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Rio de Janeiro, Brazil

Pinto V.M.,Federal University of Rio Grande do Sul | Hartmann L.A.,Federal University of Rio Grande do Sul | Wildner W.,CPRM
International Geology Review | Year: 2011

Native copper is widespread in the Lower Cretaceous Parana basaltic province, southern Brazil, both as films in fractures and as massive balls in amygdules. The focus of this investigation is on the large concentration of occurrences (n = 85) that forms the Vista Alegre district in the border region of Rio Grande do Sul and Santa Catarina states. The high average of 220 ppm Cu content of the basalts resulted in ore of native copper, Cu oxides, abundant chrysocolla in the top of mineralizations, and minor malachite and azurite. Native copper is associated with dioctahedral and trioctahedral smectites, zeolites (heulandite and clinoptilonite), quartz, and calcite, typical of a low-T (100-150C) hydrothermal alteration assembly. The PGE distribution shows enrichment in Pd in relation to Pt both in basalts and in native copper, supporting the hypothesis of hydrothermal origin of the mineralization. No evidence was found of direct precipitation of copper from the lava; based on field and petrographic evidence, integrated with BSE images, EPMA analyses, EGP contents of native copper, and bulk rock analyses, this is an epigenetic hydrothermal copper mineralization, followed by supergene enrichment. © 2011 Taylor & Francis.

In the central portion of the Transversal Zone (Borborema Province, northeastern Brazil), feld geological mapping combined with interpretation o airborne gammaspectrometric, magnetic and satellite imagery (Landsat 7 ETM+) data were performed in order to evaluate the importance (especially, continuation a depth) of shear zones as potential candidates to represent terrain boundaries and sutures. This integrated analysis emphasized three deeper structures which separat crustal blocks, including as such the Serra do Caboclo and the Congo-Cruzeiro do Nordeste shear zones. A deep anomaly, located between those two structures, ma represent the continuation, horizontally displaced, of one of them, or a third, non outcropping but interconnected shear zone. The favored interpretation considers thi sector of the Transversal Zone as formed by deep-rooted, northeast-southwest trending shear zones (with tops of anomalies reaching 6 to 16 km depths), cut by shallowe structures (tops of the anomalies reaching, at maximum, 2 to 6 km depth), with east-west trends. In this context, the importance and continuity at depth of the Serra d Caboclo shear zone is highlighted, and as such inferred to be the suture between Piancó-Alto Brígida and Alto Pajeú terranes, in accordance with feld geological data. © 2011 Sociedade Brasileira de Geofísica.

Rodrigues M.C.N.D.L.,Federal University of Parana | Trzaskos B.,Federal University of Parana | Lopes A.P.,CPRM
Journal of Structural Geology | Year: 2015

This study presents a qualitative and quantitative analysis of porosity in deformation bands by applying X-ray micro-computed tomography in conjunction with microstructural analysis. Samples of compactional cataclastic bands and shear compactional bands identified in Early Cretaceous aeolian sandstones of the Paraná Basin were analyzed. The application of X-ray micro-computed tomography expanded the view of features in the porous framework of each type of deformation band studied and provided information that are not clear or was not observable with optical microscopy. The compactional cataclastic bands and shear compactional bands differ in geometry, thickness, microstructures and, mainly, in the distribution, shape and orientation of the remaining pores. Porosity analysis was also performed by comparing values of porosity (total, open and closed pores) of the parental rock and the deformation band in each sample. Results of these analyses show a reduction of total porosity and open pores and therefore an increase in the amount of closed pores in all types of deformation bands in relation to parental rock. In addition, it is observed that changes in porosity characteristics are related to the effect of different deformation mechanisms that operated in each type of deformation band. © 2015 Elsevier Ltd.

Lopes C.G.,CPRM | Lopes C.G.,Federal University of Rio Grande do Sul | Pimentel M.M.,University of Brasilia | Philipp R.P.,Federal University of Rio Grande do Sul | And 3 more authors.
Journal of South American Earth Sciences | Year: 2015

