Introduction - The Mining District

In Belgium, there are three areas where lead-zinc deposits were exploited, the most important district being that of the Verviers Synclinorium located in Eastern Belgium. (Dejonghe, 1998). In this area, lead and zinc deposits have indeed been intensively exploited in the past, since prehistoric times and the Middle Age, but mainly during the XIX^th century. Its apogee was between 1850 and 1870. The last mine (Roer) was closed in 1936. Their main geological and mining features are described by Dejonghe & Jans (1983). Shortly, most of the ore deposits are vein-type deposits located in fractures transverse to the folding and the faulting. These fractures are related to the Rhine graben tectonics. Most of them are crossing the sedimentary carbonate Devonian and Dinantian formations. Some also cross the Famennian and Namurian detrital rocks. In many cases, irregular bodies (the so-called “amas” in French) were connected to the lodes and were located at lithostratigraphic and tectonic contacts between various lithological formations. Some deposits are linked to the Palaeozoic-Mesozoic unconformity and would originate from the reworking on short distance of pre-existing ore-deposits, likely in karstic traps. An extensive and detailed description of each of the ore deposits existing between Liège and Aachen is given by Dejonghe et al. (1993). Petrographical, mineralogical, geochemical data, stable isotopes (C and O) and fluid inclusions characterizing the “calamine” non-sulfide Zn-Pb deposits of Belgium are specified by Boni et al. (2007) and Coppola et al. (2008).

La Calamine (Moresnet) ore deposit

The region situated between Liège and Aachen produced about 1 260 000 tons of zinc (metal) and 133 000 tons of lead (metal). The largest deposit, that of La Calamine (Kelmis in German), yielded about a total of 760 000 tons of zinc (metal) during all its mining activity (Dejonghe et al., 1993, p. 443). Its zinc production topped that of all other Belgian deposits altogether. The ore body was exhausted in 1879 (small activities lasted up to 1884). It was one of the most important European mining areas during the Middle Ages up to the onset of the industrial revolution. Its importance is illustrated by paintings of Bastiné and Maugendre, respectively dating back to 1843 and 1850 (Figs. 1 & 2).

Figure 1

La Calamine deposit was a huge lenticular body of 450 to 500 m in length and 65 to 100 m in width which was exploited down to 110 m at depth. The ore body is genetically related to the Palaeozoic-Mesozoic unconformity through karstic process of concentration. Main features are:

• The ore deposit has produced only zinc while most of other Belgian deposits delivered a mixed production of zinc and lead;
• The ore was completely oxidized down to the depth of 110 m (base of the ore deposit), while most of other deposits presented a dominant sulphide nature below the current water-table. The ore was named “calamine” by the miners. It is a mixture of several minerals: Zn carbonate (smithsonite, ZnCO₃) and Zn silicate {hemimorphite, Zn₄Si₂O₇(OH)₂.H₂O} or the assemblage of smithsonite, hemimorphite, hydrozincite {Zn₅(CO₃)₂(OH)₆} and (willemite, Zn₂Si0₄), locally associated with Fe-oxyhydroxydes and clays;
• The mineralized body rested on the detrital Famennian formations by means of an irregular intermediary layer of silicified dolomite of Tournaisian age.

Figure 2

La Calamine ore deposit, also known by other names (Altenberg-, Vieille-Montagne-, Grande-Montagne- or Moresnet-ore deposit), was also the object of political tensions. After the defeat of Napoleon I, the Vienna Congress created in 1815 the kingdom of The Netherlands including current Holland and Belgium. But a part of the border between this new country and Prussia was disputed, more particularly where the ore deposit of La Calamine was situated. The problem was solved in 1816 by the Treaty of Limits creating the neutral territory of Moresnet (Fig. 3). The history of this small territory of 350 hectares, which lasted from 1816 to 1919 with a lot of privileges, is described by Defourny (2016). There, the Vieille-Montagne society, which exploited the La Calamine deposit, had a very important social role (see also Ladeuze et al., 1991 for an historical account of the role of the Vieille-Montagne in Eastern Belgium).

Figure 3

After closure of the Pb-Zn mines

Dates of closure and productions of the three main ore deposits are shown in Table 1. More details on the periods of exploitation of the various Pb-Zn deposit of Eastern Belgium are given by Dejonghe & Jans (1983, fig. 1) and by Dejonghe et al. (1993).

Table 1

Name	Date of closure	Zn	Pb	Maximum depth (m)
La Calamine	1884	760,000	0	110
Schmalgraf	1931-1932	157,835	13,763	290
Bleiberg	1882	60,675	80,500	194 (shaft)

Features of the main ore deposits (figures of Zn and Pb productions are in tons of metals).
 
Caractéristiques des principaux gisements (les chiffres des productions de Zn et de Pb sont exprimés en tonnes de métaux).

Roer, the last Pb-Zn mine of the district, was closed in 1936, after having produced 6084 tons Zn and 864 tons Pb. See also Polrot (2014) for a detailed description of the mining activity in the Bleiberg (Plombières) area. After closure of the metal mines, no particular attention has been provided to their geological and mining heritage. Today, traces of the former mining exploitations have nearly completely disappeared from the landscape in this area. Some vestiges (ruins) of the former mining surface buildings still exist at Schmalgraf, the deepest metal mine in Belgium (290 m) employing up to 120 miners. The ore extracted from Schmalgraf was sent by railway to the processing plant of La Calamine. This railway situated in the Hohn valley following the river Hohn (Hohnbach also named Lontzenerbach, a tributary of the Gueule river) is at present a touristic path. The Oskar gallery, of 500 m long, connected the Schmalgraf underground mine to the Hohn valley to evacuate waters pumped in the mine. This gallery, also known under various names (e.g., Max Stollen gallery), was dug between 1862 and 1867 starting from the mine level of – 42 m. Today, the exit of this gallery is still visible (Fig. 4) and preserved to facilitate access to the bats. The exit of another gallery, the Auenberg gallery (Fig. 5), is also kept for the same purposes.

