AQUEOUS SOLUTIONS AND THE HYDROSPHERE - Chemical Fundamentals of Geology and Environmental Geoscience - page 75

emplacement in mineral veins and ore bodies. The com-
positions and temperatures of ore-forming hydrothermal
fluids can be established from the study under the micro-
scope of the fluid inclusions left behind in the ore and
gangue minerals (Box 4.6). It is clear that simple ionic
solubilities of ore minerals (solubility products; Table 4.1)
are many times too low to explain the dissolved-metal
concentrations actually measured in such fluids, which
are often in the 100-500 ppm range for metals like copper
(Cu) and zinc (Zn). The discrepancy reflects the dramatic
increase in the solubility of such metals brought about by
complexing in highly saline hydrothermal fluids (see
Chapter 7, 'The co-ordinate bond').
Determining the chemical forms - or speciation - of
metals dissolved and transported in the hydrothermal
fluids that circulate in the Earth's crust is an important
step in understanding the formation of hydrothermal
ore deposits. The complexes formed depend on the
temperature and pH of the solution, and on the ligands
present. The most important ligands in typical ore-
forming fluids are probably Cl − , H 2 S and HS − . The
sulfide ion itself (S 2− ) is unlikely to be an important
ligand in the neutral or mildly acidic fluids believed to
be typical of ore-forming systems.
Because ore-forming fluids are known to be highly
saline, chloride complexes dominate the hydrothermal
transport of many important metals. Thus lead (Pb)
may be present as PbCl + or PbCl 4 2− depending on the
salinity of the host solution:

experiences a significant change in any of these varia-
bles, there may be a drastic reduction in the solubility of
certain metals present, leading to their deposition as
sulfide ore. This can happen if there is a drop in tem-
perature (Box 4.2), mixing with other solutions of higher
pH or lower salinity, or reaction with wall rocks.

Oxidation and reduction: Eh -pH diagrams

Two components - protons (H+ ions) and electrons - are omni-
present in the natural environment; their activities can be meas-
ured electrometrically as pH and redox potential Eh; these activities
may be plotted one against the other in an Eh-pH diagram. The
total area of such a diagram contained by (field) measurements
delineates the natural (aquatic) environment. (Baas Becking
et al., 1960)

Oxidizing power and acidity 9 are the two most impor-
tant parameters of any sedimentary environment,
jointly delineating the limits of stability of minerals
that are found there. They are expressed in terms of the
redox potential ( Eh -see Box 4.7) and the pH (Appendix
B) values of solutions with which the minerals coexist.
Where several alternative minerals may crystallize
depending on the conditions, it is logical to map their
stability fields on Eh -pH diagrams. As an illustration,
Figure 4.1a shows the stability fields of various copper
minerals, together with relevant dissolved Cu species
(shaded field), that are stable in the presence of water,
chloride, a sulfur and carbon dioxide.
What do the sloping lines at the top and bottom of
this diagram signify? Above the top line lie conditions
so strongly oxidising as to cause water to decompose
and liberate oxygen:

2 + − +

Pb +→ Cl PbCl lowsalinitysolution (4.31)

2 + − 2 −

Pb + → 4 Cl PbCl high salinitysolution (4.32)

4

These species enable solutions in contact with galena
(PbS) to contain as much as 600 parts per million of
lead (by weight), whereas pure water saturated with
PbS contains only 4 × 10 −9 ppm.
Experiments and calculations suggest that the prin-
cipal zinc species in hot saline fluids is neutral ZnCl 2 0 ,
that silver is transported chiefly as AgC1 2 − , and tin as
SnCl + . Copper forms the complex (CuCl) 0 , which dom-
inates its hydrothermal solution chemistry above
250 °C, but at lower temperatures the bisulfide com-
plex Cu(HS) 3 2− may play a significant role.
The stability of such complexes is very sensitive to
the temperature of the fluid, to its pH and to the salinity.
It follows that if a sulfide-bearing hydrothermal fluid

− (4.33)

1
2

HO OHe
2

 ++ 2

+

2

2

As the forward reaction does not take place in nature,
the line represents a formal upper limit for Eh values
found in natural waters. This upper boundary is
inclined because the value of Eh required for the for-
ward reaction to occur depends on the pH (since H + is
among the products).

9

Oxidizing power can be thought of as the capacity to remove elec-
trons from atoms, while reducing power reflects the power to
donate electrons. Acidity, on the other hand, reflects the capacity
of a solution to donate hydrogen ions (H + ions = protons).