J. SE Asian Appl. Geol., 2015, Vol. 7(1), pp. 1–9 CHARACTERISTICS OF HYDROTHERMAL ALTERATION IN CIJULANG AREA, WEST JAVA, INDONESIA Myo Min Tun*1, I Wayan Warmada2, Arifudin Idrus2, Agung Harijoko2, Reza Al-Furqan3, and Koichiro Watanabe4 1Department of Geology, Mandalay University, Mandalay, Myanmar 2Geological Engineering Department, Faculty of Engineering, Gadjah Mada University, Yogyakarta, Indonesia 3PT. Eksplorasi Nusa Jaya/Freeport-McMoran Copper & Gold 4Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Abstract advanced argillic, argillic and propylitic alteration outward from the silica core has resulted from the Characterization of hydrothermal alteration in the progressive cooling and neutralization of hot acidic Cijulang area (West Java, Indonesia) was carried magmatic fluid with the host rocks. out using shortwave infrared spectroscopy. Hy- drothermal alteration in the Cijulang area occurs in Keywords: Cijulang, High-sulfidation, Alteration the calc-alkaline volcanic and volcaniclastic rocks. minerals, Shortwave Infrared Spectroscopy Shortwave infrared spectroscopic measurements of reflectance for altered rocks and minerals were car- 1 Introduction ried out by ASD-FieldSpec and the laboratory spec- Shortwave infrared reflectance (SWIR) spec- tra acquired were then analysed with “The Spec- troscopy has been widely used in hydrothermal tral Geologist” software program. Shortwave in- alteration mapping of various types of min- frared spectroscopy is capable of detecting most fine- eral deposits, such as high- and low-sulfidation grained alteration minerals from different hydrother- epithermal, mesothermal, porphyry, sediment- mal alteration zones. Characteristic alteration min- hosted gold and copper, uranium, volcanogenic erals identified from the SWIR technique include py- massive sulfide (VMS) and kimberlite. SWIR rophyllite, alunite, kaolinite, dickite, illite, montmo- spectroscopy involves detection of the en- rillonite, polygorskite, gypsum, epidote, paragonite, ergy generated by vibrations within molecular and muscovite. Most of the spectra show mixture of bonds in the 1300- to 2500-nm (SWIR) range alteration minerals and only a few display pure spec- of electromagnetic spectrum (Thompson et al., tra of single mineral. The crystallinity of kaolinite 1999). Most minerals exhibit characteristic spec- from the samples was also determined from the re- trum and major diagnostic absorption features flectance spectra and show moderately to high crys- in this SWIR region as a result of bending and tallinity. Alteration system of the Cijulang prospect stretching of certain molecules and radical, in- is similar to others documented high-sulfidation ep- cluding OH, H O, NH , and Cation-OH bonds ithermal deposits, such as Rodalquilar (Spain), Sum- 2 4 (such as AlOH, MgOH and FeOH) (Pontual et mitville (Colorado), and Lepanto (Philippines). A al., 1997a; Thompson et al., 1999). The spectral characteristic alteration sequence and zonation of absorption features associated with different sil- *Corresponding author: M.M. TUN, Department of icates occur at or near 1400nm (OH and water) Geology, Mandalay University, Mandalay, Myanmar. E- and 1900nm (water) whereas other important mail: [email protected] 1 TUN et al. and diagnostic spectral absorption features re- radiometer which collected the reflectance in lated to cation-OH bonds such as AlOH, FeOH the 350 to 2500nm spectral range. The re- and MgOH occur at or near 2200nm, 2250nm flectance spectra acquired by ASD device were and 2330nm respectively (Figure 1). The ab- then analyzed with “The Spectral Geologist™ sorption features that represent these bonds or (TSG)” software. The TSG provides automated mineral groups are characteristic of hydrother- assistance in the mineral identification and mal alteration and mineral groups belonging statistical analysis of the spectra. Mineral iden- to kaolinite, halloysite, pyrophyllite, smectite tification was conducted by spectral matching clays, dickite, micas, chlorites, alunite, jarosite, with reference library spectra. The Spectral calcite (Pontual et al., 1997). Assistant (TSA) in The Spectral Geologist uses Reflectance spectra can be acquired by field a library of 551 spectra of pure minerals and portable spectrometers, such as GER-IRIS, water. The software identifies a maximum of ASD-FieldSpec and PIMA or by airborne re- three minerals assigning relative weights and mote sensing. Application of spectrometers an error of matching. The relative weight does allows rapid identification of the fine-grained not represent the abundance of minerals in the hydrothermal alteration minerals which are samples but it reflects the relative proportion of generally difficult to identify by conventional one