GENESIS OF THE VALENTINES IRON FORMATION, EASTERN URUGUAY, AND SIGNIFICANCE FOR THE REDOX CONDITIONS OF THE EARLY EARTH OCEANS

Banded iron formations (BIFs) are Archean-Proterozoic marine chemical sedimentary rocks that provide evidence for early Earth’s oceans chemistry and redox state. The Valentines Iron Formation in eastern Uruguay, presumably formed ~2.5 Ga, is currently being explored for potential mining. However, no detailed geochemical studies of the IF are available. We present a detailed petrographic, mineral chemistry, and bulk-rock major and trace element study of the Valentines IF with the ultimate goal to determine the physicochemical (fO₂, temperature, chemistry) conditions of formation, including the oxic or anoxic character of the ocean during their deposition. Core samples of the IFs and interbedded rocks from two different localities were used for the study. The IF rocks, locally referred to as Valentinesite, are dominantly banded, medium grained, and medium-grey to black with white, green, grey, and red bands ranging from 1 mm to 3 cm in thickness. Mineralogically they consist of magnetite, quartz, augite, accessory hematite and ilmenite, and trace apatite and garnet, and have experienced medium-high grade metamorphism and intense deformation by folding. Valentinesite is interbedded with felsic volcanic rocks and andesite and cross-cut by mafic dikes. Magnetite in the Valentinesite is almost pure Fe₃O₄ with very low amounts of TiO₂ and MnO (0.4 and 0.17 wt.%, respectively), and has increasing Ti contents with proximity to ilmenite. Valentinesite has very low Al/(Al+Fe+Mn) ratios (<0.1) typical of BIFs. Shale-normalized REE patterns of Valentinesite have high to low light to heavy REE (LREE/HREE) ratios, in contrast to those of the 2.5 Ga Kuruman and Brockman IFs, which have steep, low LREE/HREE patterns. Some Valentinesite samples have positive Eu anomalies and all lack distinct true Ce anomalies, similar to those of the Kuruman and Brockman IFs. A few IF samples have no or negative Eu anomalies. These geochemical indicators suggest that the IF formed from ocean water that had a high to medium hydrothermal component. The absence of Ce anomalies suggests that the ocean water was also anoxic and lacked an oxide shuttle across a redoxcline. Further mineralogical and geochemical studies, including age dating of associated rocks, will shed light on the genesis of these important IFs and the redox state of the ancient oceans.