Metamorphism, Metasomatism And Mineralization At Lagoa Real, Bahia, Brazil

Uranium deposits cumulatively in the 100,000 tonne U(,3)O(,8) range occur within ductile shear zones transecting Archean basement gneisses of the Sao Francisco Craton, at the Lagoa Real region of south-central Bahia, Brazil. The gneisses, dated at 2.6-3.0 Ga, are at amphibolite and granulite facies and overlie to the west, the Proterozoic Espinhaco metasedimentary sequence along a thrust fault.;Petrography and mineral chemistry show that in the zones of alteration/mineralization, the original k-feldspar + quartz + albite/oligoclase + hastingsite assemblage, is replaced by albite + aegirine-augite + andradite + hematite assemblages, with or without uraninite. This information along with oxygen isotope, whole rock geochemistry and fluid inclusion studies indicate that the alteration process involves removal of Si, K, Rb, Ba and addition of Na under oxidizing conditions. V, Pb, Sr were introduced along with U via interaction with saline CO(,2)-rich, isotopically light fluids under varying water/rock ratios and at temperatures of 500-550(DEGREES)C. (\u2787)Sr/(\u2786)Sr systematics suggest that it is unlikely that Sr, and by extension uranium, were introduced by fluids originating from the basement gneisses.;Geological constraints and the general alteration pattern are consistent with the release of the mineralizing fluids in response to overloading of the basement rocks onto the sedimentary Espinhaco via a thrust mechanism

Datação U-Pb in situ por LA-ICPMS em zircões detríticos da Formação Cercadinho, Supergrupo Minas

In this paper, new geochronological data obtained for the detrital rocks of the Cercadinho Formation, basal unit of the Piracicaba Group, Minas Supergroup, are presented. U-Pb Laser Ablation Induced Coupled Mass Spectrometer (LA-ICPMS) analysis in detrital zircon from quartzite samples provided a new maximum depositional age for the Cercadinho Formation. The youngest zircon population, among the dated samples, provided an average age of 2680 ± 24 Ma. These rocks have an expressive contribution of Meso- to Neoarchean zircons, with ages between 2812 ± 19 and 2909 ± 19 Ma, and older populations between 3212 ± 18 and 3272 ± 16 Ma, which occur mainly in the core of younger zircon grains. Comparing the U-Pb ages obtained in this work with previously published geochronological data for the basal units of the Minas Supergroup (Moeda Formation) an aging of the source for the detrital rocks of Cercadinho Formation can be observed, with a major contribution of zircons from TTG rocks crystallized between the Meso- and Neoarchean.Nesse trabalho são apresentados novos dados geocronológicos obtidos para rochas detríticas da Formação Cercadinho, unidade basal do Grupo Piracicaba, Supergrupo Minas. Análises U-Pb por Laser Ablation Induced Coupled Mass Spectrometer (LA-ICPMS) em zircões detríticos provenientes de amostras de quartzito permitiram obter uma nova idade máxima para a deposição da Formação Cercadinho, sendo que a população mais jovem entre as amostras datadas forneceu uma idade média de 2680 ± 24 Ma. Essas rochas possuem uma expressiva contribuição de zircões Meso- e Neoarqueanos, com idades entre 2812 ± 19 and 2909 ± 19 Ma, e populações mais antigas entre 3212 ± 18 e 3272 ± 16, que ocorrem principalmente no núcleo de zircões mais jovens. Comparando-se esses resultados com idades anteriormente obtidas por outros autores para unidades basais do Supergrupo Minas (Formação Moeda), percebe-se um envelhecimento da área fonte para rochas de nível estratigráfico superior, com uma maior contribuição de zircões provenientes de TTGs cristalizados entre o Meso- e o Neoarqueano

Datação U-Pb in situ por LA-ICPMS em zircões detríticos da Formação Cercadinho, Supergrupo Minas.

Nesse trabalho são apresentados novos dados geocronológicos obtidos para rochas detríticas da Formação Cercadinho, unidade basal do Grupo Piracicaba, Supergrupo Minas. Análises U-Pb por Laser Ablation Induced Coupled Mass Spectrometer (LA-ICPMS) em zircões detríticos provenientes de amostras de quartzito permitiram obter uma nova idade máxima para a deposição da Formação Cercadinho, sendo que a população mais jovem entre as amostras datadas forneceu uma idade média de 2680 ± 24 Ma. Essas rochas possuem uma expressiva contribuição de zircões Meso- e Neoarqueanos, com idades entre 2812 ± 19 and 2909 ± 19 Ma, e populações mais antigas entre 3212 ± 18 e 3272 ± 16, que ocorrem principalmente no núcleo de zircões mais jovens. Comparando-se esses resultados com idades anteriormente obtidas por outros autores para unidades basais do Supergrupo Minas (Formação Moeda), percebe-se um envelhecimento da área fonte para rochas de nível estratigráfico superior, com uma maior contribuição de zircões provenientes de TTGs cristalizados entre o Meso- e o Neoarqueano.In this paper, new geochronological data obtained for the detrital rocks of the Cercadinho Formation, basal unit of the Piracicaba Group, Minas Supergroup, are presented. U-Pb Laser Ablation Induced Coupled Mass Spectrometer (LA-ICPMS) analysis in detrital zircon from quartzite samples provided a new maximum depositional age for the Cercadinho Formation. The youngest zircon population, among the dated samples, provided an average age of 2680 ± 24 Ma. These rocks have an expressive contribution of Meso- to Neoarchean zircons, with ages between 2812 ± 19 and 2909 ± 19 Ma, and older populations between 3212 ± 18 and 3272 ± 16 Ma, which occur mainly in the core of younger zircon grains. Comparing the U-Pb ages obtained in this work with previously published geochronological data for the basal units of the Minas Supergroup (Moeda Formation) an aging of the source for the detrital rocks of Cercadinho Formation can be observed, with a major contribution of zircons from TTG rocks crystallized between the Meso- and Neoarchean

