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Lithos 115 (2010) 153–162
Contents lists available at ScienceDirect
Lithos
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / l i t h o s
Hydrothermal alteration of deep fractured granite: Effects of dissolution
and precipitation
Shoji Nishimoto a,⁎, Hidekazu Yoshida b
a
b
Nagoya City Science Museum, 2-17-1 Sakae, Naka, Nagoya, 460-0008 Japan
Nagoya University Museum, Furocho, Chikusa, Nagoya, 464-8601 Japan
a r t i c l e
i n f o
Article history:
Received 21 April 2009
Accepted 27 November 2009
Available online 6 December 2009
Keywords:
Hydrothermal alteration
Granite
Water–rock interaction
Fracture
Orogenic belt
a b s t r a c t
This paper investigates the mineralogical effects of hydrothermal alteration at depth in fractures in granite. A
fracture accompanied by an alteration halo and filled with clay was found at a depth of 200 m in a drill core
through Toki granite, Gifu, central Japan. Microscopic observation, XRD, XRF, EPMA and SXAM investigations
revealed that the microcrystalline clays consist of illite, quartz and pyrite and that the halo round the fracture
can be subdivided into a phyllic zone adjacent to the fracture, surrounded by a propylitic zone in which Fephyllosilicates are present, and a distinctive outer alteration front characterized by plagioclase breakdown.
The processes that result in these changes took place in three successive stages: 1) partial dissolution of
plagioclase with partial chloritization of biotite; 2) biotite dissolution and precipitation of Fe-phyllosilicate in
the dissolution pores; 3) dissolution of K-feldspar and Fe-phyllosilicate, and illite precipitation associated
with development of microcracks. These hydrothermal alterations of the granite proceed mainly by a
dissolution–precipitation process resulting from the infiltration of hydrothermal fluid along microcracks.
Such infiltration causes locally high mobility of Al and increases the ratio of fluid to rock in the alteration
halo. These results contribute to an understanding of how granitic rock becomes altered in orogenic fields
such as the Japanese island arc.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Hydrothermal alteration affects the geochemical properties of
granitic rocks (Ferry, 1979; Boyce et al., 2003). Granitic rocks in the
Japanese orogenic field are altered more than continental granite
because they are part of an active geothermal system with a higher
density of fractures (Yoshida et al., 2005). However, granitic rock
underlying a geothermal field obtained from a depth of 3000 m below
the surface was found to be remarkably fresh and not permeable
enough to allow hydrothermal circulation (Fujimoto et al., 2000). This
suggests that fracture development is an important factor in the
establishment of hydrothermal circulation and the facilitation of
hydrothermal alteration by the interaction between fluid and rock
within the granitic body. Although numerous studies related to
hydrothermal alteration have been carried out, most are concerned
with porphyry copper deposits, epithermal gold deposits or geothermal fields lying outside the body of the granite (e.g. Adams and
Moore, 1987; Hedenquist et al., 1996; Doi et al., 1998).
Within the body of granitic rocks, the interaction between the rock
and external fluids has, however, been studied in the context of hot
dry rock (HDR) power generation (Savage et al., 1987; Bando et al.,
⁎ Corresponding author. Tel.: +81 52 201 4486.
E-mail address: [email protected] (S. Nishimoto).
0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2009.11.015
2003), the underground storage of carbon dioxide (Ueda et al., 2005;
Suto et al., 2007) and radioactive waste disposal (Yoshida et al., 2005;
Sandström et al., 2008; Yoshida et al., 2009). In the case of HDR
reservoirs used for electricity generation, flow-path stability controlled by fluid–rock interaction is important for the performance of
reservoirs through their lifetime (Savage et al., 1987; Richards et al.,
1992). The interaction of rock and fluid containing CO2 at elevated
temperatures has been studied in order to better understand a
possible underground storage of CO2 (Suto et al., 2007). Fractures in
granitic rocks and the minerals that fill them are also critical in their
effect on the isolation of radioactive waste in sites of deep geological
disposal. The frequency, geometry and filling of fractures are significant
factors that control solute migration in geological environment (Steefel
and Lichtner, 1994; Mazurek, 1994; Yoshida et al., 2000). The long-term
stability of structural features and the interaction between fluid and
rock should therefore be taken into account in modeling the processes of
element mobility in deep granitic bodies. In order to develop such a
model, the way fractures are generated and subsequently stabilized
needs to be better understood. So also does the way in which each
mineral in granite is altered by hydrothermal fluid along fractures, a
change made more complex to understand because of the actions of
multiple alteration processes.
