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Dames & Moore, 1999 - USDA Forest Service

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6.10 CONCLUSIONS<br />

Based on the results of the fate and transport analysis in conjunction with the current conceptual site<br />

model, conclusions are listed below; specific conclusions have also been presented in each main<br />

subsection of Section 6:<br />

The primary sulfide minerals in the Holden Mine ore deposit include pyrite, pyrrhotite,<br />

sphalerite and chalcopyrite.<br />

The Holden Mine deposit is hosted by the Buckskin Schist, which is a quartz amphibole<br />

schist sequence with at least two horizons of intermittent marble beds and calcareous<br />

schists. The dominant silicates are plagioclase and biotite (aluminum-based).<br />

Host rock mineralogy is the primary factor affecting water chemistry at the Site.<br />

Weathering of these minerals, especially sulfide minerals, dominates Site water<br />

chemistry. Non-sulfide mineralogy of the tailings is expected to be - dominated by<br />

minerals contained in the ore and in diabase dikes whereas the mine wall rocks are<br />

dominated by biotite schist.<br />

Secondary mineralization and precipitates produced by weathering processes are visibly<br />

evident throughout the Site, including orange brown iron stains (iron oiyhydroxides) on<br />

waste rock and tailings, white precipitates (amorphous aluminum hydroxide) in the 1500-<br />

level main portal drainage, green stain (copper carbonate) on marble waste rock in the<br />

waste rock piles, and efflorescent crusts (metal sulfates) in the mill building and where<br />

seepage emerges along the toes of the tailings piles.<br />

Consistent geochemical processes are occurring across the Site including iron sulfide<br />

mineral oxidation, oxidation of sphalerite and chalcopyrite, and metal attenuation.<br />

Specific controls include the release of heavy metals (iron, copper, zinc, cadmium), the<br />

release of metals exerting pH control (iron, aluminum), and seep chemistry for different<br />

facilities reflecting different rock types (mine vs. tailings). This dictates the difference<br />

between water chemistry in the east and west parts of the Site. The underground mine,<br />

waste rock piles and mill building area are dominated by the effect of residual zinc and<br />

copper mineralization, whereas the tailings piles are dominated by concentrated iron<br />

sulfides and associated iron alumino-silicates.<br />

The oxidation of sulfide minerals is releasing iron and acid to surface water drainages.<br />

Buffering of acidity is occurring by the reaction of waters with alumino-silicates. This<br />

limits the solubility of some metals (e.g., iron) but also allows pH to be low enough to<br />

solubilize copper. However, since alumino-silicates are abundant, buffering occurs close<br />

to the source of acid generation.<br />

Within Railroad Creek, complete neutralization of acid drainage occurs causing<br />

precipitation of iron, aluminum and copper as flocculent. Zinc and cadmium are likely<br />

not precipitated appreciably within Railroad Creek.<br />

Comparison of sulfate and aluminum supports the general conclusion of buffering by<br />

alumino-silicates. Aluminum concentrations are lowered by aluminum hydroxide<br />

precipitation. .<br />

\\DM-SEAl\VOLI\COMMON\WP\~W~~2\nW.doc<br />

6-5 1<br />

17693-005-019Uuly 27, <strong>1999</strong>;4:11 PMDRAFT FINAL RI REPORT

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