Structural and hydrothermal evolution of the shear-zone-related iron-oxide enrichment in metavolcano-sedimentary rocks of the intracontinental araçuaí orogen: The case of the espírito santo iron deposit

Abstract

The Rio São João metavolcano-sedimentary sequence is located in the eastern border of the northern Espinhaço fold thrust belt, in the intracontinental Araçuaí-West Congo Orogen. Mafic schist, quartz-, amphibole- and carbonate itabirites, as well as a carbonate–silicate rock, occur in the study area. These contain mineral assemblages compatible with the amphibolite-facies metamorphic conditions. Magnetite- and hematite-rich itabirites form a series of iron deposits in the region near Igaporã-Caetité, Bahia state, Brazil. The Espírito Santo iron deposit has compact hematite layers truncating quartz itabirite with magnetite varying from 10 to 55 vol% of the rock volume; siliciclastic and mafic rocks also host iron-rich domains, variably enriched in magnetite and hematite. The deposit is hosted in compressional duplexes with structural top towards the SW. These structures have an Sn-1//Sn foliation represented by compositional banding and parallel schistosity that is observed at all scales. Other common features are S/C/C, boudins, pinch-and-swell structures, as well as sheath, syn-tectonic folds, and a mineral stretching lineation (Lxn). Hydrothermal alteration was contemporaneous to the development of duplex and the Sn-1//Sn foliation. A distal alteration zone is dominated by a potassic (muscovite and-or biotite) ± chlorite assemblage and the intermediate zone by carbonate development. The proximal alteration zone constitutes the iron oxide (magnetite and hematite) domains; sulfide minerals as well as quartz veins (silicification) in association with other phases also formed. Hydrothermal magnetite is associated with C’ structures, which grows either including silicates and skeletal carbonates, or forming corrosion borders in these minerals. Lamellar hematite forms polycrystalline aggregates, characterizing the S/C structure in the Sn-1//Sn foliation, as well as the axial planar foliation (Sn) in intrafolial folds and the mineral stretching line Lxn. Data obtained by the LA-ICPMS technique show that, in general, in quartz-rich and amphibole itabirites hydrothermal magnetite is richer in REE than an early-stage magnetite. Hydrothermal magnetite REE pattern is close to that of the country rock of the mineralization. The formation of hematite- and magnetite-rich domains is related to the percolation of hydrothermal fluids that leached all components of the original silicates, quartz and the carbonates of all host rocks, and precipitated new hydrothermal phases taking advantage of the Ediacaran shear structures.