Edi Mendes Guimarães

Recent discoveries of Proterozoic microfossils in south-central Brazil

Abstract Twenty-two Precambrian microfossiliferous localities, seven newly reported here, are known from the Sao Francisco Block and associated Brasilia fold belt of south-central Brazil, reported from outcrop and drill core samples of Mesoand Neoproterozoic cherts, carbonates, and siliciclastics of the ∼1170 to ∼950 Ma old Paranoa Group and the ∼950 to ∼700 Ma old Sao Francisco Supergroup (composed of the Jequitai Formation and the overlying Bambui Group). Representatives of nearly 30 genera of Proterozoic microfossils, chiefly cyanobacteria and subsidiary acritarchs, have been reported from these strata, preserved mostly within carbonaceous black chert in flat-laminated and high-relief stromatolites, and represent a wide spectrum of environments evidently ranging from nearshore lagoonal to supratidal, intertidal, subtidal, and offshore marine settings. Contrary to previously held views, the microfossils are generally more abundant and better preserved in the older unit (Paranoa Group) than in the younger one (Sao Francisco Supergroup). Nevertheless, among the newly reported assemblages, one from the lower part of the Bambui Group stands out as probably the best preserved and most diverse microbiota known from any Precambrian unit of South America. Although paleobiologic studies of the Brazilian Proterozoic are still in a formative stage, recent discoveries are highly promising, such as the detection in the work presented here of possible differences: (1) between the microfossil sizes in stromatolites of the Paranoa and Bambui Groups; (2) in the abundance and size of acritarchs with respect to lithology and facies; and (3) in the microfossil assemblages preserved in morphologically distinct stromatolites.

SAPONITE-RICH BLACK SHALES AND NONTRONITE BEDS OF THE PERMIAN IRATI FORMATION: SEDIMENT SOURCES AND THERMAL METAMORPHISM (PARANÁ BASIN, BRAZIL)

Shales and claystones in the Permian Irati Formation consist of Al-rich or Fe-Mg clay minerals in its southern/central and northern parts, respectively. The constrasting compositions indicate particular geological and paleo-environmental conditions. The purpose of this study was to determine the conditions of formation by characterizing the black shales and claystones from different sections of the northern edge of the basin, some of which reveal the presence of intruded diabase sills.Black shales consist of saponite or saponite-talc mixed layers, talc, lizardite, nontronite, and quartz. Green claystones are nontronite-rich but also contain lizardite, talc, and quartz. The chemical compositions of the black shale and claystones, except for one sample (POR-56), exhibit a positive correlation of the TiO2, Cr, and P2O5 contents with Al2O3, which typically results from weathering processes. The presence of saponite, nontronite, and some accessory minerals (spinel, pyroxene, native silver) suggests altered basic-ultrabasic rocks as sediment sources, consistent with the rare earth element (REE) composition being less than the Post-Archean Average Shale (PAAS) or North American Shale Composite (NASC) levels and with negative Ce and Eu anomalies. Sample POR-56 consists largely of nontronite and is anomalously rich in zircon, monazite, and apatite. Chemically, sample POR-56 is different from the black shales and claystones, being richer in Al2O3-Fe2O3, MgO-poor, and having greater REE contents than the PAAS or NASC standards. The POR-56 bed is probably a bentonite resulting from the alteration of volcanic ash in sea water (strong, negative Ce anomaly). The Zr/TiO2vs. Nb/Y relation indicates that the magmatism was andesitic. During the Upper Permian, intermediate to basic volcanic activity was recorded in the Mitu Group of the Central Andes.Close to the diabase sill, the black shales and claystones contain saponite, talc, and lizardite but nontronite is absent. Saponite and talc crystals, however, exhibit a larger coherent scattering domain size (CSDS) and are randomly oriented with respect to the sedimentary bedding. The thermal metamorphism effect is confirmed by the presence of secondary enstatite-augite and albite crystals.

Maritime Antarctica soils studied by Mössbauer spectroscopy and other methods

Soil samples from the King George Island, Antarctica, have been studied by 57Fe Mossbauer spectroscopy, X-ray diffractometry, radiometry, neutron activation analysis and chemical analytical methods. X-ray diffractometry measurements have identified soils containing different volume ratios of quartz, feldspar, chlorite as well as hematite. The difference in the phase composition and in the iron distribution among the crystallographic sites of iron-bearing minerals (chlorite, magnetite and hematite) of samples from two different depths was derived from the complex Mossbauer spectra. The differences in the mineral composition, iron distribution, concentration of water soluble salts, pH and radioactivity of certain radionucliedes indicate the occurence of chemical weathering of minerals.

