Abstract The Umbuzeiro Doce skarn target in the Seridó Belt (NE Brazil) was studied using reflectance spectroscopy and petrography. Interpretation of the 61 reflectance spectra obtained in the study established the following zones: marble zone, with diagnostic absorption features at 2,340 and 2,475 nm (C-O); tremolite marble zone, with main absorption at 1,393 (OH−) and 2,313 nm (Mg-OH); garnet-vesuvianite zone, with the main absorption at 2,215 nm (OH−); diopside-hornblende zone, marked by a broad absorption at 1,153 nm (Fe2+); late wollastonite zone, which lacks a diagnostic spectral signature; and a local violet vesuvianite-bearing marble, characterized by potential gemological vesuvianite marked by Cr3+ absorption features (~548 and 680 nm). Centimeter-sized molybdenite aggregates occur at the interface of the marble and calc-silicate alteration zones. Spectral indices were produced to automate identification of potential zones to explore Mo, W, and gemological vesuvianite. Indices for the zones of marble, tremolite marble, garnet-vesuvianite, diopside-hornblende and violet vesuvianite were produced and tested. This methodology appears as a powerful exploratory guide for Mo, W, and violet vesuvianite occurrences or deposits that show a mineral zoning like the Umbuzeiro Doce target.

Abstract The Paraná deposit, located at Southwestern Rio Grande do Norte state, in Brazil, is one of the few emerald deposits found at Borborema Province. The mineralization occurs in phlogopite schists and actinolite-phlogopite schists associated with pegmatites and albitites within the Portalegre Shear Zone. Unlike other well-known Brazilian emerald deposits, the mineralogy of Paraná emeralds has remained poorly investigated for the last 40 years. In this study, we conducted mineralogical characterization of theses emeralds through gemological testing, mineral chemistry, absorption and reflectance spectroscopy, and thermal analysis. The Paraná emeralds are bluish-green colored, characterized by high refractive index, several two-phase fluid inclusions and mica is the main mineral inclusion. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry analyses detected the presence of Fe2+ (0.43-1.94 wt.% FeO) and Cr3+ (0.04-0.14 wt.% Cr2O3) as the main chromophores replacing octahedral Al3+ in the crystal structure. In addition, substantial amounts of MgO (0.40-2.72 wt.%), Na2O (0.50-1.81 wt.%), and Cs2O (0.07-0.44 wt.%) were also identified. The main causes for its coloration were attributed to Cr3+ absorption features in visible spectral range, which were corroborated by absorption and reflectance spectra. The presence of types I and II H2O at channel-sites was recorded in Fourier-transform infrared spectra and demonstrated by dehydration processes observed in different thermal and thermogravimetric analyses.

Abstract We present a combined analysis of airborne geophysical data (magnetics and gamma-ray spectrometry) and field aspects of Itapetim lode gold district region, aiming to identify structurally deformed and hydrothermally altered zones, which are suggestive of gold mineralized sectors. The main geophysical lineaments are oriented in the NE-SW and E-W directions and display a clear correlation with the major gold-bearing structures in the region. In the magnetic maps, the key pathfinder for gold occurrences is their association with strongly magnetized areas positioned along the Itapetim Shear Zone and outer contour of Teixeira Batholith. In the field, such structures are materialized in protomylonitic and mylonitic rocks that present evidence of ductile and brittle deformation, whereas kinematic criteria are suggestive of a transpressional tectonic regime. In addition, the main radiometric pattern is characterized by K enrichment, which is correlated with a hydrothermal mineral assemblage dominated by quartz, potassium feldspar, and tourmaline. Based on the obtained data integration, we produced an integrative map and located ten new target areas of possible gold mineralization, which is correlative to other well-known lode deposits in Northeast Brazil and Africa.

Abstract reflectance spectroscopy is a fast tool for mineral identification and classification. We conducted spectral characterization of elbaite samples (n = 30) from Seridó Pegmatite Province, Borborema Province, NE Brazil, using a FieldSpec portable instrument and geochemical data from electron probe microanalysis. Visual interpretation of reflectance spectra reveals the absorption features in the visible to near infrared (350-1,200 nm) range related to the electronic transition processes of iron (e.g., ~720, ~1,180 nm), copper (e.g., ~694, ~922 nm) and manganese (e.g., ~400, ~520 nm) ions. In general, electron probe microanalysis data show correlation with the absorption features related to chromophore elements (e.g., Cu, Fe, Mn), but the complex crystal structure of the tourmalines also accounts for the generation of absorption features. Vibrational processes in the short-wave infrared (1,200-2,500 nm) spectral range appear associated with Mn-OH, OH, Al-OH, H2O and B-OH bonds. A statistical approach, using principal component analysis, was chosen for distinguishing tourmalines of different composition. We discriminated tourmalines characterized by green, blue, blue-green, red/purple colors, as well as the colorless ones. This included the Paraíba tourmaline, a high value variety of elbaite that is the focus of exploration carried out in Seridó Pegmatite Province.

