Origin and evolution of high-salinity, Zn–Pb mineralising fluids in the Variscides of Belgium |
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Authors: | Wouter Heijlen Philippe Muchez David A Banks |
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Institution: | (1) Fysico-chemische Geologie, K.U.Leuven, Celestijnenlaan 200C, 3001 Heverlee, Belgium e-mail: wouter.heijlen@geo.kuleuven.ac.be Fax: +32-16-327981, BE;(2) School of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK, GB |
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Abstract: | High-salinity, Na–Ca–Cl-rich fluids (˜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
fluids 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 fluids 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 the fluids with that of an evolved evaporitic
brine enables the recognition of major water–rock interactions that modified 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-feldspar probably controlled the K-content of the fluids. 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 fluids were finally expelled
along extensional faults, resulting in the formation of Zn–Pb deposits.
Received: 26 April 2000 / Accepted: 22 November 2000 |
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