Abstract. A study has been made of the response on TM imagery of a suite of undisturbed sub-economic prophyry copper deposits occurring in the coastal desert of Peru. The deposits are associated with low-grade, mainly propylitic alteration but give rise to strong anomalies on a Landsat Thematic Mapper (TM) band-ratio image. Field and mineralogical studies indicate that normal weathering within these alteration areas has been augmented by oxidation of sulphides and acid leaching of silicate minerals to produce clays and sulphates, including gypsum and natroalunite. It is this supergene assemblage, rather than the hypogene alteration minerals, that is responsible for the strong absorption in band 7 of the TM.