Mineralogical Characteristics of Hydrothermally-Altered Andesite in Kalirejo Village and the Surrounding Areas, Indonesia

Journal of Applied Geology, vol. 3(2), 2018, pp. 99–108 DOI: http://dx.doi.org/10.22146/jag.48598

Mineralogical Characteristics of Hydrothermally-altered Andesite in Kalirejo Village and The Surrounding Areas, Indonesia

Diyan Aditya Putra Pratama, I Gde Budi Indrawan,* and I Wayan Warmada

Department of Geological Engineering, Faculty of Engineering, Gadjah Mada University, Yogyakarta, Indonesia

ABSTRACT. Type and intensity of hydrothermal alterations affect rock engineering properties and slope stability. Identiﬁcation of mineralogical characteristics of rocks is essen- tial in determination of rock slope failure mechanism in a hydrothermal alteration zone. This research was conducted to identify mineralogical characteristics of hydrothermally- altered andesite in Kalirejo Village and surrounding areas, Indonesia. The research was conducted by ﬁeld observation and laboratory analyses involving petrographic and X-ray Powder Diffraction (XRD) analyses. The results showed that the research area was dominated by argillic alteration type and high alteration intensity implying high susceptibility to slope failures. Keywords: Hydrotermal alteration · Mineralogical characteristics · Andesite · Kalirejo Vil- lage · Indonesia.

1I NTRODUCTION more detailed zonation of areas susceptible to The research area was located in Kalirejo Vil- slope failures. lage and the surroundings. It was adminis- Stability of rock slopes in tectonically active tratively located in Kulon Progo Regency (Yo- and tropical regions is controlled by several fac- gyakarta Special Province) and Purworejo Re- tors, such as slope geometry, material proper- gency (Central Java Province), Indonesia. The ties, groundwater, surface cover, and active fac- regional geological map produced by Rahardjo tors involving earthquake and rainfall. Previ- et al. (1995) indicated that the research area ous publications (e.g., Bell, 2007; Pola et al., was composed of intrusive igneous rock group 2012) have shown that hydrothermal alteration of andesite. In addition to weathering, the an- may change engineering properties of rocks, in- desite near the research area had also been un- ducing slope failures. As the intrusive igneous dergone hydrothermal alteration (Ansori and rocks in the research area had undergone hy- Hastra, 2013). The 1:100.000 scale map pro- drothermal alteration, the change in their en- duced by Center of Volcanology and Geolog- gineering properties due to hydrothermal alter- ical Hazard Mitigation (PVMBG, 2010) indi- ation could partly be responsible for a number cated that the research area had a medium to of slope failures. high landslide susceptibility level. Although re- This paper presents preliminary results of a search on landslide susceptibility has been con- study conducted to investigate factors control- ducted on a regional scale, mechanism of slope ling slope failures in the research area. The ge- failures in the research area was not well under- ology and mineralogical characteristics of the stood and required further studies. Better un- hydrothermally-altered andesite are described derstanding of slope failure mechanism allows and hydrothermal alteration type and intensity of the intrusive igneous rocks in the research area are highlighted. *Corresponding author: I G.B. INDRAWAN, Depart- ment of Geological Engineering, Gadjah Mada Univer- sity. Jl. Graﬁka 2 Yogyakarta, Indonesia. E-mail: [email protected]

