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

Abstract

Solutions draining the Alta Mine, Jefferson County, MT, were contaminated by acid sulfate waters (ASW) generated from anthropogenic exposure of meteoric waters to sulfidic underground mine workings and a waste-rock pile. In 1999, a remediation effort was initiated in an attempt to improve the quality of water draining the site through removal of the waste-rock pile with which these solutions come in contact. ASW were sampled in the mineshaft prior to entering the waste-rock pile and upon discharge from the waste-rock pile aquifer near the pile toe. ASW composition changed as solutions flowed through the waste-rock pile due to sulfide and silicate weathering and schwertmannite precipitation. 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.