Epithermal gold deposits

Abstract Epithermal ore deposits form in the shallow portions of hydrothermal systems, from the surface to less than about 1-km depth. The hydrothermal activity is associated with contemporaneous volcanism and related magma intrusions, and the ore is hosted typically by volcanic rocks. There have been many major exploration successes since the late 1970s, e.g., El Indio (Chile), Hishikari (Japan), Ladolam (Papua New Guinea) and Yanacocha (Peru). As a result, epithermal deposits have become increasingly important producers of gold during the past 20 years and are now one of the main targets of gold exploration in volcanic belts of any age. Compared to other types of gold deposit, epithermal deposits are among the best understood in terms of diagnostic characteristics, variations in styles of mineralization and genetic processes. Part of this understanding, particularly of the basic geological and mineralogical features, comes from early studies that were detailed and insightful (e.g., Ransome, 1907; Lindgren, 1933). However, our understanding of the nature of the fluids responsible for metal transport, and the processes that lead to epithermal mineral deposition and associated wallrock alteration is due to recent detailed geological and geochemical studies both of ore deposits and, importantly, active hydrothermal systems and volcanoes. For example, hydrothermal systems that have been explored and developed for their geothermal energy potential provide direct evidence on the composition, distribution, and flow of fluids, the patterns of temperature and pressure, and processes such as boiling and mixing (Henley and Ellis, 1983). More recently, the study of discharges from volcanoes and