2G EOLOGIC SETTING Table 1: Classification of hydrothermal alteration intensity (Gillis et al., 2014). According to Rahardjo et al. (1995), Kulon Progo Mountains regional stratigraphy from Alteration Secondary mineral to old to young is composed of Nanggulan For- Intensity primary mineral (%) mation, Old Andesite Formation, Jonggrangan Formation, Sentolo Formation, and Alluvial Fresh rock <2 Deposits. In addition there are intrusive rock Weak 2 – 9 groups in the form of andesite, diorite and Moderate 10 – 49 dacite intrusive rocks (Figure 1). The research area belongs to the Andesite Intrusion Rock High 50 – 95 Group. This group of rocks has a composition Intensive >90 ranges from hypersthene andesite to hornblende augite andesite and trachyandesite. 4R ESULTS AND DISCUSSION Judging from the Regional Geological Struc- ture, according to Van Bemmelen (1949), the 4.1 Lithology Kulon Progo Mountains region has a struc- The geological map shown in Figure 2 shows ture characterized by the existence of ancient that the research area consisted of andesite. The volcanic complexes that have undergone sev- rock type observed in this research was con- eral tectonic phases. As a result, geologi- sistent with that in the Rahardjo et al. (1995). cal structures formed in the form of normal The andesite could be further divided into three faults with dominant shear components and rock units, namely hornblende andesite intru- upward faults (Rahardjo et al., 1995). The struc- sion 1, hornblende andesite intrusion 2, and tures formed relatively have an orientation hornblende andesite intrusion 3. In general, northeast-southwest and west-east (Figure 1). the rock units had similar characteristics and were dominated by secondary mineral group 3M ETHODOLOGY of hornblende (Figure 3). The outcrops showed A 1:25,000-scaled field mapping, which in- that the rock units had grey color, weak to high volved observation of lithology, geological alteration conditions, porphyro aphanitic tex- structure, and hydrothermal alteration, and ture with <1–3 mm crystal size, and jointed laboratory analyses, which included petro- structures. In thin sections, the rock units had graphic and X-Ray Diffraction (XRD) analyses, a porphyritic texture and were composed of were conducted in this research. Genetic clas- phenocrysts of pyroxene, hornblende, plagio- sification of the intrusive igneous rocks in the clase (labradorite), and opaque minerals and research area was carried out following the the groundmasses of plagioclase and clay min- IUGS classification proposed by Streckeisen erals. The rock units were distinguished each (1978, in Le Maitre, 2002). Hydrothermal alter- other by the weathering degree and relative ation type of the intrusive igneous rocks was age. The unit of hornblende andesite intrusion determined by natural outcrop observation 1 had a slightly to moderately weathering de- and detailed petrographic and XRD analyses. gree, the unit of hornblende andesite intrusion Meanwhile, hydrothermal alteration intensity 2 had a moderately weathering degree, and the of the intrusive igneous rocks was determined unit of hornblende andesite intrusion 3 had a by comparing the amount of secondary min- highly to completely weathering degree. erals to the primary minerals observed in the The geological structures observed in the petrographic analyses of thin sections, follow- research area were shear and tension joints ing the procedure described in Byers (1990). and sinistral strike-slip and normal faults (Fig- The percentages of the hydrothermal alteration ure 4). Generally, shear joints has an orientation intensity were then classified following classifi- direction northeast-southwest and northwest- cation proposed by Gillis et al. (2014) (Table 1). southeast, while the tension joints generally has a north-south orientation direction. In the tension joints found a filling structure in the form of calcite and quartz veins. This joints structure

