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Hidrotermal Processes and Mineral System - Franco Pirajno

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producing changes as the result of disequili-
brium, largely due toH+andOH– and other volatile constituents (e.g. B, CO2, F).
In essence, hydrothermal fluids chemically attack the mineral constituents of the
wall rocks, which tend to re-equilibrate by forming new mineral assemblages that
are in equilibrium with the new conditions. The process is a form of metasoma-
tism, i.e. exchange of chemical components between the fluids and the wall-rocks.
Therefore, it is also likely that the fluids themselves may change their composition
as a result of their interaction with the wall rocks. The main factors controlling
alteration processes are: (1) the nature of wall rocks; (2) composition of the fluids;
(3) concentration, activity and chemical potential of the fluid components, such as
H+,CO2, O2, K
+,H2S and SO2.Henley and Ellis (1983) suggested that alteration
products in epithermal systems do not depend so much on wall-rock composition
but more on permeability, temperature and fluid composition. They cited, for
example, that in the temperature range of 250–2808C, similar mineral assemblages
(e.g. quartz-albite-K-feldspar-epidote-illite-calcite-pyrite) are formed in basalts,
sandstone, rhyolite and andesite. Other workers, however, emphasised the funda-
mental role played by the nature and composition of wall rocks in hydrothermal
alteration processes, particularly in porphyry systems.
The action of hydrothermal fluids on wall rocks is by infiltration and/or
diffusion of chemical species (Rose and Burt 1979). Hydrothermal circulation
and related alteration, generally involve large quantities of fluids that pass
through a given volume of rocks, which therefore must have considerable perme-
ability in the form of fractures, or connected pore spaces. Small quantities of
fluids have lesser, or even negligible effects, as exemplified by metamorphic
hydrothermal systems in which the amount of fluids in relation to the rock, i.e.
the water/rock ratio (w/r; defined as the total mass of water that passes through
the system, in the unit time, divided by the total mass of rock in the system
considered), is small, and the resulting mineral deposits have small or negligible
wall-rock alteration. Thus the interaction between H2O and rocks, and the
intensity of alteration is, inter alia, a function of the water/rock ratio (w/r).
This ratio is an important parameter because it affects the degree of exchange
with the wall- rocks. In hydrothermal systems, w/r ratiosmay range from 0.1 to 4,
90 2 Hydrothermal Processes and Wall Rock Alteration
with a lower limit obtained when all free water is absorbed as hydrous minerals
(Henley and Ellis 1983). Exchange of oxygen isotopes during water/rock inter-
action allow to calculate the w/r ratios, as discussed by Taylor (1997) for various
granitic rocks, in which meteoric waters circulate through a very large volume of
rocks. Within this volume the w/r ratio is calculated at between 0.1 and 3.0.
2.3.1 Hydrogen Ion Metasomatism (Hydrolytic Alteration)
and Base Cation Exchange
Hydrolysis and hydration are introduced in Chapter 1. Here, these terms
are defined in the context of hydrothermal alteration processes. Hydrolysis,
or hydrogen ion metasomatism or hydrolytic alteration, is a very important
phenomenon involving the ionic decomposition of H2O into H
+ and OH–. In
hydrothermal alteration, H+ (or OH–) is consumed during reaction with the
silicate minerals, so that the ratio H+/OH– changes. The source of H+ ions can
be subsolidus reactions during alkali metasomatism, water, or acids in the
hydrothermal solution. The conversion of anhydrous silicates to hydrous ones
(e.g. micas or clays) is a reaction which consumes H+ and releases metal ions
into the solution. This in turn affects the pH of the solution and its power to
dissolve or to keep cations in solution. This is related to the dissociation of
complexes containing H+, the degree of association of compounds such as
NaCl, and consequently the formation of chloride complexes and the solubility
of metallic elements (Guilbert and Park 1986). A typical example of hydrolytic
decomposition of feldspar is:
1:5KAlSi3O8
K-feldspar
þH2O! 0:5KAl3Si3O10ðOHÞ2
K-mica
þKþ þ 3SiO2
quartz
þOH�;
Hþ þOH� ! H2O
The sum of the first and second reactions gives:
1:5KAlSi3O8 þHþ ! 0:5KAl3Si3O10ðOHÞ2 þKþ þ 3SiO2:
It can be seen from this reaction that K+ is released and H+ is consumed.
