Trace metal retention through the schwertmannite to goethite transformation as observed in a field setting, Alta Mine, MT

Schwertmannite and goethite were both sampled in the waste-rock pile where a distinct field relation was observed between the two minerals. Schwertmannite was always in contact with actively flowing ASW, while goethite was never in direct contact with ASW and was generally above the waste-rock water table. Goethite is hypothesized to be re-dissolved/re-precipitated schwertmannite that was deposited under higher flow conditions and subsequently transformed to goethite through exposure to wet/dry cycling associated with seasonal fluctuations in the amount of water moving through the hydrogeologic system. Trace metal concentrations in ammonium oxalate extracts of these minerals provides the first published data on the behavior of multiple trace metals through this phase transformation, which has important ramifications for considering schwertmannite as a long term metal sink due to its known metastability with respect to goethite. A relative retention scale through this phase transformation of Pb > Zn, Mn > As, Al, Cu is potentially applicable to other ASW systems.