100 Journal of Applied Geology MINERALOGICAL CHARACTERISTICSOF HYDROTHERMALLY-ALTERED ANDESITEIN KALIREJO VILLAGE

FigureFigure 1: (A) 1. (A) Regional Regional geological geological map mapof the ofKulon the Progo Kulon Mountains. Progo (B) Mountains. Stratigraphic column (B) Stratigraphic of the Kulon Progo column Mountains area (Rahardjo et al., 1995 with modifications). The research area is marked with a black box that belongs of theto Kulon the Andesite Progo Intrusiv Mountainse Rock Group. area (Rahardjo et al., 1995 with modifications). The research area is marked with a black box that belongs to the Andesite Intrusive Rock Group. acts as a pathway for hydrothermal fluid to get type of alteration was characterized by the out to the surface which eventually forms the presence of smectite, kaolinite, quartz, illite vein structure. Analyses of fault data (Table 2) and illite/montmorillonite mixed layer min- indicated that the sinistral strike-slip faults in erals. According to Pirajno (1992), this type the research area were estimated to be devel- of hydrothermal alteration generally has a oped by two main stress, which were north- low forming temperature (<200–250°C) and east east-southwest west and southeast east- forms near the ground surface. In the field, the northwest west directions (Figure 5). hydrothermally-altered andesite having argillic alteration appeared to be relatively soft and the 4.2 Hydrothermal alteration rock structures were easily destroyed by fin- 4.2.1 Types of hydrothermal alteration gers. In thin sections, they were characterized Determination of hydrothermal alteration type by a high content of clay minerals (Figure 7). is done by direct outcrop mapping method in Propylitic alteration the field and supported by petrographic analysis. Based on the composition of the mineral The XRD analysis results showed that the composition in general, alteration types in the propylitic alteration was characterized by study area can be grouped into two, namely: the presence of chlorite, epidote, and calcite argillic alteration types and propylitic alteration minerals. This type of hydrothermal alter- types. The map of hydrothermal alteration ation has a relatively higher formation tem- types is shown in Figure 6. perature as compared to the argillic alteration (about 250°C) and generally forms at a Argillic alteration deeper location (Pirajno, 1992). In the field, The argillic alteration was more dominant and the hydrothermally-altered andesite having developed in almost 70 % of the research area. propylitic alteration appeared to be harder than The XRD analysis results showed that this those having argillic alteration. In some places,

Journal of Applied Geology 101 PRATAMA et al.

Figure 2. Geological map and Figurecross sections 2: Geological map and cross sections.

102 Journal of Applied Geology

MINERALOGICAL CHARACTERISTICSOF HYDROTHERMALLY-ALTERED ANDESITEIN KALIREJO VILLAGE

Figure 3: PhotographsFigure 3: Photographs of the andesite of the hornblende andesite hornblende intrusion unit. intrusion (A) Natural unit. (A) outcrop Natural. (B,C) outcrop. Thin (B,C) sections Thin (XPL). Hb: hornblende;sections Cly: clay; (XPL). Opq: Hb: opaque hornblende; mineral; Cly: Px: clay; pyroxene; Opq: opaque Pl: plagioclase; mineral; Px: Chl: pyroxene; Chlorite; Pl: plagioclase;Qz: quartz. Chl: Chlorite; Qz: quartz.

Table 2: Fault measurement data.

Geological Structure Strike/Dip (N...◦E/...◦) Striation (◦) Kali Plamping sinistral strike slip fault 150/72 30 NE Tepus sinistral strike slip fault 245/78 30 NE Duren Sari sinistral strike slip fault 75/85 40 W

Journal of Applied Geology 103 PRATAMA et al.

Figure 4: PhotographsFigure 4: Photographs of geological of geologicalstructures structuresin the research in the area. research (A) area. Sinistral (A) Sinistral strike slip strike fault. slip (B) fault. Normal (B) fault. (C) Tension jointsNormal with north fault.- (C)south Tension orientation. joints with (D) north-southShear joints orientation. with orientation (D) Shear of northeast joints with-southwest orientation and of northwest- southeast. northeast-southwest and northwest-southeast.

Figure 5: Stress analyses of Figurethe sinistral 5: Stress strike analyses-slip faults of the. sinistral strike-slip faults.

104 Journal of Applied Geology

MINERALOGICAL CHARACTERISTICSOF HYDROTHERMALLY-ALTERED ANDESITEIN KALIREJO VILLAGE

Figure 6: Map of hydrothermal alterationFigure 6: type. Map of hydrothermal alteration type.

FigureFigure 7: (A)7: Photograph (A) Photograph of argillic of alteration argillic alteration in the field in. (B) the Photograph ﬁeld. (B) Photograph of argillic alteration of argillic in thin alteration section in(XPL). Opq: thinopaque section mineral; (XPL). Pl: plagioclase; Opq: opaque Cly: mineral; clay. Pl: plagioclase; Cly: clay.

