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Estudo geoquímico e mineralógico das crostas de Fe-Mn no Atlântico Norte
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As crostas hidrogenéticas de Fe-Mn (crostas de Fe-Mn) formam-se no fundo do mar e apresentam concentrações elevadas em diversos metais com interesse económico, como é o caso do Co, Ni e dos elementos do grupo das terras raras. As crostas de Fe-Mn que apresentam maiores enriquecimentos em Mn, Ni e Co formam-se no oceano Pacífico. Por outro lado, as crostas que se formam no oceano Atlântico, por norma, apresentam maiores enriquecimentos em Fe e em elementos do grupo das terras raras (REE), devido ao elevado acarreio terrígeno pelos rios e poeiras do Sahara, a terem taxas de crescimento superiores às observadas no oceano Pacífico e por o oceano Atlântico apresentar zonas de oxigénio mínimo pouco anóxicas (20-40 μmol/kg de oxigénio). Contudo, alguns autores demonstraram que as crostas de Fe-Mn presentes no oceano Atlântico também apresentam elevado potencial, semelhante às crostas de Fe-Mn presentes no Pacífico. Durante a campanha realizada em 2007 pela EMEPC (EMEPC/Açores/G3/2007) ao Atlântico NE, foram recolhidas diversas amostras de crostas de Fe-Mn perto da Zona de Fractura Este dos Açores (ZFEA) e dos montes submarinos Atlantis, Plato, Small Hyeres e Great Meteor. Estudos preliminares mostraram que as crostas de Fe-Mn têm concentrações de Fe e Mn semelhantes à média para o oceano Atlântico, e concentrações de Co semelhantes à média para o oceano Pacífico. Com o intuito de complementar os resultados anteriormente obtidos, foram estudadas 11 amostras de crostas de Fe-Mn destas regiões. Este estudo consistiu na caracterização petrográfica detalhada das 11 amostras, mapeamentos por SEM e LA-ICP-MS, e análises pontuais de microssonda electrónica. O estudo geoquímico das amostras demonstrou que são essencialmente constituídas por Fe e Mn, apresentando concentrações médias de Co e Ni que podem chegar aos 1.57 e 0.60 wt%, respectivamente. Os REE analisados foram La, Ce, Nd e Yb, ocorrendo o Ce em maiores concentrações, com médias que podem chegar aos 0.26 wt%. As amostras em estudo foram todas classificadas como sendo de origem hidrogenéticas, contudo o seu ambiente de formação pode variar de local para local. As amostras recolhidas na ZFEA e no monte Atlantis formaram-se num ambiente típico das crostas de margem continental, enquanto que as amostras recolhidas nos montes Small Hyeres e Great Meteor tendem a apresentar ambiente de formação misto de margem continental e de oceano aberto. Por fim, a maioria das amostras recolhidas no monte Plato formam-se num ambiente de oceano aberto. Comparando com outros montes e bancos presentes no Atlântico NE, onde foram recolhidas crostas de Fe-Mn, verifica-se que os montes Small Hyeres, Great Meteor, Plato e Nameless tendem a apresentar concentrações de Co e de Ni comparáveis com a média do oceano Atlântico e, como tal, seriam os locais mais propícios para futura prospecção. Comparando com os depósitos terrestres, as crostas presentes nos montes Small Hyeres, Great Meteor, Plato e Nameless apresentam teores elevados de Co mas baixa tonelagem.
