DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
Navegar
8 resultados
Resultados da Pesquisa
Item Iron-bearing phases in a peat-derived duricrust from Brazil.(2007) Varajão, Angélica Fortes Drummond Chicarino; Costa, Geraldo Magela da; Varajão, César Augusto Chicarino; Meunier, Alain; Colin, FabriceThe formation of aluminium-substituted iron-minerals is still subject to debate. We report a case study in a Humic Gleysol soil profile, developed on a sedimentary saprolite, from a basin floor on the Uberaba Plateau (Minas Gerais State, Brazil) where iron-phases are crystallized by a ferruginization process. The mineralogical and chemical properties were investigated by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and Mo¨ ssbauer spectroscopy. The specific surface area (BET method) and cation exchange capacity (CEC) were also determined. The soil profile is mainly composed of gibbsite, kaolinite and amorphous alumino-silicate phases, the latter being more frequent in the H horizon where the organic carbon content is greater. The surface area and CEC values are also greater in the H horizon (58.6 m2g 1 and 4.65 molckg 1, respectively) which indicates an increase in porosity caused by the presence of 20.8% of organic matter and amorphous materials. Goethite occurs as a secondary mineral in the H horizon and as the main mineral in the duricrust in association with haematite. The omnipresence of aluminium in the environment (24.6–46.2% of Al2O3) resulted in Al-substitution in all ironbearing phases but the goethite from the H horizon has the greatest Al-for-Fe substitution, with at least 20 mole per cent of aluminium. In spite of the greater microporosity and wetness of the H horizon, the immediate contact with the richest Al-source (gibbsite) favours the precipitation of unusually greatly Al-substituted goethite instead of haematite in this horizon.Item Study of the erosion rates in the upper Maracujá Basin (Quadrilátero Ferrífero/MG, Brazil) by the in situ produced cosmogenic 10Be method.(2007) Salgado, André Augusto Rodrigues; Varajão, César Augusto Chicarino; Colin, Fabrice; Braucher, Régis; Varajão, Angélica Fortes Drummond Chicarino; Nalini Júnior, Hermínio AriasThe present work quantifies the erosive processes in the two main substrates (schists–phyllites and granites–gneisses) of the upper Maracujá Basin in the Quadrilátero Ferrífero/MG, Brazil, a region of semi-humid tropical climate. Two measuring methods of concentration were used: (i) in situ produced 10Be in quartz veins (surface erosion rates) and (ii) 10Be in fluvial sediments (basin erosion rates). The results confirm that (i) erosion tends to be more aggressive close to the headwaters than in the lower parts of the basin and (ii) the region is now affected by dissection.Item Estudo da evolução da paisagem do Quadrilátero Ferrífero (Minas Gerais, Brasil) por meio da mensuração das taxas de erosão (10Be) e da pedogênese.(2009) Varajão, César Augusto Chicarino; Salgado, André Augusto Rodrigues; Varajão, Angélica Fortes Drummond Chicarino; Braucher, Régis; Colin, Fabrice; Nalini Júnior, Hermínio AriasGeomorfologicamente o Quadrilátero Ferrífero é uma região conspícua, onde raízes de estruturas metassedimentares proterozóicas, apresentando feições de um relevo jovem, encontram-se em destaque sobre um mar de colinas de rochas cristalinas do Arqueano. O presente trabalho teve como objetivo o estudo da evolução da paisagem do Quadrilátero Ferrífero por meio da análise integrada dos dados quantitativos das taxas de erosão (10Be) e dos tipos de perfis de solos desenvolvidos a partir dos principais litotipos da região. A mensuração da concentração de 10Be extraído do quartzo de veios, de quartzitos e de sedimentos fluviais foi obtida utilizando um espectrômetro de massa por acelerador. A razão média de erosão de 7 m por milhão de anos coloca em evidência um importante soerguimento epirogenético da região em estudo. Concordantemente, os estudos macromorfológicos, mineralógicos e micromorfológicos de todos os perfis de solos investigados por meio de trabalhos de campo, da difração de raios X e da microscopia óptica mostram perfis imaturos e autóctones. Esses resultados sugerem que o relevo do Quadrilátero Ferrífero é um produto de constante e intenso processo erosivo.Item O papel da denudação geoquímica no processo de erosão diferencial no Quadrilátero Ferrífero/MG.