To date, four (4) samples of tailings material (predominantly Horquilla) have been generated for the proposed Rosemont Copper Project. All of the samples were tested for acid-generating capacity and metals release using standard static and short-term leaching procedures, including: acid-base accounting (ABA), net acid generation (NAG) pH testing, kinetic (humidity cell testing), synthetic precipitation leaching procedure (SPLP), meteoric water mobility procedure (MWMP), and whole rock analysis. Results from the testing indicate that the tailings material generally contains less than 0.01% sufide-sulfur, can be classified as inert with respect to acid generation, and possess a high capacity for acid neutralization. Humidity cell testing was used to accelerate the weathering and release of various constituents from the tailings and the results provided no indication for the onset of acid generation or leaching of significant metals concentrations for tests lasting 20 weeks. The tailings were also subjected to short-term leaching tests (SPLP and MWMP) which produced only very low metal concentrations in the resulting leachates.