Geochemistry of natural chromium occurrence in a sandstone aquifer in Bauru Basin, Sao Paulo State, Brazil

Anomalous concentrations of Cr(VI) occur in groundwaters of the Adamantina Aquifer, in a large region in the western state of Sao Paulo, sometimes exceeding the potability limit (0.05 mg L(-1)). To identify the possible geochemical reactions responsible for the occurrence of Cr in groundwater in Urania, borehole rock samples were collected in order to carry out mineralogical and chemical analyses. In addition, multilevel monitoring wells were installed and groundwater samples were analyzed. Analyses of the borehole rock samples show the occurrence of a geochemical anomaly of Cr in the quartzose sandstones (average concentrations of 221 ppm). Chrome-diopside is one of the main minerals contributing to this anomaly, having an average Cr content of 1505 ppm. Sequential extraction experiments indicated weakly adsorbed Cr in the order of 0.54 ppm, and this quantity is enough to provide the Cr concentrations observed in groundwater. Groundwaters from the monitoring wells proved to be stratified, with the highest concentrations of Cr(VI) (0.13 mg L(-1)) being associated with high redox and pH values (over 10) and high concentrations of Na. Geochemical reactions that may explain the release of Cr from the solid phase to groundwater involve the release of Cr(III) from minerals (like chrome-diopside and Cr-Fe hydroxide), followed by oxidation of Cr(III) to Cr(VI), probably related to the reduction of Mn oxides present in the aquifer. Then cation exchange occurs and dissolution of carbonates which increases the pH of groundwater, resulting in the desorption and mobilization of Cr(VI) into groundwater. (C) 2011 Elsevier Ltd. All rights reserved.

Petrology and geochemistry of the banded iron formation in the Eastern Sierras Pampeanas of San Luis (Argentina): Implications for the evolution of the Nogoli Metamorphic Complex

The metamorphosed banded iron formation from the Nogoli Metamorphic Complex of western Sierra de San Luis, Eastern Sierras Pampeanas of Argentina (Nogoli area, 32 degrees 55`S-66 degrees 15`W) is classified as an oxide facies iron formation of Algoma Type, with a tectonic setting possibly associated with an island arc or back arc, on the basis of field mapping, mineral and textural arrangements and whole rock geochemical features. The origin of banded iron formation is mainly related to chemical precipitation of hydrogenous sediments from seawater in oceanic environments. The primary chemical precipitate is a result of solutions that represent mixtures of seawater and hydrothermal fluids, with significant dilution by maficultramafic volcanic and siliciclastic materials. Multi-stage T(DM) model ages of 1670, 1854 and 1939 Ma and positive, mantle-like xi Nd((1502)) values of +3.8, +1.5 and +0.5 from the banded iron formation are around the range of those mafic to ultramafic meta-volcanic rocks of Nogoli Metamorphic Complex, which are between 1679 and 1765 Ma and +2.64 and +3.68, respectively. This Sm and Nd isotopic connection suggests a close genetic relationship between ferruginous and mafic-ultramafic meta-volcanic rocks, as part of the same island arc or back arc setting. A previous Sm-Nd whole rock isochron of similar to 1.5 Ga performed on mafic-ultramafic meta-volcanic rocks led to the interpretation that chemical sedimentation as old as Mesoproterozoic is possible for the banded iron formation. A clockwise P-T path can be inferred for the regional metamorphic evolution of the banded iron formation, with three distinctive trajectories: (1) Relict prograde M(1)-M(3) segment with gradual P and T increase from greenschist facies at M(1) to amphibolite facies at M(3). (2) Peak P-T conditions at high amphibolite-low granulite facies during M(4). (3) Retrograde counterpart of M(4), that returns from amphibolite facies and stabilizes at greenschist facies during M(5). Each trajectory may be regarded as produced by different tectonic events related to the Pampean? (1) and the Famatinian (2 and 3) orogenies, during the Early to Middle Paleozoic. The Nogoli Metamorphic Complex is interpreted as part of a greenstone belt within the large Meso- to Neoproterozoic Pampean Terrane of the Eastern Sierras Pampeanas of Argentina. (C) 2009 Elsevier Ltd. All rights reserved.