The Passo Feio complex (PFC) is a sequence of metapelite, amphibolite, metavolcanic/metavolcanoclastic rocks, marble, calc-silicate rocks, quartzite and magnesium schist. It is part of the São Gabriel Terrane, a Neoproterozoic juvenile arc formed during the early stages of evolution of the Neoproterozoic Dom Feliciano Belt (DFB), in southern Brazil. This belt corresponds to the southernmost portion of the Mantiqueira Province, an important Neoproterozoic orogenic system exposed in the NNE direction along the southeastern coast of Brazil. The geotectonic significance of the original Passo Feio basin in the tectonic evolution of São Gabriel Terrene is not well understood. It has been considered as part of a passive margin sequence or as a back-arc sequence. Geochronological and isotopic data are very scarse for the Passo Feio rocks and this has hampered the better understanding of its significance in the Neoproterozoic tectonic evolution of southern Brazil. In the present study the age and significance of metasedimentary rocks of the PFC were investigated. The provenance study was carried out in four metapelite samples from the southern and northern Passo Feio complex, using U-Pb dating of detrital zircon. The results showed varied provenance patterns and zircon ages range from 3637 to 803 Ma. Paleo- and Mesoproterozoic detrital zircon grains are present in all samples, but an important Neoproterozoic population has been identified in one of them. 3.5 Ga old zircon grains form the oldest population of detrital zircon ever reported in southern Brazil. Paleoproterozoic/Archean terranes within the Rio de la Plata Craton may represent the main source of detrital sediments and this suggests that part of the Passo Feio complex might have been a passive margin sequence, developed along the northeastern margin of that paleocontinent. However, the presence of a Neoproterozoic zircon population is not consistent with derivation solely from the craton and indicates contribution from younger sources, such as the Neoproterozoic São Gabriel Arc itself. This Neoproterozoic zircon population with ages ranging from 948 to 803 Ma, suggests that the onset of the magmatic/tectonic evolution of the São Gabriel Arc might have started as early ca. 0.95 Ma. © 2014 Elsevier Ltd.

Rodrigues J.B.,CPRM | Rodrigues J.B.,University of Brasilia | Pimentel M.M.,University of Brasilia | Dardenne M.A.,University of Brasilia | Armstrong R.A.,Australian National University
Journal of South American Earth Sciences | Year: 2010

The Brasília Belt is one of the best preserved Neoproterozoic orogens in Brazil. It comprises a thick Meso-Neoproterozoic sedimentary/metasedimentary pile including the Canastra and Ibiá Groups, which are the object of this study. The Canastra Group constitutes a regressive sedimentary sequence made mainly of greenschist-facies metapelitic and metapsammitic rocks, including phyllite, sandy metarhythmite and quartzite, with minor intercalations of limestone, as well as carbonaceous and carbonatic phyllite. The Ibiá Group is formed of a basal diamictite followed upwards by phyllites and calc-schists. It rests on an erosional unconformity on top of the Canastra Group. A provenance study based on U-Pb zircon geochronology on a selection of seven samples helped to establish the various source areas and maximum depositional ages of the original sediments. In addition, seven new Sm-Nd analyses are presented and discussed together with previously published data. LAM-ICP-MS U-Pb dating of detrital zircon grains indicates a maximum depositional age of the Canastra and Ibiá Groups of ca. 1030 and 640 Ma, respectively. The provenance signature of the Canastra Group comprises a wide range of detrital zircon ages with a significant Paleoproterozoic component (∼1.8 and ∼2.1 Ga) and an important Mesoproterozoic source (1.1-1.2 Ga), especially for the Paracatu Formation, indicating the São Francisco-Congo Craton as main source. These provenance data, in particular the absence of Neoproterozoic zircon grains, typical of the active margin of the Brasília Belt, allied with the homogeneous Paleoproterozoic TDM values are consistent with the previous interpretation that the Canastra Group represents a sedimentary sequence deposited on a passive margin setting. Zircon grains from the diamictite of the Ibiá Group yielded ages ranging from 936 to 2500 Ma. In contrast, the overlying calc-phyllite of the Rio Verde Formation reveals a dominant Neoproterozoic provenance pattern with important peaks at 665, 740 and 850 Ma. The São Francisco-Congo Craton and Goiás Magmatic Arc are, most probably, the two main source regions for the Ibiá Group which may represent, therefore, a former fore- or back-arc sedimentary sequence. Tectonically, therefore, the Ibiá Group is equivalent to the Araxá Group exposed in central Goiás and both represent syn-orogenic sedimentary sequences formed with important detrital contributions derived from the Neoproterozoic Goiás Arc. The provenance data presented here indicate that the Cubatão Formation is most possibly representative of a Marinoan or younger glacial event. © 2009 Elsevier Ltd. All rights reserved.

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