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

2001, Mineralium Deposita

High-salinity, Na±Ca±Cl-rich¯uids ($20 wt% salts) in inclusions in gangue and ore minerals from Mesozoic Mississippi Valley-type (MVT) deposits 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 $110°C while the Cretaceous vein calcites were precipitated at temperatures <50°C. Their Cl±Br content (Cl/Br ratio between 246 and 458) suggests that the¯uids probably originated by the evaporation of seawater during basin development at the southern margin of the Caledonian Brabant Massif in the Late Palaeozoic. The Na±Ca±K content (Na: 29,700±49,600 ppm, Ca: 25,700±46,200 ppm, K: 1,000± 5,620 ppm) is similar to that of the mineralising¯uids in other Pb±Zn districts, interpreted to be of evaporative origin (e.g. Newfoundland, East Tennessee, Polaris). Furthermore, comparison of the Na±Ca±K content of thē uids with that of an evolved evaporitic brine enables the recognition of major water±rock interactions that modi-®ed the¯uid composition. It indicates that the ambient uids 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-feldspar probably controlled the K-content of the¯uids. A model is proposed where the bittern brines migrated down into the deep subsurface because of their density during extension. After the Variscan orogeny, these¯uids were ®nally expelled along extensional faults, resulting in the formation of Zn±Pb deposits.