Geochemistry of BIF in the Quadrilátero ferrífero, Brazil, as a proxy to neoarchean paleoenvironmental and depositional conditions

• Published in: Journal of South American earth sciences Vol. 143; p. 105009
• Main Authors: Mozart, Mariana Sathler, Vasconcelos Corrêa Neto, Atlas, Brando Soares, Mariana, Pereira Silva-Alves, Gabriela
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-09-2024
• Subjects: Great oxygenation event; Paleo-redox conditions; Rio das velhas greenstone belt; Trace metals; True negative Ce anomalies; True negative Ce anomalies; Paleo-redox conditions; Rio das velhas greenstone belt; Trace metals; Great oxygenation event
• ISSN: 0895-9811; 1873-0647
• DOI: 10.1016/j.jsames.2024.105009

Precambrian Algoma-type banded iron formations (BIFs) are biochemical metasedimentary rocks formed in the deep water environment relatively close to centers of volcanic expansion and can be characterized on the basis of their paleodepositional environment and the associated rock types. These rocks may contain geochemical signatures indicative of changes in the redox conditions of the Archean ocean. Such variations in the Meso- and Neoarchean are crucial in understanding the processes that led to the gradual increase of oxygen in the early atmosphere, culminating in the Great Oxidation Event (2.4–2.3 Ga). Oxide- and carbonate-facies BIFs were intercepted by drill cores in the Pilar gold deposit in the NE sector of the Quadrilátero Ferrífero (QF), Brazil, located in the Neoarchean Rio das Velhas greenstone belt. These rocks are interbedded with mafic and ultramafic metavolcanic rocks and carbonaceous phyllites. In light of the potential to reconstruct paleodepositional evidence indicative of the Earth's atmospheric redox history based on this stratigraphy, we have considered specific proxies from geological and geochemical observations. Pilar BIFs geochemistry exhibits positive anomalies of La, Eu, and Y, indicating characteristics inherited from seawater and high-temperature hydrothermal fluids. In addition, a superchondritic Y/Ho ratio further supports the influence of seawater. The signatures of the major elements Al2O3 and other immobile elements typical of detrital sediments corroborate the interpretation that these rocks were deposited in a deep marine environment with low clastic influence. Furthermore, negative Ce/Ce*SN anomalies in some sections of the Pilar BIF indicate limited oxygen availability which, together with Mo, U, V, and Zn trace-element enrichment factors, indicate deposition under suboxic/euxinic conditions. This research, together with other similar BIF data from the NW QF provide new insights into the paleoenvironmental conditions prevailing during the formation of the Archean volcano sedimentary basins that compose the southern São Francisco craton (SFC).
•PAAS normalized REE + Y from BIFs of the Pilar gold deposit reflect Fe chemical sedimentation under the carbonate stability field with minor detrital contribution.•Evidence of negative (Ce/Ce*)SN anomalies supports oxygen whiffs in the Neoarchean preceding the Great Oxygenation Event.•Trace metals (Mo, U, V and Zn) indicate deposition under suboxic/euxinic conditions.•Pilar BIFs suggest that the sedimentary registry of the QF result from highly evolved crust and basins with higher O2 levels if compared to other neoarchean records worldwide.