Circulation of hydrothermal solutions through the host rocks of ore bodies generates important mineralogical changes and, consequently, the original magnetic properties of these rocks could be strongly modified. In the Escondida porphyry copper deposit, each lithological-alteration rock unit shows highly distinctive magnetic properties; potassic and chloritic altered rocks display the largest bulk magnetic susceptibility (kbulk) values, whereas phyllic and argillic altered rocks exhibit the lowest kbulk ones. Hysteresis ratios show that magnetic grain size is related to hydrothermal alteration. Rocks showing pervasive alterations yield single (SD) and pseudosingle (PSD) magnetic grain sizes probably related to destruction and decreasing of the magnetic grains in acid conditions. In contrast, rocks showing selective alterations yield multidomain (MD) magnetic grain sizes and can be associated with relict magmatic magnetic grains or new hydrothermal minerals precipitated from more neutral or alkaline solutions. These results provide a basic frame on where future magnetic studies can help to determine aeration zones during both operations and exploration campaigns.