Journal of Applied Geology 105

PRATAMA et al. quartz and calcite-filled veins were observed strike-slip faults, allowed the intrusive ig- in the hydrothermally-altered andesite. In thin neous rocks in the research area to undergo sections, this type of propylitic alteration was hydrothermal alteration. Two types of hy- characterized by the presence of chlorite miner- drothermal alteration developed in the research als (Figure 8). area, namely argillic and propylitic alterations. The propylitic alteration was characterized 4.2.2 Intensity of hydrothermal alteration by the presence of calcite, epidote quartz, Petrographic analysis results for measurement and chlorite minerals, while the argillic alter- of the hydrothermal alteration intensity are pre- ation had more dominant spatial distribution sented in Table 3 and the map of hydrothermal and was characterized by the presence of il- alteration intensity is shown in Figure 9. The lite/montmorillonite, illite, smectite, quartz, andesite in the research area had undergone and kaolinite minerals. Three hydrothermal three different hydrothermal alteration intensi- alteration intensities, which were weak, mod- ties, namely weak, moderate, and high alter- erate, and high alteration intensities, were de- ation intensities. High alteration intensity had veloped. However, most of the andesite in the dominant spatial distribution and about 75 % research area had undergone high intensity of of andesite in the research area were highly hydrothermal alteration. altered. Aproximately 15 % of the andesite, particularly in the western and northern parts of the research area, were moderately altered, REFERENCES while approximately 10 % of the andesite, par- Ansori, C. and Hastria, D. (2013) Studi Alterasi dan ticularly located in the southern part of the re- Mineralisasi di Sekitar Gunung Agung, Kabu- search area, were weakly altered. Presence of paten Kulon Progo – Purworejo, Buletin Sumber geological structures, particularly faults, allow Daya Geologi, Vol. 8, No. 2, p. 75–86. hydrothermal fluid to flow upward and to alter Bell, F. G. (2007) Engineering Geology, 2nd Edition: mineral compositions of the host rock near the Oxford, Elsevier Ltd., 593 p. ground surface. The more intensive the pres- Byers F. M. (1990) Procedure For Determination of Volume Constituents in Thin Section of Rocks, ence of the geological structures is the higher in- Los Alamos National Laboratory, Yucca Moun- tensity of hydrothermal alteration will be. The tain Project, 8 p. zone of high hydrothermal alteration intensity Center of Volcanology and Geological Hazard Mit- in the research area appeared to be influenced igation (PVMBG) (2010) Peta Zona Kerentanan by presence of the strike-slip and normal faults. Gerakan Tanah Kabupaten Purworejo, Provinsi Other faults might exist and contribute to the Jawa Tengah, Center of Volcanology and Geolog- dominant high intensity of hydrothermal alter- ical Hazard Mitigation, Geological Agency, Min- ation in the research area. However, identifica- istry of Energy and Mineral Resources. Gillis, K.M., Snow, J.E., Klaus, A., and the Expe- tion of other faults that was hindered because dition 345 Scientists (2014) Hess Deep Plutonic the andesite had also undergone weathering. Crust: Exploring the Plutonic Crust at a Fast- The area having argillic type and high inten- Spreading Ridge: New Drilling at Hess Deep, sity of hydrothermal alteration were likely to Proceedings of the Integrated Ocean Drilling Pro- have high landslide hazard potential. As slope gram, Volume 345, College Station: TX. failure was not dictated only by material prop- Le Maitre, R.W. (2002) A Classification of Igneous erties, further study was conducted to deter- Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences mine the mechanism of slope failures in the re- Subcommission on the Systematics of Igneous search area by considering other controlling fac- Rocks, Cambridge, Cambridge University Press. tors of slope failures, such as slope geometry, 236 p. groundwater, and land use. Pirajno, F. (1992) Hydrothermal Mineral Deposits, Principles and Fundamental Concepts for the Ex- 5C ONCLUSION ploration Geologist: Berlin, Springer – Verlag, 709 The research area consisted of andesite that p. could be divided into three lithological units Pola, A., Crosta, G., Fusi, N., Barberini, V., Norini, G. (2012) Influence of Alteration on Physical Proper- based on the weathering degree and relative age. The presence of faults, particularly the

