Lead isotope evidence for metal dispersal at the Callahan Cu–Zn–Pb mine: Goose Pond tidal estuary, Maine, USA

Abstract

The abandoned massive sulfide Callahan mine, coastal Maine, is unique because the open pit operation was sited in a drained marine estuary. About 5million tons of sulfide-bearing waste and unprocessed ore remain on-site bordering ponds and tidal flats that comprise the estuary proper (US-EPA Superfund site). The prominence and position of the waste relative to the tidal estuary constitute key features representing point sources of heavily contaminated debris. The massive sulfides are hosted by bimodal volcanic rocks of the Cambrian Castine Volcanics (basalt and rhyolite). Sulfide minerals include pyrite, sphalerite, chalcopyrite, and minor galena.A total of 275 analyses of Pb isotopes were done to fully characterize barren volcanic and hydrothermally-altered and mineralized rocks, massive sulfides and disseminated sulfides, waste rock (and tailings) and pond sediments from drill cores, pit water, seeps, and aquatic biota (lobsters, crabs, and bivalves). Lead isotope compositions show a wide range in 206Pb/204Pb (17.972–20.239), 207Pb/204Pb (15.525–15.664), and 208Pb/204Pb (37.591–40.000) and extend from relatively unradiogenic massive sulfides to more radiogenic country rocks and sediments. Tailings, waste rocks, and pond sediments plot in a triangular area in standard Pb isotope diagrams, limited by the trend for the relatively unradiogenic massive sulfides (nearly constant and low 206Pb/204Pb ratios and somewhat variable but low 207Pb/204Pb) and a poorly defined but relatively more radiogenic component (high 206Pb/204Pb and 207Pb/204Pb). Estuary sediments adjacent to the tailings pile and waste rocks have high metal abundances and exhibit the closest resemblance to the massive sulfide isotopic compositions. Bivalve shells obtained from estuary core sediments have growth zones with contents of Pb (5–76ppm), Zn (6–443ppm), and Cu (0–262ppm) that do not vary systematically from zone to zone, and that display no correlation to geographic distribution. The bivalves incorporated lead and other metals by ingesting suspended fine sulfide and silicate particles in addition to dissolved metals in the contaminated waters of the estuary.Water from the open pit and from seeps draining the tailings pile (206Pb/204Pb=17.972–18.504, 207Pb/204Pb=15.501–15.590) overlaps the massive sulfide ore. The pit water maintains its isotopic resemblance to the massive sulfides partly by the influx of seeps draining the rock waste and tailings piles. Galena and other sulfides weather as acidic meteoric water filters through the waste piles. Fractures provide avenues for in-situ oxidation, redistribution of Pb++ and SO4−−, and precipitation of fine-grained anglesite and/or cerussite (depending on the activities of H+, HCO3− and sulfate) in veinlets. A rise in pH above 4.5 and reaction of fluid with calcite in the tailings results in removal of more than 65% of Pb (including anglesite precipitate) by sorption to iron-oxyhydroxide precipitates. Pb transport in the Callahan system is thus likely to occur dominantly within solid phases (galena→anglesite+Fe-oxide→cerussite/carbonate+Fe-oxide) rather than dissolved in solution. Mass wasting of such fine Pb-bearing solid particulates from the waste and tailing piles likely accounts for the largest fraction of the Pb load in the estuary.Lead-bearing water from the Callahan mine is transferred out of the site and available to lobsters and crabs; the aquatic biota thus partly reflect compositions (206Pb/204Pb=18.021–18.539, 207Pb/204Pb=15.497–15.627) within the range of the ores and tailings. The contaminating effects radiating from the mine are intense adjoining the tailings piles and waste rock and adjacent to the site. These effects are significantly attenuated and essentially undistinguishable from natural background in a short distance (~1km).