DEPOSITION OF THE CARBONIFEROUS AMSDEN FORMATION IRONSTONE HAS BEEN LINKED TO UPWELLING OF DEEP FERRUGINOUS HYDROTHERMALLY INFLUENCED SEAWATER

Paleoproterozoic Granular Iron Formations (GIFs) and Phanerozoic ironstones (ISs) have been episodically deposited since the Great Oxidation Episode (ca. 2.4-2.1 Ga). Although each share similar mineralogy, sedimentary textures, and depositional environments, different iron sources have been inferred for GIFs and ISs. It is generally recognized that Paleoproterozoic GIF deposition was episodic, synchronous with Volcanogenic Massive Sulfide deposition and submarine Large Igneous Provinces emplacements that caused short-term perturbations to the redox state of the oceans. Therefore, Fe²⁺ for GIF deposition is thought to be sourced from the deep oceans, where hydrothermal fluids leached Fe and Mn from volcanics. In contrast, Fe²⁺ for ISs is assumed to be sourced from continents during periods of intense chemical weathering. Although the Proterozoic and Paleozoic deep oceans were both anoxic and ferruginous at least episodically, smaller reducing submarine hydrothermal fluxes and higher seawater sulfate content in the Paleozoic may have supported a stronger seawater oxidant buffer preventing long-distance Fe transport. Continental weathering has therefore been considered a viable process to supply Fe²⁺ to the oceans for IS deposition. In this study, oolitic ISs from the Carboniferous Amsden Formation, a transgressive sequence deposited in Wyoming, were analyzed for trace and rare earth elements (REEs) to resolve their Fe²⁺ source. Hematite cortices, likely reflecting precipitation from seawater or shallow porewaters were targeted using LA-ICP-MS. Negative Ce-anomaly and LREE depletion support Fe-oxyhydroxide precipitation at or above the redoxcline. Low concentrations of Al, Ti, Nd, and Hf indicate that detrital contribution that could have diluted the seawater signature of precipitates was minimal. Importantly, strongly positive Eu-anomaly (up to 2.72) suggests an Fe-source from submarine hydrothermal activity rather than from continental weathering. These data indicate that the Amsden Formation IS was deposited in a similar fashion to Paleoproterozoic GIFs via long-distance transport and upwelling of deep-ocean ferruginous waters in association with intense submarine hydrothermal activity that led to short-term deep ocean deoxygenation and low seawater sulfate content.