Hidrothermal Alteration Study of Tangkuban Perahu

Proceedings Indonesia International Geothermal Convention & Exhibition 2015 Jakarta Convention Center, Indonesia – August August 19th – 21 21st, 2015

Hidrothermal Hidrothermal Alteration Study of Tangkuban Perahu Craters, and Its Implication to Geothermal Geothermal Conceptual Model Afy Syahidan ACHMAD 1, SURYANTINI 1

1,2

, Arif SUSANTO 1, Astin NURDIANA 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 [email protected] , [email protected] [email protected],, [email protected] , [email protected]

Keywords: alteration, advanced argillic , Tangkuban Perahu, XRD, sulphur, crystobalite, tridimite, alunite, pyrite

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).

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 XRay 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 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 permeability in subsurface subsurface which can be emerged 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.

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 S edimentary 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 basalticandesitic 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 northeastsouthwest orientation. It is featured with river valley lineaments, abrupt slope lineament, and manifestations spreading (Nasution et.al., 2004).

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.

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.

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 km 2. Geothermal field development of this area were divided into 3 exploration priority areas 1

Proceedings Indonesia International Geothermal Convention & Exhibition 2015 Jakarta Convention Center, Indonesia – August 19th – 21st, 2015

Figure 1. Tangkuban Perahu Map (Modified Saputra and Suryantini, 2015) 4. Results 4.a. Manifestations

4.a.2. Domas Crater and Jarian Crater

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.

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).

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).

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.

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 sulphursmelling. Rocks from surrounding wall altered to white in color properties. 2


Figure 2. Tangkuban Perahu Manifestation and Alteration Map; and Photo

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Proceedings Indonesia International Geothermal Convention & Exhibition 2015 Jakarta Convention Center, Indonesia – August 19th – 21st, 2015 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.

Figure 4. Photo and Petrography analysis from KWU-01

Fi ure 3. XRD anal sis result

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.

Figure 5. Photo and Petrography analysis from KWR-11

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.

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

This alteration zone was formed at the temperature ranging between 20 o – 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 20 o C. Last boundary from temperature plotting is constrained by tridimite at 50o C.

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Proceedings Indonesia International Geothermal Convention & Exhibition 2015 Jakarta Convention Center, Indonesia – August 19th – 21st, 2015 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. Figure 7. Photo and Petrography analysis from KWD-20

determined as the crystobalite at 80o C.

presence

of

secondary

mineral,

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). Figure 6. Photo and Petrography analysis from JLD-11

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).

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.

5. Conclusions Kesimpulan

According to temperature plotting table, it can be considered that the last alteration zone was formed at the temperature ranging between 20 o – 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



5

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 8. Tangkuban Perahu Conceptual Model



Ibrahim, H.D., Artono, A.R.T., Triyono, S., Exploration Dirlling on the TPGP Tangkuban Parahu Concession, West Java, Indonesia, Proceedings World Geothermal Congress, Melbourne, Australia (2015).

Hydrothermal alteration zone of the research area are divided into 2 type according to mineral assemblages: o

o

Sulphur+crystobalite+tridimite±pyrite alteration zone, alteration temperature ranging between 20o50o C

Morrison, K., Important Hydrothermal Minerals and Their Significance, Geothermal and Mineral Services Division, Kingston Morrison Limited (1997).

Crystobalite+alunite alteration zone, alteration temperature ranging between 0o-80o C.

Nasution, A., Kartadinata, M. N., Kobayashi, T., Siregar, D., Sutaningsih, E., Hadisantono, R., Kadarstia, E.: Geology, Age Dating and Geochemistry of the Tangkuban Perahu Geothermal Area, West Java, Indonesia, J Geotherm. Res. Soc. Japan, Vol. 26, No. 3 (2004), 285-303.

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 westeast orientation, agree with manifestation and alteration area distribution.

Nicholson, K.: Geothermal Fluid - Chemistry and Exploration techniques, Springer-Verlag Berlin Heidelberg, Berlin, ISBN 3-540-56017-3 (1993). Saputra, M.P., Suryantini, A Correlation Study between Volcanic Activities and Thermal Water Changes in Tangkuban Perahu Hydrothermal Prospect, Jawa Barat, Indonesia, Proceedings World Geothermal Congress, Melbourne, Australia (2015).

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’.

Situmorang, J., Karakteristik Tanah Beruap di Kawah Domas Tangkuban Perahu, Tesis Magister Teknik Panas Bumi, Fakultas Teknik Pertambangan dan Perminyakan, Institut Teknologi Bandung (2012).

References

Van Bemmellen, R. W.: The Geology of Indonesia. 2 edition Martinus Nijhoff, Vol. 1 (1949).

Achmad, A.S., Geologi dan Studi Alterasi Hidrotermal Gunungapi Tangkuban Perahu, Jawa Barat, Tugas Akhir Program Sarjana Teknik Geologi, Fakultas Ilmu dan Teknik Kebumian, Institut Teknologi Bandung (2015). Hendri, R., Prabowo, B.S., Perdana, M., Korelasi Intensitas Pola Kelurusan Tangkuban Perahu terhadap Keberadaan Lokasi Manifestasi di Permukaan, Proceedings the 12 th Annual Indonesian Geothermal Association Meeting & Conference, Bandung (2012). 6

Hidrothermal Alteration Study of Tangkuban Perahu

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