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Previous issue date: 2017-02-10 / As ocorr?ncias de rochas ferr?feras na regi?o central do Rio Grande do Norte
correspondem essencialmente a pequenos dep?sitos que aparentemente n?o apresentam
potencial econ?mico. Em contrapartida, nos ?ltimos anos, pequenos jazimentos desse
bem mineral tem sido explotados em algumas ?reas do Estado, especialmente os
inseridos na regi?o do Dom?nio Rio Piranhas-Serid?. As forma??es ferr?feras ocorrem
como intercala??es decim?tricas a algumas dezenas de metros em gnaisses da Forma??o
Jucurutu, na base da sequ?ncia metavulcanossedimentar do Grupo Serid?. Menos
frequentemente est?o associadas ?s rochas metam?ficas encaixadas em gnaisses do
Complexo Caic?. Mapeamentos sistem?ticos na escala de 1:100.000 (folhas Lajes e
Currais Novos) e levantamento de recursos minerais executados pela Companhia de
Pesquisa de Recursos Minerais identificaram uma quantidade significativa de ocorr?ncias
minerais (cadastro de 48 ocorr?ncias de rochas ferr?feras). Neste trabalho ser? realizado
uma caracteriza??o petrogr?fica macro e microsc?pica de l?minas delgadas e se??es
bipolidas e polidas de rochas ferr?feras, al?m da discuss?o dos dados de litoqu?mica,
atrav?s da an?lise dos elementos maiores, assinatura dos elementos tra?os e terras raras
para defini??o de prot?litos e ambiente tect?nico, quando for o caso, bem como o uso de
MEV-EDS para estudo da composi??o qu?mica qualitativa de magnetitas e anfib?lios.
Basicamente s?o rochas compostas de quartzo, ?xidos e hidr?xidos de ferro, anfib?lios
(da s?rie cummingtonita-grunerita e actinolita-tremolita principalmente), de granula??o
fina a m?dia e textura granonematobl?stica, por vezes com bandamento composicional.
Normalmente apresentam teores de Fe2O3 variando entre 46 e 59% e de SiO2 entre 33 e
50%. Petrograficamente identificou-se dois grupos distintos: rochas ferr?feras granulares
e rochas ferr?feras bandadas, encaixadas respectivamente em rochas correlatas ao
embasamento gn?issico-migmat?tico (Complexo Caic?) e rochas supracrustais correlatas
a Forma??o Jucurutu. A litoqu?mica permitiu concluir que os grupos de rochas ferr?feras
granulares e bandadas apresentam comportamentos ora semelhantes, ora distintos. S?o
rochas bimodais, compostas essencialmente de Si e Fe, com pequena propor??o de elementos tra?os e terras raras. Os par?metros qu?micos indicam ambiente
dominantemente oxidante e grupos de rochas em posi??es intermedi?rias a mais
pr?ximas das fontes hidrotermais oce?nicas. Quanto a natureza da forma??o das rochas
ferr?feras, gr?ficos bin?rios com as rela??es entre Th, Zr e Ti permitiram identificar que
s?o rochas puras quimicamente, com poucos n?veis de terr?genos. Enquanto diagramas
bin?rios de SiO2 e Al2O3 e tern?rios de Al-Fe-Mn indicam que a totalidade das amostras
de forma??es ferr?feras bandadas e granulares configuram sedimentos qu?mico-exalativos
hidrotermais. Diagramas espec?ficos para identifica??o dos ambientes tect?nicos para
forma??o das rochas ferr?feras granulares e bandadas estudadas na Faixa Serid? indicam
principalmente cadeias oce?nicas localizadas nas imedia??es de margens continentais
ativas transicionais a arco de ilha. O estudo das forma??es ferr?feras ? importante n?o
apenas por concentrarem a maior fonte econ?mica de ferro nas rochas, como tamb?m por
serem particularmente essenciais para o entendimento da evolu??o atmosf?rica e a
composi??o qu?mica dos oceanos no per?odo de sua deposi??o. / Ferriferous rocks occurrences in Rio Grande do Norte central region essentially
correspond to small deposits without apparent economic potential. In contrast, during the
last years, small deposits of this mineral has been well exploited in some areas of the
region, especially within the Rio Piranhas-Serid? Domain. The iron formations occur as
