ABSTRACT
The Questa molybdenum mine is located in the Sangre de Cristo mountains in Taos County, northern New Mexico. Currently, the mining operations consist of underground block caving; however, between 1965 and 1983 lower grade molybdenum ore (0.185% MoS2, 74 million tonnes) was recovered by open pit mining methods, with some 320 million tonnes of waste rock produced. A comprehensive physical and geochemical characterization of the mine rock piles has been carried out at the site over the last few years to evaluate the current conditions and predict the future conditions. This paper describes the results of a drilling program completed as part of a larger, site-wide characterization program. The drilling program was carried out in phases with the objective of characterizing the geochemical and physical properties of the mine rock at surface and at depth and the oxygen and temperature variations with depth in the piles. Testing included (i) geochemical testing of drill cuttings (moisture content, paste pH/EC, ABA, leach extraction and forward acid titration testing); (ii) physical testing of mine rock samples (grain size analysis, moisture retention, permeability), and (iii) in-situ monitoring of temperature and oxygen/carbon dioxide in 10 boreholes. The results of the characterization study indicate that the majority of the mine rock, where it has been tested, is potentially acid generating, but, after more than 20 years, the mine rock may not have reached a mature state of oxidation and acid mine drainage. The particle size and moisture content of the mine rock is quite variable and the in-situ temperature and oxygen monitoring suggest that there is on-going sulfide oxidation and advective airflow (chimney effect) within the rock piles. The results of this rock pile characterization and monitoring have been used to calibrate an air transport and ARD production model of the mine rock piles (see Lefebvre et al., 2002).
