Irece Basin Research Articles

Quantitative evaluation of the geosites and geodiversity sites of João Dourado Municipality (Bahia—Brazil)

Geological aspects of certain sites can help understand the history of a region and reveal discoveries in regard to the planet. Geodiversity in situ is expressed in geosites, with occurrences of geological features of scientific value, and geodiversity sites, places where other values prevail, such as education, culture, and tourism. In addition, such locations can generate socioeconomic benefits for the local population through management based on education and geotourism. Joao Dourado is a municipality located in the central north of the state of Bahia, in Chapada Diamantina. It is inserted in the Irece Basin, coinciding with lithotypes of the Salitre Formation, which presents carbonate rocks of a shallow marine environment. The municipality is yet to be the subject of a detailed study carrying out inventory and assessment of its geodiversity. The objective of this study was to investigate the geodiversity of the municipality through quantitative evaluation and classification of the relevance of geosites and geodiversity sites. In this context, the inventory and quantification of sixteen sites of interest were carried out using the GEOSSIT application of the Geological Survey of Brazil (CPRM). Five geosites were found, one of international relevance and four of national relevance, as well as eleven geodiversity sites, whereby nine are of national relevance and two of regional/local relevance. Understanding and disseminating the geodiversity of Joao Dourado provides a basis for territorial management actions by public authorities and brings the population into closer contact with geosciences.

Concepção de um índice elétrico de carstificação

A pesquisa enfoca o comportamento elétrico dos aquíferos cársticos quando submetidos a uma corrente elétrica contínua. Na prática profissional, as dificuldades da aplicação do método elétrico de resistividade nesses aquíferos induziram a busca de uma abordagem de campo mais adequada e critérios mais eficientes de interpretação das medições elétricas para fins de locação de poços produtores. Entre 1999 e 2004, seis estudos geofísicos por eletrorresistividade foram realizados no Aquífero Jandaíra desenvolvido em sedimentos carbonatados cretáceos da Bacia Potiguar/RN. Foi utilizada a técnica do caminhamento elétrico 1D. O dispositivo de medição adotado foi o quadripolo linear simétrico de Schlumberger com linha AB de emissão de corrente definida a partir de SEVs previamente obtidas. Verificou-se que as zonas mais intensamente carstificadas são eletricamente resistivas e exibem sempre uma pronunciada variabilidade elétrica. Esse comportamento, ratificado pelo confronto das medições elétricas com os resultados dos poços locados, levou à concepção de um Índice Elétrico de Carstificação (IEC) que enfatiza e quantifica a variabilidade elétrica, permitindo eleger objetivamente locais mais favoráveis à captação de água subterrânea em calcários. O IEC de uma estação de medição i, em perfis lineares, é calculado como a média da soma das variações relativas às estações (i+1) e (i-1). São apresentadas e discutidas quatro variantes do IEC dependentes do comportamento geológico e hidroquímico da área investigada. Os resultados podem ser apresentados sob a forma de perfis lineares ou, se a densidade dos perfis permitir, sob a forma de zoneamentos. São discutidas aplicações da abordagem no Aquífero Jandaíra da Bacia Potiguar (RN e CE), no Aquífero Salitre da Bacia de Irecê (BA), e no Subgrupo Paraopeba Indiferenciado/MG. A aplicação do IEC mostrou-se útil em caracterizar, local e regionalmente, zonas potencialmente aquíferas.