Occurrence and geochemistry of metamafic rocks from the Forquilha Eclogite Zone, central Ceara (NE Brazil): geodynamic implications

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Deciphering the weathering processes using environmental mineralogy and geochemistry: Towards an integrated model of laterite and podzol genesis in the Upper Amazon Basin

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Geochemistry of the Guaxupe Granulites, Minas Gerais, Brazil

Geochemical data for granulite terrain are presented from the northernmost portion of the Guaxupe Massif, at Mantiqueira Province, SE Brazil. Several types of granulites are recognized in the area: basic, intermediate and acid granulite. Major and trace elements (including REE) point to only one ma,oma source for these granulites generated at different times. Geochemical data point to plagioclase and apatite fractionation as responsable in the REE behaviour in intermediate and more basic rocks. Overall composition of the Guaxupe Granulites is similar to average composition of the lower crust.

Isotope geochemistry of the mafic dikes from the Vazante nonsulfide zinc deposit, Brazil

The Vazante Group, located in the northwestern part of Minas Gerais, hosts the most important zinc mine in Brazil, the Vazante Mine, which represents a major known example of a hypogene nonsulfide zinc deposit. The main zinc ore is represented by willemite and differs substantially from other deposits of the Vazante-Paracatu region, which are sulfide-dominated zinc-lead ore. The age of the Vazante Group and the hosted mineralization is disputable. Metamorphosed mafic dikes (metabasites) that cut the metasedimentary sequence and are affected by hydrothermal processes recently were found and may shed light on the geochronology of this important geological unit. Zircon crystals recovered from the metabasites are xenocrystic grains that yield U-Pb conventional ages ranging from 2.1 to 2.4 Ga, so the basement of the Vazante Group is Paleoproterozoic or has metasedinientary rocks whose source area was Paleoproterozoic. Pb isotopes determined for titanite separated from the metabasites have common, nonradiogenic Ph compositions, which prevents determination of their crystallization age. However, the Pb signatures observed for the titanite crystals are in agreement with those determined for galena from the carbonate-hosted Zn-Pb deposits hosted by the Vazante Group, including galena from minor sulfide ore bodies of the Vazante deposit. These similarities suggest that the metalliferous fluids that affected the metabasites may have been those responsible for galena formation, which could imply a similar lead source for both nonsulfide and sulfide zinc deposits in the Vazante-Paracatu district. This common source could be related to deep-seated, basin-derived, metalliferous fluids associated with a long-lived hydrothermal system related to diagenesis and deformation of the Vazante Group during the Neoproterozoic. (c) 2005 Elsevier Ltd. All rights reserved.

Environmental changes in the western Amazônia: morphological framewor, geochemistry, palynology and radiocarbon dating data

Os sedimentos do lago Coari, de ambiente de terra firme esculpido nos depósitos do Plio-Pleistocenos, e o Acará, típico lago de várzea e ambos formados nos sedimentos quaternários da planície de inundação do médio Solimões, no oeste da Amazônia, Brasil, foram estudados para investigar as condições ambientais durante sua formação. Este estudo inclui dados da composição mineralógica, química, isótopos de Pb, palinologia, datações de radiocarbono e a configuração morfológica dos lagos obtida por imagens SRTM. As condições geológica e ambiental dos lagos variam e sugerem que suas evoluções refletem processos autogenéticos em condições de floresta úmida e chuvosa. Embora caulinita, quartz, muscovita, illita e esmectita sejam os principais minerais em ambos os lagos, a geoquímica indica fonte distinta, os sedimentos do lago Acará têm maior concentração de Al2O3, Fe2O3, FeO, CaO, K2O, MgO, Na2O, P2O5, Ba, V, Cu, Ni, Zn, Pb, Sr, Li, Y e La e têm mais Pb radiogênico que os sedimentos do lago Coari. As idades de radiocarbono sugerem que há aproximadamente 10160 anos AP o lago Coari iniciou o desenvolvimento devido a avulsão do rio Solimões, enquanto o lago Acará foi formado devido ao abandono de meandro do rio Solimões e retendo o domínio das gramíneas nas suas praias há aproximadamente 3710 anos AP.

