Characterization of an REE-enriched black substance in fractured bedrock in the Ytterby Mine

Abstract: A black substance seeping from fractured bedrock was observed in tunnels leading to the mainshaft of the Ytterby mine on Resarö, Sweden. These are dry tunnels at shallow depth, +5 mabove sea level and 29 m below ground surface, resulting from the reconstruction of the mineinto a fuel deposit for the Swedish Armed Forces during the Cold War era. To keep the tunnelsdry, the groundwater level is forced below its natural level which has resulted in oxidizingconditions in a previously anoxic environment. Thus, the deposition of this substance occurs ina dark and moist environment which has been exposed to changing redox conditions.Geochemical analysis and scanning electron microscopy (SEM) analyses show that this is a MnandCa-bearing substance highly enriched in rare earth elements (REE) with concentrationsbeing one to two orders of magnitude higher than the surrounding rocks. A minor phase thatincludes fluorine is also present. The organic content is low. Based on X-ray diffraction patterns,the mineral assemblage is suggested to primarily consist of poorly crystalline birnessite,vernadite and pyrolusite. The high calcium concentration of these manganese oxides-hydroxidesimplies a terrestrial origin. If they were marine then they would be enriched in magnesium.Scanning electron microscopy revealed three different manganese microstructures in the driedmatter: dendritic or shrub-like, microspherolitic/botryoidal and wad-like spheres frequentlycovered by filaments of varying thickness. The observed internal lamination of one of thesemanganese oxides implies an iterative change in production.Despite the well documented mineralogy of the Ytterby pegmatite, there are no manganeseminerals reported from the area, but there are a number of minerals in which manganese likelyconstitutes a minor component. Previous results show that the REE occurrences in Ytterby arefound in the quarry pegmatite and that they are highly localized within it. It is thereforesuggested that manganese colloids, suspended in the local groundwater, work as metal traps andlikely contribute to the mobility of the REEs. The black substance acts thus as a sink for thesemetals in the Ytterby mine area. The marine influence on the investigated substance is visiblein the δ13C signature, the carbon to nitrogen ratio and to a certain extent in the identified lipids.Even though the organic carbon content is low, the influence of microorganisms in theaccumulation of manganese oxides appears to be important. Lipid biomarkers provide evidenceof bacterial presence and also suggest that that this presence is caused by in situ production oflight independent eubacteria. An electron paramagnetic resonance (EPR) analysis was done inan attempt to distinguish between abiotically and biotically precipitated manganese. Resultsimply a two, or multiple, component substance where at least one part has a biogenic signature.