CROSSING THE REDOXCLINE: AN INVESTIGATION INTO GEOCHEMICAL CHANGES THAT DIRECTLY LINK TO TEXTURAL TRANSITIONS IN THE ~1.9 GA BIWABIK IRON FORMATION, MN

Iron-rich sedimentary deposits record Earth’s surface oxygenation and the major and trace element response to this fundamental, directional shift. To inform the evolving Paleoproterozoic redoxcline structure, we analyzed samples spanning the complete stratigraphy of the ~1.9 Ga sub-greenschist facies Biwabik iron formation (IF) preserved in drill core LWD-99-1 (239.3 m) for paired textural and mineralogical data (53 samples), and whole-rock major and trace element geochemistry (42 samples). The IF divides into lower and upper sections, defined by texture as either “cherty” - due to the prevalence of silica-cemented sand-sized grains termed ‘granules’, or “slaty” - where such grains are less common. There are four recognized units - the Lower Cherty (LC), Lower Slaty (LS), Upper Cherty (UC), and Upper Slaty (US). Throughout the LC, chert and minnesotaite granules are most common, often cemented by minnesotaite and/or mega-quartz. Upper LC mineralogy changes to greenalite granules interbedded with talc, stilpnomelane, minnesotaite, quartz, and siderite. The Intermediate Slate defines the boundary between the LC and LS, afterwards shifting to thin (~10 cm or less) silica-cemented granular horizons interbedded with mud-sized material of stilpnomelane, minnesotaite, and chamosite composition. A horizon of oncolites and ooids marks the boundary between the LS and the UC. Texturally, the UC consists of poorly sorted, mega-quartz cemented granular units, which contain abundant ooids and large > 1 cm scale fragments, indicating potential re-deposition. Reaction rims around fragments often contain reduced phases (siderite, magnetite) relative to ooid material (dominantly hematite). The US contains more abundant carbonate phases, and transitions into a limestone near the top. Of interest geochemically, 2 LC samples and 1 from the LS display negative Cerium (-Ce) anomalies, while 1 LS sample and 5 from the UC display positive Cerium (+Ce) anomalies. Previous work suggests true -Ce values reflect formation in oxic waters, while +Ce values indicate deposition in anoxic to sub-oxic waters. Applied to the Biwabik IF, +Ce values in the UC would represent the transport and subsequent re-deposition of material below the chemocline, which matches textural observations.