Zinc–lead deposits of Belgium

Abstract

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 10 8 Pa (<2×10 7 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 Liège, invented the first industrial process to transform the calamines into malleable zinc. This was patented by Napoléon 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.