decimeters to tens of meters layers intercalated with the Jucurutu Formation gneiss, at the
base of Serid? Group metavolcano-sedimentary sequence. They may be associated, but
less frequently, with metamafic rocks hosted in the gneiss and possible orthoderived
metamorphic rocks from Caico Complex. Systematic mapping at the scales of 1:100,000
(Lajes and Currais Novos geological maps) and mineral resources survey carried out by
the Companhia de Pesquisa de Recursos Minerais (CPRM) identified a significant
amount of mineral occurrences (registered 48 occurrences of ferriferous rocks). In this
work will be performed a macroscopic and microscopic petrographic characterization of
thin sections, bipolid and polished sections of banded iron rocks, besides the discussion
of lithochemistry data by analyzing the major elements, trace signing and rare earth
elements to defining protolith and tectonic environment, where appropriated, as well as
the use of MEV-EDS to study the qualitative chemical composition of magnetites and
amphiboles. Basically, these rocks are composed of quartz, iron oxides and hydroxides,
amphibole (cummingtonite-grunerite and actinolite-tremolite series), fine- to medium
grained and granonematoblastic texture, sometimes with compositional banding. They
are formed by Fe2O3 content ranging between 46 and 59% and SiO2 content ranging
between 33 and 50%. Petrographically it were identified two distinct groups of
ferriferous rocks: granular and bandeds. They occur interlayered respectively, into the
migmatitic gneisses from Caic? Complex and in the supracrustal rocks correlated to the
Jucurutu Formation. The lithochemistry permited to conclude that groups of granular and
banded iron rocks sometimes present a similar behavior, sometimes not. The bimodal
rocks are mainly composed of Si and Fe, with small proportion of trace and rare earths
elements. The chemical parameters indicate dominantly oxidant environment and groups
of rocks, since intermediate positions to closest the oceanic hydrothermal vents. As for the nature of the formation of the iron rocks, binary graphs with the relations between Th,
Zr and Ti allowed to identify that they are chemically pure rocks, with few levels of
terrigenous. While binary diagrams of SiO2 and Al2O3 and Al-Fe-Mn ternaries indicate
that all samples of ferriferous banded and granular formations form hydrothermal
chemical-exhalation sediments. Specific diagrams for the identification of the tectonic
environments for the formation of granular iron rocks and banded iron rocks studied in
the Faixa Serid? indicate mainly oceanic chains located next to active continental
margins transitional to island arc. The iron formation study is important not only because
it is the most economical source of iron in the rocks, but is particularly essential to
understanding the atmospheric evolution and to the oceanic chemical composition in the
period of their deposition.
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufrn.br:123456789/23403 |
| Date | 10 February 2017 |
| Creators | Dantas, Alexandre Ranier |
| Contributors | 90459261487, http://lattes.cnpq.br/5356037408083015, Souza, Raquel Franco de, 24043427204, http://lattes.cnpq.br/5181465668193577, Medeiros, Vladimir Cruz de, 28943180497, http://lattes.cnpq.br/9369889068441214, Nascimento, Marcos Antonio Leite do |
| Publisher | PROGRAMA DE P?S-GRADUA??O EM GEODIN?MICA E GEOF?SICA, UFRN, Brasil |
| Source Sets | IBICT Brazilian ETDs |
| Language | Portuguese |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/masterThesis |
| Source | reponame:Repositório Institucional da UFRN, instname:Universidade Federal do Rio Grande do Norte, instacron:UFRN |
| Rights | info:eu-repo/semantics/openAccess |
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