Cambrian banded iron formation set up evidence for microbially recycled iron in the Bambuí Group, East Brazil

Early Paleozoic banded iron formations (BIFs) have been described in recent years, challenging the hypothesis of rapid and widespread ocean oxidation at the end of the Ediacaran and early Cambrian. The Phanerozoic ocean is known to have biogeochemical characteristics very different from those of the Precambrian or Archean. Banded iron formations are considered bioelemental sediments and they can be intimately involved in the evolution of life. Models for the genesis of iron formations require a source of ferrous iron (Fe²⁺), an oxidative mechanism, and a sedimentary process suitable for the accumulation and preservation of iron-rich chemical sediments. In addition, diagenetic and metamorphic modifications of banded iron formations involve many mineral reactions and new textures that can obscure primary data. The occurrence of banded iron formations in the Lagoa Formosa Formation, upper Bambuí Group, is an important geological archive because it is one of the youngest banded iron formations on Earth (ca. 520 Ma). Also, unlike other Paleozoic records, the Lagoa Formosa BIFs are of pure sedimentary association (i.e., Lake Superior-type), making their Cambrian ferrous iron source, oxidation mechanisms, and post-depositional processes of great scientific importance. The BIFs are lenticular (up to 5 m high) and interbedded with siltstone, shale, and some fine sandstone of the Lagoa Formosa Formation, upper Bambuí Group. The rocks are are slightly deformed but without signs of metamorphism. They occur as centimeter-scale beds, composed of gray iron-rich (hematite) and red silica-rich (microcrystalline chert) intervals. Chert layers are red in color, globular in texture, and have clusters of nanometric hematite inclusions. Iron-rich layers show micrometric lamellar hematite embedded in a chert matrix. There are rare layers with some magnetite crystals. Petrography suggests the following paragenetic sequence: (1) deposition of siliceous and ferruginous mud-like sediment; (2) compaction and iron migration; (3) transformation of ferrihydrite to nanometric hematite inclusions in chert layers and to micrometric bladed hematite and some magnetite in iron-rich mesobands. Rare earth elements data acquired by LA-ICP-MS from each layer show that the chert bands have distinct positive Eu anomalies (2.2 ± 0.2), while the iron-rich layers are all enriched in middle rare earth elements (MREE: Sm, Eu, Gd). Both are LREE depleted, have positive Ce anomalies (1.5 ± 0.2) and low to moderate Y/Ho ratios (32 ± 6). Isotopic data from bulk samples are εNd_{(520 Ma)} = -2.2 ± 0.5, and δ⁵⁶Fe = -0.19 ± 0.02. Detrital contamination is low and the purest samples suggest a complex pathway of iron deposition and reworking. Petrographic evidence suggests that the globular chert with nanometric hematite inclusions is the most primary feature. This early iron phase was remobilized during shallow diagenesis and recrystallized as coarser hematite in iron-rich mesobands. Dissimilatory iron reduction (DIR) in the seafloor may be responsible for microbial reductive dissolution of early ferrihydrite and release of REE and especially MREE into pore waters. The released ferrous iron was almost completely oxidized back to ferric iron (Fe³⁺) and precipitated as micrometric hematite and some magnetite in mesobands, probably during lowering of the redoxcline or migration of ferrous fluids to more oxidizing zones. In summary, positive Ce anomalies indicate active Ce cycle and oxygenated surface water. The slightly negative values of δ⁵⁶Fe suggest efficient iron redox reactions, and the combination of negative εNd_(t), positive Eu anomalies in chert layers, and MREE enrichments in iron-rich layers support fluvial and hydrothermal input of iron that undergoes bacterial recycling in pore waters. This work is supported by Universidade Federal de Minas Gerais and its dean of research (PRPq), CNPq (310438/2022-5) and Fapemig (APQ-02240-21).