2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): SMITH, Kathleen S., DESBOROUGH, George A., MEEKER, Gregory P., SUTLEY, Stephen J., LEINZ, Reinhard W., and ANTHONY, Michael W., U.S. Geological Survey, M.S. 973, Box 25046, Denver, CO 80225-0046, ksmith@usgs.gov
Keywords: mining, lead, anglesite, metal mobility
In the study of potential impact of mined sites on water quality and biota, it is important to understand the source(s) of possible metal contaminants and the processes controlling their release into the environment. We examined the solubility of lead in a weathering polymetallic mine-waste dump. A sequence of eight surficial (upper 15 cm) composite samples was collected from the historical polymetallic May Day mine-waste dump near Silverton, Colorado. Each sample was leached using the Synthetic Precipitation Leaching Procedure; the pH of the leachates varied between 3.1 and 3.6, and dissolved lead concentrations varied between 0.05 and 7.6 mg/L. Polished grain mounts of the samples were mapped for lead, sulfur, and iron with wavelength dispersive spectroscopy on an electron microprobe. Areas rich in lead were identified from the elemental maps and randomly selected lead-rich grains were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry. The most abundant lead-bearing phase in the samples is lead-rich jarosite. Abundant fine-grained particles (< 5 micrometers) of anglesite were observed in a sample with high-leachable lead; this anglesite appears to be the source of the leachable lead in this mine-waste material. Anglesite forms from the weathering of galena in the waste. SEM observations of weathering sequences in low-leachable lead samples indicate: galena => anglesite => lead-rich jarosite => potassium/lead jarosite, and lead appears to be sequestered in the jarosite phases. In a related study, samples from polymetallic mine-waste dumps collected from 113 sites in the headwaters of the Animas River, Colorado, and Boulder River, Montana, were leached with deionized water. The 38 sites with detectable anglesite in bulk samples or heavy-mineral concentrates produced eight to nine times more water-leachable lead than the 75 samples with no detectable anglesite. Thus, anglesite appears to be a significant source of leachable lead in polymetallic mine-waste materials. Anglesite also is known to precipitate from mine-waste pore waters and can serve as a sink for lead.
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