Disponível on-line no endereço www.igc.usp.br/geologiausp -31 -RESUMO O greenstone belt de Guarinos é uma das três faixas de rochas supracrustais de baixo grau metamórfico marginadas por ortognaisses do extremo norte do Bloco Arqueano de Goiás. A investigação da estratigrafia daquelas faixas iniciou no final da década de 1970, com modificações ao longo dos anos em decorrência de mapeamentos geológicos de detalhe. A proposta estratigráfica mais recente em vigor do greenstone belt de Guarinos reúne as rochas supracrustais no Grupo Guarinos, subdividido nas formações Serra do Cotovelo (metakomatiitos), Serra Azul (metabasaltos), São Patricinho (metaturbiditos máficos), Aimbé (BIF) e Cabaçal (filitos carbonosos e metarenitos impuros). O mapeamento de detalhe e os resultados de sondagens do programa de exploração mineral na área pela Yamana Desenvolvimento Mineral S.A. possibilitaram redefinir a Formação Cabaçal e desdobrá-la em um membro inferior de filitos carbonosos com intercalações de derrames de basalto e lentes de gondito; um intermediário de horizonte de gondito e formação ferrífera com metachert e barita maciça; e um superior de filito carbonoso com eventuais lentes de metachert. Os metarenitos impuros, por seu turno, são propostos sob a denominação de Formação Mata Preta, a qual está lateralmente interdigitada com as rochas da Formação Cabaçal. Dados geocronológicos U-Pb obtidos por LA-ICP-MS em cristais detríticos de zircão dos turbiditos máficos da Formação São Patricinho e dos metarenitos da Formação Mata Preta indicam área fonte dominada por rochas do Sideriano e Riaciano, com parcial participação de fonte Arqueana. A natureza dos litotipos das formações Cabaçal e Mata Preta e as relações de contato entre ambas sugerem que o estágio de bacia que representam evoluiu a partir de importante evento anóxico oceâni-co mundial de 2,2 a 2,06 Ga e responsável pela Formação Cabaçal. O ambiente euxênico foi contemporâneo com um arco magmático que atuou como fonte da carga clástica dos arenitos da Formação Mata Preta.Palavras-chave: Greenstone belt de Guarinos; Goiás; Revisão estratigráfica; Geocronologia; Evolução de bacia. ABSTRACTThe Guarinos greenstone belt is one of the three low metamorphic grade supracrustal rocks assemblage rimmed by orthogneisses of Central Brazil's Archean Block northern limits. The investigation of the stratigraphy of those greenstone belts started by the end of 1970's and underwent improvements during the years as a result of detailed geological mapping. The latest and accepted stratigraphic model for the Guarinos greenstone belt refers its supracrustal rocks under the Guarinos Group, which is subdivided into the Serra do Cotovelo (metakomatiites), Serra Azul (metabasalts), São Patricinho (mafic metaturbidites), Aimbé (BIF) and Cabaçal (carbonaceous phyllites and impure metarenites) formations. Detailed geologic mapping (1:10,000) and drill-cores during an exploration program by Yamana Desenvolvimento Mineral S.A. in the area allowed a better definition of the metasedimentary package of...
ABSTRACT:The Faina Greenstone Belt is located in the southern sector of the Goiás Archean Block and has been investigated since the 18 th century because of its gold deposits. Recent studies have revealed the polymetallic potential of the belt, which is indicated by anomalous levels of Ag, Cu, Fe and Co in addition to Mn, Ba, Li, Ni, Cr and Zn. This study was developed based on a detailed analysis of two selected target sites, Cascavel and Tinteiro, and multiple data sets, such as airborne geophysics, geochemistry and geological information. These datasets were used to create a final prospectivity map using the fuzzy logic technique. The gold mineralization of Cascavel target is inserted in an orogenic system and occurs in two overlapping quartz veins systems, called Mestre-Cascavel and Cuca, embedded in quartzite with an average thickness 50 cm and guidance N45º-60ºW/25ºSW with free coarse gold in grains 2-3 mm to 3 cm. The prospectivity map created for this prospect generated four first-order favorable areas for mineralization and new medium-favorability foci. The Tinteiro area, derived from studies conducted by Orinoco do Brasil Mineração Ltda., shows polymetallic mineralization associated with an iron oxide-copper-gold ore deposit (IOCG) system posterior to Cascavel target mineralization. Its prospectivity map generated 19 new target sites with the potential for Au, Cu and Ag mineralization, suggesting new directions for future prospecting programs. KEYWORDS:Faina Greenstone Belt; aeromagnetometry; gamma spectrometry; gold mineralization; polymetallic mineralization; fuzzy logic. RESUMO: O Greenstone Belt INTRODUCTIONGlobally, mining companies seek to utilize prospective efficient methods that decrease costs during mineral exploration stages. To achieve such goals, they must use techniques that efficiently aggregate the data generated in the different stages of a given study by focusing on models that have a greater likelihood of estimating deposits (Robert et al. 2007). Magnetic surveys coupled with high-resolution gamma spectrometry data represent available technology in airborne geophysics for the detection of relevant structures for mineral exploration, and they have been shown to be efficient in the selection of follow-up target sites in tropical terranes (Silva et al. 2012, Carrino et al. 2011.The Faina Greenstone Belt is the target of this study, and, along with the Pilar de Goiás, Guarinos, Crixás and Serra de Santa Rita belts, it is part of the granite-greenstone terranes of the Archean-Paleoproterozoic terrane of Goiás, which is located in the northern sector of the Brasília Belt (Pimentel et al. 2004, Jost et al. 2005, Jost et al. 2013. This region has been investigated since the time of the Bandeirantes explorers (18 th and 19 th centuries) and shows the potential to host gold (Au) mineralizations. Several companies have mined in the area, including the well-known Metais de Goiás S.A. (METAGO), Western Mining Co., Amazônia Mineração and Troy Resources Brazil.The discovery of Sertão, a low-tonnag...
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