This study has, therefore, focused on the mineralogical changes to
granite caused by the penetration of hydrothermal fluid along fractures in the deep underground environment. We have investigated a
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S. Nishimoto, H. Yoshida / Lithos 115 (2010) 153–162
simple alteration halo in granite along a fracture filled with compacted clay from a drill hole excavated at the Mizunami Underground
Research Laboratory (MIU) in central Japan. The presence of a clayfilled fracture without subsequent deformation implies that it has
been stable since its formation. This makes it an ideal site for
investigating how hydrothermal fluid affects the rock as it infiltrates
fractures, and therefore the long-term stability of sealed fractures in
the deep underground environment. In this paper we show the
chemical and mineralogical changes across the alteration halo and
discuss the process of hydrothermal alteration that results from
fracturing and microcracks in granite. In broader terms, this study is a
contribution to the understanding of the long-term stability of
fractures in granitic rocks.
such granitic rock in which a considerable part of the granite body is
concealed beneath the Late Cretaceous ash-flow deposits of the Nohi
Rhyolite (Kawada et al., 1961; Yamada et al., 1971), forming several
cauldrons (Koido, 1991). The similarity of the petrology between the
Nohi Rhyolite and the surrounding Naegi–Agematsu granite indicates
that they were formed as a series of large magmatic events on the
eastern margin of the Eurasian continent (Sonohara and Harayama,
2. Geological setting
Late Cretaceous–Paleogene granitoids are widely distributed in
central Japan (Fig. 1). In Nakajima's (1994) interpretation, based on
studies of the isotopic ages of granitic and metamorphic rocks, these
granitoids were formed during the subduction of the Kula–Pacific
ridge beneath the Eurasian continent. Naegi–Agematsu granite is one
Fig. 1. Geological map showing location of the Toki granite body part of the Naegi–
Agematsu granite intruding into Nohi rhyolite. MIU: Mizunami Underground
Laboratory.
Simplified from Geological Survey of Japan (2007) and Ishihara and Wu (2001).
Fig. 2. Illustration of the alteration halo and chemical compositions of mineral
assemblage. a) Paragenetic sequence of secondary minerals. b) sample photo; c) SXAM
compositional maps of Ca, Fe and K; d) bulk compositional of Fe2O, CaO, Na2O, K2O and
LOI within the alteration halo. *ff: fracture filling.
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S. Nishimoto, H. Yoshida / Lithos 115 (2010) 153–162
155
Table 1
Bulk composition of individual zones within the hydrothermal alteration halo.
Sample number
1
2
3
4
5
6
7
8
Unaltered
SiO2
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
78.1
0.16
12.63
1.52
0.01
0.11
0.16
3.86
3.82
0.05
0.57
100.97
77.02
0.17
12.3
1.69
0.02
0.12
0.16
3.42
4.23
0.05
1.2
100.38
76.98
0.19
12.76
1.86
0.02
0.16
0.2
3.11
4.45
0.05
1.52
101.3
75.84
0.17
13.16
2.17
0.02
0.17
0.23
3.12
4.57
0.05
1.64
101.15
74.55
0.21
13.46
2.69
0.03
0.22
0.33
2.83
4.73
0.05
1.61
100.71
74.96
0.21
13.33
2.93
0.05
0.28
0.61
3.11
4.43
0.05
1.22
101.15
74.4
0.2
13.21
2.5
0.07
0.29
0.82
3.31
4.87
0.05
0.6
100.31
73.01
0.19
13.37
2.15
0.07
0.27
1.06
3.43
4.92
0.05
0.75
99.26
73.7
0.17
13.28
2.06
0.07
0.26
1.02
3.16
4.86
0.04
0.77
99.38
73.15
0.18
13.39
2.01
0.07
0.28
0.97
3.17
5.06
0.04
0.83
99.15
Sample numbers are shown in Fig. 2c. LOI: loss on ignition. Fe2O3*: total Fe as Fe2O3.
2007). They are interpreted as a volcano-plutonic complex formed in
a shallow crust at the active continental margin.