Hydration, the transfer of molecular water from the fluid to a mineral, often
accompanies hydrolysis.
Reactions where a cation is replaced by another in a mineral are called base
exchanges, as for example in the conversion of microcline to albite, Na replaces
K, which goes into solution:
KAlSi3O8
microcline
þNaþ! NaAlSi3
albite
O8 þKþ
2.3 Hydrothermal Alteration 91
In summary, hydrogen ion metasomatism, hydration and base exchange
control the stability of silicate minerals, the pH of the solution, and the transfer
of cations into the solution. They are responsible for propylitic, argillic, sericitic
or phyllic, and potassic mineral assemblages, which are so typical of hydrother-
mal mineral deposits. Areas of intense hydrolytic decomposition, or hydrogen
ion metasomatism, of silicates, are usually surrounded by propylitic alteration
in which hydration phenomena (addition of water and CO2) are dominant.
2.3.1.1 Reactions in Feldspars and K-Mica
Hemley and Jones (1964) conducted experimental studies with aqueous chlor-
ide solutions on K- and Na-bearing systems. These experimentally determined
reactions are described below:
(a) K2O-Al2O3-SiO2-H2O system:
The formation of sericite can be expressed by the following:
3KAlSi3O8
microcline
þ 2Hþ ! KAl3Si3O10ðOHÞ2
K-mica
þ 2Kþ þ 6SiO2
quartaz
0:75Na2CaAl4Si8O24
andesine
þ 2Hþ þKþ ! KAl3Si3O10ðOHÞ2 þ 1:5Naþ þ 0:75Ca2þ þ 3SiO2
sericite
Other reactions are:
KAl3Si3O10
K-mica
ðOHÞ2 þHþ þ 1:5H2O! 1:5Al2Si2O5ðOHÞ4 þKþ;
kaolinite
KAl3Si3O10
K-mica
ðOHÞ2 þ 3SiO2 ! 1:5Al2Si4O10ðOHÞ2 þKþ
pyrophyllite
;
(b) Na2O-Al2O3-SiO2-H2O system:
1:5NaAlSi3O8
albite
þHþ ! 0:5NaAl3Si3O10
paragonite
ðOHÞ2 þ 3SiO2 þNaþ;
NaAl3Si3O10
paragonite
ðOHÞ2 þHþ þ 3SiO2
quartz
! 1:5Al2Si4O10ðOHÞ2 þNaþ;
pyrophyllite
1:17NaAlSi3O8
albite
þHþ ! 0:5Na:33Al2:33Si3:67O10ðOHÞ2
Na-montmorillonite
þ 1:67SiO2 þNaþ;
3Na:33Al:33Si3:67O10
kaolinite
ðOHÞ2 þHþ þ 3:5H2O! 3:5Al2Si2O5ðOHÞ4 þ 4SiO2
Na-montmorillonite
þNaþ:
92 2 Hydrothermal Processes and Wall Rock Alteration
All of these reactions consume H+ and release cations such as Na+ and K+,
as well as other metallic elements that may substitute for them in the lattices of
the altering silicates. These reactions are sensitive to pressure and temperature
changes and the ratios of the components’ activities. The overall patterns of
wall rock alteration containing dominant feldspars and quartz are shown in
Fig. 2.6. The silica derived by the hydrolytic alteration of silicates does not
crystallise at the site of alteration, but diffuses towards the channelways, while
parts of it may remain in the zone of sericite development.
(c) K2O-Al2O3-SiO2-H2O-SO3 system:
Oxidation of H2S leads to the formation of sulphuric acid (H2SO4), a power-
ful leaching agent, particularly active in the lower temperature regimes of
volcanic and subvolcanic environments. Acid leaching is responsible for argillic
alteration commonly seen in epithermal mineral deposits and many porphyry
systems. Hemley et al. (1969) experimentally studied the above five-component
system, involving the stability relations of K-feldspar, muscovite, kaolinite and
alunite as a function of H2SO4 and K2SO4 activities. Alunite is a key mineral in
aH+
a cations
kaolinite+qtz
alunite (if
SO4-2 present
sericite+qtz montmoril-lonite
alkali-
feldspar
rock
A
B
alteration decreases
zone of
feldspar