| Abstract |
The term epithermal derives from the genetic classification scheme for hydrothermal ore deposits proposed by Lindgren (1933). On the basis of stratigraphic relationships in volcanic sequences, and by analogy with mineral and metal occurrences and mineral textures in active hydrothermal systems, Lindgren inferred that epithermal deposits formed at more than 200°C and more than 100 atmospheres (~100 bars). In New Zealand, Japan, Philippines, United States, and other countries, the demand for alternative sources of electricity encouraged geothermal exploration drilling and development from the 1980’s. Temperatures and pressures similar to those in the epithermal environment were encountered at depths of less than 1 km (e.g., White, 1981; Henley and Ellis, 1983), and precious and base metals were found deposited in springs, wells, and surface pipes (e.g., Weissberg, 1969, Hedenquist and Henley, 1985a; Brown, 1986; Krupp and Seward, 1987). The rapid increase in understanding at the time was such that the first two volumes of Reviews in Economic Geology focused on the nature of epithermal environments (Henley et al., 1984; Berger and Bethke, 1985). Thus, by the mid 1980s, genetic models were formulated to explain the occurrence and zonation of metals and minerals, to define the physical-chemical conditions of ore deposition in several epithermal deposits, and to provide a basis for speculation on the sources of fluids and metals (e.g., Barton et al., 1977; Kamilli and Ohmoto, 1977; Sawkins et al., 1979; Buchanan, 1981; Berger and Eimon, 1983; Henley and Ellis, 1983; Hayba et al., 1985; Heald et al., 1987; Stoffregen, 1987). In these models, hydrology was seen to be an essential factor in producing ore deposits, with boiling and fluid mixing being recognized as causative agents for metal deposition. It was early recognised (e.g., Lindgren, 1933; White, 1955) that clear parallels exist between the near surface (~500 m) depositional environment of these deposits and that of modern hot spring systems and these were emphasized by the results of exploration activity through the Western U.S.A. (e.g., McLaughlin deposit, California). |