As part of a study of mechanisms that produce acid mine drainage, we made laboratory<br>and field studies of two mine dumps near Silver-ton, CO. Water that drains from both dumps<br>during spring to mid-summer had been found to be quite acidic. The dumps contained broken-up<br>material excavated from Pb-Zn-Ag prospects. In both laboratory and field measurements we<br>found that the resistivities of dump material varied substantially, while their spectral induced<br>polarization (SIP) characteristics were weaker than those expected for sulfides interacting with<br>porewaters. We speculate that after 40-plus years of residence in mine dumps, the grains of dump<br>material have become coated with oxidized minerals which inhibit the surface electrochemical<br>reactions that give rise to strong SIP response. If this is so, it implies that the mechanism for<br>generation of acid waters in the mine dumps we studied is probably not the oft-cited oxidation of<br>pyrite at grain surfaces, but rather some alternative process(es) such as those involving oxidized<br>minerals.


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