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Over the past several decades there have been numerous studies of fluids and

fluid inclusions in both active terrestrial geothermal systems and their fossil equivalents,

the epithermal precious metals deposits (Roedder, 1984; Hedenquist et al., 2000;

Albinson et al., 2001; Simmons et al., 2005). There is now a large database of fluid

properties in these systems that documents the close association between boiling and

mineralization in the epithermal environment. Once boiling begins at depth, the fluid will

usually continue to boil to the surface (Figure 1), assuming that the system is composed

of interconnected open fractures and is at hydrostatic pressure (Fournier, 1985; Henley

and Brown, 1985; Vikre, 1985; Cline et al., 1992). Above the boiling horizon low to

moderate salinity liquid and a low density vapor coexist. Fluid inclusion assemblages

trapped from the boiling horizon to the surface are characterized by coexisting liquid-rich

and vapor-rich inclusions that generally homogenize at =300°C near the base of the

boiling zone (Fig. 1) (Bodnar et al., 1985; Simmons and Christenson, 1994), to <220°C

near the top of the system (Albinson et al., 2001), and provide a valuable tool in

exploration for epithermal deposits (Roedder and Bodnar, 1997). At depths beneath the

boiling horizon, fluid inclusion assemblages are characterized by inclusions with

consistent liquid-to vapor ratios that homogenize to the liquid phase, often at

temperatures greater than 300°C (Bodnar et al., 1985). Note that hydrothermal systems

associated with the formation of epithermal deposits are dynamic systems, and the

boiling horizon likely shifts upward and downward over time as the fluid temperature,

flow rate, fracture apertures, etc., vary through time.


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