Léon Dejonghe

Zinc-lead deposits of Belgium

The exploitation of Belgian zinc–lead deposits dates back to prehistoric times. Its apogee was between 1850 and 1870 and the last mine was closed in 1946. During the period 1837–1945, during which time production levels were recorded, the tonnage of exploited metals reached about 1 500 000 tons Pb+Zn with a Zn/Pb ratio of 8–9. The mineralogy of the deposits is generally simple: sphalerite, galena, pyrite or marcasite, and locally Ni- and Co-sulfides. Several ore deposit types may be distinguished, including syndiagenetic-, vein and connected flat-, and paleokarstic types. The syndiagenetic deposits are made up of numerous disseminated-type deposits with low grades and small tonnages. They are linked to reef facies of Frasnian (=Upper Devonian) carbonate rocks. The largest exploited vein-type deposits are hosted in Dinantian (=Lower Carboniferous) carbonate rocks. Lead isotopic studies reveal that the main primary source of the metals is in Middle and Upper Devonian sedimentary rocks. Hydrothermal solutions precipitated their metals at temperatures <180°C (110°C in the Verviers Synclinorium) and pressures of 108 Pa (<2×107 Pa in the Verviers Synclinorium). Salinities of the fluids were often between 10 and 23 equiv. wt.% NaCl. The origin of the vein-type deposits is related to the migration of underground waters, likely gravity-driven, that leach the metals of the surrounding sedimentary rocks (mainly, those of Devonian age, after Pb isotopic data). The La Calamine paleokarstic deposit was the largest Zn ore body in Belgium. It was continuously exploited from the 14th century up to 1884 and produced more than 600 000 tonnes of zinc metal. It was at La Calamine that the chemist Dony, from Liege, invented the first industrial process to transform the calamines into malleable zinc. This was patented by Napoleon I in 1810. Because Belgium occupies an area of the Variscan foreland between Germany and Ireland, a comparison between deposits of these countries is performed. Belgian Zn–Pb syn-diagenetic deposit are however of types different from the volcanogenic massive sulfide deposits of Germany, as well as from the strata-bound lenses of Ireland. Prospecting by private companies during the past 50 years has increased the tonnage of reserves adjacent to known deposits. Lithogeochemical prospecting, however, has not led to the discovery of new economic ore deposits. The Chaudfontaine stratiform deposit, situated above a Frasnian biostrome, is the only recent drilling discovery.

Isotope geochemistry (S, C, O, Sr, Pb) of the Chaudfontaine mineralization (Belgium)

Investigations on S, C, O, Sr, and Pb isotopes in ores, gangue, and country rocks of the Chaudfontaine ore deposit confirm that interbedded barite and white sparry calcites are cogenetic. They emphasize the marine origin of S and the nonmarine origin of the metals. They underline the important role played by the bacterial reduction at the site of deposition. Hypotheses concerning the source of the metals and the nature of the brines involved in their transportation are briefly discussed.

Paleogeographic and diagenetic context of a baritic mineralization enclosed within Frasnian peri-reefal formations: Case history of the Chaudfontaine mineralization (Belgium)

Abstract The Chaudfontaine ore deposit, which is composed mainly of barite, is hosted in the Frasnian shale and carbonate formations of Belgium. A detailed description of the sedimentary structures involving barite crystals shows that many of these crystals have developed under gravity control. Two populations of barite were formed: crystals which grew in brine above the sediment-water interface, and crystals which developed in the sediment during early diagenesis. Each population has specific size, habits and structures, and the size and abundance of the crystals may be expressed as a function of the degree of barium supersaturation. Morphological similarities were found between Chaudfontaine barite and gypsum from evaporites. The formation of the ore deposit is then replaced in its sedimentological and palacogeographical context: a sea-level drop affecting a semi-restricted marine basin. A predominantly burial diagenesis is responsible for the good preservation of the ore deposit.