STABLE ISOTOPE INSIGHTS INTO THE EVOLUTION OF MINE DRAINAGE AT THE ELIZABETH COPPER MINE SUPERFUND SITE, VERMONT

Sulfur, oxygen, and hydrogen isotopes from sulfide minerals, water, and dissolved sulfate from the Elizabeth copper mine Superfund site provide insights into the physical and geochemical evolution of mine drainage. The abandoned mine was developed in a pyrrhotitic massive sulfide deposit hosted by metamorphosed siliciclastic and amphibolitic rocks. Waste at the site can be divided into hydrologically and geochemically distinct settings: thin (< 9 m thick) waste-rock piles, thick (< 37 m thick) tailings piles, and unmined wallrock, including flooded mine workings, and an open pit filled by a small lake.

The d³⁴S values of drainage and tailings are generally homogeneous (8.9 ± 0.5 ‰ and 8.0 ± 0.7 ‰, respectively), but isolated lower values for tailings near the base of the pile (-11.3 to -0.6 ‰) suggest that bacterial sulfate reduction may be locally important. The d¹⁸O and dD of waters from the flooded mine pool (-10.8 ± 0.1 ‰ and -72.4 ± 0.1 ‰, respectively) and waters seeping from the base of the tailings pile (-10.0 ± 0.3 ‰ and -64.9 ± 3.1 ‰, respectively) show limited annual variations, indicating a long residence time for these waters, whereas waters from the waste-rock piles, pit lake, and impacted streams show significant variations (d¹⁸O=-11.2 to -9.0 ‰; dD=-81.1 to -64.7 ‰), consistent with seasonal changes in meteoric water compositions. Isotopic variations in the shallow pond atop the tailings pile (d¹⁸O=-12.0 to -5.4 ‰; dD=-81.3 to -47.4 ‰) document the importance of snow melt in the spring and evaporation in the summer. The d¹⁸O of dissolved sulfate reflects the relative importance of dissolved oxygen and ferric iron as oxidizing agents throughout the site. The d¹⁸O of dissolved sulfate in Copperas Brook (-5.6 ± 0.2 ‰) indicates that ferric iron is the dominant oxidizing agent throughout the site. However, isotopic data suggest that dissolved oxygen is locally important, particularly in the flooded mine workings and the ground waters in the tailings pile. Thus, the stable isotope data refine previous models for the geochemical evolution of mine waters at the site based on major-, minor-, and trace-element chemical variations.