Origin and evolution of high-salinity, Zn-Pb mineralising fluids in the Variscides of Belgium

High-salinity, Na-Ca-Cl-rich fluids (similar to 20 wt% salts) in inclusions in gangue and ore minerals from Mesozoic Mississippi Valley-type (MVT) dep osits in the Verviers Synclinorium (eastern Belgium) and in Cretaceous vein calcites at the Variscan front were investigated by microthermometric and crush-leach analysis. The MVT deposits formed at temperatures of similar to 110 degreesC while the Cretaceous vein calcites were precipitated at tempe ratures < 50 <degrees>C. Their CI-Br content (Cl/Br ratio between 246 and 4 58) suggests that the fluids probably originated by the evaporation of seaw ater during basin development at the southern margin of the Caledonian Brab ant Massif in the Late Palaeozoic. The Na-Ca-K content (Na: 29,700-49,600 p pm, Ca: 25,700-46,200 ppm, K: 1,000-5,620 ppm) is similar to that of the mi neralising fluids in other Pb-Zn districts, interpreted to be of evaporativ e origin (e.g. Newfoundland, East Tennessee, Polaris). Furthermore, compari son of the Na-Ca-K content of the fluids with that of an evolved evaporitic brine enables the recognition of major water-rock interactions that modifi ed the fluid composition. It indicates that the ambient fluids participated in the early diagenetic dolomitisation of Upper Palaeozoic carbonates and also in the albitisation of plagioclase in Lower Palaeozoic siliciclastics of the Caledonian basement. Illitisation of smectites or dissolution of K-f eldspar pro bably controlled the K-content of the fluids. A model is propos ed where the bittern brines migrated down into the deep subsurface because of their density during extension. After the Variscan orogeny, these fluids were finally expelled along extensional faults, resulting in the formation of Zn-Pb deposits.