Characteristics of Hydrothermal Alteration in Cijulang Area, West Java, Indonesia

Home , Dickite, Mandalay University, Muscovite

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) spectra 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 samples 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 CuKα 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 3μm beam diameter.

Figure 1: Major spectral absorption features in SWIR range (1300-2500nm) (Pontual et al., 1997a).

Figure 2: Location map of Cijulang area, Garut Regency, West Java, Indonesia.

3 Results and Discussion tra with other mineral such as dickite, pyrophyllite 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 [Al2Si2O5(OH)4]. Kaolinite gener- lite spectra show absorption features at 1391– ally occurs as pure mineral spectra or mix spec- 1396nm and 1411–1414nm due to vibration of

Figure 3: Reflectance spectra of alteration mineral mixtures from different hydrothermal alteration zones (a) advanced argillic and (b) argillic and propylitic. Kln-kaolinite, Prl-pyrophyllite, Dck- dickite, Alu-alunite, Ill-illite, Mnt-montmorillonite, Chl-chlorite, Ms-muscovite. hydroxyl ions (OH−) and water molecules (H- group minerals are recognized by shifts in the O-H), a doublet absorption feature at 2168nm 1480nm spectral position, with value ranging and 2208nm due to vibrational processes asso- from ∼1461nm (NH4), to ∼1478nm (pure K), to ciated with Al-OH bonds. Pyrophyllite is either ∼1496 nm (Na) to 1510nm (Ca) (Thompson et seen as pure spectra or mixture with kaolinite al., 1999). Alunite from the Cijulang prospect (well crystalline) and dickite in the advanced is only found in the drill core section and argillically-altered samples (Figure 3a). generally associated with pyrite and quartz. Alunite is identified as potassium end-member Smectite Group Minerals (K-alunite) and its spectra have major characteristic alunite absorption features: hydroxyl Montmorillonite [(Na,Ca) (Al,Mg) (Si O ) 0.33 2 4 10 (OH) stretching doublet occurring at 1428nm (OH) ·nH O]. Montmorillonite spectra have 2 2 and 1480nm, diagnostic absorption feature at characteristic sharp minima and asymmetric 1766nm and the Al-OH diagnostic absorption shape of water absorption features at 1409– features at 2171nm, 2208µm and 2324nm (Fig- 1411nm and 1911–1914nm (Figure 3b). A broad ure 3a). Alunite generally occurs as mix spectra diagnostic Al-OH absorption feature gener- with other minerals, such as dickite and kaolin- ally occurs at 2208nm, a typical feature of ite. aluminum-bearing clays (Hunt, 1979). Illite Group Minerals APS Group Minerals Muscovite [KAl (AlSi O )(F,OH) ]. White Alunite [(K,Na)Al (SO ) (OH) ]. Alunite is a 2 3 10 2 3 4 2 6 micas generally have absorption features occur- diagnostic mineral of high-sulfidation epither- ring in the range between 2180nm to 2228nm mal system. Compositional variation in alunite- due to the Al-OH bonds (Figure 3b). Varia-

© 2015 Department of Geological Engineering, Gadjah Mada University 5 TUN et al. tion in chemical composition strongly affects Kaolinite Crystallinity the position of absorption in white micas; Na- Kaolinite is the most common and abundant rich micas have absorption features around mineral in the hydrothermal alteration of the 2200nm, whereas Mg-FeOH-rich and potas- Cijulang area. So, it is important to determine sic micas have similar features occurring in the crystallinity of kaolinite. Kaolinite crys- the ranges between 2216–2228nm and 2200– tallinity determination was based on the anal- 2208nm respectively (Pontual et al. 1997). Mus- ysis of shape and wavelength spacing of the covite generally occurs as mixture with well Al-OH band doublets absorption features (Fig- crystalline kaolinite has the following spec- ure 4a) by using slope parameters in the 2160– tral characteristic; hydroxyl (OH) absorption 2180nm wavelength region which is generally features at 1410nm, water (H O) absorption 2 referred to as slope ‘2160’ and slope ‘2180’ pa- feature at 1917nm and Al-OH absorption fea- rameters. Evaluation shows that the kaolinite ture at 2202nm. It is classified as Na-rich type from the Cijulang is characterized by moderate by its Al-OH value occurring at 2200nm (Figure to high crystallinity (Figure 4b). 3b). 3.3 Hydrothermal Alteration Illite [K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2, (H2O)]. Illite has similar spectral characteris- Hydrothermal alteration in the Cijulang area tic to muscovite. Illite can be distinguished covers about 1×2 km2 area of N–E trending from muscovite by its deep absorption feature elongate zone and hosted by andesitic lava, occurring at 1900µm due to molecular water lapilli tuff and hydrothermal breccia. The within its crystal lattice (Figure 3b) (Pontual distribution of alteration mineral assemblage et al. 1997). Absorption feature occurring at indicates that Cijulang alteration is classified ∼2200nm reflects compositional variation in il- as epithermal high-sulfidation styles in which lite, those near 2190nm are more sodic whereas the silicic core is surrounded outward alter- these feature close to 2206nm are more potassic ation mineral assemblages of advanced argillic, (Halley, 2010). Illite shows hydroxyl (OH) ab- argillic and propylitic (Figure 5). sorption feature at 1409–1411nm, water (H2O) Silica alteration develops as both vuggy and absorption feature at 1911-1914nm and Al-OH massive silica. This alteration is easily recog- absorption feature centered at or near 2200nm, nized in the outcrop by weather-resistant bod- which is a diagnostic absorption feature of ies. Vuggy zone comprises residual silica re- white micas. Illite from the Cijulang area is maining after loss of its reactive components by identified as paragonitic illite (Figure 3b). intense acid leaching. Massive silica is formed by secondary deposition of silica and later re- Other Minerals crystallize to form quartz. Advanced argillic alteration is the most Other minerals such as Fe-chlorite, epidote, widespread of alteration and identified by the gypsum, palygorskite and goethite occur as presence of alunite, kaolinite, dickite, pyro- mixture with other minerals in the analyzed phyllite, illite and pyrite in variable amount. samples. No pure spectra were obtained for The common mineral assemblages identi- these minerals and it was difficult to charac- fied from SWIR spectroscopy include pyro- terize their spectral characteristics. Anhydrous phyllite+kaolinite, dickite+pyrophyllite, alu- silicates and sulfides such as quartz and pyrite nite+dickite, alunite+kaolinite and kaolin- are difficult to identify because they show ite+illite. Pyrophyllite, kaolinite and dickite no absorption features in the shortwave in- are the dominant minerals of advanced argillic frared wavelength region. In addition, minerals facies in the outcrop whereas alunite is ob- present in amount less than 5% in the samples served only in the drill core section. Diaspore were not identified by the SWIR spectroscopy. occurs locally in the advanced argillic alteration The presence of such minerals was confirmed and generally associated with pyrophyllite. by the X-ray diffraction analysis.

