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Proceedings Indonesia International Geothermal Convention & Exhibition 2015
Jakarta Convention Center, Indonesia – August 19th – 21st, 2015
1
Hidrothermal Alteration Study of Tangkuban Perahu Craters, and Its Implication to
Geothermal Conceptual Model
Afy Syahidan ACHMAD 1, SURYANTINI
1,2, Arif SUSANTO
1, Astin NURDIANA
1
1 Geology Engineering Study Program, Faculty of Earth Science and Engineering, Institut Teknologi Bandung
2 Geothermal Engineering Study Program, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung
afy.syahidan@gmail.com, suryantini@gc.itb.ac.id, arifsgea@gmail.com, astin.nurdiana@gmail.com
Keywords: alteration, advanced argillic, Tangkuban Perahu,
XRD, sulphur, crystobalite, tridimite, alunite, pyrite
ABSTRACT
Tangkuban Perahu is located in West Java, Indonesia. It is
active stratovolcano type which is still showing
hidrothermal activity. The main purpose of this study is to
determine the correlation between subsurface structure, and
the hidrothermal activity on the surface. Using topographic
map, SRTM images, and field observation, geological
condition and alteration area was mapped. Alteration
samples were analyzed trough petrographic analysis and X-
Ray Diffraction (XRD). Altered rocks in study area showing
white-yellowish white colour and texture changing variation
especially in hardness properties from softening to
hardening because of sillica and sulphur alteration.
According to mineral assemblages, alteration zone are
devided into two types, sulphur+crystobalite+tridimite±
pyrite zone and crystobalite+alunite zone. Whole mineral
assemblages generally indicate advanced argillic alteration
type with west-east alteration area orientation. Alteration
area distribution correlates with manifestations appearance
such as steam vents, fumarole, solfatar, steaming ground,
and warm to boiling pools. Most of the manifestations
appear in main crater; Ratu Crater and Upas Crater, and
parasitic craters; Domas Crater, Jurig Crater, Baru Crater,
and Jarian Crater. These manifestations indicate
permeability in subsurface which can be emerged trough
structural process from same orientation. Implication for
conceptual model is possibility that upflow distribution area
follow the west-east orientation trough geology structure
such as fault or joint created by magma vents. For further
study, geophysical method such as gravity and resistivity
can be required to find permeability zone pattern in
Tangkuban Perahu subsurface.
1. Introduction
Tangkuban Perahu is an active stratovolcano which is
located in West Java, Indonesia. Location is approximately
30 km northwards from Bandung City. Tangkuban Perahu
Volcano has high-entalphy, liquid-dominated geothermal
system (Ibrahim, et.al., 2015). This volcano has two main
craters in its summit that are Upas Crater and Ratu Crater.
Beside the main craters, there are one big parasitic crater;
Domas Crater, and three smaller parasitic craters; Jurig
Crater, Baru Crater and Jarian Crater.
Reconaissance study about Tangkuban Perahu Volcano
were carried out by Pertamina at 1970. During 2010-2011,
the Energy and Geoscience Institute “EGI”-USA, working
under a USTDA, awarded PT. Indonesia Power to took fluid
geochemical sampling, gas survey, and Magneto-Telluric
(MT) within range of 80 km2. Geothermal field development
of this area were divided into 3 exploration priority areas
that are P1, P2, and P3 (Ibrahim, et.al., 2015). This study
were carried out to support field data development,
particularly P3 area which is in the western part of the
volcano. Overall aim of this study is to support volcano
conceptual model development, and to determine the
suitable point for drilling exploration in P3 area (Figure 1).
2. Geological Setting
Tangkuban Perahu Volcano is a part of Sunda Volcanic
Arc. It was formed as a result of Indo-Australia plate and
Eurasia plate subduction that generated volcanic arcs
troughout Sumatra and Java. It is one of three great andesitic
Sunda Volcano body which caldera remnant can be observed
around Tangkuban Perahu Volcano. The present magmatic
activities are freatic – freatomagmatic eruption, whereas the
other two volcano bodies, Sunda and Burangrang, are not
showing any volcanic activities anymore (Nasution, et.al.,
2004).
Generally, Nasution, et.al., (2004) divided the
stratigraphy of Tangkuban Perahu Volcano into five major
units; Tertiary Sedimentary Basement, Pre-Sunda Volcanics,
Sunda Volcanics, Old Tangkuban Perahu Volcanics, and
Young Tangkuban Perahu Volcanics. Each unit had been
dated using 14C and arranged from older to younger.