(2004) Salgado, André Augusto Rodrigues; Colin, Fabrice; Nalini Júnior, Hermínio AriasO presente trabalho investiga a contribuição da denudação geoquímica nos processos erosivos diferenciais do Quadrilátero Ferrífero/MG, área de relevante interesse geomorfológico por possuir, em seu modelado, importantes marcas desse tipo de processo. A fim de cumprir este objetivo foram utilizadas análises físico-químicas e químicas de águas superficiais coletadas em 18 (dezoito) pontos de monitoramento, localizados em quatro sub-bacias hidrográficas que drenam a área de estudo. A área em questão possui clima tropical semi-úmido com estações seca (inverno) e úmida (verão) bem delimitadas. A coleta das amostras de água foi realizada nestes dois períodos distintos. Os parâmetros analisados foram: pH, sólidos totais dissolvidos (TDS), sílica, ferro, alumínio, sódio, cálcio, magnésio, manganês e potássio. Os resultados demonstraram que a denudação geoquímica é um dos fatores responsáveis pela erosão diferencial, uma vez que é possível verificar a existência de três comportamentos denudacionais geoquímicos distintos: (i) elevadas taxas de denudação geoquímica nas áreas moldadas sobre rochas carbonáticas; (ii) taxas médias de denudação geoquímica nas áreas modeladas sobre granitos, gnaisses, xistos e filitos e; (iii) baixas taxas de denudação geoquímica nas áreas mais elevadas (quartzitos e itabiritos) em relação às depressões adjacentes (xistos, filitos, granitos e gnaisses). Tal fato comprova para a área investigada que as águas fluviais regionais são capazes de marcar a assinatura geoquímica dos litotipos que drenam. Foi possível ainda concluir que, além dos litotipos, as áreas das bacias hidrográficas e as altitudes das mesmas em relação ao nível de base regional constituem fatores que também interferem na intensidade da denudação geoquímica.Item Peatland ferruginization during late Quaternary in the Uberaba Plateau (South-Eastern Brazil).(2013) Meunier, Jean Dominique; Varajão, Angélica Fortes Drummond Chicarino; Varajão, César Augusto Chicarino; Colin, Fabrice; Grauby, OlivierDue to the poor preservation of old peat formations and the limited research developed on them, the contribution of peat oxidation to the global C cycle at geological scales is poorly understood. Iron duricrusts containing abundant well-preserved plant structures have been reported above Humic Gleysols in the Uberaba Plateau (Brazil).We show that the iron accumulation results from an in-situ impregnation of peat, fast enough to preserve the plant structures. The formation of iron oxides results from two processes: precipitation in the pores and C/Fe replacement. The iron duricrusts were probably triggered by oxidation of the peatland following dry climatic events during the last 50 kyr. The large amount of iron dissolved in peatland waters was immobilized contemporaneously with the destruction of organic matter. The oxidation of organic matter from the lower peat, dated at ca 24e27 kyr BP, may have released between 0.08 and 2.26 kg CO2 m 2 yr 1 in the atmosphere. These rates are in a good agreement with present-day measurements of CO2 release from drained peatlands. Although peatland formation has been identified as a significant contributor to the global CO2 uptake, our findings suggest that natural peatland oxidation should also be considered as a source of atmospheric CO2 during past climate change.Item Denudation rates of the Quadrilátero Ferrífero (Minas Gerais, Brazil) : preliminary results from measurements of solute fluxes in rivers and in situ-produced cosmogenic 10Be.