Geothermal System of Pariangan, West Sumatera based on Hydrothermal Alteration and Hot Springs Geochemistry Studies Herza Nurkusumariani 1, EuisTintinYuningsih 1, Aton Patonah 1, Dedi Kusnadi 2, Mochamad Nur Hadi 2 1 Geological Engineering Faculty, Padjadjaran University, Jl.Raya Bandung-Sumedang KM 21, 45363, Sumedang Regency,West Java, Indonesia 2 Pusat Sumber Daya Geologi, Jl. Soekarno-Hatta No.444, 40254, Bandung, West Java, Indonesia Email: herza120.geounpad10@gmail.com Abstract The research area is located atpariangan, Tanah Datar, West Sumatera Province. The purpose of this study is to determine the geothermal system in Pariangan based on hydrothermal alteration and fluid geochemistry (hot springs). The lithology of research area is dominated by andesitic-basaltic lava and pyroclastic rocks (Quaternar y). Surface manifestation of geothermal activity such as hot springs, warm springs and alteration minerals are identified in research area. Type of alteration developed in research area are dominated by argillic type characterized by the presence of illite, kaolinite, montmorilonite, dickite, and halloysite; propylitic type characterized by the presence of chlorite, epidote and, quartz; and phyllic type characterized by the presence of sericite, quartz, and chlorite. The result of fluids geochemistry analysis-concludes that all fluids classified as bicarbonate water which indicated volcanomagmatic environment source and has been mixing with meteoric water, thus suspected outflow zone of geothermal system. Based on analysis of conductive quartz geothermometry, estimated reservoir temperature ranges from 166 to 168 C and classified as moderate temperature geothermal system. Keywords: alteration, fluid geochemistry, geothermal,geothermometry Introduction Sumatera is formed through subduction between the Pacific Plate and the Eurasian Plate which resulted in the formation of a magmatic arc, Bukit Barisan Mountains. This tectonic order caused the volcanic activity or volcanism along Sumatera, indicates that there is considerable potential of geothermal energy in Sumatera. This raises the chances of geothermal energy development in Indonesia, hence it is very important to know the geothermal system that exists in an area. Geothermal system in the area is determined by hydrothermal alteration and fluid geochemistry (hot springs), this will indicate the characteristics of manifestations in the surface, the type of alteration and zoning, type and origin of the hot water environment, and the estimated subsurface temperature (reservoir), using geothermometry. Pariangan geothermal field is located in Tanah Datar District, West Sumatera Province, Indonesia (Fig. 1). The lithology in this area consists of slate and meta-limestone(permian-carbon), granite (Triassic), quartz sandstone (Oligocene-Miocene), conglomerate (Miocene), lava, pyroclastic flows, andpyroclastic fallout as the result of volcanic activity. The structure developed in this region is influenced by a large fault that extends through Sumatra from Banda Aceh to the Gulf Semangko in Lampung and also the pattern of radial faults that follow the development of KomplekMarapi volcanism. Based ondigital Elevation Model (DEM) analysis and data in the field, the main pattern of fractures and faults has a trend of northwestsoutheast and the southwest-northeast as the next order (PSDG, 2014). 239
Sample T manif T air ph Debit DHL ( o C) ( o C) (L/sec) (µs/cm) ADTA 25,65 27,50 6,25 2 180 ADTP 23,40 23,32 7,40 10 155 Research Area Figure 1. Research area map Geothermal Manifestations Surface manifestation exist in the research area are hot springs, warm springs and the presence of alteration minerals. Hot springs and warm springs found in six locations, those are hot water of Pariangan 1 (APPA 1), hot water of Pariangan 2 (APPA 2), hot water of Pariangan 3 (APPA 3), warm water of Sopandidih (AHSD), warm water of Batubasa (AHB), and warm water of Galogandang (AHGG). In addition, two cold spring samples aretaken such as Tampang (ADTP) and Taluak(ADTL)cold waters. These are used for temperature comparisons between the hot springs. The manifestation characteristics are shown in Tables 1 and 2. Sample Table.1 Characteristics of