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Hidrotermal Processes and Mineral System - Franco Pirajno

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42 1 Water and Hydrothermal Fluids on Earth
Table 1.8 Composition of modern and ancient hydrothermal solutions. Data from various
sources published in Skinner (1979).All values in parts permillion (ppm) unless stated otherwise
Element Salton sea Cheleken Ancient hydrothermal solutions determined from fluid
inclusions
Cl 15.5% 15.7% 8.7% 4.65% 2.95%
Na 5.04% 7.61% 4.04% 1.97% 1.52%
Ca 2.8% 1.97% 8600 7500 4400
K 1.75% 409 3500 3700 6.7%
Sr 400 636 – – –
Ba 235 – – – –
Li 215 7.9% – – –
Rb 135 1.0 – – –
Mg 54 3080 5600 570 –
B 390 – <100 185 –
Br 120 526 – – –
I 18 32 – – –
NH4 409 – – – –
SO4 5 309 1200 1600 1.1%
Fe 2290 14 – – 8000
Mn 1400 46.5 450 690 –
Zn 540 3.0 1.09% 1330 –
Pb 102 9.2 – – –
Cu 8 1.4 9100 140 –
Table 1.9 Analyses of geothermal waters. Data from various sources published in Ellis and
Mahon (1977). (1) Iceland; (2) Philippines; (3) Japan; (4) New Zealand; (5) Mexico; (6)
Taiwan; (7) Italy. All values in parts per million (ppm), unless stated otherwise
Element 1 2 3 4 5 6 7
Cl 197 1.44% 1219 1625 1.6% 1.34% 4.28%
Na 212 7800 846 950 9062 5490 7.89%
Ca 1.5 219 9.9 28 520 1470 106
K 27 2110 1005 80 2287 900 4.83%
Li 0.3 40 4.5 12.2 38 26 380
Rb 0.04 12.5 1.8 0.8 – 12 450
Mg 0.0 0.28 0.02 – 1 131 17
Mn 0.0 – 0.0 0.02 – 40 0.1
Fe 0.1 – 0.5 0.1 0.3 220 0.7
F 1.9 – 3.8 0.8 2 7.0 100
Br 0.45 – 2.5 – 31 – –
SO4 61 32 214 17 6 350 16.3%
As – 28 2.3 – 0.5 3.6 8.3
B 0.6 313 20.51200 14 106 2650 –
SiO2 480 995 425 460 1250 639 –
NH3 0.1 6.4 0.1 46 21 36 82
CO2 55 27 56 61 56 2 5850
8C 216 324 200 2340 340 245 250
pH 9.6 6.7 8.4 7.4 7.7 2.4 8.5
Depth(m) 650 1947 350 585 1285 1500 1415
1.4 Hydrothermal Fluids 43
Ca are in almost all cases the major components of the solutions; minor
components are Sr, Fe, Zn, Mg, Fe, Mn, CO2, SO2, H2S and NH3; (2) with
few exceptions, the most striking feature is that the actual concentration of ore-
formingmetals in these waters is generally low. From these Tables it can thus be
deduced that metal concentrations in the hydrothermal fluids need not neces-
sarily be high in order to form an ore deposit, and therefore the critical factors
for ore deposition must therefore be time and deposition rate. Although diffi-
cult to identify with absolute certainty, it is fair to assume that the source of
these constituents can be the cooling magmas and/or the rocks through which
the solutions pass. The case for Pb is instructive. In a classic study by Doe and
Delevaux (1972) of the sources of Pb in galena ores in southernMissouri, based
on the isotopic compositions for this metal, it was found that the Pb is probably
derived from the Lamotte Sandstone, which is the main aquifer for the hydro-
thermal solutions in the district. The Pb is thought to have been transferred
from solid solution in the feldspars to the hydrothermal fluid by rock-hot water
interactions. Other metals such as Zn, Cu, Sn and W are present in various
amounts in micas, pyroxenes and amphiboles. Sn and W concentrations of up
to 500 ppm have been found in biotites and muscovites (Ivanova 1969). The
release of the metals may take place either during specific reactions with
production of a newmineral phase from the original host or simply by a process
of ion-exchange reactions.
In summary, evidence suggests that hydrothermal fluids acquire their dis-
solved constituents by one or both of two fundamental processes, where: (1) the
constituents are released to a fluid by a crystallising magma, and (2) constitu-
ents derive from the rock through which the hot aqueous solutions circulate.
