Alvaro Penteado Crósta

Hydrothermal alteration mapping at Bodie, California using AVIRIS hyperspectral data

Abstract AVIRIS data covering the Bodie and neighboring Paramount mining districts, eastern California, were used to map hydrothermal alteration minerals. Two spectral analysis algorithms were used: Spectral Angle Mapper (SAM) and Tricorder. The objective was to compare the performance of the two algorithms where no a priori ground or atmospheric information is available. AVIRIS data were calibrated to apparent surface reflectance using a modified MODTRAN radiative transfer model in conjunction with laboratory and in-flight calibration data. Both algorithms appear to produce satisfactory results for geologic reconnaissance and mapping applications, but Tricor- der generally classified more pixels and identified more mineral species than SAM. For some minerals, such as Na-montmorillonite, results from Tricorder and SAM matched reasonably well, but major differences appeared in how the two algorithms classified kaolinite group minerals and ferric oxides. Laboratory spectra of rock samples from five localities were compared to reference spectra of minerals identified by the classification algorithms for corresponding pixels, with the results matching to some extent.

Geological mapping using Landsat Thematic Mapper imagery in Almeria Province, south-east Spain

Abstract Abstract. This paper describes the use of Landsat Thematic Mapper (TM) imagery for geological mapping in a tectonically-complex area of south-east Spain. The terrain includes the metamorphic basement rocks of the Betic Cordillera and the sedimentary strata of an intermontane basin. TM bands 1, 3 and 5 were used to produce a colour composite. Experimentation showed that this was the most effective band combination to discriminate rock types in the semi-arid study region. A combination of spectral and textural characteristics was used to identify and map rock types in the Nevado-Filabride and Alpujarride basement complexes, sedimentary strata in the Neogene Tabernas-Sorbas Basin and Quaternary drift deposits. Structural features were best shown by digital convolution using a directional filter. The enhanced images revealed a previously unreported fault and fracture belt on the northern side of the Tabernas Basin. The geological interpretation of enhanced TM imagery was locally checked in the field.

Impact Structures in Brazil

Studies carried out in the last ten years resulted in the recognition of several circular structures in the Brazilian territory which have, as common characteristics, the presence of similarities with well-known structures in other countries and planets attributed to impact with celestial bodies, and the absence of evidence for a terrestrial endogeneous origin. Those structures presently number six, ranging in diameter from three to 40kilometers. Two of them, Araguainha and Serra da Cangalha, were studied in some detail and presented enough evidence for their impact origin, including impact metamorphism features and morphologic/structural styles. The other four, Colonia, Sao Miguel do Tapuio, Vargeao and Riachao, are still poorly known, but the available data are very suggestive for their impact origin.

Mapping an uncertainty zone between interpolated types of a categorical variable

Categorical data cannot be interpolated directly because they are outcomes of discrete random variables. Thus, types of categorical variables are transformed into indicator functions that can be handled by interpolation methods. Interpolated indicator values are then backtransformed to the original types of categorical variables. However, aspects such as variability and uncertainty of interpolated values of categorical data have never been considered. In this paper we show that the interpolation variance can be used to map an uncertainty zone around boundaries between types of categorical variables. Moreover, it is shown that the interpolation variance is a component of the total variance of the categorical variables, as measured by the coefficient of unalikeability.

Fusão de imagens altimétricas e aeromagnetométricas como ferramenta de interpretação geológica, exemplo da Província Mineral de Carajás (PA)

The Carajas Mineral Province is located in the eastern portion of the Amazonian Craton, in the Para state, north of Brazil. It comprises two main tectonic domains: Carajas and Cinzento strike-slip systems. A significant proportion of the mineral deposits and occurrences of this Province is geologically related to structures belonging to these two fault systems. This paper presents the results obtained by the interpretation of merged altimetric and magnetometric data, as an auxiliary tool for understanding the structural framework of the Carajas Fault. Satellite digital elevation data acquired by the Shuttle Radar Topographic Mission (SRTM) were used in combination with aeromagnetometric data acquired by the Brazil-Canada Geophysical Project (PGBC). A method was developed for digital processing and merging the data acquired by these two surveys, based on geophysical and remote sensing processing methods. The results obtained allowed to establish the relationship between features interpreted from the merged image, with the main geomorphologic and magnetic domains known in the region. These results demonstrate the potential of applying these methods for assessing areas with similar metallogenetic characteristics located elsewhere in Amazon, thus facilitating and guiding regional exploration programs.