Lithology of the research area generally consist of basaltic-
andesitic lava, tephra, and various grain size pyroclastics.
Two major linements can be observed in research area
and its surrounding (Figure 1). Approximately 22 km length
scarp slope were observed in the southern part of Tangkuban
Perahu Volcano with west-east orientation. It extending
from Gunung Palasari at the east to Cisarua at the west. It is
identified as Lembang Fault that formation related to
Pleistocene Sunda Volcano activities (van Bemmelen, 1949,
op.cit., Hendri, et.al., 2012). Other major fault was found in
the northeastern part of research area, specified in Ciater so
called Ciater Fault which was normal fault with northeast-
southwest orientation. It is featured with river valley
lineaments, abrupt slope lineament, and manifestations
spreading (Nasution et.al., 2004).
3. Methods
Research methods are began with literature study about
Tangkuban Perahu Volcano using prior publications, SRTM
images, and topographic maps. Furthermore, manifestations
and alteration zones in its volcanic area were mapped.
Mapping area consist of three big craters, those are Ratu
Crater, Upas Crater, and Domas Crater with some small
craters within. Some samples were taken and used for
laboratory analysis comprise of petrography and XRD
analysis. Analysis results were used to consider the
alteration zones as supporting data for mapping results.
Proceedings Indonesia International Geothermal Convention & Exhibition 2015
Jakarta Convention Center, Indonesia – August 19th – 21st, 2015
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4. Results
4.a. Manifestations
Manifestations that appeared in research area consist of
steam vents, fumarole, solfatar, steaming ground, warm to
boiling pools, and mud pools. Manifestations commonly
appear inside craters area despite some are found in the
outside (Figure 2). Generally, manifestation zones
distribution has west-east orientation. Rocks that surround
manifestation points are altered shown by changing of rocks
properties.
4.a.1. Ratu Crater, Upas Crater, and Baru Crater
Some manifestations are emerged as fumarole, solfatar,
and steaming ground in Ratu Crater and Upas Crater. Steam
and tephra colour changes into yellowish can be observed at
fumarole in Ratu Crater (Figure 2a). The fumarole
temperature ranging from 90 o – 100 o C (Saputra and
Suryantini, 2015). Some rocks colour were altered to white
because of steaming ground activity at southwestern of Upas
Crater (Figure 2b). There is fumarole manifestation remnant
that had been stopped steaming at the border area of Ratu
Crater and Upas Crater. There is also yellowish solfatar near
the border with non-altered in hardnes properties (Figure
2c). Active steaming ground was also appear at the border in
certain times and it was altered surrounding rocks into
blackish. This steaming ground emerge in the abrupt wall
about 6 m high that interpreted as fault between Ratu Crater
and Upas Crater (Figure 2d).
Steaming ground also appear at Baru Crater, parasitic
crater in the western wall of Upas Crate (Figure 2e).
Vegetation is absent, the steam is observable and sulphur-
smelling. Rocks from surrounding wall altered to white in
color properties.
4.a.2. Domas Crater and Jarian Crater
Fumarole, solfatar, steaming ground, warm to boiling
pools, and mud pools were found in the Domas Crater zone.
The highest manifestations intensity are emerged at the
western part of Domas Crater. Fumarole appear with solfatar
deposit around steam outgassing vent output. It has
temperature ranging between 70 o – 100 o C (Saputra and
Suryantini, 2015). The outgassing hole diameter is about 15
cm (Figure 2f). Rocks in the surrounding altered to
yellowish because of solfatara appearence. Steaming ground
emerge resemble a small hill that appear at the each end part
of minor fault in the crater (Figure 2g). Warm to boiling
pools and mud pools are emerged at the southwest area of
Domas Crater contiguously with diameter about 2,5 m
(Figure 2h). Boiling pools are in the temperature ranging
between 88o-92o C, whereas warm pools ranging between
54,8o - 68,1o C (Situmorang, 2012).
Jarian Crater is parasitic crater in the southeastern part of
Domas Crater. Fumarole, solfatar, and warm boiling pool
were found. Small warm pool outlet has diameter about 5
cm, and form small stream to eastern part of Jarian Crater
(Figure 2i). Warm boiling pools diameter are approximately
2 m.