manifestation T manif T air ph Debit DHL ( o C) ( o C) (L/sec) (µs/cm) APPA1 48,66 22,56 6,25 2 2900 APPA2 48,44 22,94 6,18 2 2700 APPA3 48,25 23,40 5,95 1 3040 AHSD 34,60 25,90 6,08 2 1900 AHB 32,00 24,67 6,80 2 560 AHGG 30,59 25,30 6,25 0,5 550 Table 2. Characteristics of cold springs The lower manifestation temperaturefromappa 1 to AHGG indicates the temperature decreases from the north to the south of the research area. Alteration minerals in the research area are well developed so the alteration mineral mapping is done to determine the distribution and type of alteration and estimating subsurface temperatures (paleotemperatur). The chemical composition and alteration mineral zoning is determined based on petrographic analysis conducted on six samples and PIMA analysis on five samples. Alteration mineral occured in the Pariangan geothermal field includes secondary quartz, carbonates mineral, epidote, chlorite, sericite, illite, kaolinite, dickite, montmorillonite, hallosite, paragonite, phengite, and nontronite. Based on alteration mineral associations encountered in Pariangan geothermal field, the type of alteration in the research can be grouped into 4 types of alteration which refers to Corbett and Leach (1997), those are: a. argillic b. propyllitic c. phyllic d. silicification The presence of alteration minerals is mostly product of the volcanic rocks alteration whichare andesitic lava and pyroclastic rocks with weak to strong alteration intensities. Argillic alteration types is characterized by the presence of clay minerals such asillite, kaolinite, montmorillonite, dickite, nontronite and hallosite and a small amount of chlorite, quartz and carbonate minerals. Based on the minerals appearance, this type has a temperature range between 180-200 C (Henley and Ellis, 1983 in White and Hedenquist, 1995; Corbett and Leach, 1997; Lawless et al. 1998) 240
with ph from acidic to neutral, spread in the northwest and south of research area. Propylitic alteration types in the research area are characterized by the presence of alteration minerals such as epidote, chlorite, and quartz, and also a small amount ofillite, paragonite, sericite andphengite. Based on the minerals appearance, this type has a temperature range between 200-300 C (Henley and Ellis, 1983 in White and Hedenquist, 1995; Corbett and Leach, 1997; Lawless et al. 1998) with a neutral ph, spreading in northeastern and western part of the research area. Phyllic alteration types in the study area are characterized by the dominant presence of quartz, sericite, phengite and paragonite, and a small amount of chlorite. Based on the minerals appearance, this type has a temperature range between 200-250 C (Henley and Ellis, 1983 in White and Hedenquist, 1995; Corbett and Leach, 1997) with a ph of acidic to neutral. Meanwhile, silicification is characterized by the dominant presence of quartz mineral that has a temperature range of between 150-300 C, located on the southwestern part of the research area. Hot Water Chemistry Chemical analysis of hot water aims to determine the content of the main cations such as Na, K, Ca, Mg and major anions such as Cl, HCO 3, SO 4, and neutral compounds such as SiO 2, and B, which are then use to determine the characteristics of the geothermal fluid, the type and origin of the hot water, and subsurface temperature prediction with calculation of geothermometry. The results of the chemical analysis are shown in Table 3. Table 3. Water chemistry analysis results Analysis Parameter APPA 1 APPA 2 APPA 3 AHSD AHB AHGG ADTP ADTL ph 6.25 6.18 5.95 6.08 6.80 6.25 7.40 6.25 SiO 2 163.28 165.72 160.17 23.72 27.52 23.98 60.83 69.94 B 11.01 1.55 1.55 1.06 0.26 0.14 9.00 0.09 Al 3+ 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.03 Fe 3+ 1.16 0.98 1.18 2.59 2.16 <0.01 <0.01 <0.01 Ca 2+ 158.80 132.20 127.50 66.10 83.40 80.50 10.53 10.47 Mg 2+ 164.57 169.72 174.01 98.57 10.50 9.59 6.61 9.65 Na + 289.80 308.70 288.90 247.80 26.40 24.00 7.97 9.24 K + 60.58 66.22 57.49 42.23 2.30 2.46 3.55 4.63 Li + 0.24 0.25 0.26 0.29 0.03 0.02 0.02 0.01 As 3+ 0.19 0.20 0.17 <0.01 <0.01 <0.01 <0.01 <0.01 NH 4 + 1.51 23.00 0.00 2.31 0.92 0.91 0.87 1.00 F - 0.36 0.17 0.20 0.12 0.14 0.33 0.17 0.38 Cl - 249.72 224.83 222.75 108.40 6.10 3.29 3.46 3.32 SO 4 2-658.54 744.90 613.58 318.08 9.25 10.24 17.68 21.45 HCO 3 920.85 914.63 902.43 768.77 360.83 334.53 64.32 75.32 Sum cation(%) 35.78 37.03 34.79 23.53 6.37 5.97 1.56 1.89 Sum anion (%) 35.87 36.85 33.86 22.29 6.29 5.81 1.53 1.79 Ion balance (%) 0 0 1 3 1 1 1 3 Cation groups a. Sodium and Potassium (Na and K) 241
