Characterization and prevention of acid rock drainage processes at the Greens Creek mine, Admiralty Island, Alaska
University of Oregon, Eugene
Geophysical Institute Library: THESIS NOT UAF
Some rock types from the Greens Creek Mine near Juneau, Alaska are potentially acid generating, but most contain significant buffering capacity in the form of carbonate minerals. Acid-base accounting analyses, shake flask extraction tests, inductively coupled plasma spectroscopy, X-ray diffraction, electron spectroscopy and transmitted and reflected light microscopy were used to describe the chemical composition, mineral content and physical characteristics of fresh and weathered rocks. Variability in rock composition, texture and oxygen and water exposure produce lag times to acid generation ranging from less than two, to more than 20 years, with approximately 15 years being typical. Pyrite morphology and degree of silicification have the largest influence on the material's weathering performance including the buildup of weathering products and evaporative salts and their subsequent release during rain storms. Results of numerical modeling of water/rock interaction and evaporation compare favorably with field data for sulfate, calcium, magnesium, silica, zinc, iron and aluminum. The modeled system predicts a lower pH than is currently observed in the field, but the results are consistent with the acid generating potential of the rock used in the model. Evaporation modeling precipitates alunite and gypsum but ionic strengths where magnesium sulfate minerals are likely to precipitate are too high to model. Waste site design improvements supported by this study include encapsulation of sulfide-rich rock; construction of one-foot, compacted lifts; development of a soil cover to reduce water and oxygen ingress; and installation of ditches and drains to divert water from the piles. Field-scale test plots were set up to evaluate two alkaline amendment strategies for the short-term prevention of acid rock drainage. Application of lime extends the lag time to acid generation, reduces zinc, nickel and cadmium mobility and reduces the rate of sulfide oxidation. Elevated pH causes an increase in the concentration of copper, lead, selenium and chromium to levels above federal water quality standards. Despite increasing the flux of some metals, application of polyacrylamide during lime treatment reduces the flux of zinc, nickel, copper and chromium and produces drainage that meets water quality standards in more cases than drainage from plots treated with lime alone.
Minerals Data and Information Rescue in Alaska (MDIRA)