Resumo em Inglês:

Abstract The Moeda Formation (Caraça Group, base of Minas Supergroup) registers the initial rift stages of the Minas Basin in the Quadrilátero Ferrífero, Southern São Francisco craton, SE-Brazil. We present stratigraphic and U-Pb detrital zircon dating analyses of the Moeda Formation that contribute to the comprehension of its sedimentary evolution in the southernmost Gandarela and Ouro Fino synclines (Central-Eastern Quadrilátero Ferrifero). The Moeda Formation sequence consists of a basal proximal alluvial fan with clast-supported conglomerates, grading upward into an alluvial floodplain with quartz arenites, and upper massive sandstones from distal fluvial fans. Because the Moeda Formation is normally limited by similar lithologies at both its base (Nova Lima Group, Rio das Velhas Supergroup) and top (Batatal Formation, uppermost Caraça Group) and stratigraphic inversion has occurred in the study area, U-Pb detrital zircon dating was also extended to these units to differentiate them stratigraphically. The youngest clusters of detrital zircon ages in the Nova Lima Group and Moeda and Batatal formations were 2716, 2777 and 2786 Ma, respectively. Based on our data and relevant literature, the depositional age of the Moeda Formation was interpreted as between 2716 and 2520 Ma with the Mesoarchean continental crust, the Rio das Velhas Supergroup and the Archean TTG complexes as its main source areas.

Resumo em Inglês:

Abstract We present airborne magnetic and radiometric characteristics of some igneous suites from the Ponta Grossa Arch Alkaline Province (southern Brazil), namely the Bairro da Cruz, Banhadão, Barra do Itapirapuã, Mato Preto and Tunas complexes, Barra do Teixeira Phonolite, Itapirapuã Nepheline syenite, and José Fernandes Gabbro. We applied magnetic susceptibility 3D inversions using the total magnetic anomaly data together with its analytic signal of the vertical integral and the vertical integral of the analytic signal for the Bairro da Cruz, José Fernandes, and Tunas intrusions. Gamma-ray spectrometric methods involved analyses of basic, ternary, and ratio maps, thorium-normalized parameters, and radioelement concentrations at sampled data points. Stacked profiles from geophysical data were generated for each rock. The Bairro da Cruz, José Fernandes, and Tunas intrusions are associated with dipolar anomalies with normal polarity whereas their inversions show low magnetic susceptibility values. Gamma-ray spectrometric results allowed us to verify that all complexes are enriched mainly in eTh. Carbonatite bodies showed the highest contents of eTh and eU. Our findings are consistent with geophysical responses of alkaline bodies and impact the understanding of geophysical signatures, especially radiometric ones, of alkaline provinces in Brazil.

Resumo em Inglês:

Abstract The Guarani (GAS) and Serra Geral (SGAS) aquifer systems are the main groundwater sources in southern Brazil. We investigated the crucial relationship between the primary, intergranular pore system in the GAS and the secondary pore system associated with fractures in the SGAS in order to understand better the compartmentalization and connection, water storage, and productivity of both aquifers. Data from 375 wells that produce from both aquifers was used to evaluate the hydrodynamic characteristics of the GAS-SGAS contact. The integration of data using remote sensing, structural data, and thin section analysis support interpretations about the compartmentalization and potential connection of the aquifers. Fault zones trending NW, EW, and NNE demarcate six structural blocks in which the GAS-SGAS contact varies more than 400 m in elevation. Thin section analysis of sandstone texture and composition allowed us to determine the relationship between the type of porosity and cement type and content. The surface demarcating the Guarani-Serra Geral contact is highly productive in 93% of the wells that reach both aquifers. The integration of our results allowed us to propose a conceptual hydrogeological model for the GAS and SGAS aquifers in the study area.

Resumo em Inglês:

Abstract The oldest petroleum discoveries on the Brazilian offshore margin are in Paleogene reservoirs of the Guaricema and Dourado fields in the structural Mosqueiro Low, the Southern Sergipe-Alagoas Basin. Study of well logs and biostratigraphic data of the Eocene Series allowed its subdivision into three stratigraphic intervals representing the Ypresian, Lutetian-Bartonian, and Priabonian stages. The Ypresian depocenter is in the Dourado Trough, whereas the Lutetian-Bartonian and Priabonian depocenters are in the Vaza-Barris Trough, controlled by halokinesis in the first one and basement deformation in the others. The Ypresian interval mainly comprises retrogradational deposits related to the Early Eocene Climate Optimum. The Middle-Eocene Basal Unconformity is the main erosive feature in the Series. The Lutetian-Bartonian and Priabonian depositional systems are progradational. Their deposition is related to basement uplift due to both intense deep-water magmatism and to the Incaica Phase of the Andean Orogeny.