Ratio calculation results of Na / K shows that the six hot and warm springs samples have a ratio lower than 15, indicating the fluid springs are influenced by short time surface water transportation process. b. Lithium (Li) The calculations show that the value of Li in the six hot springs samples ranges between 0.01-1 mg / kg, which shows that the subsurface lithology can be andesitic or basaltic (Nicholson, 1993). This is supported by geological data of research areas that have basaltic lava to andesitic lava lithology. The value of B / Li ratiois seen that the value increases to AHGG and AHB, this suggests that fluid interaction with the side rock or lateral flow increases to the south and east. c. Calcium (Ca) The Na / Ca ratio from six hot springs sample is low and the value decline towards AHB and AHGG. This indicates that further to the south and to the east, the fluid is affected by increasing lateral flow and interaction with host rock is more intensive. d. Magnesium (Mg) The calculation shows the six hot and warm springs have a high concentration of Mg,(between 9.5-174 mg / kg). This indicates that there are two possibility process such as washing and leaching (dilution) of the geothermal fluid or the interaction with meteoric water, while the ratio of Cl / Mg indicates the absence of the possibility of mixing with seawater. Anion groups a. Fluorine (F) Water chemistry analysis on the sixth hot springs shows a low F concentration of less than 1 mg / kg and associated with a slightly high Ca content. The low concentration of Fluor indicates its association with andesitic-basaltic volcanic rocks. b. Chloride (Cl) Water chemistry analysis at the four hot springs ofappa 1,2,3 and AHSD shows that the concentration of Cl is more than 100 mg / kg, this can indicate that the hot springs are a manifestation of the mixing process and minimal conductive cooling along with dissolution by groundwater. c. Bicarbonate (HCO 3 ) HCO 3 concentration in the six hot springs is very high. The higher concentrations from APPA 1 to AHGG could be an indication of outflow manifestation due to the reaction of the fluid reservoir with host rockresulting the formation of HCO 3, while the concentration is affected by the permeability and laterally flow. The HCO 3 / SO 4 ratiohas a tendency to increase from APPA 1 to AHGG and the Cl / HCO 3 ratio which has tendency to declinefromappa 1 to AHGG, which means that the influence of the lateral flow or interaction with the hostrock more increasingto the south and to the east,which indicates that it is increasingly in the direction of outflow zone. d. Sulfate (SO 4 ) That concentration of SO 4 tends to decrease fromappa 1 to AHGG which indicates that the steam condensation process is dominantly increased to northwest and north, which indicated closer to upflow zone. Neutral groups a. Silica (SiO 2 ) Silica concentration of hot springs in Pariangan ranges between 23-166 mg / kg with a tendency of increasingconcentrations approaching toappa 2. This indicates that the further to the northwest, the more the fluid is derived directly from the reservoir (Nicholson, 1993). b. Boron (B) When the host rock is igneous rocks, the concentration of B will be higher in andesitic or rhyolitic compared to basaltic rock. The Cl / B ratio is often used as an indication of source reservoirsimilarity (Nicholson, 1993). Hot Water Type Determination of the type of hot water is using the ternary plot diagram (Nicholson, 1993), based on the relative content of anion chloride (Cl), sulfate (SO 4 ) and bicarbonate (HCO 3 ), see Table 4. Chemical analysis shows that all the hot and warm springs have major anion of HCO 3 and Cl concentrations that lower than SO 4. 242
