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Abstract

Lead-zinc deposits of Belgium all are of telethermal type chiefly fillings along transverse, nearly vertical faults. Mineralization affects only the folded Devonian and Carboniferous of the "Massif des Ardennes," western part of the "Rheinische Schiefergebirge." Cambrian and Silurian anticlinal structures are not mineralized, nor are the transgressive Mesozoic and Cenozoic sediments except in the lowermost Triassic conglomerates.

Economic mineralization is limited to the intersection of faults with limestones and dolomites of the middle and upper Devonian and of the Carboniferous limestone.

Irregular "replacement deposits" develop where cross-cutting veins intersect contact planes between the Carboniferous limestone and the upper (Coal Measures) and lower (upper Devonian) shales. Despite their strata-bound extension, these are not true replacements but are fillings of solution cavities. Comparable stratiform lead-zinc disseminations exist where veins cut the dolomitized reefs of the lowermost Upper Devonian: galena and sphalerite fill pores and vugs in coarse-grained secondary dolomite.

The northern, and richest, mineralized belt follows the east-west Namur syncline, from Charleroi to Liège, then the Vesdre syncline. crossing into Germany at the famous Moresnet mine and reaching the border faults of the lower Rhine rift-valley. There the basal conglomerates and porous sandstones of the Triassic are mineralized forming the important stratiform lead deposits of Maubach and Mechernich.

The Belgian lead-zinc mines, closed since 1945. afford little opportunity for study along modern lines, but their geological situation, well known, limits rather strictly the possible theories of origin. Lindgren. in 1933, cited the "Moresnet type" among the lead-zinc deposits independent of igneous activity. Clearly, their origin is connected with the deep circulation of connate and/or meteoric water through a "plumbing system" whose main pipes were the cross-cutting, living faults. After obstruction by precipitated sulfides they frequently were re-brecciated with additional cavity formation in the wall rocks by acid marcasite-forming solutions.

The Carboniferous Limestone bears lead-zinc deposits only where its lower part is dolomitized. Anhydrite beds in the transgressive Middle Devonian were revealed by two deep boreholes in 1960. Deep waters of the Carboniferous Limestone in Northern Belgium presently are Na-Ca-Cl brines.

Isostatic uplift, with erosion, during the arid Permo-Triassic, constitutes a paleo-hydrogeological evolution which, in our opinion, is essential. The marine formation waters could move only when erosion had removed parts of the thick, impervious cover of Coal-Measures. These waters then were replaced progressively by meteoric waters, modified by the residual salinity of the permeable horizons. They must have been able to dissolve geochemical lead and zinc, reprecipitating some in the ascending parts of their deep channelways, i.e., the fault breccias and still porous rocks.

Mineralization by this mechanism may have taken many millions of years. It ceased when the Ardennes Mountains became a peneplain, before invasion by the Upper Cretaceous sea.

Dissolved CO2 certainly played a significant role by retarding the reprecipitation of the heavy cations until the lowered pressure near the surface permitted its escape.

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