Figure 4: (a) Kaolinite crystallinity spectra shape and parameter guide (Pontual et al., 1997a) and (b) Reﬂectance spectra kaolinites from advanced argillic alteration zones of Cijulang prospect showing moderate to high crystallinity.

Argillic alteration is characterized by quartz, terize the distinct alteration zonation which is illite, paragonite, muscovite, kaolinite, smectite, significant in the determination of certain min- illite-smectite and chlorite. The common as- eral deposit type. semblage is illite+montmorillonite. Propylitic Alteration system of the Cijulang prospect alteration is composed of quartz, chlorite, epi- is similar to other systems that occur in well- dote, illite, smectite, pyrite, hematite, goethite, documented high-sulfidation epithermal de- carbonate, zeolite and magnetite. Phenocrysts posits of the world (e.g., Rodalquilar, Spain; and groundmass of host rock andesite lava and Summitville, Colorado; Lepanto, Philippines). lapilli tuff are replaced by chlorite, sericite (il- A characteristic alteration sequence and zona- lite), smectite, epidote, calcite, and albite. Chlo- tion of advanced argillic, argillic and propylitic rite and epidote tends to replace the mafic phe- alteration outward from the silicic core has nocrysts whereas as calcite and albite selec- resulted from the progressive cooling and neu- tively replaces the plagioclase feldspar. Smec- tralization of hot acidic magmatic fluid with tite generally replace the plagioclase and the the host rocks (Arribas, 1995). Formation of groundmass is replaced by fine-grained chlo- vuggy silica requires the fluids of at least pH rite, sericite, and epidote. ≤2 and at a temperature of ∼250°C (Stoffre- gen, 1987) to leach all the minerals from the 4 Conclusion volcanic hosts except quartz. High-sulfidation epithermal system, which is characterized by The application of reflectance spectroscopy en- advanced argillic alteration, has developed un- ables to identify most hydrothermal alteration der relatively oxidized, acidic, SO4-dominated minerals related to the high-sulfidation miner- environment. Dominant pyrophyllite-kaolinite alization in the Cijulang prospect. The minerals assemblage in the study area indicates that hy- identified from SWIR spectroscopy help charac-

Figure 5: Hydrothermal alteration of Cijulang area, Garut Regency, West Java, Indonesia.

8 © 2015 Department of Geological Engineering, Gadjah Mada University CHARACTERISTICS OF HYDROTHERMAL ALTERATION IN CIJULANG AREA, WEST JAVA drothermal fluids responsible for the alteration spectra of alteration minerals: potential for were silica-saturated and the temperature of use in remote sensing: Geophysics, v. 44, p. formation may have been >260°C (White and 1974–1986. Hedenquist, 1990). The presence of pyrophyl- Pontual, S., Merry, N., and Gamson, P. (1997a) lite is an indication of deep-seated formation of G-Mex Vol. 1, Spectral interpretation field the hydrothermal system. manual: AusSpec International Pty. Ltd. Pontual, S., Merry, N., and Gamson, P. (1997b) Acknowledgement G-Mex Vol. 2, Practical applications hand- book: AusSpec International Pty. Ltd. The current research is supported by Pontual, S., Merry, N., and Gamson, P. (1997c) AUN/SEED-Net (JICA). Thanks are given to G-Mex Vol. 4, Epithermal alteration systems: the PT Aneka Tambang (Persero) Tbk for per- AusSpec International Pty. Ltd. mission to conduct research works in the con- Stoffregen, R. (1987) Genesis of acid-sulfate cession area and help during field investigation. alteration and Au-Cu-Ag mineralization at Summitville, Colorado: Economic Geology, v. References 82, p. 1575–1591. Thompson, A.J.B., Hauff, P.L. and Robitaille, Arribas, A., Jr. (1995) Characteristics of high A. (1999) Alteration Mapping in Exploration: sulfidation epithermal deposits and their re- Application of Short-Wave Infrared (SWIR) lation to magmatic fluid. Mineralogical As- Spectroscopy SEG Newsletter, 1999, Number sociation of Canada Short Course, v. 23, p. 39, published by the Society of Economic Ge- 419–454. ologists, Littleton, Colorado. Halley, S.W. (2010) Mapping the footprints White, N.C. and Hedenquist, J.W. (1990) Ep- of hydrothermal systems: Geological Sur- ithermal environments and styles of miner- vey of Western Australia Record 2010/18, p. alization: Variations and their causes, and 264–265. guidelines for exploration: Journal of Geo- Hunt, G.R. (1979) Near-infrared (1.3-2.4µm) chemical Exploration, v. 36, p. 445–474.