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The salinity of the hydrothermal fluid in epithermal deposits varies over a broad

range and can be related to the metal budget of the system. The salinity above the boiling

horizon is generally <23 wt.% NaCl equivalent, and often <5 wt.%, whereas the salinity

at bottom of the system is usually between 10-20 wt% NaCl equivalent (Albinson et al.,

2001). The salinity is typically lower in gold-rich (or gold-only) systems and increases

with both increasing Ag/Au ratio and base metal / precious metal ratio (Albinson et al.,


The boiling horizon not only represents a change in the fluid inclusion types that

are observed, but also represents a change in ore metal distribution with depth. Beneath

the boiling horizon many systems are characterized by higher base metal and lower (or

absent) precious metal grades, whereas above the boiling horizon precious metals are

more common. The highest gold grades are often found immediately above the boiling

horizon (Buchanan, 1981; Cline et al., 1992; Hedenquist et al., 2000). Albinson et al.

(2001) note that ore zones in boiling epithermal systems often show an economic bottom

that is characterized by a dramatic change from high-grade ore to barren rock over a few

meters depth, whereas in the less common non-boiling epithermal deposits the economic

bottom of the deposit occurs due to the gradual decrease in ore grade with depth.

The spatial relationship between boiling, fluid inclusion characteristics and

precious metal mineralization provides a potentially valuable tool in exploration for

epithermal precious metals deposits. Thus, the presence of fluid inclusions indicative of

boiling in surface outcrops suggests that the base of the boiling zone, and the location in

the hydrothermal system where highest precious metal grades are most likely to occur if

the system is mineralized, will be at depth. Conversely, if an epithermal quartz vein at the


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