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Dames & Moore, 1999 - USDA Forest Service

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Subsection 6.4 presents general Site wide evidence for the processes described in Subsection 6.3 and<br />

reduces the variations in water chemistry to mechanisms that are common to the whole Site.<br />

Subsection 6.5 presents variations in the processes as they apply to the different components of the Site.<br />

Subsection 6.6 discusses impacts to Railroad Creek and presents a mass balance for the chemical loads to<br />

determine if chemical loads can be accounted for by the known sources.<br />

Subsection 6.7 compares Site with two other similar mine 'sites to determine if processes at the Holden<br />

Mine resemble processes elsewhere in similar environmental settings.<br />

Subsection 6.8 discusses the formation of flocculent and ferricrete in Railroad Creek and the downstream<br />

transport of flocculent.<br />

Subsection 6.9 discusses the metals data and transport mechanisms for sediment in Railroad Creek.<br />

Subsection 6.10 provides conclusions for Section 6.<br />

To assist in better understanding the Site conditions, Table 6.0-1 has been prepared which presents'a key<br />

of Site features and media sampling/data collection locations as it relates to Figure 6.1-la.<br />

6.1 SITE CONDITIONS<br />

6.1.1 Bedrock Geology and Mineralogy<br />

Bedrock mineralogy is the underlying fundamental control on mine site water geochemistry. This section<br />

summarizes the mineralogy of the host rocks and ore deposit at the Site, as discussed in Section 4.2.3.<br />

6.1.1.1 Host Rocks<br />

The Holden Mine assemblage, which is the regional host for the Holden Mine deposit is dominated by<br />

hornblende bearing rocks that include amphibolite, hornblende gneiss, and hornblende-biotite schist.<br />

Calc-silicate rock, leucocratic gneiss, and plagioclase-biotite schist are less abundant constituents, and<br />

marble, pelitic schist, and metaconglomerate occur locally (Dragovich and Derkey, 1994). This latter<br />

assemblage includes the Martin Ridge and Buckskin schists, and the Fernow Gneiss of Youngberg and<br />

Wilson (1952).<br />

The deposit itself is hosted by the Buckskin schist which is a quartz amphibole schist sequence, with at<br />

least two horizons of intermittent marble beds and calcareous'schists (Youngberg and Wilson, 1952).<br />

Typical specimens have the following composition (Dubois, 1954):<br />

Layers of almost pure quartz<br />

Biotite-rich layers of 15 percent quartz (Si02), 45 percent plagioclase (NaAISi0308 to<br />

CaA12Si208), 30 percent biotite (K(Mg,Fe)3(AISi3010)(OH)2), .and .10 percent<br />

hornblende ((Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,A1)2022(OI-I) and diopside (CaMgSi206)<br />

\u)M-SEA I\VOLI\COMMO~WP\~W)~~~\M).~~~<br />

6-2<br />

17693005019Uuly 27.<strong>1999</strong>;4:42 PM-DRAFT FINAL RI REPORT

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