GEOTHERMAL SYSTEMS AND EPITHERMAL MINERAL DEPOSITS: CHARACTERISTICS AND SIGNIFICANCE

Geothermal systems have long been recognized as modern analogs of epithermal mineral deposits formed at depths <~3 km (e.g., Lindgren, 1933; White, 1955; Henley and Ellis, 1983; Simmons and Browne, 2000; Coolbaugh et al., 2011). Indeed, some electricity producing geothermal systems in Nevada started out as Au-Ag epithermal exploration plays, including Blue Mountain, McGinness Hills, and the Don Campbell geothermal power plants. In another case, significant gold mineralization was discovered as a byproduct in developing a geothermal power plant at Noya, Japan.

Geothermal systems can be grouped into two categories— magmatically heated and amagmatic (or extension-related) systems (Coolbaugh et al., 2005). Examples of mineralized magmatically heated geothermal systems include the Hishikari gold deposit (~15 Moz Au) and the giant Ladolam gold deposit (~50 Moz Au) in New Guinea. Young (<5 Ma) amagmatic mineral deposits in Nevada include the Florida Canyon Au-Ag deposit (~3.2 Moz Au) and adjacent Au-bearing Humboldt House geothermal system and the Hycroft mine (~1.6 Moz Au), where many wells encounter water temperatures ranging from 38°C to >65°C. Empirically, mineralized magmatically heated systems have higher grade and volume compared to amagmatic systems (Simmons et al., 2016a). Notable deposition of Au-Ag in active geothermal systems appears to require repeated episodes of disequilibrium conditions, such as rapid boiling and cooling through a limited volume of rock to produce a potentially economic Au-Ag deposit (e.g., Simmons et al., 2016b). Those conditions appear enhanced in magmatically heated systems, suggesting those fossil geothermal systems having spatially and temporally related intermediate to silicic intrusions are the most prospective (e.g., the Sleeper mine, NV). During relatively steady-state conditions of a geothermal system, any metals dissolved in the fluids precipitate in a limited and dispersed fashion, making for subeconomic concentrations of Au-Ag as found in many explored paleogeothermal systems.