Resumo em Inglês:

Abstract The Serra da Saudade Formation, Ediacaran/Cambrian Bambuí Group, cropping out along the ridge of the same name in Central-Western Minas Gerais, Brazil, contains important deposits of glauconitic siltstones associated with phosphatic rhythmite as well as other siliciclastic, volcaniclastic and carbonate lithofacies. Lithofacies indicate deposition by proximal and distal storm waves on a continental marine shelf. The basal gray siltstone and part of the green siltstone represent a transgressive systems tract. The glauconitic siltstone indicates sea level rise, with the maximum flooding surface located approximately at the interval of highest K2O content. Biogenic carbonate and an upward increase in sandstone beds are interpreted as a highstand systems tract. Gradual lateral facies changes reflect the transition from foredeep (to the west) to forebulge (to the east) deposition in a foreland basin as the final stage of a shallowing-upward second-order sequence. Black organic-rich limestone has δ13C values between -8.78 and 9.79‰, negative δ18O values from -11.50 to -9.35‰, and 87Sr/86Sr around 0.7075. This isotopic signature, similar to that of other Bambuí carbonates, indicates a probably-restricted depositional environment with organic sedimentation, methanogenesis and high evaporation rates from the Ediacaran through the early Cambrian period.

Resumo em Inglês:

Abstract This paper analyzes mineralogical, geochemical, and geochronological aspects, along with the effect of hydrothermal/metasomatic overprints, to identify the presence of primary phosphate as well as depositional and paleoenvironmental conditions in marble and calcsilicate sequences recrystallized under transitional amphibolite-granulite metamorphic conditions in the Tanque Novo-Ipirá Complex within the Salvador-Curaçá Orogen, northeastern São Francisco Craton, state of Bahia, Brazil. Petrographic studies have identified up to 10 vol.% disseminated apatite and whole-rock P2O5 contents up to 3.2 wt.%. Post-depositional events affected the lithofacies to varying degrees. Late hydrothermalism did not modify the rare earth element and yttrium (REEY) patterns considerably. When normalized to Post-Archean Australian Shale (PAAS), these lithofacies are marked by flat pattern REEY, true negative Ce anomalies, and positive Y and Gd. The highly variable Eu anomalies were inherited from the source composition but may have been affected by interaction with fluids. U-Pb LA-ICP-MS (laser ablation multicollector inductively coupled plasma mass spectrometry) ages indicate a maximum depositional age of 2128 Ga, as well as Paleoproterozoic and Neoarchean sources. Samples with anomalous phosphorus show Y/Ho ratios >30 and Ce/Ce* anomalies between 0.53 and 1.0 with an average of 0.70, suggesting a sub-oxic environment for phosphate precipitation.

Resumo em Inglês:

Abstract The hypothesis that geomorphometric parameters and structural-geological features are diagnostic of reactivation of the Blumenau-Soledade Lineament in central-western Santa Catarina, Brazil was tested by confronting data on asymmetry, elongation, morphological markers, drainage patterns, and river anomalies of the Pelotinhas catchment and structural data for the Serra Geral Group (Paraná Basin). The results indicate reactivation of the lineament as a dextral strike-slip fault, as the cause of an associated morphostructure characterized by an escarpment along the left bank of the river as well as being responsible for catchment asymmetry and morphometric anomalies. Although fault data indicate paleostress correlatable to regional tectonic events described in the literature, we propose that the earliest paleostress pulse possibly was active during the Paleocene-Eocene transition, with σ1 oriented NE-SW and σ3 NW-SE. In the early Paleogene, the stress system evolved as an extensional pulse. During the Paleogene and early Neogene, the paleostress field rotated with reorientation of the σ1 axis between NNW and NNE. The current tectonic regime was established between the Late Pleistocene and Holocene, with compressional stress σ1 oriented approximately WNW-ESE and σ3 NNE-SSW, as it generally occurs in the South American Plate.

