Dames & Moore, 1999 - USDA Forest Service

Monitoring of seeps from drill holes, fractures and backfill inside the workings prior to installing the
plugs indicated that most seeps had pH between 3 and 3.5. As a result, acid generation within the Baker
Mine has reached the most reactive stage and mine drainage is not expected to worsen.
Figures 6.7-2 to 6.7-4 compare drainage chemistry for the Baker Mine with the Holden Mine. P-5 was
used rather than P- 1 because it has a larger dataset. For comparative purposes, data for different years of
monitoring are superimposed on one graph depicting one year. The Baker Mine dataset begins in May
prior to the snow melt. The drainage pH is near 7. As the melt event begins, pH drops to 4.4, then as the
portal discharge decreases pH increases until pH is near 7 in October. Almost exactly the same pH trend
is seen at the Holden Mine 1500-level main portal drainage (P-5). During maximum flows, the pH of the
Baker Mine discharge is low, but then steadily increases through the summer, reaching neutral conditions
by September. I
The mechanism involved is identical at both the Baker Mine and Holden Mine. The melting of the snow
pack results in flushing of weathering products accumulated during the previous winter. As the melt
event moderates, pH increases due to reduced leaching of acidic weathering products. The Baker Mine
drainage contains a buff precipitate similar to the precipitate observed in the Holden Mine portal drainage.
The lowermost' extreme of pH values is controlled at both the Baker Mine and Holden Mine by the
precipitation of aluminum hydroxides. The host rocks at the Baker Mine were altered to clay alumino-
silicates during mineralization. The acid solutions formed by pyrite oxidation attack these minerals,
resulting in the release of aluminum.
The comparison of copper concentrations indicates very similar concentrations and trends for the Baker
Mine and Holden Mine. Copper concentrations are relatively low for the majority of the year for the
Baker Mine, but increase as pH decreases during spring snowmelt (Figure 6.7-3). The almost identical
copper concentrations indicate a common chemical control, which could be copper carbonates, or co-
precipitation with aluminum hydroxide. Comparison of zinc concentrations indicates order-of-magnitude
higher concentrations at the Holden Mine (Figure 6.7-4). Unlike the Holden Mine, the Baker Mine does
not have zinc mineralization, and zinc concentrations are not constrained by secondary mineral formation
at these pHs. The differences between the Baker Mine and the Holden Mine in terms of source zinc
availability, therefore, become apparent.
6.7.2 Sullivan Mine
The Sullivan Mine is a relatively large underground massive sulfide lead-zinc deposit located in the
Canadian Rocky Mountains. The area also experiences cold winters during which snow pack
accumulates. The snow pack melts in April and May and summers are hot and dry. The Sullivan Mine
has been operating since 19 10 and consists of an active underground mine, an abandoned open pit, several
waste rock piles and a relatively large tailings disposal area resulting from conventional flotation (the
method utilized at the Holden Mine Site) of lead and zinc sulfides from a sulfide ore composed of
pyrrhotite and pyrite.
6.7.2.1 Underground Mine
The underground development of the Sullivan Mine is accessed by a single portal. Drainage chemistry is
summarized in Figures 6.7-2 to 6.7-4. The pH of the drainage is continuously low, approximately around
17693M)S019Uuly 27,1999.4:ll WRAFT FINAL RI REPORT