W. De Vos

Heavy-metal contamination in overbank sediments of the geul river (east belgium) - its relation to former pb-zn mining activities

Overbank sediments of the Geul River (East Belgium) are highly contaminated by the heavy metals Pb, Zn, and Cd due to former Pb-Zn mining activities in the drainage basin. Geochemical variations in vertical overbank sediment profiles sampled 1 km north of the mine tailings of Plombiéres allow metal fluxes back to the 17th century to be reconstructed. The vertical profiles are subdivided into three major units corresponding to different industrial periods based on sedimentological criteria as well as on the distribution of contaminants. Alluvial sediments with the highest heavy metal concentrations correspond to the major period of mining activity of the 19th century. The fact that Zn mining at the La Calamine open mine started before large-scale mining of the PbS-ZnS subsurface exploitations is reflected in the vertical profiles by an increase in Zn content before a marked increase in Pb and Cu. The regional extent of contamination in the alluvial deposits was evaluated on the basis of the geochemical analysis of sediments at depths of the 0–20 cm and 80–100 cm. Most of the upper samples are extremely contaminated. Significant local variations in heavy metal concentration in the lower samples are interpreted in terms of which overbank sediment horizon has been sampled at a depth of 80–100 cm. This indicates that “blind” sampling of overbank sediments to characterize the degree of contamination in shallow boreholes can give very erratic results.

The Cambrian to mid Devonian basin development and deformation history of Eastern Avalonia, east of the Midlands Microcraton: new data and a review

Abstract A review is given of recently published and new data on Avalonia east of the Midlands Microcraton. The three megasequences from Cambrian to mid Devonian described in Wales and Welsh Borderland are also present east of the Midlands Microcraton (Brabant Massif, Condroz, Ardennes, Remscheid and Ebbe inliers, Krefeld high). The three megasequences are caused by a tectonic driving mechanism and are explained by three different geodynamic contexts: an earlier phase with extensional basins or rifting and rather thick sequences, when Avalonia was still attached to Gondwana; a second phase with a shelf basin with moderately thin sequences when Avalonia was a separate continent and a later phase with a shelf or foreland basin development and thick sequences. Deformation of the megasequences 1 and 2 or 1 to 3 varies between areas. In Wales and the Lake District the Acadian phase is long-lived and active from early to mid Devonian. In the Ardennes inliers a deformation is active between the late Ordovician and the Silurian (Ardennian Phase), with a similar intensity as the core of the Brabant Massif, when present erosion levels are compared. The Brabant Massif is partly deformed by the long-lived Brabantian Phase from late Silurian till early mid Devonian. Both the Ardennes inliers and the Brabant Massif are not classic orogenic belts, only slate belts where no more than the epizone is reached at present erosion levels. Areas supposedly close to the microcraton or basement are nearly undeformed (SW Brabant Massif and central Condroz). A model of anticlockwise rotation of Avalonia of about 55° from Caradoc to Emsian is proposed to explain the deposition setting of megasequence 3 and the subsequent Acadian and Brabantian deformation. Immediately after the Avalonian microcontinent touched Baltica in Caradoc times it created a short-lived subduction magmatic event from The Wash to the Brabant Massif and soon after the magmatism ended a foreland basin developed. Possibly during and after that development a long-lived and slow compressional event occurred, leading to the deformation of the Anglo-Brabant Deformation Belt. In the early Devonian, contemporaneous with the shortening of the Anglo-Brabant Deformation Belt, extension occurred in the Rheno-Hercynian Zone, possibly caused by the same slow rotation of Avalonia. More evidence emerges that Avalonia east of the Midlands Microcraton comprises not one but probably two terranes: the remnant of the palaeocontinent Avalonia, and what is called the palaeocontinent Far Eastern Avalonia; the latter is only occasionally observed in the few deep boreholes into the Heligoland-Pomerania Deformation Belt, in southern Denmark, NE Germany and NW Poland, with scant available indirect data in between indicating only Proterozoic basement and no Caledonian deformation. For Far Eastern Avalonia a similar palaeogeographical history is postulated as Avalonia, with rifting from Gondwana in Arenig or earlier times, collision with Baltica before the mid-Ashgill and deformation between the late Ordovician and latest Silurian. The Avalonia concept might need to be expanded to an ‘Avalonian Terrane Assemblage’ with cratonic cores and small short-lived oceans as in the Armorican Terrane Assemblage.

