Geoscience Reference
In-Depth Information
and increases again, but much more rapidly, in the vapor domain. Let's now repeat the
experiment at higher pressure and therefore at higher temperature: a similar sequence of
events is obtained, but the difference between the molar volumes of the vapor and the
liquid decreases with pressure, until we reach the critical pressure of 22 MPa. Then the
volumes of both phases are identical and we pass continuously from the liquid field into
the vapor field without seeing any interface. The temperature of the critical transition also
is fixed at 374
◦
C. In nature, water is rarely pure and salts are dissolved, sometimes in
large proportions (highly saline solutions are called brines). Adding salt to the solution
increases both the temperature and pressure of the critical point. For example, a brine with
10 weight percent NaCl has a critical point at 700
◦
C and 120 MPa. Brine boiling produces
rather light and dilute vapors and other brines, dense and even more concentrated in salt
than the original solution. Highly concentrated brines with several tens of percent salt are
important because they can dissolve enormous amounts of metals such as transition ele-
ments and rare-earth elements which, upon cooling, produce ore bodies of occasionally
economical value. A hydrothermal fluid rich in Cl
−
, most likely because it derives from
seawater, must contain a heavy load of cations simply to equilibrate the negative charges.
Another reason why hot brine chemistry is different from that of low-temperature solution
is the low dielectric constant of water which enhances the recombination of ionic species,
such as Na
+
,H
+
, and Cl
−
, into molecular species, such as NaCl and HCl. Such molecular
species are true complexes and increase the apparent solubility of metals in hydrothermal
solutions. Hydrothermal deposits may form, however, from more diluted solutions at rela-
tively low temperatures: this is the case of the Pb-Zn (base metal) Mississippi Valley-type
ore deposits.
Balancing hydrothermal reactions requires a combination of cation-exchange reactions
between the rock and the solution and the principle of electrical neutrality. First we need
to draw up an inventory of proton/cation exchange reactions such as
(7.22)
, which can be
simply re-written:
2H
+
2Na
+
2NaAlSi
3
O
8
+
+
H
2
O
⇔
Al
2
Si
2
O
5
(OH)
4
+
4SiO
2
+
(albite)
(solution)
(kaolinite)
(silica)
(solution)
For pure mineral phases, this equation leads to:
ln
Na
+
solution
=
H
RT
+
H
+
constant
(10.9)
At a given temperature, this ratio is constant. The enthalpy
H
of the reaction varies from
one reaction to another. A solution rich in H
+
therefore displaces cations from the rock
more efficiently than a high-pH solution. This phenomenon is heavily dependent on tem-
perature. Moreover, the positive charges must balance the negative charges, which are very
often dominated by Cl
−
, an anion scarcely involved in any mineral reactions and whose
concentration remains virtually constant. The chemistry of the solution and therefore its
capacity to modify the chemistry of the rock depends essentially on all the thermodynamic
properties of exchange reactions and on temperature.
Among all the parameters of hydrothermal activity that need to be understood, tem-
perature is, along with Cl concentration, the most important. For thermometry, it is
