Anomalous Au silicifications in the upper carboniferous of the French Massif Central: timing and temperature constraints by combined geochemical and organic-matter studies

Abstract : Hydrothermal silicification in some of the Carboniferous rocks at Lugeac and Saint-Etienne (French Massif Central) is noteworthy for its significant Au and Ag anomalies. Two types are distinguished according to the hydrothermal mode of formation: one associated with geyser deposits (St-Etienne) and the other associated with fluid circulation through the rocks following diagenesis (St-Etienne and Lugeac). In the absence of analysable fluid inclusions, we determined the temperatures of the fluids involved in the hydrothermal events from parameters used in ranking organic matter, i.e. 100 °C at Lugeac and 150-165 °C at St-Etienne. Silicification at Lugeac was accompanied by barite and adularia, and so it can be associated to the barite/fluorite vein complex of the Paulhaguet District where the Lugeac Carboniferous deposits are located. Thus, the age of 209-8 Ma (Hettangien) assigned to the Paulhaguet mineralization is taken as the age of silicification in the Lugeac Carboniferous. The circulation of Au/Ag mineralization fluids occurred several times, starting at around 330 Ma, and apparently continued at least up to the Lias.