(2006) Salgado, André Augusto Rodrigues; Braucher, Régis; Colin, Fabrice; Nalini Júnior, Hermínio Arias; Varajão, Angélica Fortes Drummond Chicarino; Varajão, César Augusto ChicarinoThis paper investigates the denudation rates in the Quadrila´tero Ferrı´fero, Minas Gerais State (Brazil). The aim is to compare chemical weathering rates from measurements of solute fluxes in rivers and long-term mean erosion rates deduced from in situproduced cosmogenic 10Be concentrations measured in fluvial sediments. Both water samples and sediments were collected in fifteen stations (checkpoints) located in four hydrographic basins with low anthropogenic perturbations. Depending of the type of substratum, three degrees of chemical denudation rates from water samples are observed: (i) high rates in marbles; (ii) medium rates in schists, phyllites, granites, gneisses and migmatites; (iii) low rates in quartzites and itabirites. Preliminary results of long-term erosion rates deduced from in situ-produced 10Be are comparable with those of chemical rates.Item Early weathering of palladium gold under lateritic conditions, Maquiné Mine, Minas Gerais, Brazil.(2000) Varajão, César Augusto Chicarino; Colin, Fabrice; Vieillard, Phillip; Melfi, Adolpho José; Nahon, DanielSince the 80's, studies have shown that Au is mobile in supergene lateritic sur®cial conditions. They are based either on petrological, thermodynamic studies, or experimental works. In contrast, few studies have been done on the mobility of the Pt group elements (PGE). Moreover, at the present time, no study has addressed the di€erential mobility of Au, Ag and Pd from natural alloys in the supergene environment. The aim of this study is to understand the supergene behavior, in lateritic conditions, of Au±Ag±Pd alloys of the Au ore locally called Jacutinga at the Maquine Mine, Iron Quadrangle, Minas Gerais state, Brazil. The ®eld work shows that the host rock is a ``Lake Superior type'' banded iron formation (BIF) and that the Au mineralization originates from sul®de-barren hydrothermal processes. Primary Ag±Pd-bearing Au has developed as xenomorphous particles between hematite and quartz grains. The petrological study indicates that the most weathered primary Au particles with rounded shapes and pitted surfaces were found, under the duricrust, within the upper friable saprolite. This layer, however is not the most weathered part of the lateritic mantle, but it is where the quartz dissolution resulting porosity is the most developed. The distribution of Au contents in the weathered rocks are controlled by the initial hydrothermal primary pattern. No physical dispersion has been found. Most of the particles are residual and very weakly weathered. This characterizes early stages of Au particle weathering in agreement with the relatively low weathering gradient of the host itabiritic formations that leads essentially to the development of isostructural saprolite lateritic mantle. Limited dissolution of primary Au particles issued from the friable saprolite induces Pd±Ag depleted rims compared to primary Au particle Pd±Ag contents.In addition, limited very short distance in situ dissolution/reprecipitation processes have been found at depth within the primary mineralization, as illustrated by tiny supergene, almost pure, Au particles. The supergene mobility order Pd> Ag> Au as re¯ecting early weathering stages of Au±Ag±Pd alloys under lateritic conditions is proposed.Item Phytoliths as indicators of pedogenesis and paleoenvironmental changes in the Brazilian cerrado.(2006) Roschel, Miriam Borba; Alexandre, Anne; Varajão, Angélica Fortes Drummond Chicarino; Meunier, Jean Dominique; Varajão, César Augusto Chicarino; Colin, FabriceThis paper presents the preliminary results of phytolith analyses of a peat located in the cerrado of the Uberaba municipality, State of Minas Gerais, Brazil. The phytolith soil sequence is discussed by comparison with phytolith assemblages extracted from dominant plants (Cyperaceae and Poaceae). Increasing mean age of phytolith assemblages with depth is assumed. Poaceae Cuneiform bulliform cell, Parallelepiped bulliform cell and Elongate smooth long cell types dominate up to 80 cm, rapidly drop to 42% at 80 cm and regularly decrease from 42% to 2% upwards. Cyperaceae Rondel concave type shows the inverse trend, being dominant in the upper part of the profile. This pattern can be assigned to increasing selective dissolution of the Cyperaceae phytolith type with depth, or/and to a decrease of water stress suffered by the grasses, leading to a decrease of bulliform cell silicification. Soil processes and paleo-environmental changes hypotheses are discussed.