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

Geochemical and Mineralogical Characterization of Mine Dumps on BLM Lands, Upper Animas River Watershed, Colorado: Plans and Preliminary Results

By J.T. Nash,¹ G.A. Desborough,² and D.L. Fey³

Geochemical and mineralogical studies of mine dumps in the upper Animas River watershed are underway to help the U.S. Bureau of Land Management (BLM) prioritize sites and to suggest possible methods for remediation. In the summer of 1997, approximately 200 BLM dump sites were briefly examined; 82 sites were studied further because they met the following criteria: (1) location within 3 kilometers of major streams, (2) size greater than about 100 tons, and (3) access reasonable for cleanup work. Of the more than 300 BLM sites identified by previous investigations, the majority are smaller than 100 tons, and thus, were deemed unlikely to be significant contributors to the watershed and were not considered further in this study. Onsite work included brief description of rock types, rock alteration, and ore mineralogy; an estimate of size; and collection of a representative sample of the entire dump. Composite samples were collected from 30 subsites uniformly distributed over the top and sides of each dump; at each subsite, dump rocks from a 0.5-square-meter area excavated to a depth of 10 centimeters were mixed, and a portion was passed through a 2-millimeter (mm) sieve to yield about 100 grams. The <2mm fraction is deemed most likely to be involved in short-term reactions and to contribute metals to runoff. Water draining from portals and dumps was noted at 29 sites; pH and specific conductivity were measured at all sites, and at some sites, a single reconnaissance water sample was collected for chemical analysis and comparison with results of leach tests described below.

A preliminary list of BLM problem sites was made at the end of field work in September 1997, based chiefly on visual impressions of dump mineralogy, dump size, and potential mobility of metals in acidic drainage. This working list, which closely resembled that of the U.S. Bureau of Mines and BLM, was dominated by sites at which mine or surface drainage was interacting with dump rocks. Our first impressions lead us to support the recommendations of many other workers that management of mine drainage and surface waters should be a priority in mine-dump mitigation.

Laboratory studies were initiated in November 1997 to describe the bulk mineralogy and chemistry of the <2-mm samples and their reactions in deionized water; only a few results were available to include in this abstract. At this time, we are working on a suite of 120 samples, including some dump samples from privately owned sites and some representative samples of mill tailings and unmined mineralized rocks, for comparison with the BLM dump samples. X-ray diffraction (XRD) studies have been made to determine the major minerals in the <2-mm dump samples. The XRD studies confirm the presence of many sulfate minerals that are not reliably identified in macroscopic examinations: 50 percent of the samples contain jarosite-family minerals, 10 percent contain anglesite, 3 percent contain alunite, and 3 percent contain gypsum. Sulfide minerals are common: 25 percent of samples have pyrite, 30 percent have sphalerite, and 30 percent have galena. Calcite, an important acid-buffering phase, was detected by XRD in only three samples. Quartz is present in most samples, and aluminosilicates such as feldspars, micas, and kaolinite are present in 45 to 60 percent of the samples. The aluminosilicate pyrophyllite was detected in 10 percent of the samples. The aluminosilicate minerals are normally inert, but at very low pH conditions, they can release significant dissolved aluminum that may significantly degrade water quality as a colloid. The mineral suite alunite-pyrophyllite-enargite is found only in the vicinity of the Red Mountains. Because the XRD method can not detect minerals present at less than about 1 to 3 percent (but these minor phases can be important contributors of acidity or metals), a micro-panning method was used to concentrate heavy minerals, and these were examined by microscope and analyzed by nondestructive methods. This method revealed small but important amounts of anglesite, pyrite, and other minerals that were not detected by the XRD method.

Concentrations of arsenic, cadmium, copper, iron, lead, and zinc were determined by energy dispersive X-ray analysis and are highly variable in our suite of 120 samples. Maximum concentrations of some key metals of interest are: arsenic, 11,000 parts per million (ppm); cadmium, 16 ppm; copper, 37,000 ppm; iron, 22 weight percent, lead, 96,000 ppm; and zinc, 27,000 ppm. A preliminary review of the results suggests that arsenic concentrations greater than 100 ppm only occur in the vicinity of the Red Mountains. No spatial or geologic associations are evident for copper, lead, and zinc, which have highly variable concentrations even between neighboring dumps. High concentrations (thousands of ppm) of base metals in dump samples seem to reflect methods of mining and ore processing more than ore type and geology.

Two kinds of laboratory leach tests will be made to provide information on acid generation and metal mobility. Leaching of samples under static conditions in deionized water will mimic short-term conditions similar to spring 'flush' and runoff. We also will undertake leaching tests using end-over-end agitation similar to the EPA 1312 protocol. Based on our experience with leach tests of dump samples from diverse ore types in other areas of Colorado, Montana, and Nevada, we anticipate that jarosite and alunite rather than pyrite will dominate short-term acid production. Because few of our samples from the Animas River watershed contain calcite, we anticipate that most dump samples will have low buffering capacity. Laboratory work that is underway, and additional sampling planned for the summer of 1998, should permit us to better characterize individual dump sites and allow us to estimate the contribution of these dumps to the upper Animas River watershed.

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

²U.S. Geological Survey, MS 905, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (gdesboro@usgs.gov)

³U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dfey@usgs.gov)

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