Neutral mine drainage water-quality impacts from a former taconite mine

摘要

Surface waters at the site of a former Minnesota taconite mine were reported to have solute concentrations elevated with respect to water-quality standards. Waste rock and ore generated from past mining were primarily from open pit mining of the Biwabik Iron Formation (BIF). The BIF is a variably bedded iron formation composed of inter-bedded cherty and slaty iron silicate and iron carbonate rich beds. A geochemical characterization was conducted to identify potential constituents of interest (COI), facilitate understanding of mechanisms controlling their environmental behavior at the site, and guide future site activities. Primary COI were determined to be SO4, hardness (predominantly from Mg), alkalinity, Fe, Mn, and Al. BIF waste rock from the Lower Slaty member appears to be the primary source for the identified COI. Mechanisms of release are primarily attributed to pyrite oxidation and subsequent neutralization by dissolution of mixed-composition (Ca-, Mg-, Fe-, and Mn-bearing) siderite and ankerite.