RELATIONSHIP OF GEOLOGIC AND HYDROLOGIC SETTING TO DISTINCT EVOLUTIONARY PATHS OF MINE DRAINAGE AT THE ELIZABETH COPPER MINE SUPERFUND SITE, VERMONT

Drainage at the Elizabeth mine evolves to different compositions due to differences in hydrologic setting and physical characteristics of the wastes. These vary due to historical changes in targeted commodities and mining technology, despite a fairly uniform primary mineralogy of ores, wall rocks, and wastes. The mine exploited a pyrrhotitic Besshi-type massive sulfide deposit with minor chalcopyrite and sphalerite; the host rock consists of quartz, albite, muscovite, biotite, garnet, amphibole, and minor carbonate. Waste at the site can be divided into thin (< 9 m thick) waste-rock piles comprising sand- to boulder-sized material, thick (< 37 m thick) tailings piles comprising sand-sized material, and unmined rock. Unmined rock settings include discharging, flooded mine workings, and an open pit filled by a small lake.

Drainage from the waste-rock has lower pH (2.1 to 3.2), and higher dissolved Fe (440 to 1,000 mg/L), Al (7.0 to 236 mg/L), Cu (0.06 to 200 mg/L), Zn (1.4 to 38.0 mg/L), and sulfate (4,500 to 4,900 mg/L) than that from the tailings, which has higher pH (6.1 to 6.9) and lower dissolved Fe (14.0 to 904 mg/L), Al (<0.01 to 0.48 mg/L), Cu (<0.005 to 0.020 mg/L), Zn (0.005 to 0.100 mg/L), and sulfate (1,300 to 3,800 mg/L). Waters in the pit lake have a lower, but slightly overlapping pH values (3.4 to 3.8) compared to those from underground workings (3.2 to 5.3). However, differences in the dissolved constituents of waters from the pit lake (Fe: 0.11 to 0.50 mg/L; Al: 2.7 to 3.1 mg/L; Cu: 0.97 to 1.10 mg/L; Zn: 0.46 to 0.50 mg/L; sulfate: 250 to 270 mg/L) and underground workings (Fe: 45.1 to 74.0 mg/L; Al: 0.64 to 6.36 mg/L; Cu: 0.044 to 0.390 mg/L; Zn: 0.44 to 0.74 mg/L; sulfate: 530 to 920 mg/L) do not vary systematically with pH. Differences in the geochemical evolution of waters at the site reflect the complex interplay of factors such as grain size, access to oxygen, neutralization by carbonates, pH-dependence of metal sorption, and photoreduction of Fe.