Carbon and oxygen isotope geochemistry of Ediacaran outer platform carbonates, Paraguay Belt, central Brazil

Após a glaciação do final do Criogeniano, a região central do Brasil foi palco de extensa deposição de carbonatos plataformais do Grupo Araras. Este grupo inclui na sua base uma seqüência de capa carbonática sucedida por depósitos transgressivos de calcilutitos betuminosos e folhelhos de plataforma profunda. Dados de fácies e estratigráficos combinados com análises isotópicas de carbono e oxigênio da seção mais completa desses depósitos transgressivos, expostos no sinclinal da Guia, foram empregados para avaliar o paleambiente deposicional e para testar a correlação desses depósitos ao longo da faixa e também com outras unidades de outros continentes. A sucessão estudada consiste de 150 m de espessura de camadas tabulares de calcilutitos e folhelhos cinza a pretos com valores de d13CPDB negativos, entre -2,5 e -1 . O perfil de d13CPDB do sinclinal da Guia mostra clara correlação com a porção superior da Formação Guia da região de Cáceres, 200 km a sudoeste. O perfil de d13CPDB do Grupo Araras é comparável aos perfis de d13CPDB de outras unidades ediacaranas da parte sul da Faixa Paraguai, oeste do Canadá, e crátons do Congo e do Kalahari. Além disso, a distribuição de fácies, estratigrafia e o perfil isotópico do Grupo Araras são equiparáveis à porção média do Subgrupo Tsumeb na Namíbia, o que reforça a atribuição de idade Ediacarana para o Grupo Araras.

Petrography, magnetic susceptibility and geochemistry of the Rio Branco Granite, Carajás Province, southeast of Pará, Brazil

Petrografia, suscetibilidade magnética e geoquímica do Granito Rio Branco, Província Carajás, sudeste do Pará, Brazil. O Granito Rio Branco é um stock paleoproterozoico intrusivo no biotita-monzogranito arqueano Cruzadão. Ocorre a oeste da cidade de Canaã dos Carajás, nas proximidades da mina de cobre do Sossego na Província Carajás. É constituído por sienogranitos não deformados e isotrópicos, hololeucocráticos, em geral de granulação média. A mineralogia é formada por feldspato alcalino pertítico, quartzo e plagioclásio. A biotita, intensamente cloritizada, é a principal fase máfica, acompanhada por flluorita, allanita, zircão, pirita e calcopirita como minerais acessórios. Albitização e, com menor intensidade greisenização, afetaram o granito, sendo a mineralogia secundária albita, fluorita, topázio, clorita, muscovita, siderofilita e óxidos e/ou hidróxidos de ferro. O Granito Rio Branco apresenta valores sistematicamente baixos de suscetibilidade magnética (SM) variando de 1,3 x 10^-5 a 6,96 x 10^-4 (SI). Geoquimicamente, é metaluminoso a peraluminoso, possui altas razões FeOt/(FeOt + MgO) e mostra afinidades com granitos ferrosos, tipo-A do subtipo A2. Os padrões dos ETR revelam um ligeiro enriquecimento de ETR leves em relação ao ETR pesados e anomalia negativa acentuada de Eu (Eu/Eu* = 0,08 - 0,13), resultando feição em "gaivota", característica de granitos evoluídos. O conjunto de dados obtidos demonstra o caráter evoluído do Granito Rio Branco e sua derivação a partir de líquidos reduzidos e enriquecidos em voláteis, causadores das transformações hidrotermais tardias. O estudo comparativo deste corpo com aqueles das suítes anorogênicas da Província Carajás sugere que o Granito Rio Branco possui maior afinidade com os granitos das suítes Velho Guilherme e, em menor grau, Serra dos Carajás. Por outro lado, é claramente distinto da Suíte Jamon. Embora apresente características similares às dos granitos especializados em estanho, não há mineralizações desta natureza associadas ao corpo.

