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Hidrotermal Processes and Mineral System - Franco Pirajno

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destruction
kaolinite+qtz kaolinite montmoril-lonite
plagioclase
rock
Ch
an
ne
lw
ay
Fig. 2.6 Diagrammatic representation of hydrothermal alteration in rocks containing (A)
dominant alkali-feldspar and (B) dominant calcic plagioclase (After Hemley and Jones 1964)
2.3 Hydrothermal Alteration 93
the system considered, being an important and common component found in
high-sulphidation epithermal systems. Alunite is therefore commonly found asso-
ciatedwith hot springs,mudpools and fumaroles in volcanic terranes, where it forms
veins, lenticular bodies and masses of rocks almost entirely replaced by it. Specta-
cular examples of alunite formation can be seen in the Phlegrean fields near Naples
and at La Tolfa north of Rome (Italy). Alunite is generally associated with opal,
kaolinite, dickite, sericite, pyrophyllite anddiaspore. Because of the intense leaching
associated with the presence of alunite, a leached zone may be present that is
typically porous and siliceous. This vuggy silica, however, is residual and should
not be confused with sinter material (see Chapter 5). Relevant reactions are:
KAl3Si3O10
K-mica
ðOHÞ2 þ 4Hþ þ 2SO2�4 ! KAl3ðSO4Þ2
alunite
ðOHÞ6 þ 3SiO2;
3Al2Si2O5
kaolinite
ðOHÞ4 þ 2Kþ þ 6Hþ þ 4SO2�4 ! 2KAl3ðSO4Þ2
alunite
ðOHÞ6 þ 6SiO2 þ 3H2O;
2KAl3Si3O10
K-mica
ðOHÞ2 þ 2Hþ þ 3H2O! 3Al2Si2O5ðOHÞ4
kaolinite
þ 2Kþ:
2.3.2 Styles and Types of Hydrothermal Alteration
The terms used to describe and classify hydrothermal alteration can be expressed
as a function of: (1) recognised mineral assemblage(s) and (2) chemical changes.
In the former, the recognition of mineral assemblages is primarily carried
out through extensive thin section studies. This leads to a listing of minerals in
order of abundance, as shown by Rose and Burt (1979) and Gifkins et al. (2005),
or by general descriptive terms reflecting the dominantmineralogy (assemblages),
such as argillic, potassic, sericitic etc. Chemical changes indicating the type of
chemistry of the fluids involved in the alteration process would include hydrogen
ion metasomatism, alkali metasomatism, fluorine and boron metasomatism etc.
In addition, the style of alteration takes into account the intensity, form and
character of the phenomenon. Here the terminology becomes a little confusing
because of its inherent subjectivity. Terms, such as weak, moderate, strong,
extensive, pervasive, non-pervasive, are well known and frequently used. These
terms essentially refer to the state of preservation of the original rock, how far the
alteration process has advanced, both at the single mineral scale and at the
regional scale, the overall geometry of the alteration halo and so forth. Gifkins
et al. (2005) recommended a ‘‘multi-faceted’’ approach for describing alteration
facies in volcanic rocks, based on four variables mineral intensity, distribution,
texture and mineral assemblage, as follows:
4þ 3þ 2þ 1
94 2 Hydrothermal Processes and Wall Rock Alteration
Where, the intensity variable (4) can be subtle, weak, moderate, strong or intense, the
distribution variable (3) can be local or regional, on the footwall, hanging wall, pipe,
stratabound etc., the texture variable (2) is usually described from hand specimen
or thin section and includes shape, grainsize, fabric and can be selective, pervasive or
vein halo. Finally, the mineral assemblage variable (1) lists the minerals and in
order of decreasing abundances, for example quartz > sericite > K-feldspar.
Alteration intensity refers to how much a rock has been affected by alteration.
The intensity of alteration, though perhaps a little more subjective, is generally
used as a convenient field term. Weak, or low, intensity would mean that only a
few of the original minerals have been replaced with little or no modification of
the original textures. Qualitative estimates can be made using thin sections.
