Petrography and lithochemistry of ferriferous rocks in Rio Grande do Norte central region (Rio Piranhas – Seridó Domain, Borborema Province Northeast, Brazil)

Abstract

Ferriferous occurrences in the state of Rio Grande do Norte, Brazil, essentially correspond to small deposits without apparent economic potential, but small deposits of this mineral have been well exploited in some areas of this region. The iron formations occur as decimeters to tens of meters layers intercalated with the Jucurutu Formation gneiss, but less frequently, with metamafic rocks hosted in the gneiss from Caicó Complex. In this work, it is presented a macroscopic and microscopic petrographic characterization of banded iron rocks thin sections, besides the discussion of lithogeochemistry data and the use of MEV-EDS to study the qualitative chemical composition of magnetites and amphiboles. These rocks are formed by Fe₂O₃ content ranging between 46 and 59% and SiO₂ content ranging between 33 and 50%. Petrographically, there were identified two distinct groups: granular ferriferous rocks and banded iron rocks. The lithogeochemistry permited to conclude that groups of granular and banded iron rocks sometimes present a similar behavior, sometimes not. The 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. The tectonic environments for the formation of granular and banded iron rocks studied indicate mainly oceanic chains located next to active continental margins transitional to island arc.