The Toki granite body is one component of the Naegi–Agematsu
granite. It has a circular exposure of about 140 km2 and intruded
discordantly into the Jurassic accretionary complex of the Mino
Terrain (Wakita, 2000). The granite body mostly consists of medium
to coarse-grained biotite granite. It contains a small amount of
pegmatite but lacks economic-grade ore deposits (Ishihara and Wu,
2001). Magnetite is not observed, indicating that it belongs to the
ilmenite-series (Ishihara, 1977). The peraluminous chemical composition (A/CNK = 1.09–1.16: Ishihara and Wu, 2001) is consistent with
S-type granite (Chappell and White, 1974). The Rb–Sr whole rock age
is 72.3 Ma (Shibata and Ishihara, 1979) while the K–Ar age of the
biotite is 60–63 Ma (JNC, 2002) and the fission track age of the zircon
is 59–61 Ma (Sasao et al., 2006). The high initial 87Sr/86Sr ratio
(0.7106) compared with other granites in southwest Japan (0.706–
0.709) implies the existence of an old basement at depth in this region
(Ishihara and Matsuhisa, 2002). Toki granite is unconformably
covered with Miocene of the Mizunami Group (20–15 Ma) and
Pliocene of the Seto Group (0.7–5 Ma). There is a highly weathered
zone less than 10 m thick in the upper part of the Toki granite beneath
the unconformity. The Tsukiyoshi Fault, an E–W striking reverse fault
dipping 70° to the south, cuts the Toki granite and the Mizunami
Group (Onishi and Shimizu, 2005). The granite in the vicinity of major
faults is fractured and altered (Nakamata et al., 2007) suggesting
association of alteration with faults. Within the granitic body,
Iwatsuki and Yoshida (1999) recognized two fracture systems, an
intact, moderately fractured part and an intensely fractured part,
based on both the degree and frequency of fracturing. Mineralogical
study showed that the moderately fractured and intensely fractured
parts are associated with chlorite/montmorillonite and kaolinite
respectively as fracture fillings. Even in the intact part of the granite,
however, weak illitization of some plagioclase and chloritization of
biotite grains are observed under the microscope (Nishimoto et al.,
2008). Based on the O and C-isotopic composition of fracture-filling
calcites, it appears that the Toki Granite has been altered by three
types of fluid: hydrothermal fluids, seawater, and groundwater
(Iwatsuki et al., 2002). The present underground water in the granite
is of Na–(Ca)–Cl type and the temperatures at depths of 500 to
1000 m are about 30 °C (Iwatsuki et al., 2002).
geological disposal of high level radioactive waste. The granite is
mainly composed of brownish gray quartz, with pale pink K-feldspar,
white plagioclase, and biotite. The open fracture that we studied is
filled with unsolidified, compacted pale green clay without pores. No
slip trace was observed on the fracture surface between the infilling
clay and the wall rock. An alteration halo is symmetrically developed
on both sides of the clay-filled fracture (Fig. 2). Visually, the halo is
divided into a 3 cm wide whitened internal part and a 2 cm wide
greenish external zone. The drill core sample was divided into two
halves. One half was sliced and polished for chemical analysis. Bulk
chemical composition was measured on eight cut pieces of the core
sample near the fracture and two cut pieces of unaltered granite for
comparison by XRF (Shimadzu SXF-1200) equipped with an Rh X-ray
tube. The instruments were calibrated against rock reference samples
issued by the Geological Survey of Japan. Compositional maps of Ca, K
and Fe of the whole sliced sample surface were obtained using a
scanning X-ray analytical microscope (SXAM, Horiba XGT-2000), an
X-ray fluorescence analyzer that shows the distribution of elements
across the surface of a sample (Hosokawa et al., 1997). A high-
3. Sample description and analytical methods
The sample was collected from a borehole /v192-(6)/ drilled
horizontally at a depth of 200 m in a shaft of the MIU which is being
constructed by the Japan Atomic Energy Agency (JAEA) in order to
establish techniques for investigation, analysis and assessment of
Fig. 3. Isocon diagram for the each zone of the alteration halo along the fracture in the
Toki granite. Cunaltered granite is an average of two analysis of unaltered granite.
Caltered granite is the average analysis of each zone. Each analysis is multiplied by an
appropriate factor: 0.3 for SiO2, 1.5 for Al2O3, 5 for Fe2O3, 18 for CaO, 3 for K2O, 4 for
Na2O, 40 for TiO2, 20 for MgO, 50 for MnO and P2O5.
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S. Nishimoto, H. Yoshida / Lithos 115 (2010) 153–162
intensity continuous X-ray beam (Rh anode 50 kV, 1 mA), 100 μm in
diameter, is focused with a guide tube and directed onto the surface of
the sample on a PC-controllable X–Y stage. The second half of the drill
core sample was used for mineralogical investigation. Thin sections
were cut to observe mineral texture by polarizing microscope. Some
sections were polished to analyze chemical composition and element
distribution in plagioclase and biotite grains by electron microprobe
(EPMA, JEOL JXA-8800). Mineral compositions were acquired using a
focused electron beam of 1 µm diameter with an accelerating voltage
of 15 kV and a beam current of 12 nA. Some mineral grains were
drilled with a mini-drill to identify clay minerals. The powder X-ray
diffraction pattern was measured by diffractometer (XRD, Multiflex,
Rigaku) set at a CuKα radiation of 40 kV and 20 mA in the range of 2–
50° 2θ.
