WHOLE-ROCK GEOCHEMISTRY OF AMPHIBOLITES AT THE ELIZABETH SUPERFUND SITE, VERMONT: ACID GENERATORS AND ACID NEUTRALIZERS

First discovered in 1793, the Elizabeth mine located in east-central Vermont is one of the largest Besshi-type massive sulfide deposits in the Vermont Copper Belt (Slack et al., 2001). Elizabeth was intermittently mined for copper and copperas from the early 1800’s until the mine closed in 1958 (Annis, 1983). The deposit is hosted by the siliciclastic Gile Mountain Formation; other rock types are interlayered with the deposit and the siliciclastics including amphibolite, coticule rock, and garbenschiefer.

The deposit was mined using both open-cut and underground techniques, and had on-site smelting and copper processing which produced three tailings piles. The flooding of underground areas and water flowing through the mine tailings continues to produce acid mine drainage that feeds into the West Branch of the Ompompanoosuc River. The EPA added the Elizabeth mine to the National Priorities List as a Superfund Site in June 2001.

We are defining the chemistry of amphibolites in and around the ore bodies as input for environmental modeling of this and similar deposits. Based on their whole-rock chemistry, 16 amphibolites and 46 altered amphibolites form three groups: unaltered rocks, acid neutralizers, and acid generators. These groups are clearly defined on the ACF diagram and on a plot of Ca/[(Al-Na-K)/2] vs. Mg/[(Al-Na-K)/2], which shows original versus modified igneous composition (Schumacher, 1988).

Unaltered amphibolites from the mine area are chemically similar to metabasalts of the Standing Pond Volcanics and have the basic assemblage of hornblende + biotite + plagioclase. Acid neutralizers are CaO- and CO₂-rich relative to the other amphibolites, reflected by large modes of calcite. They plot much closer to the Ca corner of the ACF diagram, and form a calcium-rich cluster on Schumacher's plot. The acid generators are amphibolites most strongly altered during ore formation. They are characterized by overall loss of calcium and selective gain of magnesium, with assemblages containing sphalerite and chalcopyrite.

Recognizing the presence and identifying the abundance of acid neutralizers and generators at Elizabeth is an important step in defining the chemistry of the surrounding rocks at this and similar sites and quantifying surface water-rock interaction.