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

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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 prop- erties 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 domi- nated 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] 2502-2822/© 2018 Journal of Applied Geology PRATAMA et al. 2G EOLOGIC SETTING Table 1: Classification of hydrothermal alter- ation 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 horn- blende 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 ten- sion joints found a filling structure in the form of calcite and quartz veins. This joints structure 100 Journal of Applied Geology MINERALOGICAL CHARACTERISTICS OF HYDROTHERMALLY-ALTERED ANDESITE IN 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 anal- ysis. 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.
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