The “calamine” nonsulfide Zn–Pb deposits of Belgium: Petrographical, mineralogical and geochemical characterization

Abstract

The Belgian nonsulfide Zn deposits represent the historical basis for the zinc mining and smelting industry in Europe. The zinc ores were called “calamines” and consist of a mixture of Zn carbonates (smithsonite, hydrozincite) and Zn silicates (hemimorphite, willemite and Zn clays), with a variable content of Pb minerals. The “calamines” are considered to be oxidation products of primary sulfide ores. The most important was the La Calamine orebody (more than 600,000 tons of Zn metal) that was continuously exploited for several centuries. Nonsulfide ores were also exploited at Schmalgraf, Engis, Dickenbusch, Fossey, Rocheux-Oneux, Welkenraedt and in other smaller deposits (all no longer accessible for study), where the “calamines” extended from the surface to an average depth of 40 to 50 m. The primary Zn–Pb sulfide mineralization consists of post-Variscan hydrothermal veins and replacement bodies, mostly occurring in the Dinantian (Visean) limestones. The sulfides intersect the Paleozoic rocks and are truncated and unconformably covered by Late Cretaceous sediments (Santonian Aachen Fm.). The same setting has been observed for the nonsulfide ores, suggesting an Early to Mid-Cretaceous weathering, consistent with other geological and geochronological data on paleoweathering in Europe. The extensive occurrence of willemite, that appears to be the first deposited nonsulfide mineral, is a striking particularity of the Belgian “calamines”. Most fluid inclusions in willemite (80%) and all inclusions in smithsonite are monophase (aqueous). Ice melting temperatures suggest salinities between 0 and 5 wt.% NaCl equiv. Homogenization temperatures of rare two-phase inclusions (liquid + vapor) in willemite display a large variation between 80 and 190°, an interval matching the temperature ranges of other willemite ores throughout the world, so far considered of hydrothermal origin. The stable isotope variation of Belgian smithsonites and cerussites is very similar to the range measured in other supergene nonsulfide deposits similar to those of SW Sardinia. Belgian smithsonites exhibit a limited range of δ 18O values from 27.1 to 30.6‰, averaging 28.4‰ ± 0.8‰ V-SMOW. This limited range points to a relatively uniform isotope composition of the oxidizing waters and constant temperatures of smithsonite crystallization. Carbon isotope values, even from the same mine, show a considerable range from − 11.6 to − 1.6‰. This extensive range indicates at least two carbon sources: 13C-depleted carbon derived from the organic matter in the soils and 13C-enriched carbon originating from marine carbonate host rocks. Cerussites have oxygen isotope values ranging from 16.8 to 19.3‰, averaging 17.8 ± 1.0‰ and carbon isotope values of − 18.4 to − 14.7‰. The metallic deposits of Eastern Belgium have undergone a polyphase history, which started with a possibly Jurassic sulfide mineralization, followed by oxidation under high silica activities and consequently by supergene weathering. Owing to structure and mineralogical association, we do not consider the high homogenization temperatures as reliable minimum temperature estimates of willemite formation. However, it is possible that the willemite concentrations could have been derived from low-temperature, localized hydrothermal processes caused by deeply reaching oxidizing fluids. Smithsonite and hemimorphite, often developed at the expenses of willemite, have all the characteristics of supergene oxidation products.