Mapeamento geoquímico da bacia do rio Itacaiúnas, Província Mineral de Carajás: Assinatura geoquímica dos blocos crustais e implicações para recursos minerais e meio ambiente

Abstract

Multielement geochemical surveys (MGS) when combined with advanced data processing techniques and robust statistical analysis, are important tools for understanding the environment. In the last decades, its application in mineral exploration is well established and, in the last decades, it has shown remarkable relevance for environmental studies linked to sustainable territorial management, particularly in the establishment of geochemical background concentration values. The definition of background values has been widely used to demonstrate the inconsistencies of laws and regulations, which establish concentration limits for potentially toxic elements in the environment, often without taking into account the complex spatio-temporal heterogeneity of each region. In Brazil, many MGS were, and continue to be carried out mostly by the Companhia de Pesquisa de Recursos Minerais (CPRM, Geological Survey of Brazil) in different parts of the national territory, although they are still scarce in the Amazon region. The present study is associated with a large geochemical mapping project called the Geochemical Background of the Itacaiúnas River Basin (GBI) carried out by the Instituto Tecnológico Vale (ITV). The Itacaiúnas River Watershed (IRW) is located in the largest mineral province of Brazil, the Carajás Mineral Province, and it is particularly relevant for geochemical studies, because the distribution of chemical elements in this region and the influence of the great geological domains on that distribution are not known. The purpose of this research is to investigate the influence of large geological domains in the chemical composition and geochemical signature of stream sediments of the IRW. In addition, it is intended to generate geochemical maps, identify the associations and geochemical processes in stream sediments, define geochemical compartments for the whole watershed, and determine background concentrations for the analyzed elements, taking into account the geochemical compartments and/or geological domains of the study area. In order to achieve these objectives, the stream sediment geochemical data from the GBI-ITV project obtained in the whole IRW in 2017, and those from two CPRM projects in the center-south portion of the BHRI with sampling in the years 2011-2012 were used. These projects were conceptualized at different scales and sample density; however, the sampling techniques and analytical procedures are similar. Stream sediment sample collections were carried out in active water stream, at surface levels from 0 to 10 cm deep, and preferably in the middle of the channel. Sample preparation included the following steps: drying, disaggregation, quartering and sieving. Approximately 50 g of the fraction <0.177 mm was sent to accredited laboratory for chemical analysis. In the laboratory, the samples were submitted to digestion with aqua regia, and then 51 elements were analyzed (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn and Zr) via Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The geochemical data was submitted to descriptive statistical analysis and some non-parametric statistical tests. Several graphs (e.g., boxplots, histograms, probability and dispersion) were generated for the main chemical elements. Multivariate statistics (e.g., correlation matrices, cluster analysis and factor analysis) were used to investigate the main existing multi-element associations. To determine geochemical background values, modern methods widely used in the specialized literature were employed. Geospatial information was processed and managed in an environment of geographic information systems, in which different cartographic and geoprocessing techniques were used to generate geochemical distribution maps. In general, it was found that, on the scale of geochemical surveys conducted by ITV and CPRM, there is no conclusive evidence of contamination related to human activity, but very strong evidence of a marked geological contribution in the geochemistry of the stream sediments of the IRW. A consistent and replicable methodology was used to identify the main multi-element associations and to define IRW’s surface geochemical compartments. The geochemical associations identified are controlled by the geological domains, by specific lithologies in restricted areas and/or by biogeochemical factors acting in the study area. The delimitation of geochemical compartments revealed a strong similarity with the simplified geological domains of the basin. Geochemical background concentrations were determined for the IRW and its geological domains. Among the methods suggested for determining the background, the Median ± 2*Median Absolute Deviation showed the most consistent and realistic results. In addition, it is considered essential to define reference values based on geochemical compartments, or even a simple geological setting. The assumption of a single uniform reference value for a wide area is inappropriate. The background values proposed in this research may assist environmental impact studies by monitoring anomalous concentrations of potentially toxic elements, which exceed background concentrations. In addition to its scientific interest, the results presented here can be useful to assist local surveys of geochemical prospecting and in the formulation of environmental policies in the Brazilian territory.