This work consists of a geochemical study of heavy metals and Pb isotopic signature of sediments from the hydrographic basin of the Amapari river, which is the main affluent of the right edge of the Araguari river, in the central region of Amapá State. The study is more precisely located in a part of the river, close to the Serra do Navio area, between the cities of Pedra Branca do Amapari and Porto Grande. The region hosts the larger mining district of Amapá, including two important gold and iron mines at the Serra do Navio. The aim of this work is to investigate the distribution of heavy metals and other metals like Al, Fe, Th and U, together with the determination of Pb isotopic composition of recent sediments that are sediments deposited at the superior edge of the river during inundation events (overbank sediments), sampled along the Amapari river and tributaries, in order to distinguish the natural contributions from the main geological units and a possible contribution of mining activities. The sampling strategy depended on two factors: (1) the occurrence of adequate outcrops and (2) the accessibility. The sediments, collected at 25 sampling points in November, 2007, include a set of samples from the Amapari river and three sets of samples collected in tributary of the right edge (Cachorrinho river and Cupixi river) and left edge (Cupixizinho river). In all the samples, granulometric, mineralogical, chemical and isotopic analyses were performed. The granulometric analyses were effectuated in order to determine the proportions of fine material (silt + clay) and clay. The mineralogical composition was determined by Xrays Diffractometry using a X-rays diffractometer with cobalt anode tube. The determination of major end trace elements contents has been carried out at a commercial laboratory (ACME Analytic Laboratory) by ICP-EOS and ICP–MS after four acids dissolution (HF + HCl + HClO4 + HNO3) in 8 whole samples and 22 samples of the fine fraction of the sediments. The elements of major interest were: Al, Fe, Pb, Mn, Cu, Zn, Ni, As, Cd, Cr, Th e U. For the isotopic analyses, the samples suffered total digestion with HF and HNO3+HCl (20 samples) and acid leaching with HNO3 (5 samples). The Pb separation and purification were achieved by ionic exchange chromatography (1x8 DOWEX resin). The Pb isotopic compositions were determined in static mode with a multi-collector thermal ionisation mass spectrometer Finnigan MAT262 at the Pará-Iso Laboratory of the IG-UFPA. The geochemical results indicate that all heavy metals behaved similarly and are enriched in the same samples, indicating that the sediments suffer the same processes of enrichment for all investigated heavy metals, excepting cadmium that displayed a different trend. No relationship between granulometry and heavy metals content was pointed out. Thus, the proportion of clays presents in the sediments does not appear to have been preponderant for heavy metals concentrations, at the contrary of that commonly occur in bottom sediments. Not significant variation of mineralogical composition has been observed between the fine fraction and the fraction superior to 62μm that account for the similarity of the chemical compositions between those fractions. In both granulometric fractions, the same main paragenesis composed by quartz, kaolinite, gibbsite and muscovite was identified. Other minerals were recognized in some of the samples (anatase, rutile, microline, etc.), which probably belong to the primary paragenesis. The presence of these minerals does not have influence on the heavy metals concentrations. The heavy metals contents furnished a better correlation with Fe than with Al contents although Fe-minerals were not been identified in the samples. The Pb isotopic compositions displayed significant variations and linear rend that permitted to construct isochrons in the 207Pb/204Pb vs. 206Pb/204Pb diagram, which defined Paleoproterozoic ages around 2,0 Ga, (Amapari river: 1964 ± 88 Ma, mswd = 1,6), indicating that the sediments came essentially from the Paleoproterozoic supracrustal units (metassedimentary and mafic rocks) and associated granitoids. Despite its extensive exposition in the area, the Archean basement does not significantly contribute as a source of the sediments. The concentrations of metals display strong variations according to the geographical location but they are randomly distributed along the Amapari river, indicating that homogenization does not occurred and that the sediments reflect the difference in the source rocks. Therefore the processes which account for that geochemical distribution are probably naturals as no was evidenced no correlation has been evidenced between metal contents and sample locations in relation the mining areas. The comparison between metals contents of the Amapari river and tributaries and those determinate by others authors in bottom sediments of a creek close to the Fe and Au mining plants of the Serra do Navio – Pedra Branca do Amapari confirms that hypothesis.
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