Mapeamento geoquímico de baixa densidade por sedimentos de corrente em Roraima: definição de valores de referência (background/baseline geoquímico) e abordagens por índices ambientais e análise multivariada

Abstract

Stream sediments provide essential information on the distribution of chemical elements in river basins, whether because of rock weathering or the influence of human activities. In the State of Roraima, growing concerns about the impacts of mining and agriculture drive the need to define a geochemical baseline. This baseline allows distinguishing natural element concentrations from those influenced by anthropogenic activities. Such an analysis is crucial for accurately assessing environmental impacts and monitoring future changes in regional geochemistry, in accordance with the elements stipulated in CONAMA Resolution 454/2012 (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn). For this purpose, data from the "Low-Density Geochemical Survey in the State of Roraima" project were used, with samples collected between 2009 and 2013. Sampling was carried out synchronously during the same precipitation period to avoid errors due to seasonality. The study area covers 106,369.051 km², distributed across 429 micro-basins. Active stream sediments were collected by the SGB following the technical manual of the Geochemistry Area v.5.0. At each sampling site, coordinates were recorded, and the material was sieved to obtain the silt and clay fraction. The samples were collected in a composite manner over an approximately 100-meter stretch along the drainage upstream of the access point. The analytical methods protocol included drying the samples at 60°C, sieving them through a <80-mesh screen, and sending them to the SGS GEOSOL LTDA laboratory for chemical analysis. In the laboratory, the samples underwent digestion with aqua regia (HNO₃ + 3HCl) and were subsequently analyzed for 53 chemical elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, F, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Pd, Pt, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, U, V, W, Y, Zn, and Zr) using ICP-OES and ICP-MS. Exploratory data analysis employed descriptive statistics, graphs, and contingency tables to support the selection of non-parametric univariate, bivariate, and multivariate statistical techniques. Additionally, modern methods were applied to determine geochemical baseline values, including mMAD, TIF, and percentiles (98, 95, 90, 75). Anomalies identified in the singleelement maps, based on the geochemical baseline thresholds, are explained by multielement associations determined through confirmatory factor analysis. This analysis revealed six main associations that explain the distribution of chemical elements and their relationship with various geological processes. Factor 1 (20.60% of variance): Includes elements such as Ba, Cs, Rb, Sr, and Zn, associated with differentiated igneous rocks, granites, pegmatites, and hydrothermal processes linked to the Surumu domain. Factor 2 (15.76%) Highlights rare earth elements and actinides (Ce, La, Th, U, Y), concentrated in accessory minerals such as monazite and apatite, associated with the magmatism of the Orocaima Igneous Belt. Factor 3 (13.90%) Identifies an association of Mn, Nb, Ti, and Fe with tholeiitic magmatism in the Avanavero Suite and alluvial deposits. Factor 4 (11.68%) Comprises elements typical of mafic and ultramafic rocks, such as Co, Cr, Cu, Ni, and V, present in Mesoproterozoic basalts and gabbros. Factor 5 (7.51%) Relates elements like Al, Ga, Pb, and Zn to weathering processes and the formation of secondary minerals in saprolitic and alluvial environments. Factor 6 (6.01%) Associates Sc, Sn, and Nb with granite intrusions and placer deposits related to the magmatism and tectonics of the Uatumã SLIP and the Orocaima Igneous Belt. The environmental quality assessment using geochemical and environmental indices indicated that the region exhibits significant enrichment, mainly related to geology. The interpretation of results highlights the interaction between regional geology and tectono-magmatic processes in the distribution of analyzed elements. Additionally, anomalous zones with potential for targeted investigations were identified, particularly in areas impacted by human activities and land-use changes.