Geology, Geochemistry, and Geochronology (U-Pb) of the Rio Fortuna Gneiss - Serra do Baú Intrusive Suite - Paraguá Terrane - SW Amazonian Craton

O Gnaisse Rio Fortuna aflora na região da serra Santa Bárbara, nas imediações do Destacamento Militar Fortuna, na fronteira Brasil-Bolívia. Estes ortognaisses estão inseridos no Terreno Paraguá, em um setor afetado pela Orogenia Sunsás (1.0 a 0.9 Ga.). São classificados como ortognaisses de composição monzo a granodiorítica, com registros de, no mínimo, três fases de deformação. Idade U-Pb em zircão de 1.711 ± 13 Ma obtida por ablação a laser MC-ICP-MS, é considerada como correspondendo à idade de cristalização do protólito ígneo. Geoquimicamente, essas rochas constituem uma sequencia ácida formada por um magmatismo subalcalino, do tipo cálcio-alcalino de alto potássio, metaluminoso a peraluminoso.

Contrasted crustal sources as defined by whole-rock and Sr-Nd-Pb isotope geochemistry of neoproterozoic early post-collisional granitic magmatism within the Southern Brazilian Shear Belt, Camboriu, Brazil

The early phase of post-collisional granitic magmatism in the Camboriu region, south Brazil, is represented by the porphyritic biotite +/- hornblende Rio Pequeno Granite (RPG; 630-620 Ma) and the younger (similar to 610 Ma), equigranular, biotite +/- muscovite Serra dos Macacos Granite (SMG). The two granite types share some geochemical characteristics, but the more felsic SMG constitutes a distinctive group not related to RPG by simple fractionation processes, as indicated by its lower FeOt, TiO2, K2O/Na2O and higher Zr Al2O3, Na2O, Ba and Sr when compared to RPG of similar SiO2 range. Sr-Nd-Pb isotopes require different sources. The SMG derives from old crustal sources, possibly related to the Paleoproterozoic protoliths of the Camboriu Complex, as indicated by strongly negative epsilon Nd-t (-23 to -24) and unradiogenic Pb (e.g., Pb-206/Pb-204 = 16.0-16.3; Pb-207/Pb-204 = 15.3-15.4) and confirmed by previous LA-MC-ICPMS data showing dominant zircon inheritance of Archean to Paleoproterozoic age. In contrast, the RPG shows less negative epsilon Nd-t (-12 to -15) and a distinctive zircon inheritance pattern with no traces of post-1.6 Ga sources. This is indicative of younger sources whose significance in the regional context is still unclear; some contribution of mantle-derived magmas is indicated by coeval mafic dykes and may account for some of the geochemical and isotopic characteristics of the least differentiated varieties of the RPG. The transcurrent tectonics seems to have played an essential role in the generation of mantle-derived magmas despite their emplacement within a low-strain zone. It may have facilitated their interaction with crustal melts which seem to be to a large extent the products of reworking of Paleoproterozoic orthogneisses from the Camboriu Complex. (C) 2012 Elsevier Ltd. All rights reserved.

Whole-rock geochemistry and Sr-Nd isotopic composition of the pre-rift sequence of the Camamu Basin, northeastern Brazil

Whole-rock geochemistry, combined with Sr-Nd isotopic composition of pelitic sedimentary rocks, have been considered to be useful parameters to estimate not only their provenance but also to make inferences about their depositional environment as well as the weathering processes they have been through. The basal sedimentary units of the basins of the northeastern Brazilian continental margin, particularly those of the pre-rift sequence, have been subject of interest of studies based on chemical and isotopic data, since they lack fossil content to establish their age and, therefore, stratigraphic correlations are difficult. The major and trace element contents as well as Sr-Nd isotopic compositions of whole-rock shale samples from five outcrops attributed to the pre-rift supersequence of the Camamu Basin were analyzed with the purpose of characterizing and obtaining further information that would allow a better correlation between the sites studied. The geochemical data suggest that the rocks exposed in the studied outcrops are part of the same sedimentary unit and that they might be correlated to the Capianga Member of the Alianca Formation of the Reconcavo Basin, exposed to the north of the Camamu Basin. The chemical index of alteration (CIA) suggests conditions associated with a humid tropical/subtropical climate at the time of deposition. Nd isotopic compositions indicate provenance from the Paleoproterozoic rocks of the Sao Francisco craton. The results presented here, therefore, show that the combined use of chemical and isotopic analyses may be of great interest to characterize and correlate lithologically homogeneous clastic sedimentary sequences. (C) 2012 Elsevier Ltd. All rights reserved.

