Aqueous geochemistry of the "other" pit lake in Butte, Montana, USA

The Continental porphyry Cu-Mo mine, located 2 km east of the famous Berkeley Pit lake of Butte, Montana, contains two small lakes that vary in size depending on mining activity. In contrast to the acidic Berkeley Pit lake, the Continental Pit waters have near-neutral pH and relatively low metal concentrations. The main reason is geological: whereas the Berkeley Pit mined highly-altered granite rich in pyrite with no neutralizing potential, the Continental Pit is mining weakly-altered granite with lower pyrite concentrations and up to 1-2 % hydrothermal calcite. The Continental Pit waters are near equilibrium with a number of carbonate, sulfate, and molybdate minerals.