Landsat multispectral scanners, Landsat-4 thematic mapper. Three porphyry copper deposits used as test sites. Landsat data useful for delineating areas of iron oxide on the surface. Limited spatial resolution of MSS hampered mapping of some critical geologic relationships. Synoptic view well suited for examining regional tectonic patterns. Landsat-4 thematic mapper is an improved mapping tool compared to the previous Landsat scanners.--Modified journal abstract.