Hidrogeoquímica do baixo e médio rio Madeira e de seus principais tributários, Amazonas – Brasil

Abstract

The present work addresses the Madeira River along with its effluents’ waters and sediments, from the city of Porto Velho right down its mouth into the Amazonas river, aiming to determine the compositional and mineralogical influence brought about by sediments carried by the large Amazonian rivers with headwaters, in the Andes, originating from cratonic rocks. Major ions, trace elements, including REEs, and Sr isotops were analyzed in the waters during three hidrological periods, low water, high water and transition, between 2009 and 2010. On sediments we carried out a study based on granulometric, mineralogical, chemical, including Pb isotop, analyses. There is an apparent hidrological cycle control on the chemistry of the waters, yet it acts in a differentiated mode according to the dissolved element order of greatness, since in both the Madeira river and its tributaries water, most trace elements present greater dilution in high water, whereas for REEs, this occurs in low water, along with the Sr isotopic ratio showing to be lower in the latter. There is influence of both vegetation and geology on the chemistry of the waters, which becomes quite evident in the difference between the Madeira river water, originating from the Andes, and that of its tributaries originating from the southern Amazonian stable cratonic environment, presenting low erosive capacity, therefore generating more diluted waters. Data shows the Madeira River’s bed sediments to be granulometrical, mineralogical and chemically distinct from those of its main affluents. Madeira River sediments are sandier, present larger quantity of quartz, lower content of Al2O3, Fe2O3, K2O, MgO, Na2O, TiO2, P2O5, MnO and trace elements, and their maturity increases downriver. Among tributary sediments, those from from the Machado and then Marmelos Rivers are the ones showing to be closer to Madeira River’s sediment composition, while Jamari sediments, with higher TiO2, Zr, Y, Nb, Ga, Hf, U, Ta and REE are the most distinct ones. Despite the intensive weathering and erosion environment, in the humid tropical conditions, to which rocks drained by Madeira River basin, are submitted to, Th/Co, Th/Pb, Th/Yb, Al/Pb, Zr/Co ratios and Pb isotopic composition values indicate distinct sources for the studied sediments. Madeira River sediments would originate mainly from mafic rocks, while tose from its tributaries would essentially have felsic rocks as their source.