The main styles of alteration are ‘‘pervasive’’, ‘‘selectively pervasive’’ and ‘‘non-
pervasive’’. Pervasive alteration is characterised by the replacement ofmost, or all,
original rock-forming minerals. This results in the partial or total obliteration of
the original textures. Selectively pervasive alteration refers to the replacement of
specific original minerals, e.g. chlorite replacing biotite, or sericite replacing
plagioclase. In this case the original textures are preserved. Non-pervasive altera-
tionmeans that only certain portions of the rock volume have been affected by the
altering fluids. An empirical approach recommended byGuilbert andPark (1986),
proposes the use of symbols to characterise the type of alteration. Selective
pervasiveness and pervasiveness would be indicated on scales from 1 to 10, so
that 1 would mean that alteration is confined to a veinlet or a thin fracture,
whereas 10 would indicate that the entire rock is permeated by the alteration
effects. For example, a notation like S-10-4 means a rock in which the minerals
susceptible to sericitisation (S) are all affected in about 40% of the rock volume.
Clearly, then, selectively pervasive alteration falls in this category, as well as
fracture or veinlet-controlled alteration. In the latter, the alteration minerals are
confined to within a certain distance from a vein or a fracture. The alteration style
around the controlling vein or fracture can be pervasive or selectively pervasive.
Alteration types and patterns of specific hydrothermal deposits are dealt with
in the chapters that follow. Here I examine in a general way, types of alteration
resulting from the interaction of hydrothermal solutions with wall rocks as
revealed and understood from a great variety of hydrothermal ore deposits,
and therefore each type discussed may be applicable only if the proper setting
and related deposit type are considered. The effects produced on thewall rocks by
interaction with and the chemical changes in a hydrothermal solution as a result
of variations in the aK+/aH+ ratio; i.e. the activities of the K+ and H+ ions in
the system. This ratio decreases as the system evolves towards lower temperatures
and pressures. In other words, with increasing H+ metasomatism alteration
processes would move from alkalic to argillic in a theoretically continuous
evolving system. This concept is schematically shown in Fig. 2.7A, B. Conse-
quently, the types of alteration discussed will be in order of decreasing aK+/aH+
(increasing H+metasomatism), and are: (1) alkali metasomatism and potassium
silicate alteration; (2) propylitic; (3) phyllic, or sericitic, alteration and greisenisa-
tion; (4) intermediate argillic; (5) advanced argillic (Fig. 2.7B).
2.3 Hydrothermal Alteration 95
ALKALI
METASOMATISM
H+ METASOMATISM
ADVANCED +H
METASOMATISM
METEORIC INPUT
 PORPHYRY SYSTEMS; NEAR
SURFACE INTRUSIONS
I OR A TYPE MAGMAS Cu, Mo,
AND Au MAINLY
 Sn-W SYSTEMS; CRUSTAL
INTRUSIONS I-S OR S-TYPE
MAGMAS
ALBITISATION AND?OR
MICROCLINISATION
 PHYLLIC AND GREISENS
± VEINING
± METALS
ARGILLIC
(may be absent)
PHYLLIC
± VEINING
+ METALS
ARGILLIC
± VEINING
± METALS
 POTASSIC ALTERATION
PROPYLITISATION
900°
700°
500°
300°
0 2 4
MELT + XSTALS
SOLIDUS
PROPYLITIC
ARGILLIC
PO
TASSIC
PHYLLICADVA
NCE
D
AGR
.
 (P = 1 kb 3 – 4 km depth) 
T°C
LOG aK+/aH+
increasing H+ metasomatism
increasing alkali metasomatism
A
B
Fig. 2.7 Idealised evolutionary alteration sequence. (A) Illustrates types of alteration as a func-
tion of temperature, K+andH+ activities (After Guilbert and Park 1985; Burnham andOhmoto
1980). (B) Alkali metasomatism liberates H+, resulting in decreasing alkali/H+ ratios, and
subsequent destabilisation of feldspars and micas, with growth of new mineral phases (greisen
and phyllic stages). Advanced H+metasomatism is due to meteoric water input into the system,
with oxidation and further H+. Acid leaching