Ferramentas Hidrogeoquímicas e Isotópicas (δ18O e δ2H) Aplicadas no Entendimento dos Processos de Salinização da Água Subterrânea em Aquíferos Carbonáticos Proterozoicos: Evidencia do Semiárido do Nordeste Brasileiro

Groundwater in arid and semi-arid regions is frequently the only available source of water for the population. Quality decline or deterioration is considered a crucial problem for the management of groundwater resources. In this study, a hydrogeological investigation was carried out using geochemical and isotopic data in a karst aquifer, installed in the Proterozoic carbonate rocks of the Salitre Formation: southern portion of the Irece Basin, Chapada Diamantina, Bahia, Brazil. The aim was to understand the hydrogeochemical and physical processes that contribute to the salinization of groundwater. This understanding is essential for the sustainable exploitation of water resources. The main results indicated that the processes of evaporation and leaching were not very efficient in the reconcentration of mineral salts in the vadose zone of the aquifer. Hydrogeochemical and isotopic modeling showed that the salinity of the groundwater is subjected to geological processes, resulting from rock-water interaction. The integrated use of geochemistry and stable isotopes (δ18O and δ2H) in karst aquifers is an important instrument in the modeling and understanding of groundwater salinization processes and an effective tool in the evaluation and management of water resources.

O cinturão epidérmico de antepaís da Bacia de Irecê, Cráton do São Francisco[/title]: [subtitle]principais elementos estruturais e modelagem física analógica

Localizada na porção central do estado da Bahia, a Bacia de Irecê exibe as mais significativas exposições de coberturas neoproterozoicas da porção norte do Cráton do São Francisco. Apesar da grande quantidade de estudos geológicos ali realizados, restam ainda várias questões em aberto, em especial no que tange à evolução tectônica do cinturão epidérmico de antepaís que envolve as rochas da bacia. No sentido de contribuir para o entendimento de tal evolução, este trabalho revisa os principais elementos estruturais da bacia e adjacências e apresenta novos dados adquiridos por meio de modelo físico-analógico em caixa de areia. O cinturão epidérmico de antepaís da Bacia de Irecê corresponde a uma grande feição curva confinada ao longo do sinclinal homônimo e tem sua formação atribuída ao fechamento das faixas marginais do setor norte do Cráton do São Francisco. Seu desenvolvimento foi condicionado por um vetor tectônico N-S, responsável pela nucleação de dobras e falhas E-W. Nas bordas da bacia, a deformação é acomodada através de falhas direcionais responsáveis pela rotação geral das estruturas. Para o sul, o cinturão perde gradativamente sua expressividade, ocorrendo apenas as estruturas geradas previamente durante o desenvolvimento do Sistema de Dobras e Empurrões da Chapada Diamantina. O modelo físico-analógico em caixa de areia simulou com sucesso o desenvolvimento do Cinturão de Antepaís da Bacia de Irecê e indica que sua curvatura em planta resulta da interação com as bordas do sinclinal e da própria curvatura do substrato. A propagação deu-se por meio de um descolamento com baixo coeficiente de atrito, provavelmente condicionado pelo contraste reológico entre as unidades carbonáticas do Grupo Una e o substrato quartzítico do Supergrupo Espinhaço.

Geotectonic setting and metallogeny of the northern São Francisco craton, Bahia, Brazil

Abstract This paper aims at establishing a tectonic and temporal framework to characterize the metallogenic processes that contributed to the origin of the mineral provinces in the northern Sao Francisco Craton. Many Archean mineralizations (eg. massive sulfide zinc, lead, zinc and copper, besides magnesite–talc, iron–titanium–vanadium, iron, chromite and manganese) were generated before the assembly of the Craton. Deposits of chromite, nickel, gold and emerald were produced during the Paleoproterozoic orogenic cycle, when the Craton was amalgamated into the Atlantica paleocontinent. An extension event is recorded in the Neoproterozoic, during the breakup of Rodinia, associated with deposits of phosphorite and uranium. Kimberlite diamond and gold mineralization were generated during the Brasiliano orogenic cycle, coeval with the amalgamation of West Gondwana. A long-lasting and rather uniform crustal stress is recorded in the area during the Cambrian period. Resetting of the isotopic and magnetic systems that affected the Neoproterozoic sediments of the Irece Basin at about 520 Ma was attributed to the regional-scale fluid migration and mineralization in the aftermath of the Brasiliano orogenic cycle.