4. Results
4.1. Overall sequence of mineralogical and chemical changes
The development of mineral and chemical features within the
hydrothermal alteration halo is summarized in Fig. 2. Based on
paragenesis (Fig. 2a) and occurrence (Fig. 2b), the alteration halo can
be subdivided into three zones: an outer zone, a propylitic zone and a
phyllic zone, although the mineralogical sequence of the alteration
halo is less sharp. The outer zone is distinguished by plagioclase
breakdown. The propylitic zone is greenish and characterized by
secondary Fe-phyllosilicates such as chlorite, corrensite and smectite
that are typical of propylitic alteration. The phyllic zone is distinguished by whitened K-feldspar that results from the breakdown and
illitization of plagioclase. Compositional mapping by SXAM (Fig. 2c)
revealed an abrupt decrease of Ca in the outer zone, and also a
depletion of Ca in propylitic and phyllic zones. K is detected in
alteration products of plagioclase within the phyllic zone while Fe is
distributed in alteration products of plagioclase throughout the
propylitic zone.
The bulk composition of the eight cut pieces examined by XRF is
shown in Table 1. The ratios of CaO, Fe2O3(total), K2O and Na2O in the
eight portions to those in “unaltered” host granite are shown in
Fig. 2d. As shown in SXAM maps, CaO-depletion is significant in the
alteration halo. Fe2O3 is enriched in the propylitic zone and depleted
in the phyllic zone. Na2O is enriched along the fracture and K2O
progressively depleted from the outer zone to the fracture surface. LOI
increases between the phyllic and propylitic zones. Since calcite has
not been identified, LOI represents crystallochemical OH or water.
The isocon plots (Grant, 1986) of each zone, based on Table 1, are
shown in Fig. 3. The points of Ti and P which are generally considered
immobile, show indeed the least scatter of all elements. Overall, the
element distribution suggests a weak volume loss. The concentration
for Al, which is also commonly considered immobile, differs
somewhat from the isocon, suggesting that Al is mobile especially in
the phyllic zone. The loss of Si is the most in the outer zone. Ca
depletion implies a substantial dissolution of plagioclase and its
removal. The loss of Mn and Mg in the propylitic and phyllic zones
indicate that biotite dissolved during the alteration process.
The chemical composition of illite is phengitic (2–3 wt.% as FeO)
and shows no significant changes within the alteration halo (Table 2,
Fig. 4a). However, the molar Fe/(Fe + Mg) ratio of corrensite and
chlorite in the alteration halo is higher than that in “unaltered” wall
rock (Fig. 4b).
The compacted clay that fills the fracture is pale green in color and
is a mixture of illite, quartz and pyrite along with fragments of
plagioclase and quartz grains. Although it is hard to separate
fragments from secondary minerals, albite can also be identified.
Calcite, smectite and kaolinite are absent in the clay fillings.
4.2. “Unaltered” wall–rock granite
The bulk composition of unaltered granite outside the alteration halo
is similar to that given in previous reports (Ishihara and Wu, 2001;
Nishimoto et al., 2008), although it is slightly poorer in Si and Na, and
richer in Al. The molar Fe/(Fe + Mg) ratio of the biotite varies between
Table 2
Representative chemical composition of the secondary minerals.
Mineral
zone
Illite
Smectite
Mixed-layer (corrensite)
Chlorite
Fracture
Phyllica
Phyllicb
Propylitica
Propylitica
Propylitica
Propyliticb
outer
Analysis no.
95
78
77
47
22
45
56
18
SiO2
TiO2
Al2O3
FeOc
MnO
MgO
CaO
Na2O
K2O
total
49.48
0.03
32.01
2.92
0.00
1.06
0.20
0.10
6.23
92.03
50.63
0.03
33.50
2.60
0.07
0.67
0.06
0.08
7.79
95.42
49.44
0.00
34.78
1.78
0.02
0.50
0.04
0.08
7.74
94.38
48.77
0.16
29.67
3.44
0.00
1.30
0.10
0.11
6.86
90.41
31.24
0.00
21.32
28.79
0.22
2.15
0.52
0.09
1.45
85.78
26.12
0.09
17.78
33.39
1.11
1.72
0.25
0.10
0.04
80.60
27.09
0.00
17.71
33.64
0.49
1.38
0.21
0.91
0.03
81.46
25.56
0.14
16.48
34.18
1.10
6.24
0.03
0.03
0.09
83.85
3.32
0.68
1.85
0.00
0.16
0.00
0.11
0.01
0.01
0.53
0.61
3.30
0.70
1.87
0.00
0.14
0.00
0.07
0.00
0.01
0.65
0.69
3.24
0.76
1.93
0.00
0.10
0.00
0.05
0.00
0.01
0.65
0.67
3.36
0.64
1.77
0.01
0.20
0.00
0.13
0.01
0.01
0.60
0.60
2.66
1.34
0.80
0.00
2.05
0.02
0.27
0.05
0.02
0.16
0.88
2.82
1.18
1.09
0.01
3.02
0.10
0.28
0.03
0.02
0.01
0.92
2.89
1.11
1.12
0.00
3.00
0.04
0.22
0.02
0.19
0.00
0.93
3.00
1.00
1.35
0.01
3.35
0.11
1.09
0.00
0.01
0.01
0.75
Atoms per formula unitd
Si
Al(IV)
Al(VI)
Ti
Fec
Mn
Mg
Ca
Na
K
Fe/(Fe + Mg)
a
b
c
d
Present in altered plagioclase.
