Stable isotope and petrographic analyses of diagenetic calcite ("cone-in-cone" and "beef") veins from British Jurassic and Lower Cretaceous shales have been used to determine the environment of precipitation of fibrous calcites. Successive growth took place by antitaxial displacive addition at the vein margins, away from primary sedimentary laminations or early diagenetic concretions. Carbon and oxygen isotopic ratios (delta 13 C + or -0 and delta 18 -4 to -11 per thousand ) indicate a relatively late dingenetic origin for the veins after tens or probably hundreds of meters of burial, and after cessation of bacterial activity and considerable modification of the oxygen isotopic composition of the pore water. Vein growth was discontinuous and took place in waters of changing isotopic and trace-element composition. Changes are not unidirectional and are unlikely to result from the simple evolution of a single conhate pore water; reversals in isotopic trends indicate that precipitation took place during periods of renewed (lateral?) groundwater flow, tapping different sources of bicarbonate-bearing solution.