Paleobasinal to band-scale REE + Y distribution in iron formations from Carajás, Amazon Craton, Brazil

Abstract

Banded Iron Formations (BIFs) from the Carajás Mineral Province (CMP), southeastern Amazonian Craton, are the protolith of the world-class iron deposits of the CMP. Besides being one of the largest iron ore deposits in the world, they recorded important paleoenvironmental and regional geodynamic markers across the Neoarchean-Paleoproterozoic transition. We investigate the secular variation of geochemical signature, from several deposit of Carajás Province. The low contents of Al2O3 and of 19 other immobile trace elements (e.g., Zr, Th, Hf, Ti) in 26 high-resolution detritus-free BIF samples indicate their detritus- free nature. Their positive shale-normalized (subscript SN) La and Gd anomalies are similar to those of modern seawater; whereas their Y/Ho ratios are higher. These BIF samples also display large positive chondrite-normalized (subscript CN) Eu anomalies, reflecting the influence of high-temperature hydrothermal vents and bottom solutions. In contrast to modern seawater and to most other Neoarchean/Paleoproterozoic detritus-free BIFs, the Carajás BIFs do not show the typical heavy rare earth element (REE) enrichment relative to light REE; instead they do show variable light to heavy REE fractionation trends (YbSN/PrSN ratios: 0.05–4.91). The Th/U ratios of both bulk BIFs and mesobands are generally lower than the upper-crustal ratio. Although the bulk BIFs show mostly chondritic values, the individual Fe and Si bands exhibit distinctly higher ratios. The general absence of negative CeSN anomalies attests for rather reducing paleoenvironmental conditions. Unlike the great majority of the data which lack negative anomalies of Ce, a few samples do display such anomalies, which may be compatible to the oxidizing conditions shown by the low Th/U ratios of most the Carajás BIFs. Such low Th/U ratios may register terrestrial weathering conditions, which cause preferential U input into the oceans in response to the oxidation of immobile U4+ to more mobile U6+.