Influence of weathering and hydrothermal alteration on the REE and δ56Fe composition of iron formation, Cauê Formation, Iron Quadrangle, Brazil

The change in Fe isotopic composition with depth is more complex. In drill core from the OFS, negative δ56Fe values shift from −0.91‰ at depth to less negative values of −0.6‰ near the surface. The most altered samples do not lie on this trend because they are characterized by positive values of 0.30‰ and 1.33‰. The δ56Fe values from the GS increase linearly from −0.11‰ to 1.23‰ towards the surface with the most negative values −0.7‰ and −1.16‰ near the top. The most negative values from the OFS could reflect synsedimentary microbial dissimilatory Fe reduction, and the positive values from the GS may be related to partial Fe2+ oxidation between the hydrothermal Fe source and Fe-(oxyhydr)oxides. However, when interpreted together with mineralogical and REE data, these data likely record the various alteration processes that affected the Cauê Iron Formation. Such findings highlight the importance of coupling sedimentologic analysis with elemental and isotopic chemistry to understand iron formation deposition and Fe ore-forming processes.