Mineralogy, Geochemistry, and Genesis of the Valentines Iron Formation, Nico Pérez Terrane, Eastern Uruguay, and Significance for the Redox Conditions of Early Earth's Oceans

Abstract

The Valentines Iron Formation of eastern Uruguay is one of three well-defined rock units that comprise the Valentines Formation, and is commonly interlayered with felsic and intermediate granulites. The iron formations contain abundant magnetite, pyroxene, and quartz, accessory hematite, ilmenite, and feldspar, trace apatite, and rare garnet. Two varieties of iron formation, well banded and less banded, are found in the Las Palmas and Valentines study areas. The former consists of alternating, 1 mm-3 cm thick bands of magnetite, pyroxene, and quartz, while the latter shows less well-defined banding. The iron formations and the associated rocks are highly folded and have undergone granulite facies metamorphism. The iron formations have relatively simple chemical compositions. The well banded variety and less banded variety are chemically similar and mostly composed of Fe (78-12 wt. %) and Si (61-14 wt. %), and minor Ca (12- < 1 wt. %) although, the less banded variety has slightly higher aluminum content. On a molecular Al-Fe-Mn triangular diagram the banded samples from the Valentines Iron Formation plot as having higher than 90% molecular Fe and fall in the hydrothermal field, as well as in the field that includes East Pacific Rise (EPR) sediments, while the less banded samples from the Valentines Iron Formation plot having higher than 60 % molecular Fe and fall in the hydrothermal field. On a plot of Fe/Ti vs. Al/Al+Fe+Mn, both varieties of Valentines Iron Formation plot as having high hydrothermal components. The less banded variety has higher Al/Al+Fe+Mn values suggesting small detrital input. The Valentines Iron Formation plots similarly to well-studied iron formations of various ages and geographic locations. The chemical compositions of the Valentines Iron Formation suggest that the original sediments were mostly Fe oxides and silica formed from seawater influenced by hydrothermal fluids. Trace element concentrations are typically lower in the strongly banded variety than in the less banded variety. Both varieties have relatively high Ba, Rb and Sr contents (up to 1887, 339 and 140 ppm, respectively) compared to other trace elements contents. Patterns of rare earth element abundances normalized to Post Archean Australian Shale indicate a marine origin for the sediments of the Valentines Iron Formation. The patterns display positive La, Eu, and Y anomalies and generally lack Ce anomalies, reflecting precipitation of the protolith from a mixture of seawater and high-temperature (T) source hydrothermal fluids. Samples with the lowest detrital contents lack or display small negative Ce anomalies, high Y/Ho ratios (24-58), and a wide range of (Pr/Yb)[subscript]SN ratios similar to those of late Archean iron formations. These characteristics suggest that the protoliths of the Valentines Iron Formation were deposited in suboxic to anoxic waters that were influenced by high-T hydrothermal fluids. Zircons from felsic granulites interlayered with the Valentines Iron Formation were dated by U-Pb via LA-ICP-MS. Two preliminary age dates of 2039 ± 4 Ma and 2011 ± 10 Ma were obtained from 85 point analyses. Samples show similar ages and lacked distinct variation among rims and cores of individual zircon crystals. These age dates coincide with a regional metamorphic event determined in previous studies. Therefore, the age of deposition of the Valentines Iron Formation is expected to be at least 2.0 Ga, and is likely late Archean to Mesoproterozoic.