Finally, Skinner (1979) questioned whether or not a rock mass need be abnor-
mally rich in certain elements in order to serve as a source for the elements. For
those elements that have crustal abundances of 0.001–0.01% (10–100 ppm), the
rocks need not be enriched. An example is the Lamotte Sandstone, above, in
which its feldspars provided the Pb to the solutions. Volcanic piles with pre-
dominant rhyolite-dacite components will produce Pb-rich ores because of the
abundance of feldspars relative to mafic silicates; yet if andesite-basalt predo-
minate, with an abundance of olivines and pyroxenes, they will yield Cu-rich
ores. For those elements such as Sn and Ag, which have very low crustal
abundances (less than 10 ppm or 0.001%), a pre-concentration would probably
be necessary before solution extraction takes place, although a paucity of
reliable data makes this an uncertain point.
1.4.8 The Role of Complex Ions and Ligands
in Hydrothermal Fluids
A complex ion is a ‘‘charged species in which a metal ion is joined by co-ordinate
covalent bonds to neutral molecules and/or negative ions’’ (Masterton et al. 1981).
A complex can also be defined as a coordination compound, where a central
44 1 Water and Hydrothermal Fluids on Earth
atom or ion, M, unites with one or more ligands, L, to form a species MLiLjLk
(Cotton et al. 1999, cited in Rickard and Luther 2006). For example, in the
complex Cu (NH3)4
2+, where one Cu2+ ion combines with four neutral NH3
molecules, each of the NH3 contributes a pair of unshared electrons to form a
covalent bond with the Cu2+ ion. The structure is shown in Fig. 1.10A. Metals
that have the tendency to form complex ions are those that are placed towards
the right of the transition series (i.e. Ni, Cu, Zn, Pt, Au, Co, Cr, Mo, W), while
non-transition metals (Al, Sn, Pb) form amore limited number of complex ions.
The central ion in a complex is a metal cation, and the neutral molecules or
anions bonded to the cation are called ligands. The number of bonds formed by
the central ion is the co-ordination number. In the case illustrated in Fig. 1.10A,
Cu has a co-ordination number of four. Charged complex ions, such as
Cu(NH3)4
2+ or Al(H2O)6
3+, cannot exist in the solid state unless the charge
is balanced. Thus, Cu(NH3)4Cl2, for example, is a complex ion balanced by 2 Cl
ions. Therefore the complex ion acts in this case as a cation. Other examples are:
PtðNH3Þ 2þ4 þ 2Cl ¼ PtðNH3Þ4Cl2 ðcomplex cationÞ;
PtðNH3ÞCl�3 þK ¼ PtðNH3ÞCl3K ðcomplex anionÞ
If a ligand has more than one bond then it is called a chelating agent (from
the Greek chelate, meaning hard). Ligands usually contain atoms of the
NH3 Cu
3+
NH3
NH3
NH3
METAL IONLIGAND
A
Zn2+
NH3
NH3NH3
NH3
B
Me
C
A Square planar structure of Cu(NH3)2+4
B Tetrahedral structure of Zn((NH3)2+4
C Idealised octahedral complex
Fig. 1.10 Molecular
structures of complex ions.
Details in text. After
Masterton (1981)
1.4 Hydrothermal Fluids 45
electronegative elements (C,N, O, S, F, Cl, Br, I). Themost common ligands are
NH3, H2O, Cl
–, OH– and HS–. Co-ordination numbers are usually 6, 4 and 2, in
that order of frequency. Odd co-ordination numbers are very rare. The co-
ordination number also determines the geometry of the complex ions. Thus, for
complex ions in which the central ion forms only two bonds the ligands are
linear with the bonds directed at 1808.
Metal complexes with co-ordination numbers of 4 form either tetrahedral
structures or square planar structures (Fig. 1.10A, B), whereas the octahedral
geometry is obtained for ion complexes in which six ligands surround ametal ion
(Fig. 1.10C).The interpretation of the nature of the bonding in complex ions is
beyond the scope of this book; suffice it to say that two models are considered,
based on the electronic configurations of the ion in question. One is the valence
bondmodel and the other is the crystal-field model, the nature of the bonding in
the latter case being essentially electrostatic. Details on the nature of complex
ions and their role in the transport of transitional metals in hydrothermal
solutions can be found in Brimhall and Crerar (1987) and Crerar et al. (1985).