( nearly equal 5 x 10 km) includes a N30 degrees E-trending fault system a few kilometers wide. In the rocks of this fault system, the diagenetic mineralogy consists of early siderite and abundant calcite cements, followed by late quartz, minor dolomite, and chlorite. The abundant calcite cement occludes most of the porosity. In the region adjacent to the fault system, the diagenetic mineralogy consists of early siderite and minor calcite, followed by late quartz, abundant dolomite, and kaolinite. Amounts of diagenetic cement in both regions range from ten to more than forty percent of the bulk rock volume. Mass balance based on petrographic evidence and point-counting indicates that such quantities of cement cannot have been generated locally, which thus implies considerable large-scale mass transfer. delta 18 O values for coexisting (though not clearly cogenetic) dolomite and kaolinite from the kaolinite zone suggest temperatures < 100 degrees C in late diagenesis. Published evidence from fission tracks in apatites and from fluid-inclusion homogenization temperatures indicates that the temperature during early diagenesis ( nearly equal 130-120 Ma) was 200-250 degrees C or even higher. We hypothesize that both the required diagenetic mass transfer and the observed distribution of diagenetic minerals were brought about by hot water flow. This circulation, perhaps convective, was driven by the high early geothermal gradients directly related to the opening of the South Atlantic, and was especially focused along the tensional fracture systems involved in the rifting. The abundant early calcite cement that occurs in the fracture system within the study area represents the diagenetic mineral signature of the high geothermal gradient that prevailed early in the history of the Gabon rift basin.