Leaching of chalcopyrite occurs at modest temperature (less than 100 degreesC) and atmospheric pressure when nanosize silica is added to the system. Important parameters in the system are chalcopyrite particle size, oxidant type and concentration, nanosilica concentration, pulp density and temperature. Electron micrographs and EDX spectra are presented showing the chalcopyrite surface covered with sulfur after leaching in the absence of nanosilica whereas the chalcopyrite surface is covered with nanosilica (with little sulfur) after leaching in the presence of nanosize silica. (C) 2004 Elsevier Ltd. All rights reserved.