Present in altered biotite.
All Fe as Fe2+.
Calculated based on O10(OH)2 for illite and smectite, O10(OH)5 for corrensite, O10(OH)8 for chlorite.
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S. Nishimoto, H. Yoshida / Lithos 115 (2010) 153–162
157
occurs throughout the granite body and is not related to the alteration
along clay-filled fractures.
4.3. Outer zone
In the outer zone of the alteration by infiltration of hydrothermal
fluid from the fracture, breakdown is readily observed in the inner
part of plagioclase grains, even by naked eye (arrow in Fig. 2b). Ca
decreases (Figs. 2d and 3) suggesting that the dissolution of
plagioclase occurred already at this stage of the alteration. Because
of the higher solubility of Ca-rich plagioclase in water (Blum and
Stillings, 1995), the prediction is that plagioclase would be dissolved
from the Ca-rich core progressively towards the rim by the infiltrating
fluid, and would form pores in the inner part of the grains. Indeed,
selective dissolution, where there is a Ca-rich core of plagioclase
grains, has been reported in granite (Nishimoto et al., 2008). Some
biotite is partly replaced by chlorite and corrensite along the cleavage
(Fig. 5f). Such alteration of the biotite may well be developed in the
outer zone.
4.4. Propylitic zone
Fig. 4. Plots of Fe/(Fe + Mg) ratio to Si for illite and tetrahedral Al in Fe-phyllosilicates.
0.7 and 0.8. The composition of the plagioclase is Ab80–91An7–20Or1–3
with a zonal structure. K-feldspar, showing a pale pinkish color, is fresh;
its composition is Or90–92. Calcite is absent. The granite appears
unaltered by naked eye but under the microscope both plagioclase
and biotite are partly altered. Plagioclase is often altered to illite in the
central part of the grains. Epidote is also observed in the plagioclase.
Biotite is partly altered to chlorite, epidote and titanite. The presence of
epidote and chlorite implies that the formation temperature is more
than 220 °C, based on comparison with geothermal fields (Henley and
Ellis, 1983; Goko, 2000; Mas et al., 2006). The Fe/(Fe+ Mg) ratio of
chlorite is similar to that of parental biotite. This suggests that the
chloritization took place as a solid-state transformation (Altaner and
Ylagan, 1997) preserving the Fe/(Fe+ Mg) ratio and crystallographical
orientation of the parental biotite. Chloritization is ubiquitous in the
Toki granite (Nishimoto et al., 2008), implying that incipient alteration
The propylitic zone is characterized by the formation of Fephyllosilicates such as chlorite, corrensite and smectite. The assemblage of these Fe-phyllosilicates corresponds to a propylitic alteration
widely reported from hydrothermal fronts in geothermal fields or ore
deposits (e.g. Inoue, 1995). The XRD pattern of powder extracted from
replaced biotite and plagioclase grains shows an expansion from
2.9 nm to 3.1 nm upon ethylene-glycol saturation, indicating corrensite (Fig. 7). Corrensite is a trioctahedral phyllosilicate of regular, 1:1
mixed-layer chlorite–smectite. The number of octahedral cations
ranges from 4.4 to 4.7, corresponding to 44–55% smectite (assuming
that there are 3 octahedral cations per O5(OH)2 for smectite and 6 per
O5(OH)8 for chlorite). This is consistent with the designation as
corrensite. The corrensite is rich in Fe and its Fe/(Fe + Mg) ratio
reaches 0.9 (Table 2). The ubiquity of fine, Fe-rich corrensite and
chlorite in the zone suggests that the greenish color is caused by these
minerals. Corrensite is often observed as amygdaloidal or colloform
aggregates of very fine (b 100 μm) green crystals under the optical
microscope (Fig. 5e, f). The form resembles that reported in other
granitic rocks (Meunier et al., 1988; Lindqvist and Harle, 1991;
Sugimori et al., 2008; Sandström et al., 2008). It occurs not only in
plagioclase and biotite but also in the pores in K-feldspar (Fig. 6d). Its
presence, without association with a particular mineral, implies direct
precipitation from fluid rather than replacement through an in situ
fluid–mineral reaction. Its additional occurrence in fracture filling and
dissolution pores as an amygdaloidal aggregate supports the idea that
it is directly precipitated from the hydrothermal fluid in dissolution
pores.
