Dames & Moore, 1999 - USDA Forest Service

Another iron sulfide mineral at the Holden Mine is pyrrhotite @el-$). This mineral is commonly more
reactive than pyrite (MEND 1991) but the reaction products are similar. Sulfur may be incompletely
oxidized to elemental sulfur rather than sulfate.
The dominant copper mineral at the Holden Mine is chalcopyrite. It oxidizes by similar processes,
releasing acid, sulfate and copper to solution:
Chalcopyrite is less readily oxidized than pyrrhotite or pyrite (MEND 1991).
Like iron, copper will precipitate due to hydrolysis:
This reaction occurs most at a higher pH (4.0) than iron and produces a precipitate.
63.1.2 Oxidative Dissolution
Oxidative dissolution refers to the breakdown of other minerals by oxidizing their individual components
(Figure 6.3-2). It is an important mechanism for release of heavy metals from other types of sulfide
minerals (at the Holden Mine, primarily sphalerite and chalcopyrite). These minerals may not oxidize
rapidly when exposed to atmospheric oxygen alone but when exposed to the strongly acidic solutions .
generated by oxidation of pyrite can be oxidized by dissolved iron(III):
Sphaleiite can also be oxidized directly by air:
ZnS + 202 + zn2+ + SO? (6-1 1)
A third mechanism of oxidative dissolution occurs when two different minerals with different rest
potentials are in contact and form an electrical cell with one mineral acting as the cathode and the other as
the anode (Kwong, 1995). If sphalerite (Rest Potential -240 mV) is on .contact with pyrite (630 mV),
oxidation of sphalerite occurs preferentially, and pyrite is protected from oxidation. Sphalerite acts as the
anode undergoing oxidation:
ZnS + zn2' + So + 2e- (6- 12)
This process only occurs in sulfide deposits where there is a strong electrical connection between sulfide
grains.
63.13 Acid-Consuming Minerals
The water produced by weathering of sulfide minerals is typically acidic (pH