Resumo em Inglês:

Abstract The analysis of trace elements (TE) in geological materials is a valuable tool to understand geological processes, including studies in geochemistry and petrology. Among the methods applied to determine TE, Laser Ablation Inductively Coupled Plasma Mass Spectrometry ­(LA-ICP-MS) detects TE contents in the μg/g and ng/g range. The aim of this paper is to validate, at Laboratório de Geologia Isotópica (LGI) — Centro de Estudos em Petrologia e Geoquímica/Universidade Federal do Rio Grande do Sul (CPGq/UFRGS), a high-precision method for LA-ICP-MS analyses of TE in geological materials. Samples used in this work were reference Max-Planck-Institut für Chemie — Dingwell (MPI-DING) glasses. The method is used in tandem with the Electron Probe Microanalyzer — Wavelength Dispersive X-ray Spectrometer (EPMA-WDS) technique to determine the major and minor elements of the samples and check for homogeneity. Analyses were conducted in a Thermo® Element2 Inductively Coupled Mass Spectrometer (ICP-MS) coupled to a New Wave Research® Neodymium-Doped Yttrium Aluminum Garnet (Nd:YAG) (213nm) laser ablation system. 43Ca and 29Si were used as internal standards (IS). Glitter® software and in-house spreadsheets were utilized for reduction treatments. The results using 29Si as IS present a high degree of fractionation errors compared to the results using 43Ca. Uncertainties caused by matrix interferences and fractionation effects can be corrected applying a correction factor. The obtained results demonstrate an effective recovery of TE content for the analyzed reference materials. Moreover, these silicate glasses proved to be robust to create an internal database of matrix matching standards in routine silicate analyses.

Resumo em Português:

Abstract The Santa Fé Ni laterite deposit (Brazil) developed as a consequence of weathering of serpentinized dunites that produced a regolith in which the main Ni ore zone is a ferruginous saprolite with chlorite as the main ore-carrier along with Ni-bearing Fe-oxyhydroxides. The genesis of chlorite is related to hydrothermal alteration of spinel. This study found that part of the chlorite has a supergene origin. XRPD-Rietveld method and EMPA/SEM analyses coupled with geochemical and mass balance calculations have provided further information about the chemistry and mineralogy of the main ore minerals. An attempt is made to explain the mode of formation of secondary chlorite and associated ore minerals. The main Ni-bearing minerals in the green saprolite are serpentine and minor chlorite. In the ferruginous and ochreous saprolites chlorite is a major phase with Ni contents of the order of 3 and 7 wt% NiO, respectively. Fe-oxy-hydroxides (mean NiO = 1.8 wt%) are also abundant in the ferruginous and ochreous saprolites. According to the isocon method, Al and Ti are definitely enriched in the saprolite compared to dunite. The source of Al for chlorite formation probably derives from pyroxenites and peridotite. The lateral migration of Al in solution was fundamental for the supergene formation of chlorite.

Resumo em Inglês:

Abstract Two porphyritic granitoids (Huaco and Sanagasta) in the Velasco district of the Pampeana Pegmatite Province in Northwestern Argentina are recognized. They are considered the fertile granites of the beryl-bearing pegmatites and can be described as post-orogenic and peraluminous A-type granites formed in an intraplate tectonic setting during an extensive regime, whose magma source is predominantly of cortical origin and to a lesser extent, mantle-derived. The pegmatites are classified as Rare Elements of the beryl type and beryl-columbite-phosphate subtype, while the entire district shows characteristics related to the NYF (Nb-Y-F) petrogenetic family. From rocks and cogenetic minerals of an individual ‘Ismiango’ pegmatite of the Velasco district, two Rb/Sr isochrons have been constructed. They define an age of 330.3 ± 8.3 and 331.7 ± 2.3 Ma and fall in the Lower Carboniferous period, consistent with the age of the parental and host-rock, the Huaco granite. As the Ismiango pegmatite has a similar composition and structure to the other beryl mineralized pegmatites of the Velasco district, the obtained age is attributed extensively for the entire district. According to the initial 87Sr/86Sr value obtained of 0.713, the pegmatite-magmatism of the Velasco District might be mainly derived from the crust with some minor participation of mantle materials.