Figure 4

Figure 5

The calaminarian grasslands

Calaminarian grasslands are environments where the process of seral succession has been halted due to the toxicity of soils containing high levels of toxic metal ions. They can have three origins: (1) Natural primary sites directly above iron caps deposits; (2) Secondary sites where mining, ore processing, storage, manufacture of finished products and landfill of waste lead to the formation of calaminar waste dumps; (3) Tertiary sites related to the emission and atmospheric dispersion of metallic particles and acid fumes.

In Belgium, calaminarian grasslands are the most visible remnants of past Pb-Zn mining operations, sheltering an anomalous metallophyte flora (Fig. 6). These are plants which need zinc to grow and are only found were soils contain a high zinc content (e.g. above zinciferous gossans or in polluted soils by storage of extracted ore and by residues around processing plants). Pseudometallophytes are plants which grow as well on metalliferous soils than elsewhere. For more details on these plants appearing in Eastern Belgium, see Rosengarten (2010) and Hermanns (2014).

Figure 6

Six metallophytes and several pseudometallophytes are identified in Eastern Belgium (Table 2).

Table 2

Latin name	English name	French name
Metallophytes
Viola calaminaria (Fig. 6)	Zinc violet	Pensée calaminaire
Armeria maritima subsp. Halleri	Thrift	Gazon d’Olympe calaminaire
Thlaspi caerulescens subsp. Calaminare	Alpine pennycress	Tabouret calaminaire
Silene vulgaris subsp. humilis var. humilis	Bladder campion	Silène calaminaire
Minuartia verna var. hercynia	Spring sandwort or “Leadwort” locally used in Derbyshire	Alsine calaminaire
Festuca ovina subsp. Guestfalcia	Sheep’s fescue	Fétuque de Westphalie
Pseudometallophytes
Rumex acetosa	Sorrel	Oseille sauvage
Campanula rotundifolia	Harebell	Campanule à feuilles rondes
etc.

Metallophytes and pseudometallophytes identified in Eastern Belgium.
 
Métallophytes et pseudométallophytes identifiés dans l’Est de la Belgique.

The literature provides information on the presence of calaminarian grasslands in Belgian areas characterized by former lead-zinc exploitations in the Meuse, Vesdre, Ourthe and Gueule valleys. An inventory of calaminarian sites in the Walloon region was carried out by Graitson (2006). Thirty-seven sites were studied in the field. For each site, Graitson (2006), lists the detected metallophytes. But he remarks that, within their ranges, the distribution of metallophytes is increasingly fragmented by the destruction of their habitats. One can observe the typical and complete assembly of metallophytes in Wallonia no more than on 3 sites. That is why he advocates that measures allow the maintenance of these environments in a state of conservation favourable to long term.

The calaminarian grasslands already existed before any human activity because Zn-Pb veins capped by zinciferous gossans already outcropped in the Pleistocene. But the level of pollution by Pb and Zn dramatically increased during the industrial period around the surface mine sites and the ore processing plants, creating new calaminarian grasslands. Since, most of these have disappeared because they were removed to recover the ground for other purposes. Those which still exist constitute remarkable ecosystems which are now the subject of particular protection through the actions listed below (after Rosengarten, 2010, p. 8).

• CORINE: Inventory of major important sites for the conservation of nature in the European community;
• ISIWAL: Inventory of Walloon sites of great biological concern;
• SGIB: Sites of great biological concern;
• ZHIB: Humid zones of biological concern;
• Natura 2000;
• Natural reserves under the impetus of the ASBL (non-profit organisation) Ardenne & Gaume.

Since 1995, the Koul calaminarian grassland at La Calamine has enjoyed official protection status to preserve its exceptional historical significance. See also the issue published by Ardenne & Gaume in 2011 and devoted exclusively to the calaminarian grasslands. In the Netherlands, close to the Belgian border, in the Gueule valley, calaminarian grasslands also occur. They are due to an ancient pollution of the soils related to the Belgian mining activities of lead-zinc deposits. But according to Willems (2004), calaminarian grassland with Viola calaminaria, which undergoes a sharp decline, occupies less than half a hectare.

Conclusion

This paper illustrates one aspect of the post-mine context in the case of lead-zinc mines in Eastern Belgium. With the exception of calaminarian grasslands, the trace of these former mining exploitations has almost completely disappeared. These calaminarian grasslands are remarkable ecosystems regarded as of principal importance for biodiversity conservation. Therefore, they deserve special attention and protection measures due to their unique historical significance.

Acknowledgements. — I am grateful to Prof. Nathalie Verbruggen (Free University Brussels) for assisting with the English translation of the metallophytes and pseudometallophytes and to Michiel Dusar (Geological Survey of Belgium) and Didier Torz (Editor-in-chief, Société géologique du Nord) for constructive comments about the manuscript.