Skip Links

USGS Home Contact USGS Search USGS

Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Spectral Induced Polarization Studies of Mine Dumps near Silverton, Colorado

By David L. Campbell,¹ David V. Fitterman,² and Robert J. Horton³

To help study mechanisms that produce acid mine drainage (AMD), we made spectral induced polarization (SIP) measurements at the Mayday and Yukon mine dumps, in a Department of Interior Abandoned Mine Lands study area near Silverton, Colorado. Induced polarization is a geophysical method that has long been used to detect disseminated sulfides, and SIP is a refinement of that method that can characterize electrochemical processes taking place at mineral/pore-water interfaces. At Silverton, we made SIP measurements at intervals of 1.5 meters (m) at the Mayday dump and 3.0 m at the Yukon dump along profiles on the dump faces, investigating to depths of about 5 m below the surface.

We had expected the SIP surveys to locate sulfide-rich pockets that might be AMD sources within the dumps. We think that they did that at the Yukon dump. Field SIP measurements at the Mayday dump site, however, were relatively uniform, both laterally and with depth. None of the Mayday SIP spectra resembled those reported in the literature for typical sulfide deposits. Most Mayday SIP spectra looked alike, and even those taken on outcrops of "jarosite" (yellow-colored) and "goethite" (brownish) were only subtly different from each other. Observed variations in SIP phase are ascribed mostly to compositional variations in the dumps, while those in conductivity are ascribed to a combination of variations in composition, pore-water saturation, and amounts of dissolved solids in the pore water.

Ten samples of material from the surface of the Mayday dump were measured in the laboratory to further investigate their SIP characteristics. The resulting SIP spectra showed processes which we still must identify, but which seemed weaker than those expected for ground water interacting with sulfides. Examination of the samples using a hand lens showed they consist mostly of oxidized minerals, but their exact mineralogies still must be determined. Because field SIP spectra were similar to those measured in the laboratory, and because they were similar at all depths, we infer that material in the Mayday dump is generally homogeneous. We speculate that after 40-plus years of residence in mine dumps, the surfaces of grains of dump material at all depths have become well oxidized, thereby inhibiting the surface electrochemical reactions which give strong SIP responses. If this is so, it implies that the acid waters currently draining from the Mayday mine dump probably result mainly from process(es) involving already-oxidized minerals, rather than from primary oxidation of sulfides at grain surfaces.

¹U.S. Geological Survey, MS 964, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (davec@usgs.gov)

²U.S. Geological Survey, MS 964, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (fitter@usgs.gov)

OFR 98-297 Home | OFR 98-297 Table of Contents | Previous | Next

USGS Abandoned Mine Lands Intiative Home Page