Dames & Moore, 1999 - USDA Forest Service

to the underground mine workings, residual concentrates in the abandoned mill building and waste rdck
. piles.
Based on this information, chalcopyrite is considered 'the primary mineral source of copper in Site waters.
The abundance of available chalcopyrite is an important limitation on copper concentrations in seeps, as
discussed later in this section.
6.4.4 Evidence of Acid-Buffering by Minerals
Assuming that acid production is represented by sulfate, the effect of acid-buffering can be evaluated by
comparing sulfate concentrations with alkali and alkali earth elements commonly associated with acid-
buffering minerals. At the Holden Mine, fhese minerals include carbonates containing calcium and
magnesium (in marbles), calc-silicate rocks, and alumino-silicates containing magnesium (chlorite, micas,
hornblende), calcium (hornblende, plagioclase feldspars), potassium ,(biotite, sericite) and sodium
(hornblende, plagioclase, clays). Comparison of sulfate with potassium, magnesium, calcium, and
sodium, as well as inter-comparisons for the elements can indicate the types of buffering reactions
occurring.
Calcium is probably the most ubiquitous element (after silicon) as it occurs in plagioclase, diopside and
hornblende. Plagioclase is a major component of all rock types.
Calcium shows a strong correlation with sulfate (Figure 6.4-6), though tailings pile 1 shows a different
relationship than the other water sources. The calcium to sulfate ratio is approximately 0.5, except for
.IU .* .: tailirigs pile 1 (0.2). The constant ratios are consistent with leaching reactions of the type for calcic
plagioclase:
.- .n.. The.difference for tailings pile 1 suggests that leachate chemistry is controlled by different mineralogy or
: : 'that the lower pH is allowing different minerals to react with the acidity.
Magnesium is also a ubiquitous element occurring in several minerals (hornblende, biotite and chlorite).
The ratio between sulfate and magnesium is nearly constant for the whole Site (Mgt'S04-0.2, Figure 6.4-
7). Railroad Creek waters show a decreasing ratio of magnesium to sulfate, consistent with mixing of the
higher magnesium background surface waters with higher sulfate mine waters. The very strong
relationship between magnesium and sulfate indicates a ubiquitous buffering reaction by magnesium and
aluminum containing minerals (e.g., biotite), for example:
The value of x is less than 1. The same reaction could be written for chlorite or hornblende, both of
which are also common minerals at the Holden Mine Site. Evidence that biotite is an important buffering
control is shown by the relationship between potassium and magnesium (Figure 6.4-8). For the 1500-
level portal drainage, the ratio of potassium to magnesium averages 0.33 which is consistent with
buffering by biotite with x near 1 (ir., 3 moles of magnesium for each mole of potassium) or additional
1769340~419Wuly 27.1999;4:11 PMD~FINAL RI REPORT