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Dames & Moore, 1999 - USDA Forest Service

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Another iron sulfide mineral at the Holden Mine is pynhotite (Fel,S). This mineral is commonly more<br />

reactive than pyrite (MEND 1991) but the reaction products are similar. Sulfur may be incompletely<br />

oxidized to elemental sulfur rather than sulfate.<br />

The dominant copper mineral at th; Holden Mine is chalcopyrite. It oxidizes by similar processes,<br />

releasing acid, sulfate and copper to solution:<br />

CuFeS2 + 171402 + 5/2H20 -> Fe(Om + cu2' + 2H' + 2~01" (6-8)<br />

Chalcopyrite is less readily oxidized than pynhotite or pyrite (MEND 1991).<br />

Like iron, copper will precipitate due to hydrolysis:<br />

cu2' + 2H20 t, Cu(OH)2 + 2W<br />

This reaction occurs most at a higher pH (4.0) than iron and produces a precipitate.<br />

.;s<br />

63.1.2 Oxidative Dissolution<br />

Oxidative dissolution refers to the breakdown of other minerals by oxidizing their individual components<br />

(Figure 6.3-2). It is an important mechanism for release of heavy metals fiom other types of sulfide<br />

minerals (at the Holden Mine, primarily sphalerite and chalcopyrite). These minerals may not oxidize<br />

rapidly when exposed to atmospheric oxygen alone but when exposed to the strongly acidic solutions<br />

generated by oxidation of pyrite can be oxidized by dissolved iron(II1):<br />

Sphalerite can also be oxidized directly by air:<br />

ZnS + 8~e" + 4H20 -) 8~e~' + zn2' + SO:- + 8W (6- 10)<br />

ZnS + 202 + zn2++ SO? (6-1 1)<br />

A third mechanism of oxidative dissolution o&urs when two different minerals with different rest<br />

potentials are in contact and form an electrical cell with one'mineral acting as the' cathode and the othet as<br />

the anode (Kwong, 1995). If sphalerite (Rest Potential -240 mV) is on contact with pyrite (630 mV),<br />

oxidation of sphalerite occurs preferentially, and pyrite is protecte'd from oxidation. Sphalerite acts as the<br />

anode undergoing oxidation:<br />

This process only occurs in sulfide deposits where there is a strong electrical connectioli between sulfide<br />

grains.<br />

63.13 Acid-Consuming Mineials<br />

The water produced by weathering of sulfide minerals is typically acidic (pH

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