EVALUATING MECHANISMS FOR MANGANESE ENRICHMENTS IN THE PROTEROZOIC ANIMIKIE BASIN

The mechanisms responsible for the development of manganese (Mn) enrichments in the iron formations of the Proterozoic Animikie basin of Minnesota are not well understood. The high grade (up to 30% MnO) Mn-deposits in the Emily district have been suggested to represent secondary enrichment from Mn-bearing fluids. Lower grade (4-8% MnO) enrichments are also known from the Cuyuna, Emily, and southern Mesabi districts. A recent study suggested that low grade, carbonate-hosted Mn enrichments in correlative rocks in the Crystal Falls district of Michigan represent a basin-scale deep water oxygenation event. In each case, Mn mineralization is presumed to indicate strongly oxidizing conditions wherein dissolved Mn²⁺ oxidizes to insoluble MnO₂. Carbonate-hosted Mn deposits are inferred to develop diagenetically when MnO₂ reduction is coupled to organic carbon mineralization to form manganese carbonate (MnCO₃). However, recent work in modern environments suggests that MnCO₃ may also originate as a primary precipitate from anoxic waters.

To evaluate this hypothesis, we sampled cores from distal facies of the Biwabik formation, in addition to sections from the Cuyuna and Emily districts that were known to contain laminated facies with low grade (4-8%) Mn enrichments. Samples from each district contain a similar thinly bedded chert-carbonate facies consistent with deposition below wave base. Our initial bulk X-ray diffraction (XRD) and fluorescence (XRF) analysis of samples from these cores confirms that this facies hosts low-grade Mn enrichments associated with increased siderite abundance. However, sample Mn content appears to be more closely associated with Ca abundance than Fe abundance. The zone of Mn-enrichment occurring in the upper Biwabik formation may be consistent with a shift towards more oxidizing conditions in the basin, changes in carbonate saturation, or proximity to a local hydrothermal source. These scenarios may be consistent with the fine carbonate textures observed in our samples, but changes in redox conditions may also be required to enhance the potential for direct capture of Mn by carbonate phases. Additional work is ongoing to develop criteria for distinguishing between primary and diagenetic Mn carbonates, and assess the relationships between Mn enrichments and basin stratigraphy.