As formações ferríferas da Região de Conceição do Mato Dentro - MG: posicionamento estratigráfico, evolução tectônica, geocronologia, características geoquímicas e gênese dos minérios

Abstract

A new stratigraphy based on detailed mapping, geophysical data and U/Pb SHRIMP age data of detritic zircons is proposed for the banded iron-formation-bearing sequences that outcrop at the eastern border of the southern Espinhaço Range and two metasedimentary sequences were defined: the Serra da Serpentina and the Serra de São José Groups. The Serra da Serpentina Group have maximum depositional Orosirian age (youngest detrital zircon age = 1990±16 Ma) and comprises fine clastic metasediments at the base and chemical sediments, including banded iron-formations (BIFs), on top, respectively belonging to the Meloso Formation and Serra do Sapo Formation. The group represents the sedimentary deposition into an epicontinental-epeiric slow downwarping basin with little tectonic activity. The maximum depositional Orosirian age of the Serra da Serpentina Group indicated that there is no chronostratigraphic correlation between the banded iron formations of the Serra da Serpentina Group and the Cauê Formation of Minas Supergroup, as previously proposed. The younger Serra de São José Group was deposited in a continental rift-basin tectonically active, with a north-south axis and has a complete cycle of transgressive sedimentary deposits comprising from base to top: Lapão Formation; Itapanhoacanga Formation; Jacém Formation and, on the top of the sequence, the banded ironformations of the Canjica Formation. The Serra de São José metasedimentary Group is separated from the underlying Serra da Serpentina Group by a significant erosional unconformity. The Itapanhoacanga Formation has the same maximum depositional Statherian age (i.e., 1.70 Ba) of Espinhaço Supergroup basal unit (i.e., São João da Chapada Formation) from which could be considerate facies variation. Both metasedimentary sequences were affected by an east-west compressive tectonic event that generated thrust faults and associated fault-related folds. Thrusts faults could be mapped in field works and a tectonic map were produced delimiting tectonic blocks where the pre-tectonic stratigraphy were preserved and could be detailed. During the SHRIMP U-Pb dating of detrital zircons grains, overgrowth and outgrowth of hydrothermal zircon and xenotime was identified and also dated, unveiling pulses of a hydrothermal event which occurred within a wide range of time (i.e., 466 to 628 Ma), during the Neoproterozoicearly Paleozoic Brasiliano /Pan-African orogenies. The geochemical study, including analysis of rare earth elements, indicates that the deposition of the Serra do Sapo and Canjica banded iron formations occurred in stratified basins with the oxygen-rich upper portions and that iron on both units have a hydrothermal origin. The profile of rare earth elements for both sequences is similar to other Paleoproterozoic banded iron formations profiles, containing positive anomalies of Europium and Yttrium, negative cerium anomaly and a relative concentration of heavy rare earths elements.