Modelagem espacial (lógica fuzzy) foi empregada para a previsão de áreas potenciais à exploração aurífera na Província Mineral do Tapajós (PMT -Amazônia brasileira), especificamente nas regiões aqui denominadas Ouro Roxo-Cantagalo e Chico Torres. Os dados utilizados compreendem os aerogeofísicos e de imagens SAR-R99B. O processamento dos dados aerogeofísicos permitiu a geração de imagens como as do potássio anômalo, parâmetro F, razões e normalização de radioelementos e a amplitude do sinal analítico do campo magnético anômalo, enquanto que imagens SAR foram empregadas para a etapa de fotointerpretação. A partir da geração de modelos hipotéticos para mineralizações de ouro nas regiões das zonas de cisalhamento Ouro Roxo-Cantagalo e Chico Torres, imagens geofísicas e interpretações, quando convenientes, foram selecionadas para a geração de modelos prospectivos por meio da lógica fuzzy, enquanto que as ocorrências de ouro já conhecidas foram usadas para avaliar a qualidade dos resultados modelados. As principais contribuições desta pesquisa incluem a observação de assinaturas gamaespectrométricas potenciais, um refinamento interpretativo dos dados digitais e melhoramento de informações apresentadas nos mapas geológicos atuais. Os resultados das modelagens prospectivas para as regiões Ouro Roxo-Cantagalo e Chico Torres a partir da lógica fuzzy exemplificam a geração de modelos exploratórios regionais para a cartografia e seleção de áreas com potencial para mineralizações de ouro, podendo esta metodologia ser estendida a outras regiões da PMT, utilizando-se os dados aerogeofísicos atualmente disponíveis pelo Serviço Geológico do Brasil e as imagens SAR do Centro Técnico Operacional do Sistema de Proteção da Amazônia (CENSIPAM).

Spatial modeling (fuzzy logic) was used to predict key areas for gold exploration in the Tapajós Mineral Province (Brazilian Amazon), specifically in the regions here named Ouro Roxo-Cantagalo and Chico Torres. The data used were regional airborne geophysical and SAR-R99B data. The airborne geophysical data processing allowed the generation of evidence layers for modeling approaches. The best evidences in current models were anomalous potassium, F parameter, radioelement ratios, radioelement normalization, and amplitude of the analytical signal of the magnetic anomalous field. The SAR images were used in the photointerpretation stage. In the case of regions of the Ouro Roxo-Cantagalo and Chico Torres shear zones, geophysical images and interpretations (when necessary) were used to application of fuzzy logic, and the gold locations were used to evaluate the quality of the modeling results. The main contributions of this research include the observation of potential gamma-ray spectrometry signatures, the refining interpretation of the digital data, and improving the information of the current geological maps. The prospective Ouro Roxo-Cantagalo and Chico Torres modeling results using the conceptual fuzzy logic method exemplify the generation of regional exploration models for the cartography and selection of gold potential areas. This methodology can be extended to others regions of the Tapajós Mineral Province using airborne geophysical data current available by CPRM and the SAR images of the Centro Técnico Operacional do Sistema de Proteção da Amazônia (CENSIPAM).

Áreas com espessos regolitos estão amplamente distribuídas na região amazônica. Nas últimas décadas, as pesquisas sobre geologia de regolitos no Brasil têm se concentrado, principalmente, nos aspectos geoquímicos e mineralógicos e seus possíveis usos para a exploração de recursos minerais. Embora a utilização da aerogeofísica e de métodos de integração de dados tenham se ampliado na avaliação de recursos minerais, a aplicação em terrenos regolíticos ainda é recente. O objetivo deste artigo é mostrar o potencial de uma abordagem sistemática, incluindo o uso de aerogamaespectrometria regional, modelo digital de elevação e processamento digital de imagens, para a identificação de áreas de espesso regolito com potencial interesse econômico em uma área pouco explorada da porção leste do Estado do Amazonas. A aplicação de técnicas de estatística multivariada (Análise por Principais Componentes e K-médias) em dados aerogamaespectrométricos permitiu a individualização de regolitos frente às respostas provenientes de unidades geológicas da região. Como guias para a identificação de áreas de espesso regolito, os seguintes padrões foram observados: baixos valores de K (facilmente lixiviado durante o intemperismo) e da razão K/eU; altos valores de eTh, da razão eTh/K e médios de eU, além de maiores altitudes (platôs). Aplicou-se o algoritmo originalmente desenvolvido para o sensoriamento remoto hiperespectral, denominado Spectral Angle Mapper (SAM), e técnicas de modelagens espaciais (lógicas booleana e fuzzy) para a produção de mapas de favorabilidade para a ocorrência de espesso regolito que foram comparados com mapa geológico em escala 1:250.000. As relações quantitativas entre conjuntos de dados geofísicos e altimétricos mostraram grande capacidade para o link entre interpretações geológicas e geofísicas, indicando-se áreas potenciais para direcionar a prospecção mineral na área de estudo.

