Hermínio Arias Nalini Júnior

Distribuição, fracionamento e mobilidade de elementos traço em sedimentos superficiais

The gold and iron mining activities in the Velhas River yield considerable amounts of suspended solids as well as metals to the hydrological system of the region. The risks were assessed by sequential extraction procedures. They were carried out in sediment samples, aiming to evaluate the presence of Co, Cr, Cu, Ni, Zn and As. Although being at high levels in some parts of the river (e.g. As 527 mg g-1 and Cr 475 mg g-1) they are associated with the residual fraction. Mobility factors were also calculated and show that these species are enriched along the river, e.g. for As 0.02 at the spring and 0.33 at the high course of the river.

Valores de background geoquímico e suas implicações em estudos ambientais

The establishment of background geochemical values as a relative measure to distinguish anthropogenic input from natural (geogenic and/or biogenic) concentrations of elements in different environmental samples, is one of the most important subjects in recent environmental sciences. The relationship between natural and anthropogenically altered concentrations of chemical species is a question that involves many implications in the geological, toxicological, biological and other areas of knowledge. This is especially important when geochemical interpretations of toxic elements are required. Thus, the principal objective of this article is to present and discuss the reasons for which the establishment of background geochemical values has implications in environmental studies. Throughout this article some concepts of the geochemical background used by various authors is presented and the main methods used for assessing background concentration of elements (direct and indirect methods). In addition, some examples of studies developed in Brazil are presented. Studies involving the establishment of background geochemical values are important because they allow the evaluation of potential contamination risks and the identification of natural trace-element concentrations in a determined area.

ADSORÇÃO DE As, Cu, Pb E Cr NA AVALIAÇÃO DA CAPACIDADE DE FIXAÇÃO DE METAIS POR RESÍDUO DE MINERADORAS DE FERRO

Two samples of residues from iron mining plants have been investigated for their retention capacity of As, Cu, Cr, and Pb. The sample with the higher content of iron oxides showed the highest capacity to retain metals. The adsorption affinity series changes from Pb>Cu>Cr»As to As>Pb>Cu>Cr or As>Cu>Cr>Pb, depending on the material and the concentration of the initial solution. In the competitive environment, the Pb adsorption decreases and the As, Cu and Cr adsorption increases. Sequential extraction procedures, carried out after adsorption batch experiments, showed that the most important adsorption process occurs in the oxide fraction and that the major part of the absorbed metal is remobilized from exchangeable and oxide fractions.

A INFLUÊNCIA LITOLÓGICA NAS TAXAS DE DENUDAÇÃO GEOQUÍMICA DO MÉDIO ESPINHAÇO MERIDIONAL - MG

O presente trabalho visa investigar a influencia da litologia nas atuais taxas de denudacao geoquimica na media Serra do Espinhaco Meridional. As taxas de denudacao correspondem ao volume de material proveniente da alteracao das rochas retirado de uma determinada area em um dado periodo de tempo (Souch, 2004). Estas taxas sao intensamente controladas pelo substrato geologico, notadamente em bacias hidrograficas de menor extensao. O metodo adotado baseia-se na coleta de amostras das aguas superficiais de dezesseis sub-bacias hidrograficas – que apresentam baixa interferencia antropica e estao sobre substrato geologico predominantemente quartzitico – e na medicao da vazao dos canais amostrados. Analises quimicas de aguas superficiais, coletadas nos periodos umido e seco do ano, foram utilizadas no calculo da taxa de denudacao geoquimica, que, de modo geral, apresentou-se baixa devido a alta resistencia dos quartzitos - litologia predominante. Os resultados demonstraram que, apesar da relativa homogeneidade litologica, a area apresenta uma denudacao geoquimica diferencial, onde: (i) bacias que drenam rochas carbonaticas possuem taxas mais elevadas de denudacao, confirmando a alta solubilidade desse material, (ii) bacias que drenam unidades quartziticas apresentam taxas baixas de denudacao, principalmente aquelas sobre as formacoes Galho do Miguel e Sopa-Brumadinho.

Petrogenesis and tectonic of the Urucum granitic suite, Rio Doce Valley (Minas Gerais – Brazil): an example of syn to late collisional peraluminous magmatism associated with high-angle transcurrent shear zone

The Urucum suite (582 ± 2 Ma, zircon U-Pb age), situated in the Mid-Rio Doce Valley, eastern part of Minas Gerais State, is characterized by elongated, NW-SE and N-S trending granitic massifs associated with the Conselho Pena-Resplendor high-angle shear zone. It corresponds to a syn to late collisional magmatism that presents dominant solid-state foliation. Four facies are distinguished within the Urucum suite: (i) a porphyritic (Urucum); (ii) a medium- to coarse nequigranular (Palmital); (iii) a tourmaline-bearing; and (iv) a pegmatitic facies. These facies are peraluminous, with alumina saturation index varying from 0.98 to 1.38. SiO2 contents vary from 70.7 to 73.7 wt%, with K2O values ranging from 3.5 to 5.7 wt%, Na2O from 1.9 to 4.4 wt%, MgO from 0.6 to 1.2 wt%, and CaO from 0.3 to 0.9%. Harker-type diagrams show rather continuous trends from the less-evolved Urucum facies to the more evolved tourmaline-bearing and pegmatitic facies. The behavior of several major oxides and trace elements (Fe2O3, MgO, MnO, CaO, TiO2, Al2O3, K2O, Rb and Ba) reflects the role played by fractionation of ferromagnesian minerals, feldspars and accessory minerals. Initial Sr87/Sr86 ratios vary from 0.711 to 0.716, with eNd (580 Ma) values between -7.4 to -8.2, and Sm-Nd TDM model ages ranging from 2290 to 1840 Ma.

