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Supergene enrichment of Cu deposits in northern Chile is believed to have occurred from 45 to 9 Ma in a climate with significant rainfall. Atacamite, a major constituent of supergene zones of Cu deposits in the Atacama Desert, has been considered to be of primary supergene origin, i.e., pre-9 Ma. Chlorine-36 data in atacamite samples show very low (36)Cl/Cl ratios, comparable to previously reported (36)Cl/Cl ratios of deep formation waters and old groundwaters. The (36)Cl data strongly suggests that the saline waters related to atacamite formation are old in origin (>1.5 Ma), but that the atacamite is much younger than previous interpretations suggested (e.g. Sillitoe and McKee, 1996). U-series disequilibrium data obtained in gypsum intimately associated with the atacamite confirm young ages for atacamite-bearing assemblages, between similar to 240 and 75 ka. These results indicate that supergene enrichment of Cu deposits did not cease after the onset of hyper-aridity as previously thought. Therefore, supergene metallogenesis of Cu deposits in the region is a two-stage process: first, supergene oxidation of hypogene ore by meteoric water between 45-9 Ma. This was followed by a second stage of supergene alteration under hyper-arid conditions that continued until at least the Pleistocene, when deep formation waters modified the pre-existing Cu oxide minerals.