Chemostratigraphy of the Carajas banded iron formation, Brazil: A record of Neoarchean Ocean chemistry

One of the most important occurrences of banded iron formation (BIF) worldwide is situated in the Carajas Mineral Province, southeastern Amazonian Craton. The BIFs are jaspilites and are hosted in the Neoarchean (similar to 2.74 Ga) volcano-sedimentary sequence of the Grao-Para Group. They are mostly composed of cm-thick intercalations of hematite, jasper, and chert. Their primary textures and structures are still preserved, which make them an ideal archive to evaluate the paleomarine environment. Low abundance of Al2O3 (< 1.0 wt%) and HFSE (< 1 ppm) for most BIF samples indicate an essentially detritus-free depositional environment. Overall, the rare earth elements and yttrium (REY) patterns show a weak positive lanthanum (La) anomaly, and a pronounced positive europium (Eu) anomaly (Eu/Eu-PAAS = 1.86 - 5.05), although the presence of true cerium (Ce) anomaly is not evident. Stratigraphic variations in iron isotope compositions, up to 0.80% (delta Fe-56 = +1.10 to +1.90%) over tens to hundreds of meters of stratigraphic section, point to relative changes in the iron isotope composition of Carajas seawater over periods of a few million years. The jaspilites show heterogeneous distribution of Nd isotopic signature throughout the BIF sequence, and rocks from near the basaltic flows/jaspilite contact (type-II) have negative eNd (t) values (-4.97 to -0.90). In contrast, predominantly positive epsilon Nd(t) values (-0.84 to +5.40) are common in the remaining samples (type-I). The strongly positive delta Fe-56 values indicate a low degree of partial oxidation of Fe(II), which, combined with the pronounced positive Eu anomalies and the absence of Ce anomalies, hint towards that the deposition occurred mainly on a deep-sea environment with intense hydrothermal activity under anoxic and suboxic conditions, distal to continental landmasses. Locally, considerable oxygen was probably present in the ancient ocean's water masses, which led to the precipitation of BIFs. (C) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.