Areas with deep regolith profiles are widely distributed in the Amazonian region. Research on regolith geology in Brazil has concentrated, mainly, in geochemical and mineralogical aspects and where possible utilizing these in exploration. Although airborne geophysics and spatial data integration methods are being used increasingly for mineral resource assessments, the application in regolith-dominated terrains is new. The objective of this paper is to show the potential of a systematic approach, including the use of regional gamma-ray spectrometry, digital elevation model and image digital processing techniques for the identification of deep regolith areas characterized by potential economic interest in less well explored region of the eastern portion of Amazonas State. The application of multivariate statistics techniques (Principal Components Analysis and K-means) in airborne gamma-ray data allows the individualization of regoliths in comparison to the geologic units of this region. How exploratory guide, the following patterns were observed: low values of K (easily lixivied during the weathering) and K/eU ratio; high values of eTh, eTh/K ration and medium values of eU, besides the higher altitude (plateau). Based on these images, the hyperspectral algorithm named Spectral Angle Mapper (SAM) and the boolean and fuzzy logic spatial modeling techniques were applied to produce regolith potential maps that were compared with previous geological data in 1:250.000 scale. The quantitative relationships between geophysical and altimetric datasets have shown great capability for linking geological and geophysical interpretations, indicating potential areas that can direct new ways for mineral prospecting at the study area.

Este trabalho utilizou dados multiespectrais (ETM+/Landsat 7), de radar de abertura sintética (RADARSAT-1 e MAPSAR simulado) e aerogeofísicos para caracterização geológica e geração de alvos exploratórios para mineralizações de ferro no bloco Serra Leste, porção oriental da Serra de Carajás (PA). A técnica Minimun Noise Fraction (MNF) foi aplicada às seis bandas do sensor ETM+/Landsat 7, produzindo-se o realce de coberturas ricas em ferro. Dados SAR permitiram a observação da macrotopografia e lineamentos que foram também interpretados com base em dados aerogeofísicos e multiespectrais. As imagens geofísicas e SAR do RADARSAT-1 foram integradas por meio da técnica IHS (intensidade-matiz-saturação), e os produtos gerados foram usados para uma melhor delimitação de unidades geológicas e para definição de alvos para prospecção de ferro. O método de classificação supervisionada Spectral Angle Mapper (SAM) foi aplicado a um conjunto de imagens geofísicas selecionadas a partir da determinação do modelo descritivo das mineralizações de ferro e das assinaturas geofísicas verificadas no alvo Serra Leste. Utilizando-se a ocorrência documentada de ferro na região como ponto de treinamento, um mapa de favorabilidade foi produzido. Os resultados da classificação SAM mostraram-se equivalentes ou superiores quanto a sua acurácia quando comparados ao mapa geológico e produtos de realce espectral produzidos neste artigo.

This paper applied multispectral (ETM+/Landsat 7), SAR and airborne geophysical data both to aid the geologic characterization and to highlight tracts with potential for iron deposits in the Serra Leste Block, located in the Serra dos Carajás (Pará State, Brazil). The Minimum Noise Fraction (MNF) technique was applied to six bands of the ETM+/Landsat sensor/satellite, producing the enhancement of iron-bearing surfaces. SAR data allowed the observation of the macro-topography and lineaments that were also interpreted on the basis of airborne geophysical and multispectral data. Geophysical and SAR (RADARSAT-1) data were combined by means of the IHS (intensity-hue-saturation) transform and the products yielded were used to improve the limits of the geologic units and to define targets for iron exploration. The Spectral Angle Mapper (SAM) supervised classification method was applied to a set of selected airborne geophysical images based on the rationale of the iron mineralization descriptive model and geophysical signatures observed at Serra Leste. Using documented iron occurrence as training point for the SAM classification, iron favorability maps were produced and compared against several early maps, proving to be equivalent or superior in accuracy.

A região Amazônica é caracterizada por floresta equatorial densa, espessa cobertura de solos e clima chuvoso. Este contexto torna os aerolevantamentos geofísicos ferramentas-chave para mapeamentos litológicos e estruturais e para revisão de mapas geológicos pré-existentes. Este artigo apresenta um estudo de aplicação de dados gamaespectrométricos e magnetométricos na região de Serra Leste, Província Mineral de Carajás, proporcionando discussões sobre as assinaturas geofísicas de litotipos específicos e estratégias para realce de alvos favoráveis à exploração de depósitos de Au-Pd-Pt, Cr-EGP, Cu-Mo-Au e Fe.

The Amazon region is characterized by dense equatorial forest, thick soil cover and rainy weather. This makes airborne geophysics a key tool for lithologic and structural mapping and improvement of preexisting geological maps. This paper presents an application study of airborne magnetic and gamma-ray spectrometric data in the Serra Leste region, Carajás Mineral Province, providing discussions about the geophysical signatures of specific lithologies and strategies to constrain favorable targets for exploration of Au-Pd-Pt, Cr-PGE, Cu-Mo-Au and Fe deposits.