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 km 2. 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.