Gis modeling for mineral potential mapping of carbonate-hosted pb-zn deposits

Regional exploration datasets from carbonate-hosted Pb-Zn deposits in the Neoproterozoic Irece Basin (Brazil) were modeled by GIS-based (Geographic Information System) predictive, probabilistic tools that allow fuzzy logic and weights-ofevidence methods to be applied. These deposits are hosted within dolomitic shallow-water facies in the carbonate sequences of the Una-Bambui Group, and show remarkable stratigraphic control at the end of the first regressive cycle (the so-called B1 Unit, Salitre Formation). The datasets used for this study include a geological map (1:100.000 scale), airborne radiometric grids, a soil geochemical survey and two LANDSAT TM images. Twenty known stratabound Pb-Zn sulfide deposits were employed as training points. Thirteen evidential themes derived by GIS processing based on the exploration model were grouped into five factors: stratigraphic, structural, geochemical, geophysical and multispectral factor. Weights (W + , W - ) and contrast (C=W + -W - ) calculations guided the data-driven modeling, and an exploration model supported the choice of fuzzy membership functions for the knowledge-driven modeling. The most predictive factor was the geochemical factor. The geophysical maps, although not having sufficient spatial resolution to predict deposits, provide important regional screening variables.

Origin of Zn-Pb-Ag Sulfide Mineralization within Upper Proterozoic Phosphate-Rich Carbonate Strata, Irecê Basin, Bahia, Brazil

The Irece Basin is a relatively small late Proterozoic basin within the São Francisco craton of northeast-central Brazil. Stratabound Zn-Pb-Ag sulfide and phosphate-rich units occur within a 50-m-thick tidal-flat sequence of dolomitic limestone and cherty dolostone within the lower Salitre Formation of the Una Group. Phosphate is concentrated in stromatolitic structures; metallic mineralization is represented by stratiform masses and disseminations of sulfide minerals in a tidal-flat sequence that has relict evaporitic structures. Zn-Pb-Ag sulfide concentrations in the Irece area are associated with parallel laminated, silty dolostone with shallow-water sedimentary structures, including laminar stromatolites, mud cracks, tepee structures, collapse breccias, length-slow quartz nodules, and pseudomorphs after evaporitic sulfates. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite, in order of decreasing abundance. Nodular and bladed sulfide forms are pseudomorphous after evaporitic sulfate minerals. Replacement textures, such as corroded grain boundaries between sulfide minerals, are common. Sulfur-isotope analysis indicates relatively uniform heavy δ34S values. Barite has a δ34S range from +25 to +31‰, CDT. Pyrite, sphalerite, galena, and jordanite-representative of a variety of textural types-have a δ34S range of +19 to + 23‰; δ34S values for Irece barite probably represent original late Proterozoic sea-water sulfate values. The consistently heavy δ34S values of the Irece sulfides likely are the result of thermochemical reduction of a limited evaporitic sulfate source by organic matter that is abundant in the Salitre carbonates. Major Zn-Pb-Ag concentrations appear to be an overprint on the earlier iron sulfide-rich zone. Limited fluid-inclusion data suggest precipitation in the range of 140 to 200°C from formation waters with salinities ranging from 3 to 12 wt%. Deep fluid circulation may have been promoted by reactivation of basement faults originally related to the Irece Basin's development. Petrographic and fluid-inclusion data suggest that sulfide mineralization may have occurred within a mixing zone of metal-bearing, high-temperature basinal brines with meteoric water. Metallic mineralization scavenged sulfur from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. The metal and phosphate concentrations in the Irece Basin possess some similiar geologic characteristics that result from a common host-rock depositional environment. However, existing information indicates that the two deposit types are not directly related genetically, although the origin of both is related to the organic-rich nature of the host sequence.