Primary minerals of Zn-Pb mining and metallurgical dumps and their environmental behavior at Plombières, Belgium

The primary phases and minerals of the Plombières dumps include typical smelting furnace products such as metallic Fe, Pb, Cu, Zn, Fe-Zn alloys, carbides, phosphides, sulfides of Fe, Zn, Pb, Cu, Mn (alabandite), and FeAs. Spinels, mainly of Fe and Al, are common constituents of the primary assemblage; substitution by Zn, V, Cr, Ti, Mg, and Ca occurs. Primary phases also include the most common Zn-rich fayalite, Zn-rich Ca-Fe silicates, melilite, corundum, and apatite. Most of the Zn is incorporated in iron silicates, ZnO and ZnS. Lead occurs mainly as PbS, metallic lead, and is also present in coal residues. Cadmium is found mainly in metallic zinc and its alloys and in ZnO. The dumps also contain mining wastes composed of pyrite, melnikovite, and iron oxides produced by natural weathering of Zn-Pb ores. Melnikovite and iron oxides are rich in As, Pb, and Zn and possess an increased content of Tl. Leaching tests carried out on the surfaces of polished sections indicate that acid rain (solutions I and II) will mobilize mainly Zn and Cd and, to a much smaller extent, Pb and Sb. Leaching of metals by sulfate-chloride fluids present in the pore network of dumps (solutions III, IV, and V) depends on the pH, which in the dumps is controlled by the proportion of carbonates to sulfides. The more acid fluids leach both sulfides and silicates.

Geochemical mapping based on overbank sediments in the heavily industrialised border area of Belgium, Germany and the Netherlands

Abstract The geochemical distribution patterns based on overbank and stream sediments were studied in the border region of Belgium, Germany and the Netherlands. In total 34 sites were sampled over an area of approximately 16,000 km 2 . Geologically the region comprises formations of the Palaeozoic Ardennes and Eifel, and the unfolded Mesozoic to Quaternary cover. Two overbank sediment samples were collected at each site; one from the lower part of the profile and another from the upper. In addition, samples of active stream sediment were taken from the same sites for comparison and complementary geochemical information. Geochemical distribution patterns for the lower overbank sediment samples reflect the natural situation, with lithology as the main influencing factor. Natural anomalous patterns, due to Pb and Zn mineralization, are also detectable. The upper overbank sediment clearly shows the influence of PbZn mining and metallurgy, as well as other anthropogenic chemical contamination. Pollution is even stronger in stream sediment, obscuring almost completely the natural pattern. The results of these investigations demonstrate the applicability of overbank sediment as a sample medium in regional geochemical mapping. However, for an effective interpretation of the results a good knowledge of the sedimentological history and age of the sample sites is required in mining and industrial areas.

Regional geochemical mapping of Western Europe towards the year 2000

Abstract The Directors of the Western European Geological Surveys (WEGS, presently the Forum of European Geological Surveys, FOREGS) created in 1985 a Working Group on “Regional Geochemical Mapping” for the study of different aspects of regional geochemical surveys. The Working Group proposed to the Directors a systematic low sampling density regional geochemical mapping project to obtain baseline data. Following this proposal for a systematic low sampling density regional geochemical mapping programme of Western Europe a three-year (1988–1990) Pilot Project was carried out. Its objectives were (1) to prepare an inventory of regional geochemical mapping already completed in Western Europe, and (2) to assess the scope and limitations in the use of overbank and stream sediment as sample media in wide-spaced regional geochemical mapping. Based on the results obtained in the Pilot Project, the Directors decided that more research was required before taking a final decision. The Working Group was asked to carry out further research and to submit its final report in 1993. The WEGS Working Group collected and evaluated a substantial amount of information during the pilot and research projects. This paper summarises the main results of the research work carried out from 1986 to 1992.

Structural analysis of the adjacent Acadian and Variscan fold belts in Belgium and northern France from geophysical and geological evidence

Abstract Two Palaeozoic fold belts are juxtaposed in Belgium and northern France. The northern belt of the Brabant Massif, traditionally considered a part of the European Caledonides, was folded during the Acadian orogeny in the early Devonian. Although it is largely concealed, gravity and magnetic maps give a general picture of the existing structures. A curved gravity trend is interpreted as an arc-shaped succession of granitic batholiths. Structures north and south of this arc are completely different. The northern subdomain, containing magnetic lower Cambrian metasediments, shows multiple thrust faults towards the south over the granites, whereas the southern subdomain, without magnetic rocks, is moulded around the granitic core. The southern Variscan belt is characterized in general by thrusting with a north, NW and NE vergence. During the Variscan orogeny the Brabant Massif acted as an obstacle to the general transport of the Ardennes Massif to the N–NW. The oblique convergence with the Brabant Massif caused differential displacement towards the northwest, the western areas having travelled farther, as seen from a pattern of WNW-oriented strike-slip faults.

