Epithermal gold, silver and copper deposits in volcanic environments have been divided into "adularia-sericite" and "acid-sulfate" types. The adularia-sericite gold deposits are more numerous and have been well studied, their environment of formation being further clarified by studies of active geothermal systems. These deposits formed from heat near-neutral and low salinity alkali chloride waters under reducing conditions. In contrast, the acid-sulfate type gold, silver and copper deposits are relatively scarce and few have been studied in detail using modern techniques. In these deposits the enargite and the alunite appear to have formed from acid waters with relatively high sulfur and oxygen fugacities, but it is not certain if the gold was also transported and deposited under these conditions. It is possible that the gold ores formed as a result of a separate, generally later but spatially superimposed process. We intend to resolve this uncertainty by making an integrated geochemical and isotopic investigation of the El Indio (Chile) deposit, where very high grade gold ore formed after the enargite mineralization, sometimes in different structures. This will allow us to study separately the conditions of formation of the gold and the copper ores, and thus clarify if both belong to a single evolutionary process or if they result from quite different processes that coincide geographically, perhaps because both are linked to the same intrusive and volcanic locus.