Upper Proterozoic Carbonate Stratigraphy, Diagenesis, and Stromatolitic Phosphorite Formation, Irece Basin, Bahia, Brazil

ABSTRACT Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece Basin of east-central Brazil. The Irece Basin contains a basal siliciclastic sequence (the Bebedouro Formation) composed of diamictites dominantly of glacial origin that is overlain by lacustrine carbonate sediments. These are succeeded by the Una Group, a marine carbonate sequence that totals several hundred meters thick. The marine units consist of a basal laminated limestone and dolomitic limestone sequence that grades upward into dolostone and cherty dolostone; a middle sequence dominated by gray argillaceous limestones, shales, and siltstones; and an upper sequence of black organic-rich lime grainstones. Stratabound phosphate- and sulfide-rich units are present within a 50-m-thick tid l flat sequence of dolomitic limestone and cherty dolostone in the lower Una Group. Shallow-water sedimentary structures include stromatolitic laminae, columnar stromatolites, mudcracks, teepee structures, collapse breccias, length-slow quartz nodules, and other pseudomorphs after evaporitic sulfates. Conventional petrography supported by cathodoluminescence and scanning electron microscopy of the carbonate strata reveals a complex diagenetic history representing four major diagenetic environments: Marine, Reflux, Meteoric, and Burial. Carbon and oxygen isotope analyses of diagenetic carbonate generations yield distinct populations that support the environmental interpretations. Concentrations of fine-grained carbonate fluorapatite are present in stromatolitic carbonate units in the Irece-Lapao area. Three types of primary phosphate concentrations have been defined: columnar stromatolitic, laminar stromatolitic, and intraclastic. The presence of resedimented phosphatic clasts and of phosphatic units interbedded with barren dolostones suggest that the formation of stromatolitic phosphorite was an early-diagenetic process. Carbon and oxygen isotope data for calcite, dolomite, and carbonate fluorapatite, as well as sulfur isotope data from sulfates and sulfides, suggest the presence of an anoxic environment during the formation of the Irece phosphorites. Postglacial enrichment of the basin waters in phosphate is considered to be a critical aspect of phosphatizat on. Bacterial degradation of organic matter in the stromatolites probably was responsible for local phosphate enrichment of pore waters and resulted in concentration of carbonate fluorapatite by direct precipitation or by replacement of calcium carbonate.

Origin of Sulfide and Phosphate Deposits in Upper Proterozoic Carbonate Strata, Irece Basin, Bahia, Brazil

Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece basin of north-central Brazil. Stratabound sulfide- and phosphate-rich units occur within a 50-m thick tidal flat sequence of dolomitic limestone and cherty dolomite. Three types of primary phosphate concentrations are present: columnar stromatolitic, laminar stromatolitic, and intraclastic. Resedimented phosphate clasts and phosphatic units interbedded with non phosphatic dolomites suggest early diagenetic replacement of algal carbonate units. Local stratabound Zn-Pb-Ag sulfide concentrations at the Tres Irmas prospect occur within silty dolomite with shallow water sedimentary structures and local disturbed laminae, synsedimentary faults, and breccias. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite. Pyrite crystal aggregates commonly show bladed forms. Nodular aggregates of length-slow quartz are locally associated with sulfides. Sulfur isotope analyses indicate relatively uniform heavy {delta}{sup 34}S values. Barite shows a {delta}{sup 34}S range from +25.2 to +29.6{per thousand}, CDT. Pyrite and sphalerite representative of a variety of textural types have a {delta}{sup 34}S range of +20.2 to +22.6{per thousand}. Late Proterozoic evaporite sulfates show a wide range of {delta}{sup 34} S values from about +10 to +28{per thousand}. Thus, the {delta}{sup 34}S values for Irece barite could reflect original seawater sulfate values.more » However, the relatively heavy {delta}{sup 34}S values of the associated sulfides suggests that the original seawater sulfate was modified by bacterial sulfate reduction processes in shallow sea floor sediments. Textural and {delta}{sup 34}S evidence suggests that a later stage of metallic mineralization scavenged sulfur from preexisting sulfides or from direct reduction of evaporitic sulfate minerals.« less