Vertical distribution of elements in overbank sediment profiles from Belgium, Germany and The Netherlands

Major and trace element variations in overbank sediment profiles in the border area of Belgium, Germany and the Netherlands are described. 14C and pollen dating, as well as identification of artifacts, were used to evaluate whether the lowermost overbank sediments are pristine or contain anthropogenic input. The variety of lithological units in the different catchment areas is clearly reflected by the element concentrations and associations in the oldest overbank sediments. Areas where PbZn mineralization occurs can be delineated by elevated Pb and Zn values in overbank sediments. The history of PbZn mining in the study area, and the beginning of industrialisation, can be recognised on the basis of typical element associations in the overbank sediment sequence. The heavy-metal concentrations of the oldest overbank sediments can be used as background values for stream sediments in polluted areas. It is, therefore, possible to estimate the degree of anthropogenic contamination during the course of history.

Fluid flow, alteration and polysulphide mineralisation associated with a low-angle reverse shear zone in the Lower Palaeozoic of the Anglo-Brabant fold belt, Belgium

Abstract In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is related to a low-angle reverse shear zone. This shear zone has been attributed to the main early Devonian deformation event. Data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone. Macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification. In addition, chloritisation is associated with this alteration. The alteration zone may reach a thickness of 250 m. Ore mineralisation occurred synkinematically and is spatially related to the shear zone. The mineralisation consists of pyrite, marcasite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, stibnite and smaller amounts of tetrahedrite and other sulphosalts. It is concentrated in quartz–sulphide veins or occurs diffusely in the host rock. The mineralising fluids have a low-salinity H 2 O–CO 2 –CH 4 –NaCl–(KCl) composition and a minimum temperature of 250–320 °C. The δ 18 O values of quartz vary between +12.3‰ and +14.5‰ SMOW, and δ D compositions of the fluid inclusions in the quartz crystals range from −65‰ to −35‰ V-SMOW. The δ D and the calculated δ 18 O values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids. The δ 34 S values, between +4.7‰ and +10.6‰ CDT, fall outside the interval typical for I-type magmas. Important migration of likely metamorphic fluids, causing a widespread alteration and a polysulphide mineralisation along a low-angle shear zone, has, thus, been identified for the first time in the Caledonian Anglo-Brabant fold belt.

Alteration and fluid characteristics of a mineralised shear zone in the Lower Palaeozoic of the Anglo-Brabant belt, Belgium

Abstract In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is intimately related to a high strain zone. Data from drillings, completed with outcrop data allow a detailed investigation of mineralisation, alteration and fluid characteristics of this high strain zone, currently interpreted as a low-angle reverse shear zone and attributed to the main Early to early Middle Devonian Acadian deformation event. Ore mineralisation occurred synkinematically and was closely associated with the shear zone. Low saline H2O–CO2(–CH4)–NaCl fluids with temperatures >260°C were involved in the hydrothermal circulation, which caused alteration of the host rock and extensive sericitisation in the shear zone. Isotope data and the general setting indicate a metamorphic-driven system.

GEMAS: CNS concentrations and C/N ratios in European agricultural soil

A reliable overview of measured concentrations of TC, TN and TS, TOC/TN ratios, and their regional distribution patterns in agricultural soil at the continental scale and based on measured data has been missing - despite much previous work on local and the European scales. Detection and mapping of natural (ambient) background element concentrations and variability in Europe was the focus of this work. While total C and S data had been presented in the GEMAS atlas already, this work delivers more precise (lower limit of determination) and fully quantitative data, and for the first time high-quality TN data. Samples were collected from the uppermost 20cm of ploughed soil (Ap horizon) at 2108 sites with an even sampling density of one site per 2500km2 for one individual land-use class (agricultural) across Europe (33 countries). Laboratory-independent quality control from sampling to analysis guaranteed very good data reliability and accuracy. Total carbon concentrations ranged from 0.37 to 46.3wt% (median: 2.20wt%) and TOC from 0.40 to 46.0wt% (median: 1.80wt%). Total nitrogen ranged from 0.018 to 2.64wt% (median: 0.169wt%) and TS from 0.008 to 9.74wt% (median: 0.034wt%), all with large variations in most countries. The TOC/TN ratios ranged from 1.8 to 252 (median: 10.1), with the largest variation in Spain and the smallest in some eastern European countries. Distinct and repetitive patterns emerge at the European scale, reflecting mostly geogenic and longer-term climatic influence responsible for the spatial distribution of TC, TN and TS. Different processes become visible at the continental scale when examining TC, TN and TS concentrations in agricultural soil Europe-wide. This facilitates large-scale land-use management and allows specific areas (subregional to local) to be identified that may require more detailed research.