Genetic studies of red bed mineralization in the Triassic of the Cheshire Basin, northwest England

The Triassic sediments of the Cheshire Basin are host to a number of small mineral deposits characteristic of red beds. The mineralization comprises Cu and Pb with minor Zn, Ag, CO, V, Ni, As, Sb and Mn which, together with widespread barite mineralization, is closely associated with basin margin faults. The Wilmslow Sandstone Formation is cemented by authigenic quartz overgrowths enclosing iron sulphides, which predate the main sulphide cements of tennantite, bornite and sulpharsenides. Studies on late calcite-hosted fluid inclusions show homogenization temperatures ranging between 60-70 °C (regarded as the minimum temperature of mineralization) and salinities of 9-22 wt% equivalent NaCl (mean = 17 wt%), indicating that the mineralizing fluids were warm and saline. Consideration of the basin history suggests that fluid temperatures in the Permo-Triassic sequence may have reached 150°C at the time of mineralization. New data significantly constrain the genetic hypothesis for the deposits, refuting a magmatic origin. A basin brine expulsion model is favoured. The sulphur isotope data from the barite mineralization (mean δ³⁴S = +17%) suggest that the bulk of the barite sulphur was derived directly from the Upper Triassic evaporites (mean δ³⁴S = +19%), whilst the distribution of δ³⁴S values for sulphides (-1.8% to +16.2%) is consistent with ultimate derivation of sulphur from the evaporites by closed system reduction of a sulphatebearing brine. It seems likely that the reducing agent was organic material (methane) derived from the underlying Coal Measures, but it is uncertain at present whether the principal mechanism was thermochemical or bacterial reduction. The pattern of sulphide δ³⁴S favours the former process.