106 Journal of Applied Geology MINERALOGICAL CHARACTERISTICSOF HYDROTHERMALLY-ALTERED ANDESITEIN KALIREJO VILLAGE

FigureFigure 8: (A) 8: Ph (A)otograph Photograph of propylitic of propylitic alteration alteration in the field in the. (B) ﬁeld. Photograph (B) Photograph of propy oflitic propylitic alteration alteration in thin section (XPL).Cly:in thin clay; section Opq: (XPL).Cly: opaque mineral; clay; Opq: Pl: plagioclase; opaque mineral; Chl: chlorite; Pl: plagioclase; Qz: quartz. Chl: chlorite; Qz: quartz.

Table 3: Measurement of hydrothermal alteration intensity.

Mineral (%) No Tot (p) Tot (s) AI Qz (p) Qz (s) Pl Opq Px Hb Chl Cly 1 5.67 3.83 90.50 3.83 96.17 High 2 1.30 5.00 93.70 6.30 93.70 High 3 6.50 93.50 6.50 93.50 High 4 1.53 2.25 5.42 6.95 83.85 9.20 90.80 High 5 4.08 2.67 3.17 90.08 9.92 90.08 High 6 8.50 2.67 8.33 80.50 11.17 88.83 High 7 1.83 10.50 87.67 12.33 87.67 High 8 4.75 0.83 12.25 82.17 13.08 86.92 High 9 7.50 8.58 4.58 79.34 13.16 86.84 High 10 8.08 2.50 17.23 72.19 19.73 80.27 High 11 11.17 14.75 74.08 25.92 74.08 High 12 8.67 9.25 18.00 64.08 27.25 72.75 High 13 9.20 24.70 5.50 60.60 30.20 69.80 High 14 26.30 7.80 65.90 34.10 65.90 High 15 2.25 42.57 2.92 2.44 10.32 39.50 58.25 41.75 Moderate 16 65.71 1.00 4.50 9.87 18.92 81.08 18.92 Moderate 17 71.40 1.25 2.33 10.60 14.42 85.58 14.42 Moderate 18 1.25 76.50 2.00 1.20 10.30 8.75 91.25 8.75 Weak 19 81.44 0.08 2.30 8.60 7.58 92.42 7.58 Weak 20 0.67 63.00 4.58 7.00 17.92 6.83 93.17 6.83 Weak Qz (p) = primary quartz, Qz (s) = secondary quartz, Pl = plagioclase, Px = pyroxene, Opq = opaque mineral, Chl = chlorite, and Cly = clay mineral; Tot (p) = total of primary minerals, Tot (s) = total of secondary minerals, AI = Alteration intensity according to Gillis et al. (2014).

Journal of Applied Geology 107 PRATAMA et al.

Figure 9: Map of hydrothermalFigure alteration 9: Map intensity. of hydrothermal alteration intensity.

ties of Volcanic Rocks: Tectophysics, Elsevier Ltd., Van Bemmelen, R.W. (1970) The Geology of Indone- Oxford, p. 566-567. sia, Vol.1A. General Geology of Indonesia and Rahardjo, W., Sukandarrumidi, and Rosidi, H.M.D. Adjacent Achipelago, 2nd Edition: Netherlands, (1995) Peta Geologi Lembar Yogyakarta, Jawa, Martinus Nilhoff, The Haque, 766 p. Edisi 2, Pusat Penelitian dan Pengembangan, Bandung.

108 Journal of Applied Geology