Facile Method to Tune the Particle Size and Thermal Stability of Magnetite Nanoparticles

Nucleation and growth mechanism of iron oxide nanoparticles on zeolite template and their stability dependence are reported. Hyperfine field resulting from the variation of particle size indicates the effect of zeolite on particles growth; particle size decreases at lower concentration of zeolite. At higher concentration, a fraction of nano Fe3O4 experiences hyperfine field (45 and 49 T) similar to bulk particles. Effect of incubation and digestion time on the particles growth and the binding effect with zeolite are discussed. Annealing treatments show that the binding of nanoparticles with zeolite stabilizes the nanoparticles with regard to agglomeration and structural transformation. Thermogravimetry-differential thermal analysis (TG-DTA) shows that increase in dehydration temperature from 335.1 to 351.7 K results in zeolite content increasing from 0 to 1000 mg. Weight loss of the particles prepared in incubation time of 0.5 min is 9.46% and reaches 13.9% in 240 min. The weight loss remains practically constant at ca. 9% irrespective of the digestion method.

Estudo geoquímico da disposição de lodo de estação de tratamento de água em área degradada

This survey determined the physical and chemical properties of the gravel place where urban sludge from Rio Descobertos Water Treatment Plant is disposed. Physical, chemical and biological analysis of the soil samples (n=54), sludge samples (n=2), chemical coagulant (n=20) and samples from superficial waters (n=9) and water table (n=60) were performed. As results we can emphasize the horizontal distribution of mineral phases like gibbsite, organic material, exchanged Ca, available Mn and P on the soils are originated from the sludge. Some of these mobile elements could stimulate the growing of the vegetation, but they also could contaminate the water table.

Influência da pressão e temperatura na cristalinidade da illita em sequências Proterozoicas: Norte do Distrito Federal e Goiás, Brasil

The Proterozoic sequences in the North of Distrito Federal (Brazil) and in Bezerra and Vila Boa regions (Goias) comprise carbonates, pelites and sandstones from Bambui and Paranoa Groups. In Distrito Federal, such units are tectonically overlapped by metasediments of green schist facies of Canastra Group. Among the mineral constituents of the pelitic terms of all these units, phyllosilicates, particularly illite and muscovite, are predominant and are identified using X-ray diffraction with the help of reflection d(001) decomposition. The Kubler Index (Illite Crystallinity Index) complements the interpretation of local diagenesis and the possible bulk tectonic influence on the diagenesis/metamorphism that is a result of the overlap by the thrust fault of Canastra Group on Paranoa and Bambui groups, in the Distrito Federal region. Results indicate epizone conditions for the samples analyzed from Canastra Group and anchizone for samples from Bambui and Paranoa Groups. In the rocks at the North of Distrito Federal, the Kubler Index values are lower than in Bezerra - Vila Boa region. Moreover, the contents of Bambui Group rocks are smaller than Paranoas in the entire study area; therefore, the influence of tectonic bulk occurs gradually, starting from the highest load point.

Análise de dados hiperespectrais em mina de manganês laterítico, São João da Aliança (GO)

The aim this paper is to develop a methodology of lateritic manganese identification using imaging spectroscopy. In this study was used the Airborne Visible/InfraRed Imaging Spectrometer (AVIRIS) image. The AVIRIS is an imaging sensor that measures the solar reflected spectrum from 400 nm to 2500 nm at 10 nm intervals. The study area is located at Goias State and enclosed the Fazenda Extrema mine that presents lateritic manganese deposits developed on Paranoa Group rocks. Laboratory spectrometry studies conjugated with mineralogical analysis by diffractometry of X-ray demonstrated that the ore spectrum is opaque and spectrally featureless, due to the conduction band of MnO3 extending throughout this spectral range. The digital image processing can be subdivided in the following steps: (a) atmospheric correction using Green method, (b) areas selection with opaque behavior from the integral spectra calculation, and (c) lateritic manganese individualization using the Minimum Noise Fraction transformation. The method is efficient to classify areas of abandoned manganese mine that consist of priority areas for implementation of environmental recovery.