4.a.3. Outside Area of Crater
Steaming ground emerged in southwestern part of Upas
Crater, particularly at post of the security and defense affair
department outside the crater. It is steaming but not showing
sulphur-smell, and still contain some vegetation at the
surface.
Figure 1. Tangkuban Perahu Map (Modified Saputra and Suryantini, 2015)
Proceedings Indonesia International Geothermal Convention & Exhibition 2015
Jakarta Convention Center, Indonesia – August 19th – 21st, 2015
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Figure 2. Tangkuban Perahu Manifestation and Alteration Map; and Photo
Proceedings Indonesia International Geothermal Convention & Exhibition 2015
Jakarta Convention Center, Indonesia – August 19th – 21st, 2015
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4.b. Alteration Zone
Alteration zone in the research are devided into two
types, sulphur+crystobalite+tridimite±pyrite zone and
crystobalite+alunite zone (Figure 2). Both of that zones are
distinguished according to its mineral assemblages. Whole
mineral assemblage indicate advanced argillic type common
alteration that featured by mineral assemblages as alunite,
crystobalite, and tridimite. That mineral assemblages are
obtained through field mapping, petrography analysis, and
XRD analysis comprehensively.
4.b.1. Sulphur+Crystobalite+Tridimite±Pyrite Alteration
Zone
This alteration zone are featured by secondary mineral
assemblages comprise of sulphur+crystobalite+tridimite±
pyrite. Sulphur, crystobalite, and tridimite are obtained
through XRD analysis (Figure 3), whereas pyrite is apparent
in the outcrop. Physical appearance of alteration rocks are
pale grey – yellowish grey, increasing of hardness,
increasing of the rock compactness, showing vuggy quartz
texture, and strong altered in the nearby manifestations area.
Alteration appeared in the Domas Crater area, ridge between
Ratu Crater and Upas Crater; Ratu Crater, Upas Crater, and
Baru Crater. This alteration zone spreading form relatively
west-east orientation which are interpreted as
manifestation’s activities effect.
The outcrops of andesite are altered and showing various
hardness changing from harder to softer in the northern part
of ridge between Upas Crater and Ratu Crater, KWU-01
observation point (Figure 4). Some part of the rocks are
showing yellowish grey, increasing of the hardness,
increasing of the rock compactness, and abit sulphur deposit.
The thin section indicate andesite with hipocrystalline
texture, strong altered, fenocryst (3%) consist of pyroxene,
plagioclase, 0,01 – 0,5 mm in size, entire plagioclase and
some pyroxene have been leached, groundmass (50%)
consist of volcanic glass, secondary mineral (17%) that
intensively leached (30%) and comprise of crystobalite,
anatase, and sulphur. Some volcanic glass are leached until
mineral trace on the parallel nicole is difficult to observed.
In the north part of ridge between Upas Crater and Ratu
Crater, KWR-11 observation point, there is volcanic breccia
that intensively altered. Breccia matrix comprise of fine –
coarse ash grain that color was changed become yellow,
whereas fragments were changed into grey. The rock
hardness and compactness are less (Figure 5). Volcanic
breccia, poorly sorting, open fabric, grains (30%) consist of
altered lithic fragment, 1 mm – 8 cm in size, angular –
subangular, lithic fragments are andesite and tuff, matrix
consist of volcanic glass, secondary minerals (70%) are
sulphur that change volcanic glass matrix, rocks are
intensively sulphur altered. Crystobalite is identified based
on XRD analysis result. From the intensive presence of
sulphur and altered grain by crystobalite, it can be
determined that the rocks are altered by high sulphur content
fluid and the grains previously had been altered by high
sillica content fluids.
This alteration zone was formed at the temperature
ranging between 20o – 50o C based on temperature plotting
table. Temperature range for each mineral are obtained
through reference from Morrison (1997). Alteration
temperature initial boundary determination is synchronized
with research area surface temperature, about 20o C. Last
boundary from temperature plotting is constrained by
tridimite at 50o C.
Figure 3. XRD analysis result
Figure 4. Photo and Petrography analysis
from KWU-01
Figure 5. Photo and Petrography analysis
from KWR-11
Proceedings Indonesia International Geothermal Convention & Exhibition 2015
Jakarta Convention Center, Indonesia – August 19th – 21st, 2015
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4.b.2. Crystobalite+Alunite Alteration Zone
This alteration zone are featured by the presence of
crystobalite and alunite. Altered rocks physical appearance
are white – grey, rock hardness variation from harder –
softer, increasing of the compactness, and showing vuggy
quartz texture. This alteration type appear in Domas Crater
area according to lithological description and field
observation.
