Insights into the processes and controls on the absolute abundance and distribution of manganese in Precambrian iron formations

Abstract

The role of manganese as redox tracer in the context of iron formations (IF) has received renewed attention in recent years. The utility of several redox-sensitive trace metal proxies in IF (e.g., Mo) is based on the premise that simultaneously elevated manganese concentrations in the form of primary Mn(IV) oxides, would have controlled the redox behaviour and stable isotope variations of such elements during primary deposition and diagenesis. Recent research on the mineral fraction-specific geochemistry of Paleoproterozoic IF from South Africa (Griquatown and Kuruman Iron Formations) reports no preserved Mn(IV) oxides and a strong affinity of manganese (>90%) to partition into carbonate minerals as Mn(II). In this study, we exploit further this relationship by examining in detail the mineral-specific abundance and distribution of manganese across the same IF succession, with main emphasis on the Mn-enriched upper 120 m of the Griquatown IF. Dominant carbonate minerals include ankerite, siderite and occasional calcite, with MnO contents across all three minerals ranging from less than 1 to well over 10 wt%. Co-existing ankerite and siderite on the scale of individual thin sections record very similar MnO (and MgO) contents at statistically very low variance. Calcite is only observed as relic phase showing apparent textural evidence for replacement by ankerite and siderite. The calcite is Mg-rich (up to 15 wt% MgO) and contains the highest MnO contents out of the entire dataset obtained. Variability in the MnO content of calcite is also high and anti-correlates strongly with CaO content. Ankerite and siderite co-existing with calcite have similar MnO and MgO concentrations, consistent with conservative transfer during replacement reactions and only net addition of Fe(II) from pore fluids. For one of the calcite-containing samples, all three carbonate minerals were also analysed sequentially for carbon isotopes and yielded very similar δ13C values (−6.7 to −6.0‰). We interpret the above features as indicative of primary calcite being the dominant mode of manganese acquisition in the initial sediment. Diagenetic replacement reactions then homogeneously reworked Mn (along with Mg and Ca) in a largely conservative fashion into diagenetic ankerite and siderite. Our interpretation questions IF models that assume high primary Mn(IV) oxide deposition and its bacterially-mediated diagenetic reconstitution in IF during transient oxygenation events (oxygen “whiffs”); instead, it lends support to emerging ideas for primary water-column processes leading to the formation and precipitation of reduced Fe(+Mn) mineral species as precursors to IF.