Metaturbidite-hosted gold deposits, Córrego do Sítio lineament, Quadrilátero Ferrífero, Brazil

A study of geology and rocks paragenesis has been conducted at the Córrego do Sítio auriferous lineament, containing the Cachorro Bravo, Laranjeiras and Carvoaria metaturbidite-hosted lode-gold deposits located in the Quadrilátero Ferrífero Region, Minas Gerais, Brazil. These representative deposits are described to illustrate the essentially similar general character of all the deposits of lineament as well as the wide compositional and mineralogical differences in the ore of the different deposits, where, for the Cachorro Bravo deposit, a geological mapping included two underground mine galleries. The Córrego do Sítio unit is a metamorphosed turbidite in an alternating sequence of metagraywackes and phyllites, with parallel to discordant metamafic dikes and sills. The ore zone is predominantly hosted at the stratigraphic break between metasedimentary and metamafic rocks. Four deformation events affected the mine sequence. Mineralized veins and veinlets are considered to have formed within a brittle-ductile shear-zone environment and occurred in multiple episodes. Different vein types are recognized, but the most important volumetrically is a S1-concordant type characterized by smoky and milky quartz-carbonate-sulfide ± sulfosalts veins. Veins are dominated by quartz, but locally they are characterized by carbonate and a large variety of sulfide and sulfosalt minerals. Pyrite is the commonest associated sulfide mineral, followed by arsenopyrite and pyrrhotite. An extensive mineralogical study of polished sections has confirmed different generations of sulfide minerals. The sulfides and sulfosalt minerals are interrelated in the veins and disseminated on wall rocks. The data are consistent with a genetic models related to other Archean lode-gold deposits

U-Pb geochronology of the Lagoa Real uranium district, Brazil : implications for the age of the uranium mineralization.

The Lagoa Real uranium district in Bahia, northeastern Brazil, is the most important uranium province in the country and presently produces this metal in an open-pit mine operated by Indústrias Nucleares do Brasil. Uranium-rich zones are associated with plagioclase (dominantly albite ± oligoclase) -rich rocks, albitites and metasomatized granitic-gneisses, distributed along NNW/SSE striking shear zones. We have used the ID-TIMS U-Pb method to date zircon and titanite grains from the São Timóteo granitoid, and albite-rich rocks from the Lagoa Real district in order to assess the age of granite emplacement, deformation/metamorphism and uranium mineralization. The isotopic data support the following sequence of events (i) 1746 ± 5 Ma - emplacement of the São Timóteo granitoid (U-Pb zircon age) in an extensional setting, coeval with the beginning of the sedimentation of the Espinhaço Supergroup; (ii) 956 ± 59 Ma hydrothermal alteration of the São Timóteo granitoid and emplacement of the uranium mineralization (U-Pb titanite age on an albite-rich sample); (iii) 480 Ma metamorphism, remobilization and Pb loss (U-Pb titanite age for the gneiss sample), during the nucleation of shear zones related to the collision between the São Francisco-Congo and Amazonia paleoplates. The 956 ± 59-Ma mineralization age is apparently associated with the evolution of the Macaúbas-Santo Onofre rift. This age bracket may bear an important exploration implication, and should be included in the diverse age scenario of uranium deposits worldwide

Metaturbidite-hosted gold deposits, Córrego do Sítio lineament, Quadrilátero Ferrífero, Brazil

A study of geology and rocks paragenesis has been conducted at the Córrego do Sítio auriferous lineament, containing the Cachorro Bravo, Laranjeiras and Carvoaria metaturbidite-hosted lode-gold deposits located in the Quadrilátero Ferrífero Region, Minas Gerais, Brazil. These representative deposits are described to illustrate the essentially similar general character of all the deposits of lineament as well as the wide compositional and mineralogical differences in the ore of the different deposits, where, for the Cachorro Bravo deposit, a geological mapping included two underground mine galleries. The Córrego do Sítio unit is a metamorphosed turbidite in an alternating sequence of metagraywackes and phyllites, with parallel to discordant metamafic dikes and sills. The ore zone is predominantly hosted at the stratigraphic break between metasedimentary and metamafic rocks. Four deformation events affected the mine sequence. Mineralized veins and veinlets are considered to have formed within a brittle-ductile shear-zone environment and occurred in multiple episodes. Different vein types are recognized, but the most important volumetrically is a S1-concordant type characterized by smoky and milky quartz-carbonate-sulfide ± sulfosalts veins. Veins are dominated by quartz, but locally they are characterized by carbonate and a large variety of sulfide and sulfosalt minerals. Pyrite is the commonest associated sulfide mineral, followed by arsenopyrite and pyrrhotite. An extensive mineralogical study of polished sections has confirmed different generations of sulfide minerals. The sulfides and sulfosalt minerals are interrelated in the veins and disseminated on wall rocks. The data are consistent with a genetic models related to other Archean lode-gold deposits