Inferences about the likely physical and chemical nature of fluids at 
temperatures and pressures above the critical point of water (CPW) come from 
studies of hydrothermal ore deposits related to shallow magmatic intrusions, 
from studies of metamorphic rocks, and from results of laboratory experiments 
investigating (1) mechanical properties of rocks, (2) fluid-mineral reactions, and 
(3) PVT characteristics of brines and gases.  Fluid pressures that are likely to be 
encountered at drillable depths where temperatures are above the CPW are 
greatly influenced by the transition from brittle to plastic behavior of rock.  This 
occurs at about 400°C in silica-rich rocks in tectonically active regions (assumed 
strain rate 10-14 sec-1).  In contrast, in basaltic rocks the transition from brittle 
to plastic behavior occurs at about 500° to 600°C.  Thus, seismicity that opens 
new fractures and reopens old fractures that have become clogged by mineral 
deposition can occur at much higher temperatures in basaltic rocks than in silicic 
rocks.  
	A second important factor is self-sealing of fractures by mineral 
deposition that appears to occur very rapidly in most rocks at about 400°C.  
Aqueous-rich fluids and gases evolved from crystallizing magma and/or by 
metamorphic reactions, tend to become trapped beneath this self-sealed zone 
at greater than hydrostatic pressure.  In rocks that behave plastically, and 
where the least principal stress is the lithostatic load, the mechanically most 
stable configuration for brine and coexisting gas is in flat-lying sheets or 
discontinuous lenses at or near lithostatic pressure.  However, many 
epithermal ore deposits appear to have formed where there has been episodic 
rapid movement of substantial quantities of fluids from the high-temperature, 
high-pressure plastic region across the self-sealed zone into lower-
temperature brittle rock. In this process brecciation of rock occurs adjacent to 
channels of flow in the shallower and cooler brittle region. 
	Saline fluids above the CPW evolve to mixtures of hypersaline brine 
and coexisting gas.  The brine becomes more saline and the coexisting gas less 
saline either as temperature increases or as pressure decreases.  Brecciation 
occurs in the plastic source region where “boiling” of brine and expansion of 
gas are induced by a temporary drop in fluid pressure that occurs as the gas, 
or a froth of brine and gas, moves rapidly through breaches in a self-sealed 
zone.  A continued decrease in pressure results in the system entering a field 
of gas plus solid salt.  In this region significant concentrations of HCl° tend to 
be generated by hydrolysis of the precipitated salts.  However, the HCl° is a 
highly associated species which is non-reactive.  It becomes dissociated and 
reactive only where the gas moves into a cooler environment and liquid water 
condenses. 
	Above the CPW non-condensable gases, such as CO2, H2S, and SO2 
strongly partition into the “steam” phase coexisting with brine.  With 
decreasing pressure the SO2/H2S ratio in the vapor phase exsolved from a 
crystallizing magma increases.  Thus, volcanic gases tend to be relatively rich in 
SO2.  However, underground, as a hydrothermal system cools, SO2 reacts with 
water and with the surrounding rock producing H2S and sulfates.

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