Trace elements, C–O isotopes and U–Pb geochronology of the Minas Supergroup in the Segredo deposit, Quadrilátero Ferrífero, Brazil

Abstract

The metasedimentary succession of the Minas Supergroup in the Quadrilátero Ferrífero (QFe) region, eastern Brazil, contains important, up to 400 m thick Lake Superior-type Banded Iron Formation (BIF) of the Cauê Formation. These are 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 Segredo BIF deposit is located in the SW part of the QFe. The structural complexity and the discontinuity of layers challenge local studies on the sedimentary evolution of the Minas Basin. In the Segredo deposit, the stratigraphic sequence of the Minas Supergroup is exposed in the overturned limb of Fábrica Synform, that refold the Dom Bosco Synform.Based on the geological mapping and the study of a 480-m deep drill core, a lithostratigraphic review of the area is proposed, with stacking of the Itabira, Piracicaba, Sabará and Itacolomi groups, from the bottom up.The Itabira Group comprises BIF, intraformational metaconglomerate and dolostone with lateral and vertical interlayering. The chemical and isotopic data from the Cauê and Gandarela formations are defined by variations of the trace element and C-isotopic signatures. The intraformational metaconglomerate is interpreted as gravity flow products at the edges of the carbonate platform.Dolomitic BIFs show positive Eu anomalies and low terrigenous content, thus confirming the contribution of seawater hydrothermal fluids during the precipitation of BIF and carbonates. Negative Ce anomalies reflect a locally oxygenated ocean during precipitation of the Gandarela Formation carbonates.The carbon isotope stratigraphy is marked by δ13C values from around −1.5‰ in the Cauê BIF and around 0‰ in the Gandarela dolomites, indicating normal seawater conditions and deposition probably before the global Lomagundi event. The negatively fractionated values obtained for the Cauê BIF are consistent with previous studies suggesting the operation of microbial Dissimilatory Iron Reduction as an important process in BIF formation.Above the Gandarela carbonates, the occurrence of the Sabará Group through an angular unconformity is for the first time confirmed in the region by the occurrence of detrital sedimentary facies and by U–Pb geochronological data (younger detrital zircon at ca. 2.1 Ga).