The Impact Record of Southwest Gondwana

Impact cratering has been, and still is, a planetary process of utmost importance. Here, the limited impact record for Southwest Gondwana, amended by the record of Australia and the only two impact structures known from India in East Gondwana, is examined. Care is taken to distinguish between known impact events during the run-up to the completion of Gondwana—that is, prior to 1000 Ma—and in the interval 1000–500 Ma, the subsequent Pangea and Pangea-break-up phase till about 135 Ma, and in the post-135 Ma record. The overall record is simply too limited to allow a distinct evaluation of periods of possibly enhanced impact flux that, in turn, might have affected paleoenvironmental or even biological evolution. It stands to reason that at least a major part of the SW Gondwanan impact record was obliterated during the rifting stages related to the post-500 Ma break-up phase, but equally it cannot be excluded that large impact structures (or remnants of them) may still be discovered in future on the shields and in the orogenic belts/basins of Africa and South America. Finally, the SW Gondwanan impact record may be further expanded through careful observation aiming at identification of impact ejecta deposits, such as spherule layer beds, or through the discovery of shock metamorphosed zircon or monazite (derived from eroded impact structures) in the sedimentary records and the isotopic dating of these ancient impact-witness grains.

Espectrometria de raios gama de granitos pegmatíticos da Província Pegmatítica da Borborema (PPB), nordeste do Brasil

The objective of this study is the characterization of pegmatitic granites located in the southern region of the Borborema Pegmatitic Province (BPP) with aerial and ground gamma ray data. Ground gamma ray data, radiometric and ICP-MS laboratory analysis showed that these granites have uranium concentrations varying between 0.4 ppm and 7.8 ppm, thorium concentrations varying between 0.1 ppm and 21 ppm and potassium concentrations varying between 2% and 5.2%. Based on these radiometric characteristics and using individual, ratios and ternary images of radioelements of aerial gamma ray data it was possible to distinguish from others country rocks a group of granitic pegmatitic bodies and also to individualize quartzitic formations from the northern and southern portions of the area, based on their different thorium (Th) contents. Ground gamma ray data, acquired for one of these pegmatitic granites, called Galo Branco granite, reveals an irregular spatial distribution of radioelements, indicating mineralogical changes in contents and concentrations of accessory minerals during magmatic crystalization. Pegmatites intruding quartzites in the south portion of the granite were also distinguished using eU/eTh and eU/K ratios images. The results indicated the potential of these methods for the characterization and mapping of pegmatitic granites and associated pegmatites of the Borborema Pegmatitic Province (BPP).

Identificação gamaespectrométrica de placeres rutilo-monazíticos neoproterozóicos no sul da Faixa Seridó, nordeste do Brasil

Aerial gamma-ray survey data covering Neoproterozoic supracrustal sequences in the Serido Belt were processed and analyzed together with ground gamma-ray data, air photos and geological data for lithogeophysical characterization and mapping of granitic rocks, related pegmatites fields and lithological units of Serido Group. Interpretation was based on individual and ternary images of the three radio-elements and the eU/eTh and eTh/K ratios, and allowed the discovery of thorium anomalies associated with coarse-grained metarenites and metaconglomerates facies intercalated with quartzites of the Equador Formation. High contents of iron oxides, ilmenite, monazite, rutile, titanite and zircon were identified by ore microscopy of polished sections in the metaconglomerates matrix. Semiquantitative scanning electron microscopy (SEM) analyses in minerals of two samples revealed up to 79.4% of Th02 and 87.7% of REE in monazites; up to 99.2% of Ti02 in ilmenite and rutile and up to 1.81 % of HfO2 in zircon. Gamma-ray anomalies due to thorium were also identified in association with sediments of Cenozoic age in the region.

Identification of mafic-ultramafic rocks using fuzzy logic classification of airborne geophysical data in the Anapu-Tuerê region, Brazilian Amazon

The Anapu-Tuerê region, in the Brazilian Amazon, is known for the occurrence of mineral deposits associated with mafic-ultramafic rocks. The spatial distribution of these rocks was investigated by integrating airborne magnetic and gamma ray spectrometry data using Geographic Information System (GIS) and fuzzy logic classification. Highest values of relevance fuzzy were associated with mafic-ultramafic rocks mapped regionally by the Brazilian Geological Survey (CPRM). Furthermore, mineral occurrences mapped by the CPRM showed good spatial correlation with the higher values of relevance fuzzy presented in this paper.