P. O'Connor

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.

Multielement regional geochemical reconnaissance as an aid to target selection in Irish Caledonian terrains

Abstract A regional geochemical reconnaissance, based upon the minus 150 micron fraction of active stream sediment, has been conducted by the Geological Survey of Ireland (GSI) over Irish Caledonian terrains. Thirty-eight chemical elements of economic and environmental significance (Ag, As, Au, Ba, Br, Cd, Ce, Co, Cr, Cs, Cu, Eu, Fe, Hf, Ir, La, Li, Lu, Mn, Mo, Na, Ni, Pb, Rb, Sb, Sc, Se, Sm, Sr, Ta, Te, Th, U, V, W, Yb, Zn and Zr) were determined by combined instrumental techniques (AAS and INAA). To date, the multielement database for some 2500 samples from three regions covers about 7,500 km2. Data analysis employed resistant non-parametric techniques, or EDA (Exploratory Data Analysis) for objectively selecting outlier values (anomalies) and class boundaries for the distribution of each element. Such techniques are considered more appropriate to the treatment of geochemical data where assumptions about normality are rarely, if ever, achieved. Class selection was based on the resistant selected order statistic as represented in the boxplot. Special EDA mapping symbols, designed to give equal weighting to all data values, were assigned to the relatively wide boxplot class intervals. None of the original data are redundant in the resulting geochemical maps and, in many instances subtle regional variations can be recognized which are directly attributable to lithological variation. Outlier values are directly correlatable with known bedrock mineralization. The methodology was applied to data from two Caledonian terrains: the Inishowen area of northwestern Ireland and the Leinster massif of southeastern Ireland. A great diversity of metalliferous mineralization (Pb, Zn, Cu, Ba, Au, Ag, W, Li, Ta, U, Cr, Ni, Sb) is found in both terrains. The geochemical maps of Inishowen identify the most prospective gold-bearing lithologies quite effectively. In Leinster, the geochemical maps show the close spatial correlation of Li, Ta, U and W with granitic lithologies, the unequivocal volcanogenic signature of Cu, Sb and Au, an ultra-mafic Cr-Ni association and a new possible volcanogenic Zr-Hf association. While effective in identifying known bedrock mineralization, the maps also suggest possible extensions of mineralized zones and new targets for detailed follow-up surveys. The geochemical patterns defined on the maps are also of assistance in constraining metallogenic models. In addition it can be demonstrated that EDA-based techniques offer a viable and effective method of geochemical mapping.

U-Th signatures of agricultural soil at the European continental scale (GEMAS): Distribution, weathering patterns and processes controlling their concentrations

Agricultural soil (Ap-horizon, 0-20cm) samples were collected in Europe (33 countries, 5.6millionkm2) as part of the GEMAS (GEochemical Mapping of Agricultural and grazing land Soil) soil-mapping project. The GEMAS survey area includes diverse groups of soil parent materials with varying geological history, a wide range of climate zones, and landscapes. The soil data have been used to provide a general view of U and Th mobility at the continental scale, using aqua regia and MMI® extractions. The U-Th distribution pattern is closely related to the compositional variation of the geological bedrock on which the soil is developed and human impact on the environment has not concealed these genuine geochemical features. Results from both extraction methods (aqua regia and MMI®) used in this study support this general picture. Ternary plots of several soil parameters have been used to evaluate chemical weathering trends. In the aqua regia extraction, some relative Th enrichment-U loss is related to the influence of alkaline and schist bedrocks, due to weathering processes. Whereas U enrichment-Th loss characterizes soils developed on alkaline and mafic bedrock end-members on one hand and calcareous rock, with a concomitant Sc depletion (used as proxy for mafic lithologies), on the other hand. This reflects weathering processes sensu latu, and their role in U retention in related soils. Contrary to that, the large U enrichment relative to Th in the MMI® extraction and the absence of end-member parent material influence explaining the enrichment indicates that lithology is not the cause of such enrichment. Comparison of U and Th to the soil geological parent material evidenced i) higher capability of U to be weathered in soils and higher resistance of Th to weathering processes and its enrichment in soils; and, ii) the MMI® extraction results show a greater affinity of U than Th for the bearing phases like clays and organic matter. The comparison of geological units with U anomalies in agricultural soil at the country scale (France) enables better understanding of U sources in the surficial environment and can be a useful tool in risk assessments.

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.