Biotite is almost completely replaced by corrensite, chlorite, illite,
and rutile. Small (b100 μm) amygdaloidal aggregates of corrensite
and chlorite occur between the cleavage surfaces, while biotite that
has been replaced by illite preserves the parental crystal orientation
(Fig. 5f), probably due to the similarity of structure between these two
minerals.
Plagioclase has a unique zoning (Fig. 5b,c) which probably formed
during the progress of the alteration. XRD analysis revealed the
presence of corrensite and an illite/smectite interstratified mineral
(Fig. 7). The alteration zoning indicates replacement of plagioclase by
illite in the central part of the grain, surrounded by corrensite and
smectite (Fig. 6b), although some grains lack the illite core. This can
also be identified by naked eye in some plagioclase grains as an ivory
core with a grayish green mantle. Illite is seen to form lines that cut
through the corrensite within the plagioclase (Fig. 5b). Thus, the
alteration zoning in the plagioclase grains suggests that precipitation
follows a sequence from corrensite through smectite to illite in the
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S. Nishimoto, H. Yoshida / Lithos 115 (2010) 153–162
Fig. 5. Photomicrographs showing hydrothermal alteration progress in plagioclase (a–c) and biotite (d–f) within the alteration halo. a) illitized plagioclase in the phyllic zone;
b) linear arrangement of illite suggesting illitization along a microcrack in altered plagioclase in the propylitic zone; c) plagioclase showing alteration zoning with corrensite,
smectite and illite in the propylitic zone; d) dissolved chlorite and illite altered from a biotite pseudomorph with a pore filled with quartz in the phyllic zone; e) illitized and partly
dissolved corrensite and chlorite aggregates altered from biotite in the propylitic zone (without analyzer); f) biotite with chlorite and corrensite in the outer zone (without analyzer).
dissolution pores. This is consistent with the bulk composition,
assuming that the LOI and Fe-rich components (Fig. 2) correspond to
the abundance of smectite and corrensite/chlorite, respectively. Thus
the alteration zoning indicates that illitization progresses as microfractures develop, even within plagioclase grains.
4.5. Phyllic zone
Significant dissolution of K-feldspar and precipitation of illite and
quartz are striking features of the phyllic zone. Microcracks are
common in this zone and are infilled by Fe-bearing illite and quartz
(Fig. 6c) indicating that the hydrothermal fluid infiltrated along this
pathway. K-feldspar shows a whitish color and ragged breakdown,
suggesting dissolution with the loss of K, Al and Si. Biotite is absent
and even the replacing chlorite and corrensite are dissolved, while the
dissolution pores are filled with quartz (Fig. 5d). Plagioclase is
strongly illitized (Fig. 5a; Fig. 6) and surrounded by a Na-rich (Ab95)
marginal zone (Fig. 6) probably due to residual Na. The illite
composition is similar to that in the fractures, and the grains are
generally larger than those of corrensite. Na and Ca zoning of
plagioclase grains is not seen. Dissolution pores like etch pits are
commonly observed in plagioclase near the fracture. Corrensite and
smectite have not been detected in the plagioclase. Chlorite and
corrensite replacing biotite are broken down and replaced by quartz,
consistent with a gradual decrease of Fe and LOI and an increase of
SiO2 in the phyllic zone (Fig. 2d). These facts indicate that dissolution
and outflow are significant near the fracture. K is transported to form
illite, Si is added to form quartz, and then Fe is released and
dissolution of previously formed hydrous silicates occurs. The
illitization front revealed by the distribution of K in the plagioclase
(Fig. 2c) is not sharp, indicating that it progresses gradually to reach
the propylitic zone.
5. Discussion
5.1. Composition and temperature of the hydrothermal fluid
The clay minerals that fill a fracture, and the minerals that are
secondarily formed in the vicinity of the fracture, allow us to infer the
temperature and composition of the hydrothermal fluid. Many studies
have shown that illite is stable at temperatures between about 200
and 350 °C (Velde, 1985; Izawa et al., 1990; Hedenquist et al., 1996).
Applying a chlorite geothermometer (Cathelineau, 1988), the temperature is estimated approximately 260 °C. Sass et al. (1987) showed
experimentally that illite and smectite can coexist stably with
microcline at more than 200 °C. The formation temperatures of
corrensite and smectite are estimated lower than 250 °C in the fossil
geothermal field (Inoue et al., 1991; Robinson and Santana, 1999) and
in active geothermal fields (Lindqvist and Harle, 1991; Goko, 2000;
Mas et al., 2006). It is reasonable to conclude that the hydrothermal
fluid injected into the fracture we have studied was at approximately
200–300 °C.