Table 4. Hot water type and percentage of Cl-SO 4 -HSO 3 Table 5. Percentage of Cl-Li-B Hot Water Type T Compositions (%) PH ( C) Cl SO 4 HCO 3 Origin of Hot Water T Compositions (%) PH ( C) Cl Li B/4 APPA1 48.66 6.25 13.65 36.00 50.34 APPA1 48.66 6.25 45.49 4.37 50.14 APPA2 48.44 6.18 11.93 39.53 48.54 APPA2 48.44 6.18 77.91 8.66 13.43 Bicarbonate APPA3 48.25 5.95 12.81 35.29 51.90 AHSD 34.6 6.08 9.07 26.61 64.32 Volcano Magmatic APPA3 48.25 5.95 77.48 9.04 13.48 AHSD 34.6 6.08 66.14 17.69 16.17 AHBB 32 6.8 1.62 2.46 95.92 AHBB 32 6.8 39.10 19.23 41.67 AHGG 30.59 6.25 0.95 2.94 96.11 AHGG 30.59 6.25 37.43 22.75 39.82 The analysis results obtained that all manifestations of the hot and warm springs in the research area is bicarbonate water type(fig. 2), which indicates that the hot water has been affected by the meteoric water on its way to the surface. The content of HCO 3 from APPA 1,2,3 toward AHB and AHGG is increase, which indicates that washing, leaching, or transport processes of the hot water is increase which also changes the concentrations of Cl, SO 4 and HCO 3. Figure 2.The Cl-SO 4 -HCO 3 plotting (Ternary Diagram by Nicholson, 1993) Figure 3. The Cl-Li-B plotting(ternary Diagram by Giggenbach, 1991) Based on the calculation of Cl, Li, B / 4 content and plotting on the Cl-Li-Bdiagram, the origin of the hot water in the research area is from volcanomagmatic that bring HCl and dissolved H 2 S (Nicholson, 1993). It is characterized by relatively higher Cl content than Li and B which indicates that magmatic gas absorption process with the B / Clratio is low, and indicates that the hot water environment may have undergone a washing process of igneous and pyroclastic rocks or dilution is not dominant process that causes the element to be higher in Cl. Hot Water Origin The origin of hot water is determined by plotting the content of chloride (Cl), li thium (Li), and boron (B) (Table 5 and Fig.3) in the Giggenbach diagram (1991) and content of sodium (Na), potassium (K) and magnesium (Mg) (Table 6 and Fig.4) in the Giggenbachdiagram (1988). Origin of Hot Water Environment Immature Water Table 6. Percentage of Na-K-Mg T ( C) PH Compositions (%) Na K Mg /1000 /100 APPA1 48.66 6.25 2.11 4.41 93.47 APPA2 48.44 6.18 2.21 4.73 93.06 APPA3 48.25 5.95 2.06 4.09 93.85 AHSD 34.6 6.08 2.34 3.98 93.68 AHBB 32 6.8 0.80 0.70 98.50 AHGG 30.59 6.25 0.76 0.78 98.45 243
Based on the calculation of the relative content of Na / 1000-K / 100- Mg, all spring waters have a Mg content higher than Na and K and plotting on a diagram of Na-K-Mg (Giggenbach, 1988) shows all the manifestations of the hot and warm springs in the research area located in immature water zone, which means fluid from the geothermal system has not experienced equilibrium in the reservoir which then may undergo mixing with the meteoric water on its way to the surface and has low temperatures of the surface manifestations with very high Mg content, which is likely influenced by the interaction between hydrothermal fluid with chemical elements of host rock in its path. This corresponds with the occurrence of Mg-rich limestone host rock in the research area. Figure 4. Plotting of Na-K-Mg(Giggenbach, 1988) Paleo-temperatureof Alteration Mineral Reservoir temperature is affected by the formation of secondarycalc-silicate minerals that were formed in neutral to alkaline ph conditions. Type of calc-silicate minerals that found in the research area based on the results of petrographic rock samples is epidote. The presence of epidote in rock samples are associated with phyllosilicates minerals such as chlorite, illite and montmorillonite. Based on the appearance of high temperature alteration mineral of epidote in the research area with ranges between 200-300 C (Henley and Ellis, 1983 in White and Hedenquist, 1995; Lawless et al. 1998),its considered as the approximate trace reservoir temperature in the past (paleo - temperature). Geothermometry of Reservoir The estimation calculation of reservoir temperature is done with conductive quartz geothermometry, since this geothermometry is the most suitable use for water with sub-boiling conditions and are exposed by the influence of the leaching process or dissolution and the high content of SiO 2 in the six hot and warm springs compared to Ca. of Hot and Warm Springs Table 7. Calculation results of geothermometry Na-K-Mg (Giggenbach, 1988) ( C) Na-K-Ca (Fournier and Truesdell,1973) ( C) Conductive Quartz (Fournier, 1983) APPA 1 120 204 167 APPA 2 130 211 168 APPA 3 110 206 166 AHSD 100 201 70 AHB 85 0 76 AHGG 80 0 71 From the geothermometry calculation (Table 7), the temperature values that can be usedor the most appropriateas an estimate reservoir is the value of the temperature of the APPA 1, 2 and 3because APPA 1, 2, 3 has temperature and water flow greater than the other hot springs. The estimation of current subsurface temperature (reservoir) in Pariangan geothermal field based on geochemical of hot springs is ranges between 166-168 C. Geothermal System in Research Area Hot water in the research area included into the type of bicarbonate, where the origin of the hot water is associated with volcano-magmatic and has experienced mixing with meteoric water. This indicates that the research area is still in the same geothermal system because of the uniformity of the type and origin of the hot water. Bicarbonate water types characteristic and the results of chemical analysis of water in the previous subsection which are lowof Na / Ca and B / Li ratios pointed out that the research area is an indication of outflow zone in Pariangan geothermal systems (Figure 6), while the upflow zone is estimated 244
located outside from the research area in the northwest partwith Merapi Mount as a recharge area. Figure 6. Schematic geothermal system in the research area The presence of alteration minerals and relative similar Cl / B ratio of hot springs indicates that the fluid isoriginally from the same reservoir. Reservoir in the research area is probably from igneous rocks of andesite or pyroclastic rocks with propylitic alteration. Reservoir temperature determined based on a calculation of conductive quartz geothermometryis ranging between 166-168 C. Based on the estimation of reservoir temperature, geothermal field ofpariangan included into the medium temperature geothermal system, with 125-225 C (Hochestein and Browne, 2000). Conclusion Surface manifestation of the geothermalfield of Parianganare include hot springs, warm springs with temperatures ranging from 30-49 C, neutral ph and the presence of alteration minerals which mostly overprinting volcanic rocks such as andesite (lava) and pyroclastic rocks. The type of alteration that developed in the Pariangan geothermal field arephyllic which is characterized by sericite, quartz, and chlorite, silicification which is characterized by the presence of quartz, propylitic which is characterized by the presence of epidote, chlorite, quartz and advanced argillic characterized by the presence of clay minerals such as illite, kaolinite, montmorillonite, and dickite. Type of hot water in Pariangan geothermal field is bicarbonate water with high Mg content, whereas the origin of the hot water is derived from volcanomagmatic which influenced by mixing with meteoric water. Subsurface temperature forecasts based on mineral alteration indicates a temperature of 200-300 C which is indicated the temperature of the reservoir in the past (paleo -temperature), while the calculation results of quartz conductive geothermometryindicate current temperatures of reservoir, is range between 166-168 C, thus the geothermal system in the Pariangan geothermal field is included into the medium temperature geothermal system with the temperature range 125-225 C. References Corbett, G.J., and Leach, T.M. (1997) Southwest Pacific Rim Gold-Copper Systems:Structures, Alteration, and Mineralization, Short Course Manual. Hochstein, M.P., and Browne, P.R.L. (2000) Surface Manifestation of Geothermal Systems With Volcanic Heat Sources in Encyclopedia of Volcanoes, Academic Press. Lawless, et al. (1998) Hydrothermal Mineral Deposits in the Arc Setting, Kingston Morrison. Nicholson, K. (1993) Geothermal Fluids : Chemistry and Exploration Techniques, Springer-Verlag, Berlin. Pusat Sumber Daya Geologi ( 2014) Laporan Pendahuluan Survei Terpadu Geologi dan Geokimia Daerah Panas Bumi Pariangan, Kabupaten Tanah Datar, Provinsi Sumatera Barat, Bandung. White, N.C., and Hendenquist, J.W. (1995 )Epithermal Gold Deposits: Styles, Characteristics and Exploration,Special Publication in SEG Newsletter No. 23, pp 1, 9-13. 245