New Mexico Geological Society Annual Spring Meeting — Abstracts

Nine boreholes were drilled in four rock piles at the Molycorp Questa molybdenum mine as part of a characterization study in 1999. Temperature and concentration of pore oxygen and carbon-dioxide were monitored in the holes over a five-year period at montly intervals. The data collected over the five-year period were analyzed with plots of each of the three parameters against times and depth profiles of mean monthly values.

Oxygen concentrations were lower but carbon-dioxide concentrations were higher than ambient values. Concentrations of oxygen and carbon-dioxide had a negative correlation in both the depth profiles and time plots. The highest oxygen and lowest carbon-dioxide concentrations were in the colder months. Temperatures varied with the seasons at depth above 20 to 30 feet. Below 30 feet, temperatures did not show seasonal variations but decreased by up to 10 degrees Fahrenheit over the five-year period. Over the same period, mean oxygen concentrations increased by up to 10% and carbon-dioxide decreased by 1%. These observations suggest that air enters the piles through the toe and flows upward. Oxygen is used up by oxidation processes and carbon-dioxide is produced by decomposition of carbonates, which produces heat in piles. With time, the chemical processes slow due to depletion of reactants, resulting in decreasing temperatures and carbon-dioxide concentrations and increasing oxygen concentrations. The flow of air into the pile is partly controlled by the difference in temperature between ambient air and the interior of the pile and, therefore, there is greater influx of air during the colder months.