Kaolinite, which would indicate acidic conditions, is not observed.
Whereas, absence of calcite indicates that the fluid was not alkaline.
Therefore, it is reasonable to infer that the infiltrating hydrothermal
fluid was neutral or slightly acidic. Harder (1976) showed that the
formation of Fe-containing phyllosilicates is only possible under
reducing conditions by the synthesis of clay minerals. The presence of
pyrite in the propylitic zone also indicates that local conditions were
reducing during precipitation.
Preserved Na-rich marginal zone of the plagioclase suggests that
Na was not released to the hydrothermal fluid. The fluid inclusion in
quartz from the Toki granite is reported to be at a NaCl equivalent of
0.2–10.5% (Takagi et al., 2008). Fluid containing NaCl is common in
geothermal fields at deep sites (Hedenquist and Lowenstern, 1994;
Fujimoto et al., 2000). Savage et al. (1993) demonstrated that the
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159
Fig. 6. Representative EPMA compositional maps of Fe, Na and K showing the phyllic zone (a: Fe, b: Na, c: K) and the propylitic zone (d: Fe, e: Na, f: K). Symbols: ill = illite; cor =
corrensite; pl = plagioclase; Kf = K-feldspar; bi = biotite; chl = chlorite; ab = albite (plagioclase).
reaction of granite with diluted NaCl solution (0.008–0.028 M) at
200 °C produces sparsely developed illitic clay (as a secondary solid
product) and K-feldspar is much more easily attacked than plagioclase. These results are consistent with the conclusion that the fluid in
our site was diluted saline. The probable origin of the hydrothermal
fluid is circulating meteoric water, as reported in geothermal thermal
studies (e.g. Hedenquist and Lowenstern, 1994).
5.2. Progress of the hydrothermal alteration
Based on the paragenesis of each zone within the alteration halo,
a schematic model of alteration by the infiltration of hydrothermal
fluid into granite is proposed in Fig. 8. At the onset of the alteration,
plagioclase starts to dissolve, leaving pores and biotite undergoes
chloritization (outer zone). This has probably occurred under nearly
isochemical conditions at relatively low fluid/rock ratios. In the next
stage, biotite is dissolved and Fe phyllosilicates (chlorite, corrensite
and smectite) precipitated in the pores (propylitic zone). Since,
smectite is observed only within plagioclase in the propylitic zone,
the inference is that the precipitation of smectite is driven by the
local mineralogical environment. In the last stage, K-feldspar is
dissolved and illite and quartz precipitate as the Fe-phyllosilicates
break down. The result is that illite and quartz fill the microcracks.
Since K-feldspar and plagioclase break down to provide a local
source of Al, and the mobility of Al is high, it is reasonable to
conclude that Al is efficiently transported through the fluid in the
microcracks and pores. The conclusion is therefore that the
dissolution–precipitation process controls the hydrothermal alteration of granite.
The development of the microcrack network from the fracture
allows the hydrothermal fluid to infiltrate the wall rock, and the
consequent formation of pores by dissolution results in a higher
fluid/rock ratio. Previous work has shown that propylitic alteration
is common at the margins of alteration zones produced at low fluid/
rock ratios, and that the fluid/rock ratio of rocks in phyllic alteration
is higher than that in propylitic alteration (Lowell and Guilbert,
1970; Berger and Velde, 1992; John et al., 2008). Hence the
increasing fluid/rock ratio resulting from the development of
microcracks promotes the alteration. Most studies report that
corrensite is a stable mineral under hydrothermal conditions (e.g.
Kimbara, 1975; Inoue et al., 1991; Beaufort et al., 1997) and that it is
the product of a dissolution–recrystallization reaction (Meunier
et al., 1991). Schiffman and Staudigel (1995) considered that
corrensite occurs under high flux (fluid/rock) conditions. These
studies also support the hypothesis that the alteration sequence is
associated with an increase of the fluid/rock ratio. Because it is hard
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by the precipitation of Fe-pyllosilicates through the limited mobility
of elements in the stagnant fluid of dissolution pores at relatively low
fluid/rock ratios because of poor connectivity between the microcracks. Subsequent development of microcracks would result in the
precipitation of illite and quartz to form the phyllic zone.
5.3. Implications for hydrothermal alteration in orogenic belts
Our results have important implications for understanding the
hydrothermal alteration of granite and the stability of fractures in
orogenic belts. The presence of self-sealed fractures with unsolidified,
compacted clay formed by hydrothermal alteration indicates that
fractures can remain stable without deformation during cooling and
uplift of the granitic body. Studies on the subsurface underlying stock
of Quaternary granite (ca. 5 × 8 km) in the Kakkonda geothermal area
in northeast Japan provide evidence for the state of the granitic body
soon after its emplacement. Drilled samples from the hot zone
(N320 °C) at ca. 3000 m depth are fresh (Kanisawa et al., 1994; Sasaki
et al., 2003) and there is a brittle–ductile transition zone that
corresponds to this temperature (Matsushima and Okubo, 2003).
