GEOCHEMICAL EVOLUTION OF SOLUTIONS DERIVED FROM LABORATORY WEATHERING EXPERIMENTS OF SULFIDE-BEARING ORES

The chemical composition of natural waters is affected by the weathering of geologic materials at or near the surface of the Earth. The oxidation of sulfide-bearing ores are of particular interest because of their acid generating capacity and the release of potentially toxic elements to bodies of water during weathering processes. Laboratory weathering experiments of whole-rock sulfide ore from the Shoe Basin Mine (SBM) and the Pennsylvania Mine (PM) from Summit County, Colorado indicate that the composition of the ore, changes in pH, the duration of the experiment, and the formation of sorbents such as Fe and Al hydroxides affect the chemical composition of the resulting solution. Carbonate minerals provide buffering capacity to the solution and contribute to increases in the pH to enhance the formation of Fe and Al hydroxides, which sorb cations from solution. The final pH values obtained in the experiments are similar to those measured in the field (i.e., 2.8 for PM and 5.0 for SBM). However, the chemical compositions of some elements in the solutions arising from the experiments differ from the samples of mine effluent collected in the field as would be expected because the natural samples represent mixtures of mine, meteoric and groundwater. At the Pennsylvania Mine site acidic, metal-rich mine effluent is discharged into Peru Creek where it mixes with the stream water. As a result, the pH increases and Fe hydroxides including schwertmannite precipitate and sorb metal cations from the water thereby improving the quality of the water in Peru Creek.