LA MINERALISATION Ba, ...

LA MINERALISATION Ba, (Zn, Pb) DE CHAUDFONTAINE (SYNCLINORIUM DE VERVIERS, BELGIQUE)

ABSTRACT.
Zn, Pb, (Ba, F) Belgian ore deposits are distributed into three districts. However, in the past, the main productions (around 1.500.000 t of Zn + Pb) were concentrated only into one of the districts, the Verviers Synclinorium. All the mined ore deposits were vein - or paleokarst-type deposits. In 1964, a barite ore deposit, with some sphalerite and galena, was discovered at Chaudfontaine by drillhole. The stratigra- phic control of the Chaudfontaine ore body, the presence of sedimentary structures with barite and sphalerite crystals, the metaco1loidal nature of some mineralogic phases and the presence of other mineralized occurrences in other drillholes, far away from Chaudfontaine (Heibaart, Booischot, Bolland, Soumagne) have led to the conclusion that the Chaudfontaine deposit was of sedimentary origin. This conclusion has been supported by the study of the fluid inclusions of the barite and by isotopic studies (S, C, O, Sr, Pb). In particular, the Pb isotopes emphasize the role of the Givetian and Frasnian sediments as transitory traps for metals introduced during the sedimentation. Effectively, lithogeochemical anomalies have been discovered in many localities of the Verviers and Namur Synclinoria, as well in the Meso and Neodevonian shales as in the carbonates. It is, indeed, because of the discovery of huge zinciferous anomalies at Trooz, in the shales of the Aisemont Formation (Upper Frasnian), that a study by X-ray diffraction has been performed on the Frasnian shales. This has indicated the presence of many interstrati- fied minerals which can have adsorbed metals. Various researches (geological, petrographical, paleontolo- gical, geochemical, etc...) undertaken on the Chaudfontaine deposit and on the country rocks, in the Verviers and Namur Synclinoria, have permitted to define ore guides: stratigraphic position, connection to a biostromal formation, vicinity of a high and evaporites, siliceous nature of the host-rock, and surface mineral occurrences. The trap of the Chaudfontaine ore deposit consists of small depressions on the top of the second Phillipsastraea biostrome of the Aisemont Formation. They have temporary emerged and have evolved to evaporitic conditions (source of S). The metals come from the dewatering of a pelitic subsidizing basin. The precipitation of barite has occurred in a zone of mixing of two different solutions: the first, bringing the sulfur, the second, bringing the metals. Finally, a metallogenic model has been presented for all the Zn, Pb, (Ba) deposits of the Verviers Synclinorium. This illustrates the notions of permanency and inheritance. The metals from this system could come from the continental weathering. These have been carried out, in particular, by adsorption on pelites, and have been introduced into a subsidizing basin. They have been remobilized during the diagenesis, after the variscan orogeny, and eventually, at the periods of emersion. At each successive stage, the metals have precipitated at fronts of dolomitization, at fronts of mixing of various solutions and into sedimentary, karstic and tectonic traps. The study concludes with the chances of success on prospecting in the Verviers Synclinorium.

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