Discrimination of anthropogenic contamination and the effects of mineralization in soils from background patterns using multifractal modelling: Cyprus case study

Abstract

A key task in the analysis of exploration geochemical data is the selection and application of efficient classification models to discriminate mineralization-related signals from other processes affecting variation in element concentrations. Similarly in environmental or urban geochemistry one objective is to separate geochemical patterns associated with anthropogenic contamination from geogenic processes. To classify geochemical maps into background or anomalous samples or regions, a variety of mathematical and statistical models have been developed. In this study various fractal modelling has been applied to centered logratio transformed Cu, Ba, Mn, Pb, Zn, In, As, Au, and Ag contents of soil samples from the Geochemical Atlas of Cyprus. Areas with contamination have previously been shown not to display normal fractal behavior for values exceeding lithology-dependent background populations. Therefore, two new fractal methods – concentration-concentration (C-C) and concentration-distance from centroid-points (C-DC) – were applied to discriminate anthropogenic from geogenic anomalies. One of the strongest indicators of proximity to major Cu mineralization is In. The C-C model displays broad similarity between the Cu-In pairing and the raw Cu, and between its reverse in the In-Cu pairing and raw In. Of the five populations that emerge from the Cu-In fractal model, the first two (regional background and weakly anomalous) are largely restricted to the Circum-Troodos Sedimentary Succession units. The moderately anomalous population extends across all the basalts and north from the Troodos Ophiolite (TO) across the fanglomerates and more recent alluvium-colluvium that contains material shedding north off the TO. It is noted that the strongest anomalies are at the boundary between the sheeted dyke complex and basalts and on one of the major NE-trending structures that cut across the TO, but where there are only a small number of minor Cu mineralization occurrences. In the C-DC model, the centroids used to model the spatial variation of the soil geochemistry were the known mineral deposits. The Cu C-DC model delivers just two populations that are lithologically-controlled. The first spans the ultramafic TO core and the Pakhna Formation carbonates (the two extremes in the raw data geochemical compositions), and all other units, including the TO mafics, Mamonia Terrain, and the fanglomerates and alluvium-colluvium areas in the second population. The In C-DC model is somewhat similar to the In-Cu C-C model, but the second major population is more restricted to the sections of the basalts containing known Cu mineralization as well as a restricted zone in the sheeted dykes in western TO. Applying the C-DC model to the transformed scores, there are three main populations evident. The highest one contains all the known Cu mines and mineral deposits, as well as a number of NE-trending zones that cut across the sheeted dykes on the western and the eastern sides of the TO, and which also appear to follow the major sinistral faults that transect the TO.