Hydrogenetic Ferromanganese crusts (Fe-Mn crusts) form on the oceans floors and show high concentrations in several metals with economic interest, such as Co, Ni and Rare Earth Elements (REE). Fe-Mn crusts with the highest concentration in Mn, Ni and Co form in the Pacific Ocean. On the other hand, the Fe-Mn crusts formed in Atlantic Ocean, show variably higher concentration in Fe and REE, due to high input of terrigenous materials from rivers and Saharan dust, high growth rate, the oxygenminimum zone have high oxygen content (20-40 μmol/kg) and often have associated an hydrothermal component. However, some authors showed that Fe-Mn crusts from the Atlantic Ocean also have high economic potential, similar to the Fe-Mn crusts from the Pacific Ocean. During the 2007 EMEPC survey (EMEPC/Açores/G3/2007) in the NE Atlantic Ocean, several samples of Fe-Mn crusts were collected near de East Azores Fracture Zone (EAFZ) and the Atlantis, Plato, Small Hyeres and Great Meteor seamounts. Preliminary studies showed that these Fe-Mn crusts have similar Fe and Mn concentration than the average for the Atlantic Ocean, and Co concentration is similar to the average for the Pacific Ocean. In order to complement the results, previously collected, 11 samples of Fe-Mn crusts from these regions were studied. The study consisted of detailed petrographic characterization of 11 samples, SEM and LA-ICP-MS mapping and microprobe analysis. Geochemical study showed that the major elements present in Fe-Mn crusts are Fe and Mn and maximum averaged concentrations of Co and Ni of 1.57 and 0.60 wt%, respectively. The REE analysed were La, Ce, Nd and Yb, and Ce has the highest concentrations with maximum averaged of 0.26 wt%. All studies samples were classified as hydrogenetic origin, however they formation environment may vary from site to site. The samples from EAFZ and Atlantis seamount were formed in a typical continental margin environment, whereas samples from Small Hyeres and Great Meteor form in a mixed open-ocean and continental margin environment. Most samples from Plato seamount were classify as open-ocean environment. Comparing with the remaining seamounts and banks in the NE Atlantic Ocean, where Fe- Mn crusts were collected, only Small Hyeres, Great Meteor, Plato and Nameless seamounts show Co and Ni concentrations similar to the averaged for Atlantic Ocean and so the Small Hyeres, Great Meteor, Plato seamounts would be the most favorable places for future exploration. Comparing with land deposits, the Fe-Mn crusts from Small Hyeres, Great Meteor and Plato seamounts show high grades of Co but low tonnage.
Hydrogenetic Ferromanganese crusts (Fe-Mn crusts) form on the oceans floors and show high concentrations in several metals with economic interest, such as Co, Ni and Rare Earth Elements (REE). Fe-Mn crusts with the highest concentration in Mn, Ni and Co form in the Pacific Ocean. On the other hand, the Fe-Mn crusts formed in Atlantic Ocean, show variably higher concentration in Fe and REE, due to high input of terrigenous materials from rivers and Saharan dust, high growth rate, the oxygenminimum zone have high oxygen content (20-40 μmol/kg) and often have associated an hydrothermal component. However, some authors showed that Fe-Mn crusts from the Atlantic Ocean also have high economic potential, similar to the Fe-Mn crusts from the Pacific Ocean. During the 2007 EMEPC survey (EMEPC/Açores/G3/2007) in the NE Atlantic Ocean, several samples of Fe-Mn crusts were collected near de East Azores Fracture Zone (EAFZ) and the Atlantis, Plato, Small Hyeres and Great Meteor seamounts. Preliminary studies showed that these Fe-Mn crusts have similar Fe and Mn concentration than the average for the Atlantic Ocean, and Co concentration is similar to the average for the Pacific Ocean. In order to complement the results, previously collected, 11 samples of Fe-Mn crusts from these regions were studied. The study consisted of detailed petrographic characterization of 11 samples, SEM and LA-ICP-MS mapping and microprobe analysis. Geochemical study showed that the major elements present in Fe-Mn crusts are Fe and Mn and maximum averaged concentrations of Co and Ni of 1.57 and 0.60 wt%, respectively. The REE analysed were La, Ce, Nd and Yb, and Ce has the highest concentrations with maximum averaged of 0.26 wt%. All studies samples were classified as hydrogenetic origin, however they formation environment may vary from site to site. The samples from EAFZ and Atlantis seamount were formed in a typical continental margin environment, whereas samples from Small Hyeres and Great Meteor form in a mixed open-ocean and continental margin environment. Most samples from Plato seamount were classify as open-ocean environment. Comparing with the remaining seamounts and banks in the NE Atlantic Ocean, where Fe- Mn crusts were collected, only Small Hyeres, Great Meteor, Plato and Nameless seamounts show Co and Ni concentrations similar to the averaged for Atlantic Ocean and so the Small Hyeres, Great Meteor, Plato seamounts would be the most favorable places for future exploration. Comparing with land deposits, the Fe-Mn crusts from Small Hyeres, Great Meteor and Plato seamounts show high grades of Co but low tonnage.
Descrição
Relatório de estágio de mestrado, Geologia Económica (Prospecção Mineral) Universidade de Lisboa, Faculdade de Ciências, 2019
Palavras-chave
Crostas de Fe-Mn hidrogenéticas Ambiente de margem continental NE Atlântico Montes submarinos Atlantis Plato, Great Meteor e Small Hyeres Zona de Fractura Este dos Açores Relatórios de estágio - 2019