Altered tephra color is grey – pale grey, rock hardness
and compactness are less observation point is JLD-11. There
are also grains that have texture resemble as clay minerals
(Figure 6). That lithological petrography analysis reveal
lithic ash tephra characteristics, clastic texture, well sorting,
close fabric, grains (7%) consist of pyroxene, plagioclase,
lithic fragment, crystal, 0,1 – 1,7 mm in size, angular –
subangular, almost entire grains were leached (77%).
Secondary minerals (16%) comprise of quartz, crystobalite,
anatase, alunite, and opaque minerals.
KWD-20 observation point is andesite outcrop that were
changed into white, the hardness and compactness are
unchanged (Figure 7). This alteration characteristic
differences are caused differences of lithology type that
altered. Andesite thin sections are featured by
hipocrystalline, hyalophyllitic, plagioclase and pyroxene
mineral traces, and trakhitic texture trace. Fenocryst
comprise of pyroxene, plagioclase, 0,04 – 1,4 mm in size,
were leached entirely, groundmass are entirely leached
plagioclase. Secondary minerals (15%) consist of quartz,
anatase, alunite, crystobalite, opaque minerals, that most of
were leached (85%). Predominantly leached fragments and
groundmass reveal that rocks were altered by high sillica
content fluid.
According to temperature plotting table, it can be
considered that the last alteration zone was formed at the
temperature ranging between 20o – 80o C (Figure 9).
Temperature range for each mineral are obtained through
reference from Morrison (1997). Alteration temperature
initial boundary is determined as research area room
temperature, and alteration temperature last boundary is
determined as the presence of secondary mineral,
crystobalite at 80o C.
4.b. Implication to Conceptual Model
Fumarole commonly emerged in upflow surface area
trough available permeability (Nicholson, 1993). It indicate
Ratu Crater, Upas Crater, and Domas Crater still included in
upflow zone because they have fumarole manifestation
inside. Crater, fumarole, and alteration zone which directed
to west – east indicate the presence of geological feature that
serve permeability with same orientation. The implication to
present conceptual model is upflow zone distribution that
may have same orientation with manifestation area
distribution (Figure 8).
The occurring of general alteration, advance argillic, can
support the formation of cap rock that have significance role
within geothermal system (Nicholson, 1993). Surface data
that show the absence of surface manifestation in the
western part of Tangkuban Perahu Volcano and fumarole
manifestations emergence in Baru Crater indicate that the
area likely have the heated fluid, yet cap rock layer have
been developed enough so steam could not reach the surface
in such like in the Domas Crater area, in the eastern side of
volcano. Warmer fluid which circulate subsurface may
located near the volcano because usually colder recharge
supply flew form caldera wall in high-entalphy system with
caldera (IGA, 2014).
5. Conclusions
Kesimpulan
Active surface manifestations in the research area
comprise of fumarole, solfatar, steaming ground, warm
to boiling pools, dan mud pools. That manifestations can
be found in the Upas Crater, Baru Crater, Domas Crater,
and Jarian Crater.
Figure 6. Photo and Petrography analysis
from JLD-11
Figure 7. Photo and Petrography analysis
from KWD-20
Proceedings Indonesia International Geothermal Convention & Exhibition 2015
Jakarta Convention Center, Indonesia – August 19th – 21st, 2015
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Hydrothermal alteration zone of the research area are
divided into 2 type according to mineral assemblages:
o Sulphur+crystobalite+tridimite±pyrite alteration
zone, alteration temperature ranging between 20o-
50o C
o Crystobalite+alunite alteration zone, alteration
temperature ranging between 0o-80o C.
Generally, hydrothermal alteration in the research area is
advance argillic that occur at low temperature and acid
pH.
There is geological feature subsurface which serve
permeability in Tangkuban Perahu Volcano with west-
east orientation, agree with manifestation and alteration
area distribution.
Acknowledgements
The first author would like to thank Institut Teknologi
Bandung, and Ms. Suryantini for the final project
scholarship support. This study also supported by ‘Program
Riset Desentralisasi Dikti 2014’.
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Figure 8. Tangkuban Perahu Conceptual Model