DENUDAÇÃO QUÍMICA E REBAIXAMENTO DO RELEVO EM BORDAS INTERPLANÁLTICAS COM SUBSTRATO GRANÍTICO: DOIS EXEMPLOS NO SE DE MINAS GERAIS

Os interfluvios entre as grandes bacias hidrograficas do Brasil oriental correspondem, muitas vezes, a degraus morfologicos que dividem planaltos escalonados. Esses degraus sao feicoes escarpadas resultantes da diferenca de intensidade dos processos erosivos nas cabeceiras dessas grandes bacias. Esse artigo apresenta um estudo da denudacao quimica em dois degraus dessas bordas interplanalticas localizadas no sudeste de Minas Gerais: (i) o degrau de Cristiano Otoni (250m) divisor das bacias dos rios Sao Francisco (terras altas ou planalto superior) e Doce (terras baixas ou planalto inferior) e, (ii) o degrau de Sao Geraldo (450m) que divide as bacias dos rios Doce (terras altas ou planalto superior) e Paraiba do Sul (terras baixas ou planalto inferior). Para entender a dinâmica da denudacao quimica nessas bordas escarpadas, foram monitorados o total de solidos dissolvidos (TDS), o Eh, o pH, a vazao de 21 bacias hidrograficas distribuidas ao longo de ambas vertentes dos degraus (terras altas e frente da escarpa) no final dos periodos chuvoso (abril) e de estiagem (agosto) em um mesmo ano hidrologico (2009). As bacias amostradas situam-se sobre o mesmo substrato litologico (granitoides). Utilizando os dados de TDS e vazao foram calculadas as taxas de denudacao quimica (ton.ano-1.km-²) e as taxas medias de rebaixamento do relevo (m.Ma-1) para as bacias amostradas. Os resultados revelaram que as aguas fluviais dos degraus estudados apresentam assinaturas geoquimicas distintas: (i) nas terras altas, os valores de pH sao sempre acidos (≈ 6,35) e nas escarpas, levemente basicos (≈ 7,10); (ii) os valores de TDS (carga dissolvida) nas terras altas (≈ 10,00 mg.L-1) sao menores que os encontrados nos frente das escarpas (≈ 35,00 mg.L-1 para Sao Geraldo e, ≈ 18,00 mg.L-1 para Cristiano Otoni). As taxas de rebaixamento do relevo ocasionadas pela denudacao quimica sao semelhantes em ambas as terras altas (≈ 2,40 m.Ma-1). Em contrapartida, a frente da escarpa da Serra de Sao Geraldo apresenta taxa de denudacao quimica mais elevada (7,06 m.Ma-1) do que o Degrau de Cristiano Otoni. (4,25 m.Ma-1). Essa diferenca foi interpretada como diretamente relacionada a altura das escarpas, 450 e 250 m, respectivamente.

Estudo Geoquímico e Petrológico dos Diques Máficos da Região de Candeias-Campo Belo-Santo Antônio do Amparo (MG), Porção Meridional do Craton São Francisco

Four groups of mafic dykes, distinguished by their petrographic, geochemical and tectonic features and designated as A1 amphibolites, A2 amphibolites, Gabbronorites and Gabbros, intrude the Campo Belo Metamorphic Complex in southern Minas Gerais. The majority of the samples have tholeiitic affinities, but the A2 amphibolites and one gabbronorite plot in the calc-alkaline field, and some A1 amphibolites and gabbronorites fall at the boundary of the two fields. The parental magmas of the four groups display different degrees of evolution: gabbros (mg# 0.18 - 0.23), gabbronorites (mg# 0.33 - 0.35), A2 amphibolites (mg# 0.34 - 0.37) and A1 amphibolites (mg# 0.24 - 0.45). Incompatible trace-element patterns, when compared with those of garnet and spinel peridotite sources, show enrichment of the original magmas, which mainly affected the parental melt of the gabbroic suite. This feature is attributed to either an enriched mantle source or crustal contamination. Major and trace element geochemistry indicates that the four groups are unrelated. Comparison of the trace-element patterns of the studied dykes with swarms from Salvador (Sao Francisco Craton), Carajas (Amazon Craton) and Crixas-Goias (Archean Block of Goias) suggest formation in an intracratonic environment.