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

Fluvial Tailings Deposits in the Boulder River Watershed, Montana: Preliminary Results

By D.L. Fey,¹ S.E. Church,² J.S. Curry,³ and T.C. Sole⁴

In the Boulder River watershed, tailings from mill sites have been transported into stream drainages at many locations. The tailings are either part of the active stream sediment load or in such proximity as to be transported into the stream by mass wasting, stream-bank cutting, or large runoff events. Fluvial tailings are being examined to determine volume, content of heavy metals, and potential impact to streams in the study area.

In July 1997, one-inch diameter cores were collected at six sites: (1) the tailings flat below the Buckeye-Enterprise mine and mill complex in the headwaters of Basin Creek, (2) below the mill site of the Bullion Mine, including a breached impoundment and dam, on an upper unnamed tributary of Jack Creek, (3) a smaller but similar site composed of another breached impoundment and dam several hundred meters downstream from site 2 on the same unnamed tributary, (4) two separate sites on upper High Ore Creek downstream from a breached and dissected impoundment area below the Comet Mine, and (5) a small deposit of fluvial tailings capping a bar located on the Boulder River about 200 meters (m) downstream from its confluence with Basin Creek. At each site, cores were collected along the long axis of the fluvial tailings deposit and along several perpendicular traverses. Samples were collected in plastic core tubes using a stainless-steel soil probe/sampler to a depth of up to 1 m; typical depths were 30-50 centimeters. Sample spacing was 7-15 m, depending on the size of the site.

A total of 133 core samples was collected, totaling more than 50 m in aggregate length. In the laboratory, the cores were divided into nearly 700 subsamples on the basis of visual differences in mineralogy, organic content, and leaching zones. The subsamples were ground and analyzed using total digestion and inductively coupled plasma-atomic emission spectroscopy analyses. Fine-grained pyrite and other sulfide minerals were observed in fewer than 10 of the subsamples. Many subsamples, however, contained iron oxy/hydroxide minerals and sulfate-coated quartz grains. In this talk we: (1) present the geochemical data and their relation to core depth, gross mineralogy, and visible leaching, (2) estimate the mass of fluvial tailings and metals at each site, and (3) assess the potential for impacts on streamwater quality.

¹U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dfey@usgs.gov)

²U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (schurch@usgs.gov)

³Louisiana State University, Baton Rouge, LA 70803 (jscurry@unix1.sncc.lsu.edu)

⁴U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (tsole@usgs.gov)

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