NATURAL PYRITE WEATHERING RATE IN A SMALL CATCHMENT COMPARED WITH MINED CATCHMENTS

As part of the USGS background study of the Questa mine in northern New Mexico, a small catchment (Straight Creek) of about 2.67 km², was studied to determine the hydrogeochemical processes that naturally generate acidic, metal-rich surface waters and groundwaters. The predominant reaction is pyrite oxidation because there is excess pyrite (2-10 wt.%) in quartz-sericite alteration. A mass–balance calculation was performed reacting pure water to the median composition of 14 water analyses from 2000-2003 with median pH of 2.98 and median sulfate concentration of 2,030 mg/L. The most likely mass–balance model consistent with mineralogical analyses, assuming that rates are unaffected by solid/solution ratio or by evaporation, indicates oxidative dissolution of 8.66 mmol pyrite/kg_H2O. When combined with a median discharge of 37.85 L/min measured simultaneously, the weathering rate was 5.46 mmol pyrite/h. This value is larger than batch abiotic laboratory rates, about 0.01 mmol/h for Fe(III) oxidation with pH close to 2, or microbial laboratory rates, about 0.22 mmol/h based on an interlab comparison. Evaporation effects or a “pooling effect” might be the main sources of discrepancy between apparent field rates based on mass balances and laboratory rates. Similar calculations for effluent water at the Leviathan Mine, CA (pH = 1.8) result in an apparent pyrite weathering rate of 40 mmol/h, 200 times higher than expected from laboratory rates; this result could be explained by the extremely fine-grained pyrite that occurs there as well as evaporation effects. Calculations for Iron Mountain, CA result in an apparent pyrite weathering rate of 17,100 mol/h, five orders of magnitude higher than laboratory rates. At Iron Mountain pyrite weathering has produced enormous amounts of efflorescent salts that periodically dissolve and reprecipitate in the underground workings. When the mass–balance is modified to account for dissolving soluble salts, removal of some iron as goethite is required and the weathering rate is 24,800 moles soluble sulfates/h. Regardless of how the mass–balances are calculated, the field rate for pyrite oxidation is much faster than the laboratory rate suggesting that evaporation, pooling, or possibly surface areas, influence these calculations for Questa and most other mine sites.