Thus, hydrothermal alteration related to fractures within granitic
bodies is likely to occur after cooling below 320–350 °C at depths
shallower than ca. 3000 m (Doi et al., 1998). It is also the case that
hydrothermal alteration accompanied by illite and quartz occurs at
around this temperature. Because the fission track age of the zircon
(ca.60 Ma; Sasao et al., 2006) indicates an age when the temperature
of the granite had dropped to ca. 200 °C, the fracture we studied has
stayed in a relatively stable condition for about 60 million years after
hydrothermal alteration at a relatively deep level and high temperature. Although fractures are implicated in the alteration of granite by
affecting its permeability, once a fracture is sealed with clay, the
process of alteration may be inhibited.
Similar hydrothermal alteration haloes along fractures filled with
clay (illite) have previously been reported in both Japan and Korea
(Kitagawa et al., 2001) but less often in continental settings. Usually,
quartz, calcite, laumontite, chlorite, epidote and prehnite are common
as fracture-filling minerals (Yoshida et al., 2000, 2008; Sandström
et al., 2008). Yoshida et al. (2005) pointed out that the density of
fractures of granitic rock in an environment of orogenic stress is
higher by almost one order of magnitude than that in the granitic rock
of stable continental regions. Although further studies are required to
confirm the origin of clay-filled fractures in granite, we believe that
hydrothermal alteration through clay-filled fractures in granitic rock
is associated with this specific tectonic setting.
6. Conclusion
Fig. 7. XRD patterns of replacement products of plagioclase (a) and biotite (b) from the
phyllic zone, propylitic zone and outer zone within the hydrothermal alteration halo
along fracture from the Toki granite, Japan. EG: after ethylene-glycol treatment;
Symbols: ill = illite; chl = chlorite; sme = smectite; cor = corrensite; bi = biotite.
to conceive of a thermal gradient within such a limited space, the
various zones of the alteration halo probably simply reflect different
fluid/rock ratios.
The unique alteration zoning of plagioclase observed in the
propylitic zone is evidence of the progress of the alteration process.
Murakami et al. (2004) showed in a dissolution experiment under
reducing conditions that Fe-rich smectite or vermiculite is precipitated at the edge of biotite. They also demonstrated that, after biotite
dissolution, the concentration of Fe in the fluid increases by more than
one order of magnitude and that the Fe/Mg ratio is higher under
reducing conditions. So it is possible that the propylitic zone is formed
An alteration halo along a fracture filled with compacted clay at
depth in Toki granite has been investigated. Petrographic and
geochemical evidence suggests that the halo was formed by a
hydrothermal, reducing, neutral to slightly acidic fluid. Within the
alteration halo, reaction progress formed three zones: the outer,
propylitic and phyllic zones. Once the hydrothermal fluid is injected
into a fracture, it infiltrates the host granite through developing
microcracks. At the onset of the alteration plagioclase is dissolved,
thus forming pores inside the grains and facilitating the chloritization
of biotite. In the next stage, biotite is dissolved and Fe-phyllosilicates
(chlorite, corrensite and smectite) are precipitated and fill the pores.
In the last stage, K-feldspar and the Fe-phyllosilicates dissolve and
illite precipitates, causing the development of microcracks. Hydrothermal alteration is controlled by an increasing fluid/rock ratio which
is due to an increasing connectivity between microcracks and a
process of dissolution–precipitation which causes locally high
mobility of Al during uplift of the host rock. There is, however,
evidence that fractures self-sealed by the clay remain stable in the
granitic body for long periods of time. These results contribute to an
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S. Nishimoto, H. Yoshida / Lithos 115 (2010) 153–162
161
Fig. 8. Schematic diagram illustrating the hydrothermal alteration process of granite along a fracture.
understanding of how fractured granitic rock can be altered and the
way it behaves in the long term in orogenic fields such as the Japanese
island arc.
Acknowledgements
We gratefully acknowledge Drs. K. Amano and T. Tsuruta of the
Japan Atomic Energy Agency for their assistance and for discussion
during this study. We would like to thank Dr. N. Katsuta of Gifu
University and Mr. S. Yogo of Nagoya University for technical
assistance. Professors M. Adachi, M. Takeuchi, K. Yamamoto and K.
Tsukada of Nagoya University for valuable discussion. Dr. G. Clarke of
The Natural History Museum, London is also acknowledged for his
assistance during the preparation of this paper. In addition, I wish to
thank to two anonymous reviewers for valuable comments to
improve the manuscript.
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