13.01.2013 Views

Dames & Moore, 1999 - USDA Forest Service

Dames & Moore, 1999 - USDA Forest Service

Dames & Moore, 1999 - USDA Forest Service

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

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

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!