Neoarchean, Rhyacian and Neoproterozoic units of the Saquinho region, eastern Rio Piranhas-Seridó domain, Borborema Province (northeastern Brazil): implications for the stratigraphic model

Abstract

Aeromagnetic, petrographic and isotopic data coupled with information obtained from a deep stratigraphic hole (440 m), shallow drillings (<100 m), and field data from the surroundings of the Saquinho iron mine in the Rio-Piranhas-Seridó domain of the Borborema Province, northeastern Brazil, allowed to identify a Neoarchean unit while provi-ding a better understanding of the stratigraphy of the Jucurutu Formation, the host of the iron mineralization, and its basement. The drill hole allowed differentiating three compartments (upper, intermediate and lower). The upper compartment consists of paragneisses/schists, marbles and banded iron formations (top to bottom, respectively) correlated to the Jucurutu Formation (Seridó Group). The marbles provided δ13C and δ18O values ranging from 2.4 to 10.4‰ and from -9.7 to -5.3‰, respectively, which are compatible with those of marine-glacial carbonates and correlated to those of the Jucurutu Formation. The intermediate compartment consists of amphibole-bearing biotite augen gneiss, varying in composition from alkali feldspar granite to syenogranite, with U-Pb zircon dating (LAM-MC-ICPMS) of 2210 ± 13 Ma, correlating it with the Rhyacian augen gneisses described in the literature. Finally, the lower compartment consists of a sequence composed mainly of microaugen gneisses, fine banded gneisses and metamafic-metaultramafic rocks (Saquinho Complex). The U-Pb zircon dating (LAM-MC-ICPMS) provided Neoarchean ages of 2512 ± 3 Ma and 2501 ± 3 Ma, respectively, for microaugen gneisses and metamafic-metaultra-mafic rocks, which were considered as crystallization ages of the protoliths. In the lower part of the drill hole, disse-minated sulfides (pyrite, chalcopyrite, and more rarely pyrrhotite) occur, especially in the metamafic-metaultramafic lithotypes, which added to the Neoarchean age of the lithotypes, may represent new economic/metallogenic targets for the Seridó region. The geological model proposed for the iron formations of the Saquinho area involves the inte-raction of fluids with the Neoarchean mafic rocks, which percolated brittle and/or ductile/brittle structures.