Iron Quadrangle stream sediments, Brazil: geochemical maps and reference values

摘要

Geochemical mapping and the determination of reference values receive increasing attention. The need to know regional geochemical background data as well as the spatial allocation of areas with anomalous geochemical concentrations and chances to identify their major sources motivate the interest. Here, results from a high-density geochemical mapping exercise and derived related reference values are presented; 541 stream sediment samples have been collected across the Iron Quadrangle (IQ), Brazil, with a density of one sample per 13 km(2). Geochemical maps were compiled using the inverse distance weighted interpolation method. "Normal" concentrations were distinguished from anomalies with the separation-by-concentration-range technique, where positive anomalies are defined by the "boxplot" Upper Inner Fence (UIF) rule. More than 70 % of the IQ show exclusively geogenic concentration ranges for the elements studied. About 20 % represent positive anomalies, likely related to both near surface rock types and to human interference. Distinct anthropogenic anomalies, notably from mining activities explain 5-10 % of all samples. This first high-density sampling in the IQ allows delineating the role of lithology on the elemental composition of stream sediments, and to establish reference values for the main river basins-thus pinpointing localities, cities and fluvial basins that are exposed to environmental risks and need to be protected.