Sources of Metals and Acidity at the Elizabeth and Ely Mines

Abstract

Abstract The Elizabeth and Ely mines are developed in pyrrhotite-rich, Besshi-type massive sulfide deposits in the Orange County copper district of east-central Vermont. Mine waste and flotation mill tailings have been exposed to weathering for 50 years or more, resulting in acid mine drainage. The Elizabeth mine became a Superfund site in 2001; the Ely mine is proposed. Geochemical and mineralogical characterization of historical mine waste and modern flotation tailings show that despite differences in mine site history, the mines share many similarities. The Elizabeth site includes metal-rich mine waste piles associated with 19th century copperas (melanterite) production, remnants of copper smelter operations, as well as 1.5 million m3 of pyrrhotite-rich flotation tailings. Total base metal concentrations in the surface material on the historically significant Elizabeth mine wastes are higher (as much as 7,000 ppm) than in oxidized parts of flotation mill tailings piles (1,000 ppm or less). Cu >Zn>Pb throughout historic waste piles; Zn>Cu >Pb throughout much of the flotation tailings piles. Variations in Cu/Zn ratios reflect changes in efficiency of chalcopyrite processing over time. Ely ore was smelted for copper on-site after partial roasting, leaving roast beds, mine waste piles, and slag. Oxidized surface materials at both sites are goethite- and jarosite-rich, and locally hematite-rich in roast bed areas. Fine-grained, unoxidized pyrrhotite- and mica-rich tailings lie within 30 cm of the oxidized tailings surfaces. Highly soluble efflo-rescent sulfate salts (melanterite, rozenite, chalcanthite) form intermittently on historic mine waste piles and locally on flotation tailings surfaces at both mines and contribute metals and acidity to surface runoff during rainstorms and spring snowmelt. Water quality in streams draining both mine sites will improve only if the established cycles of weathering of exposed ore, efflorescent sulfate salt formation and dissolution, and release of metals and acidity to the surface runoff, are broken.