Estratigrafia e geoquímica do Supergrupo Minas na Mina de Segredo, Quadrilátero Ferrífero, Brasil

Abstract

The metasedimentary package of the Minas Supergroup in the Quadrilátero Ferrífero (QFe) region, eastern Brazil, bears important Lake Superior-type iron formations up to 400m thick in the Cauê Formation, superposed by dolomitic carbonates of the Gandarela Formation, deposited ca. 2.4 Ga ago and thus probably registering the Great Oxygenation Event (GOE) of the Paleoproterozoic. The deposit of Segredo is in the SW portion of the QFe. The structural complexity and the discontinuity of the layers are challenges to local studies on the sedimentary evolution of the Minas Basin. In this deposit, the inverted stratigraphic sequence of Minas Supergroup is arranged in the overturned limb of Fábrica Synform. Based on the geological mapping and the study of a deep drill hole core (480 m) performed at Segredo deposits, a lithostratigraphic review is proposed. The lithostratigraphic succession occurs through the stacking of the Itabira, Piracicaba, Sabará and Itacolomi groups, from the bottom up. The basal units of the Minas Supergroup (Grupo Caraça) are not exposed. The units of the Itabira Group comprise banded iron formation (BIF), diamictite and dolostone that have lateral and vertical interdigitation. The chemical and isotopic data from the Cauê and Gandarela formations are defined by variations of the chemical (REE) and C-isotopic signatures. Based on the defined lithostratigraphic configuration, an evolution of depositional tracts of 2nd order by transgressive and highstand system tracts is proposed. The diamictites are interpreted as gravity-driven flow products at the edges of the carbonate platform. Chemically, dolomitic BIFs show positive Eu anomalies and low terrigenous content, thus confirming the contribution of hydrothermal fluids during the precipitation of BIF and carbonates. Negative Ce anomalies reflect a locally oxygenated ocean during the carbonates precipitation (Gandarela Formation). In addition, the occurrence of the Sabará Group is defined in the region by the sedimentary facies and by U-Pb geochronological data (younger detrital zircon at ca. 2.1 Ga). The Sabará Group overlay directly the Itabira Group through an angular disconformity.