As sucessões carbonáticas neoproterozóicas do Cráton do São Francisco e os depósitos de fosfato: correlações e fosfogênese

Neoproterozoic Carbonate Sequences of the São Francisco Craton and the Phosphate Deposits: Correlation and Phosphogenesis. The neoproterozoic carbonate successions of the São Francisco craton are formed by thick carbonate and siliciclastic strata accumulated in epicontinental seaways, and on passive margins. At least two transgressive-regressive sea level cycles have been recognized during the evolution of the carbonate mega-sequences. These successions, represented by the Vazante, Bambuí, and Una Groups, lie above basal glacio-marine diamictites of probable Sturtian age, and an upper glacio-marine diamictite, was observed in upper units of the Vazante Group. The broad similarities of lithofacies, as well as the presence of phosphate and sulfide (Zn,Pb) deposits restricted to narrow stratigraphic intervals, suggest that the Vazante, Bambuí and Una Groups may be correlative. The Sr isotope data from well preserved carbonates and carbonate fluorapatite from each of these successions, ranging from 0.70763 to 0.70794, support the general correlation and are indicative of seawater composition around 650 Ma, although the lower 87Sr/86Sr values of 0.70614 at the Vazante carbonates may suggest that this unit is older than the Bambuí and Una carbonate rocks. Carbon isotope data of carbonate fluorapatite reveal sharp negative excursions between -3.47 and -12.25‰ VPDB in the cratonic area, and between -1.3 and -9.61‰ VPDB in the carbonate fluorapatite hosted by the passive margin Vazante carbonates, suggesting that primary P concentration was accumulated in strong anoxic environments in both geotectonic settings. Carbonate hosting the phosphate deposits in these units are moderately enriched in 13C, ranging from positive δ13C values of +2.0 and +9.6‰ VPDB in the different sections. Primary phosphate accumulations occur above glaciogenic successions. We suggest that these concentrations may be due to high organic productivity after glacial events

Correlations of some Neoproterozoic carbonate-dominated successions in South America based on high-resolution chemostratigraphy

Esta revisão incorpora novos dados isotópicos e químicos para correlação quimioestratigráfica de sucessões, predominantemente carbonáticas, na América do Sul (Argentina, Bolívia, Brasil, Paraguai e Uruguai). Estas sucessões, depositadas durante eventos de extensão, aparecem como espessos estratos carbonáticos/siliciclásticos em bacias epicontinentais ou foram acumulados sobre margens passivas em bordos de cratons (ex.: São Francisco, Amazonas, Bloco Rio Apa, Pampia e Río de La Plata) como três megasequências: glaciogênica, plataforma marinha carbonática (sobre diamictitos glacio-marinhos) e predominantemente continental siliciclástica. Em cinturões circunjacentes a cratons, sucessões carbonáticas contendo material vulcanoclástico/siliciclástico foram, na maioria dos casos, fortemente deformados. A idade precisa de sedimentação destas sucessões permanece em debate, mas recentes dados paleontológicos e geocronológicos têm consideravelmente restringidos os intervalos de deposição. Neste trabalho, reportamos trends isotópicos de alta resolução de C, O, Sr e S medidos em amostras bem preservadas e usamos principalmente isótopos de Sr e C em consonância com observações litostratigráficas/biostratigráficas, para estabelecer correlações detalhadas destas sucessões. Propor uma correlação quimioestratigráfica entre bacias Neoproterozoicas na América do Sul é o principal objetivo deste trabalho.This report reviews and incorporates new elemental and isotope chemostratigraphic data for correlation of Neoproterozoic carbonate-dominated successions in South America (Argentina, Bolivia, Brazil, Paraguay and Uruguay). These thick mixed carbonate/siliciclastic successions were largely deposited in epicontinental basins or accumulated on passive margins on the edges of cratons (e.g. São Francisco, Amazonia, Rio Apa Block, Pampia and Río de la Plata paleocontinents) during extensional events related to the rifting of the Rodinia Supercontinent. From the stratigraphic point of view, these successions occur as three mega-sequences: glaciogenic, marine carbonate platform (above glaciomarine diamictites or rift successions), and dominantly continental to transitional siliciclastics. In the orogenic belts surrounding cratons, carbonate-dominated successions with important volcanoclastic/siliciclastic contribution have been, in most cases, strongly deformed. The precise ages of these successions remain a matter of debate, but recently new paleontological and geochronological data have considerably constrained depositional intervals. Here, we report high-resolution C, O, Sr, and S isotope trends measured in well-preserved sample sets and mainly use Sr and C isotopes in concert with lithostratigraphic/biostratigraphic observations to provide detailed correlations of these successions. The establishing of a high-level and definite chemostratigraphic correlation between Neoproterozoic basins in South America is the main goal of this work

ESTIMANDO A FAVORABILIDADE PARA RECURSOS MINERAIS NA BACIA DE IRECÊ PELOS MÉTODOS LÓGICA NEBULOSA E PESOS DAS EVIDÊNCIAS

A aplicação dos métodos Lógica   Nebulosa   (FuzzyLogic) e Pesos das Evidências (Weights of Evidence) na estimativa da favorabilidade para ocorrência de depósitos  de Pb e Zn na área da Bacia  de Irecê (Bahia) aborda dois enfoques distintos da modelagem em ambiente  de Sistemas de Informações  Geográficas:   o primeiro, simulando as idéias de um especialista sobre determinado fenômeno, e o segundo, obtendo medidas estatísticas da associação espacial para usar no prognóstico de novas ocorrências do mesmo fenômeno