mineral to another which is influenced by macroscopic observation. SWIR spectroscopy grain size and distribution of the minerals. The not only detects the presence of minerals in the level of confidence in the mineral identification sample but also determines the relative abun- is indicated by the error of matching. dance of mineral within the samples. Moreover, Interpretation of spectral data from TSG soft- mineralogical and compositional variation in ware were cross-checked by spectral interpre- certain specific mineral species can also be rec- tation field manual and USGS library spectra. ognized by the SWIR technique (Thompson et Identification of the mineral is based on the fol- al., 1999). lowing spectral characteristics; wavelength po- Cijulang area is located in the Garut Re- sition, intensity and shape of the absorption gency of West Java, Indonesia (Figure 2) and the troughs and the overall shape of the entire spec- prospect has been explored by PT Aneka Tam- trum. Shifting of the wavelength positions of bang since 1994. This research aims to explore diagnostic absorption feature represents com- the characteristics of the hydrothermal alter- positional variation in minerals whereas varia- ation associated with pyrite-enargite-gold min- tion in depth or width of the absorption feature eralization in the Cijulang prospect by the ap- reflects the variation in crystallinity or the grain plication of reflectance spectroscopy. size of the relative abundance of mineral (Pon- tual et al., 1997). 2 Research methods Bulk rock powdered and clay fraction sam- ples were analyzed by Rigaku RINT-2100 Field investigation was carried out in the Ciju- Diffractometer at Laboratory of Earth Resources lang area of West Java, Indonesia during March, Engineering Department, Faculty of Engineer- 2013. A total of 70 altered rock and clay sam- ing, Kyushu University. X-ray diffraction anal- ples were collected from different hydrother- ysis was done using CuKa radiation at 40kV mal alteration zones. Study on alteration min- and 20mA. Microprobe examination of alter- eralogy was carried out by Shortwave Infrared ation minerals was carried out by SHIMADZU Spectroscopy (SWIR) aided by petrographic mi- SS-550 Scanning Electron Microscope equipped croscopy, X-Ray Diffraction and SEM-EDS anal- with a genesis-2000 EDX Spectrometer at the yses. Centre for Advanced Instrumental Analysis, Shortwave infrared (SWIR) spectroscopic Kyushu University, Japan. Experimental con- measurement of reflectance for altered rocks dition for the analysis was performed at an and clay minerals was carried out by Analytical accelerating potential of 15kV, beam current Spectral Devices (ASD-FieldSpec), a spectro- about 10mA, and 3mm beam diameter. 2 © 2015 Department of Geological Engineering, Gadjah Mada University CHARACTERISTICS OF HYDROTHERMAL ALTERATION IN CIJULANG AREA, WEST JAVA Figure 1: Major spectral absorption features in SWIR range (1300-2500nm) (Pontual et al., 1997a). © 2015 Department of Geological Engineering, Gadjah Mada University 3 TUN et al. Figure 2: Location map of Cijulang area, Garut Regency, West Java, Indonesia. 3 Results and Discussion tra with other mineral such as dickite, py- rophyllite and illite. Kaolinite spectra have 3.1 Alteration Minerals the following major absorption features: hy- Hydrothermal alteration minerals identified droxyl (OH) stretching doublet around 1400nm from reflectance spectra of altered rocks and and 1411nm, water (H2O) absorption feature at clay minerals include kaolinite, dickite, py- 1913nm and Al-OH diagnostic double absorp- rophyllite, illite, muscovite, chlorite, alunite, tion feature at 2168nm and 2208nm (Figure 3a). epidote, polygorskite, and goethite. Most of the spectra show a mixture of these alteration Dickite [Al2Si2O5(OH)4]. Dickite is one of the minerals and only a few show pure spec- kandite group minerals commonly found in the tra of single minerals. The common mineral Cijulang prospect. It occurs as pure mineral assemblages identified from the reflectance spectra or mix spectra with other minerals, such spectroscopy include pyrophyllite–kaolinite, as pyrophyllite and kaolinite (Figure 3a). Spec- dickite–pyrophyllite, alunite–dickite, alunite– tral characteristics of dickite are similar to those kaolinite, kaolinite–illite, illite–montmorillonite of kaolinite. It has hydroxyl doublet absorption and chlorite–montmorillonite (Figures 3a and features occurring around 1390nm and 1415nm. 3b). Al-OH diagnostic double absorption features exhibiting at 2172nm and 2208nm, which are 3.2 SWIR Spectral Characteristics of Alter- common characteristic of kandite group miner- ation Minerals als (Figure 3a). Kandite group minerals Pyrophyllite [Al2Si4O10(OH)2]. Pyrophyl- Kaolinite