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To evaluate the mechanics of mid-Proterozoic environmental iron transport and deposition, we coupled microscale textural and bulk rock magnetic techniques to study the similar to 1.4-Ga lower Belt group, Belt Supergroup, Montana and Idaho. We identified a pyrrhotite-siderite isograd that marks metamorphic iron-bearing mineral reactions beginning in subgreenschist facies samples. Even in the best-preserved parts of the basin, secondary overprints were common including recrystallization of iron-bearing sulfides, base metal sulfides, and nanophase pyrrhotite. Despite these overprints, a record of redox chemistry was preserved in the early diagenetic framboidal pyrite and detrital iron oxides including trace nanoscale magnetite that remained after sulfidization in anoxic and sulfidic sedimentary pore fluids. Based on these results, we interpret the Belt Basin as having oxic waters, at least in shallow-water environments, with no indication of abundant ferrous iron in the water column; this is consistent with the cooccurrence of early eukaryotic fossils within the same strata.

Plain Language Summary Observing iron chemistry and mineralogy in sedimentary rocks is one of the primary methods for understanding ancient redox environments; today, iron is poorly soluble in seawater due its high dioxygen levels, but some iron-based metrics suggest that ocean and lakes were rich in soluble iron for a billion years after the rise in atmospheric oxygen 2.3 Ga. Using a novel approach for evaluating iron environmental processes, we analyzed the iron-bearing minerals within 1.4-Ga rocks and were able to untangle which minerals were primary, containing information about the ancient environment, and which minerals were secondary, telling the story of later alteration through fluids carrying new elements into the rock or higher temperatures/pressures from burying the sediments. Primary detrital iron minerals implied that the Belt Basin waters were oxygenated, at least in shallow-water environments, contrary to prior work but consistent with the rich fossil record of eukaryotes in the same strata.