Geoscience Reference
In-Depth Information
sampling and analyses of the natural geothermal systems, their extinct analogues,
epithermal ore deposits, and other geologic environments, primarily by analyses of
fluid inclusions
[35]
. Therefore, an understanding of the mineral
water reaction
kinetics is essential to quantifying the behavior of natural and engineered earth sys-
tems. Although several studies have been carried out on the behavior of most hydro-
thermal systems, the predictions based upon rates from deionized water are unlikely
to be representative of processes occurring in complex fluids of natural systems which
contain numerous dissolved ions. These constituents have both significant rate-
inhibiting and enhancing effects on behavior, even when present in very small quanti-
ties
[36,37]
.
In nature, the most common minerals in soils and rocks are generally in contact
with water at a wide pH range. In extreme cases, the pH can be nearly 2 in the
presence of sulfides, which oxidize to give H
2
SO
4
, and as high as 10 in the pres-
ence of alkaline salts like Na
2
CO
3
. Here, the authors briefly discuss the gold depo-
sition in hydrothermal ore solutions. In this case, the major role is played by the
chloride and sulfur-containing ligands
[38,39]
. The dominant gold complexing
ligands are usually sulfide species. The stability constants for gold(I) chloride com-
plexes (e.g., at 250
C) are up to 20 orders of magnitude smaller than those of Au(I)
hydrosulfide complexes and, therefore, the latter predominate in nature
[40]
.
Despite this observation, the stability constants for Au(I) hydrosulfide complexes
under high-temperature and high-pressure environments are not yet well defined.
This is particularly true for the low pH region where no satisfactory data are
available.
Benning and Seward
[41]
have proposed three sets of experimental conditions
of pH range for gold deposition in nature:
Au
ð
s
Þ
1
H
2
S
H
2
O
H
2
ð
gas
Þ
pH
4
1
1
1
Au
ð
s
Þ
1
H
2
S
NaHS
H
2
O
H
2
ð
gas
Þ
pH
neutral
1
1
1
Au
ð
s
Þ
1
H
2
S
H
3
PO
4
H
2
O
H
2
ð
gas
Þ
pH
4
1
1
1
,
The solubility of gold increases with increasing temperature, pH, and total dis-
solved sulfur. At near neutral pH, an inverse correlation between solubility and
pressure has been observed, whereas in acid pH solutions above 150
C, an increase
in pressure increases the solubility. The equilibrium constants for the uncharged
complex AuHS show that this species plays an important role in the transport and
deposition of gold in ore-depositing environments which are characterized by low
pH fluids.
Recently, some thermodynamic modeling in the chloride systems Au
a
NaCl
a
H
2
O
and Au
H
2
O shows that the gold solubility decreases in the presence
of CO
2
due to decreasing dielectric permeability of the CO
2
-bearing solution
[42]
.
This model has been experimentally verified at 350
C and 50 MPa in 1 M
KCl
a
NaCl
a
CO
2
a
0.1 M HCl solutions in the presence of 3 M CO
2
and without CO
2
. It was
found that the gold concentration in chloride CO
2
-bearing solutions is one order
lower in magnitude than in systems without CO
2
. Similarly, the silver bearing sys-
tems show distinct influence of CO
2
in comparison with gold system. CO
2
has a
1
