Diagenetic iron-oxyhydroxides formed in suboxic to anoxic mine-impacted lake sediments near Sudbury, Ontario

Abstract

Iron-oxyhydroxides are common in lake sediments, typically forming in oxidizing conditions near the sediment-water interface. However, in the mine-tailings-impacted sediment delta at Long Lake near Sudbury, Ontario, diagenetic Fe-oxyhydroxides are formed in suboxic-anoxic conditions. Geochemical and mineralogical techniques were applied to investigate the diagenetic transformations of tailings-derived oxidation products following burial in the sediment delta. Two sets of vertical cores were collected, one from a shallow-water location in the sediment delta near the lake shore and the other from a deeper-water location, approximately 90 m north from the mouth of the active channel. One core from each location was sectioned in the field into 0.1 m intervals using an anaerobic globe bag and porewater was extracted for aqueous geochemical analysis. The second core from each location was dried anaerobically and sub-sampled for solid-phase geochemical and mineralogical analyses. The aqueous concentrations of As, Fe, and SO4 are elevated near the sediment-water interface at both locations but at the shallow location they decline to the lowest observed concentrations at depth in the natural sediment. The decline in their concentrations may be enhanced by bacterial SO4 reduction and precipitation of Fe- and As-sulfides which were observed in mineralogical analysis. The mine-impacted sediment consists of numerous fragments of tailings-derived ferric-arsenate-cemented crusts. Compared to the cemented crusts that occur in the tailings, these fragments exhibit partial-dissolution features and SEM-EDS analyses demonstrate that they have relatively low As/Fe ratios, suggesting the occurrence of selective As(V) dissolution during diagenesis. Diagenetic Fe-oxyhydroxides occur throughout the mine-impacted sediment, including zones that contain diagenetic Fe- and As-sulfides, suggesting they may have formed in suboxic-anoxic conditions. They occur proximal to, but separate from, the fragments of tailings-derived crusts and display diverse morphologies, including what may be a previously unreported spherulitic form. Individual Fe-oxyhydroxides range in size from nm to μm but they mostly occur in delicate clusters suggesting they are autochthonous. Compared to the cemented crusts from the tailings, they contain very small amounts of As. Analysis of one spherule indicates it contains lepidocrocite microcrystals with amorphous SiO2.nH2O in the interstitial spaces. The microcrystals contain a mixture of Fe(III) and Fe(II), with Fe(III)/ΣFe = approximately 0.8. We interpret them to have formed by burst nucleation from Fe(III)-supersaturated solution, with subsequent aggregation of crystallites to form spherules.