Minério de ferro de alto teor no setor norte do domínio intracontinental do Orógeno Araçuaí, Bahia, Brasil: caracterização geológica e modelo evolutivo.

Abstract

The Igaporã-Licínio de Almeida Metavolcano-sedimentary Sequence, located in the northern region of the intracontinental sector of the Araçuaí Orogen, stood out for the occurrence of hematitites with a content greater than 66% by weight in Fe. The general objective of this paper is to understand the processes responsible for the hypogenic enrichment in iron content in the itabirites of this Sequence at the Pedra de Ferro Mine (MPF). Structural, petrographic, microstructural and chemical characteristics of the iron-rich domains and their surroundings were determined, the deformational structures were defined, with interpretation of the paragenetic succession of hydrothermal alteration and the control of mineralization, in addition to a proposal for a metallogenetic evolution model. The microstructural study allowed the identification of four types of hematite, which were associated with the four identified deformation phases (Dn-1, Dn, Dn+1 e Dn+1), three of which were compressive, related to the inversion of the Paramirim Aulacogen, and one was distensive. Whole-rock geochemical studies on itabirites suggest that the protolith of these rocks were deposited in a restricted marine basin, with detrital contribution, in a suboxic environment. The geochemical signature for REE is similar to rocks with depositional age < 1.9 Ga. The distribution diagrams of rare earth elements and trace elements in whole rock were compared with data from poorly hydrothermalized quartz-itabirite from the Espírito Santo target, located north of the MPF, with the results obtained from itabirite from the Horto-Baratinha deposit (Grupo Serpentina; Espinhaço Supergroup - embedded as a tectonic slice in the Guanhães Block east of Serra do Espinhaço), and with data on hematite domains from hydrothermal alteration zones in sandstones from Eastern Chapada Diamantina, as well as from other regions of Brazil and the world. The similarity between the ETR data of the itabirite from the Espírito Santo target and the less hydrothermalized Serpentina Group with the itabirites from the study area suggests similar environmental conditions of deposition for these rocks. Chemical analyzes by LA-ICP-MS were carried out on hematites crystals of two types (oriented and cataclastic) and on magnetite. Two distinct ETR chemical signatures are noted for MPF oriented hematite: (i) predominance of ETRP in relation to ETRL, negative Y and positive Eu anomalies; lower ETRL contents in relation to the whole rock and are quite similar to the patterns observed in hematite from quartz itabirites at the Espírito Santo target; and (ii) higher ETRL and ETRP values when compared to whole rock values. Multi-element diagrams were created that demonstrate the subtle enrichment in ETR in the cataclastic generation hematite, in addition to high amounts of ETR in magnetite. Finally, it is interpreted that inter and intrastratal shear zones related to the inversion of the Paramirim Aulacogen promoted the formation of the S0//Sn-1//Sn transposition foliation, which is the control in the formation of high-concentration iron domains. The data suggest that iron enrichment occurred before the development of the folds that control the deposit's morphology. The cataclasis contributed to the formation of fragments more susceptible to weathering agents, but in ore already enriched previously. The high-grade ore forming process is hypogenic, with the participation of oxidizing fluids that led to the crystallization of hematite from magnetite and dissolution of quartz.