Geochemistry and Pb-207/Pb-206 zircon ages of granitoids from the southern portion of the Tamboril-Santa Quiteria granitic-migmatitic complex, Ceara Central Domain, Borborema Province (NE Brazil)

The Tamboril-Santa Quiteria Complex is an important Neoproterozoic granitic-migmatitic unit from the Ceara Central Domain that developed from ca. 650 to 610 Ma. In general the granitoids range in composition from diorite to granite with predominance (up to 85%) of granitic to monzogranitic composition with biotite as the main mafic AFM phase. Geochemical and Pb-207/Pb-206 evaporation zircon geochronology studies were applied in a group of these abundant monzogranitic rocks from the region of Novo Oriente in the southern portion of the Ceara Central Domain. In this area the granitoids are weakly peraluminous biotite granitoids and deformed biotite granitoids of high-K calc-alkaline and ferroan composition, which we interpreted as primary magmas (segregated diatexites) derived from the partial melting of crustal material. The close temporal relation of this magmatism with local eclogitic and regional high temperature metamorphism in Ceara Central Domain point out to an orogenic setting, arguably emplaced during the collisional stage. Subordinate coeval juvenile mantle incursions are also present. This crustally derived magmatism is the primary product of the continental thickening that resulted from the collision between the rocks represented by the Amazonian-West African craton (Sao Luiz cratonic fragment) to the northwest and the Paleoproterozoic-Archean basement of the Borborema Province to the southeast along the Transbrasiliano tectonic corridor. (C) 2011 Elsevier Ltd. All rights reserved.

Petrogenesis of syntectonic granites emplaced at the transition from thrusting to transcurrent tectonics in post-collisional setting: Whole-rock and Sr-Nd-Pb isotope geochemistry in the Neoproterozoic Quatro Ilhas and Mariscal Granites, Southern Brazil

The Neoproterozoic post-collisional period in southern Brazil (650-580 Ma) is characterized by substantial volumes of magma emplaced along the active shear zones that compose the Southern Brazilian Shear Belt. The early-phase syntectonic magmatism (630-610 Ma) is represented by the porphyritic, high-K, metaluminous to peraluminous Quatro Ilhas Granitoids and the younger heterogranular, slightly peraluminous Mariscal Granite. Quatro II has Granitoids include three main petrographic varieties (muscovite-biotite granodiorite mbg; biotite monzogranite - bmz: and leucogranite - lcg) that, although sharing some significant geochemical characteristics, are not strictly comagmatic, as shown by chemical and Sr-Nd-Pb isotope data. The most primitive muscovite-biotite granodiorite was produced by contamination of more mafic melts (possibly with some mantle component) with peraluminous crustal melts; the biotite monzogranite, although more felsic, has higher Ca, MgO,TiO2 and Ba, and lower K2O, FeOt, Sr and Rb contents, possibly reflecting some mixing with coeval mafic magmas of tholeiitic affinity; the leucogranite may be derived from pure crustal melts. The Mariscal Granite is formed by two main granite types which occur intimately associated in the same pluton, one with higher K (5-6.5 wt.% K2O) high Rb and lower CaO, Na2O, Ba and Zr as compared to the other (3-5 wt.% of K2O). The two Mariscal Granite varieties have compositional correspondence with fine-grained granites (fgg) that occur as tabular bodies which intruded the Quatro Ilhas Granoitoids before they were fully crystallized, and are inferred to correspond to the Mariscal Granite feeders, an interpretation that is reinforced by similar U-Pb zircon crystallization ages. The initial evolution of the post-collisional magmatism, marked by the emplacement of the Quatro Ilhas Granitoids varieties, activated sources that produced mantle and crustal magmas whose emplacement was controlled both by flat-lying and transcurrent structures. The transition from thrust to transcurrent-related tectonics coincides with the increase in the proportion of crustal-derived melts. The transcurrent tectonics seems to have played an essential role in the generation of mantle-derived magmas and may have facilitated their interaction with crustal melts which seem to be to a large extent the products of reworking of orthogneiss protoliths. (C) 2012 Elsevier B.V. All rights reserved.