Dames & Moore, 1999 - USDA Forest Service

zinc sulfate salts. Zinc and sulfate concentrations are correlated and the ratio is constant (Figure 6.4-2);
however, zinc is less than sulfate. This can be explained as follows:
Oxidation of iron sulfides releases iron, sulfate and acidity (Equations 6-4 to 6-6). The
acidity released is proportional to the sulfate released.
The acidity released enhances oxidation of sphalerite. The greater the acidity, the greater
the release of zinc. Hence, the zinc concentration is proportional to acidity.
As a result, sulfate, produced by oxidation of iron sulfide, is correlated with zinc
produced by oxidation of sphalerite. In this process, sulfate exceeds zinc release.
The difference between tailings and other locations is consistent with negligible sphalerite concentrations
in the tailings (due'to ore processing) but residual sphalerite in the mine workings, waste rock piles, and
the abandoned mill building.
Zinc to cadmium ratios (Figure 6.4-3) are nearly constant throughout the Site. The generally constant
values indicate that cadmium is associated with sphalerite, and that sphalerite tends to have a constant
zinc to cadmium value. Cadmium and zinc also show very similar chemical behavior in the range of
natural conditions. This is a very common observation for hard rock metal mine sites, as at the Holden
Mine where the type'of sphalerite appears to be relatively uniform.
A very strong correlation exists between manganese and sulfate (Figure 6-44), and a constant ratio
applies to the whole Site (Mn/S04 = 0.005). Manganese can be associated with both sulfides (e.g., pyrite,
sphalerite), iron oxides and as a minor phase substituting in iron silicates. Manganese is relatively
resistant to pH and Eh changes under surface conditions, hence it may remain in solution. The
relationship suggests a constant Site-wide control on manganese chemistry (i.e., leaching of pyrite and
sphalerite).
Sphalerite oxidation therefore occurs in the presence of acidic waters produced by the oxidation of iron
sulfides. This releases zinc, cadmium and probably manganese.
6.43 Evidence of Oxidation of Chalcopyrite
Copper concentrations are consistent with oxidation of chalcopyrite and production of secondary copper
sulfate salts. Several common copper sulfates have also been documented at the Site (see Table 6.1-1).
Relationships between copper and sulfate (Figure 6.4-5) are similar to zinc and sulfate (Figure 6.4-2),
although the data are more scattered. Copper to sulfate ratios are greatest for the abandoned mill building,
the east and west waste rock pile area seeps, and 1500-level main portal drainage (pHc5). The ratio is
relatively consistent for these sources (-0.03). Lower ratios are apparent for the iess acidic 1500-level
main portal drainage. This reflects the strong pH control on copper concentrations at pH>5. As the pH
increases, copper concentrations decrease but sulfate is unaffected.
The tailings pile seeps (noted on the Figure 6.4-5 as "tailings piles 1, 2 and 3") indicated a much lower
ratio (lo4) in acidic waters (pH4). This is consistent with removal of the main source of copper
(chalcopyrite) fiom the ore during processing. The tailings therefore contain little chalcopyrite compared
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17693M)S-O19Wuly 27.1999:4: 1 1 PM;DRAFT FINAL RI REPORT