Adriana Maria Coimbra Horbe

Spatial-temporal variation of dissolved inorganic material in the Amazon basin

O presente trabalho estuda a variabilidade espaco-temporal da composicao quimica do material inorgânico dissolvido (Ca2+, Na+, K+, Mg2+, Cl-, SO4 -2, HCO3 - e SiO2) nas aguas de seis estacoes hidrologicas da rede ORE-HYBAM na bacia Amazonica (Tabatinga- Nazareth, Manacapuru e Obidos no rio Solimoes Amazonas; Porto Velho e Borba-Fazenda Vista Alegre no rio Madeira e Labrea no rio Purus). Nas estacoes do rio Solimoes e Amazonas as concentracoes de Ca2+, Mg2+, HCO3 - e SO4 -2, tem distribuicao heterogenea ao longo dos oito anos estudados e nao apresentam sazonalidade. Entretanto nas estacoes do rio Madeira esses ions mostram variabilidade quimica sazonal e inversamente proporcional a vazao (efeito diluicao-concentracao). Este mesmo comportamento e observado para Cl- e Na+ nas estacoes do rio Solimoes-Amazonas e Madeira o que indica a liberacao de um fluxo quase constante ao longo do ciclo hidrologico. K+ e SiO2, apresentam concentracoes quase constantes ao longo dos anos e entre as estacoes, indicando que seus fluxos dependem da variabilidade da vazao. Identifica-se tambem que na distribuicao dos fluxos de Ca2+, Mg2+, HCO3 - e SO4 -2 nas estacoes do rio Solimoes e Amazonas intervem tanto fatores hidrologicosclimaticos como a heterogeneidade das fontes (ex. carbonatos, silicatos). No Madeira e Purus, ha menor influencia destes fatores, o que evidencia maior aporte dos silicatos com nintemperismo mais lento que os carbonatos. Portanto, a geologia e os fatores hidroclimaticos influenciam a distribuicao dos fluxos de material dissolvido. Porem como a bacia do Solimoes aporta aproximadamente 84% do fluxo total de material dissolvido na bacia e esta sob influencia, principalmente do fator hidroclimatico, pode-se dizer que a variabilidade temporal desse fator controla a temporalidade dos fluxos de material dissolvido na bacia amazonica. Este estudo tem importância para o entendimento do processo geoquimico na bacia amazonica, alem de servir de linha base para futuros estudos.

Mineralogia, geoquímica e evolução da lateritização em Apuí, sudeste do Amazonas

Chemical analysis, XRD and SEM carried out in bauxites, Fe-rich and siliceous Fe-Al duricrusts of southeast of Amazonas state, associated to relief study, aimed to discuss the process that led to the formation and preservation of crusts through the geologic time. The crusts are massive, pisolitic, fragmental and vermiform textures, and support plateaus and hills with elevation between 200 to 320 m and 140 to 200 m, and developed from igneous and sedimentary rocks of Colider and Alto Tapajos Groups and Supergroup Sumauma. The correlation among lateritization products and rocks were defined for three main geochemistry associations: (1) that includes sandstones, volcanics and siltstones due to the association SiO2-Ba-Rb; (2) gathers bauxites and soils related to Al2O3-TiO2-PF-Y-Ta-Nb-U-Th-Ga-Hf-Zr-ETR; (3) brings together the pisolitic bauxite, Fe-rich and siliceous Fe-Al duricrusts assigned to Fe2O3- Ag-As-Ni-Pb-V. Geochemistry aspects and absence of quartz pebbles were used to recognize the protolith of bauxites and lateritic duricrusts, therefore massive bauxite and the overlapped soil belong to volcanics rocks. Sandstones of Supergroup Sumauma are correlated with pisolitic bauxites and vermiform siliceous Fe-Al duricrust, however Alto Tapajos Group is related to the other crusts. The erosion was facilitated by the incision of Aripuana, Acari and Sucunduri rivers; by the conditioning of grabens and horsts; and by dry seasons in Quaternary Amazonian that promoted the formation of truncated profiles with stone line exposures, dissected relief and thin soil.

A crosta laterítica ferro-titanífera da Vila Nova Colina e a lateritização no sul de Roraima

Lateritic crusts developed upon the Guiana Shield basement rocks (e.g. granites of the Agua Branca Intrusive Suite) was recognized in the Vila Nova Colina region, located in south Roraima state. The objectives of this work were to study the mineral and geochemical characteristics of lateritic crusts and to evaluate the relationship between them and the rocks of the region, in order to understand the evolution of laterization and provide information about the economic potential. This study comprises petrography, mineralogy and litogeochemical investigations of the granites, the quartz-hornblende gabbro and the lateritic crusts formed from these rocks. The granites and the quartz-hornblende gabbro has developed, respectively, a specific type of lateritic crust: (i) the ferruginous vermiform one with high amounts of kaolinite, goethite, hematite, magnetite and anatase, higher concentration of Al2O3, Sc, Th e Zr and, (ii) the iron-titaniferous massive one with high amounts of magnetite, hematite, maghemite, goethite, kaolinite, anatase, gibbsite and ilmenite, and consequently in higher amounts of Fe2O3, TiO2, V e Cu. Each crust type indicates the effect of the lateritization in the south portion of Roraima and their topographic position. The higher topographic position of the massive crust (220 m), its mineral and geochemical composition, indicate the maturity (i.e. needed more time to be formed) and may represent a relict of an erosion phase in the region, probably associated with a subsidence of the Tacutu basin in the Miocene. Otherwise, the vermiform crust formed at a lower topographic position (140 m) with no gibbsite, suggests that it is younger than the massive one and represents the immature laterization of the Plio-Pleistocene.