A sequência portadora de formações ferríferas de Guanhães, Minas Gerais, Brasil

• Ano de defesa: 2016
• Autor(a) principal: Vitor Rodrigues Barrote
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; UFMG
• Palavras-chave em Português: Análise estrutural; Geoquímica; Formação ferrífera bandada; Guanhães; Grupo Guanhães; Geocronologia; Granitogênese brasiliana; Geoquímica Guanhães (MG); Tempo geologico; Minérios de ferro Guanhães (MG)
• Link de acesso: http://hdl.handle.net/1843/IGCC-AB6PFH

The BIF-bearing supracrustal sequence of Guanhães is a Rhyacian metasedimentary sequence that occurs as thin tectonic slices superposed to Archean TTG granite-gneissic basement rocks developed probably at the Proterozoic-Paleozoic boundary during the Brasiliano event of the Araçuaí Orogeny. The sequence is comprised of a layer of BIF bordered by two quartzite-rich units, correlatable to the Serra da Serpentina Group, metamorphosed at a high amphibolite facies. The supracrustal sequence was subjected to a two-phase deformational event (D1 and D2) responsible for the tectonic emplacement of the metasedimentary slices over the TTG by a NW-SE thrust system with an oblique component and that generated isoclinal to tight F1 folds, with an S1 axial plane foliation, and tight to open F2 folds, respectively, creating a fold interference pattern. Measurements of the anisotropy of magnetic susceptibility (ASM) of selected sites show dominance of paramagnetic minerals, as expected for BIFs. Both magnetite and hematite display oblate susceptibility ellipsoids, and the magnetic foliation imitates the BIFs banding. Mesoproterozoic mafic dikes and Cambrian Brasiliano-related granites intrude the sequence. The granites are connected to the large amount of hydrothermal fluids indicated by several veins and Cambrian alteration haloes in zircon grains. The analyses of PAAS-normalized REY (Rare Earth Elements + Yttrium) values for the Guanhães BIF, shows HREE enrichment, consistent positive Eu anomaly and virtually no Ce anomaly. Pr/Yb(SN) vs. Y/Ho and correlation between lithophile elements is evidence for detrital contamination on part of the samples. Positive correlation between Ce/Ce*(SN) vs. Y/Ho indicates fractionation of REY such as La, Pr or Sm. The non-contaminated samples display REY distribution similar to BIF from other Precambrian BIFs, particularly BIFs from the Morro-Escuro Sequence and Serro Group, without true Ce-anomaly and Y/Ho close to seawater values (45). These geochemical data indicate that the BIFs were deposited on a platform environment distant of the high T hydrothermal fluids source. Several samples exhibit positive Ce-anomaly linked to HREE enrichment and higher Eu-anomaly that are related to an igneous hydrothermal alteration from younger Granitic intrusions Two samples display LREE enrichment due to supergene.