Variations in the Composition of Epidote in the Karaha-Telaga Bodas Geothermal System

Transcription

1 GRC Transactions, Vol. 39, 2015 Variations in the Composition of Epidote in the Karaha-Telaga Bodas Geothermal System Emma Grace McConville 1, Dr. Philip Candela 2, Philip Piccoli 2, and Joseph Moore 3 1 University of Maryland 2 Advisor, University of Maryland 3 Advisor, University of Utah Keywords Indonesia, Galunggung volcano, Karaha-Telaga Bodas, geothermal, epidote, chlorite and plagioclase ABSTRACT The Karaha-Telaga Bodas vapor-dominated geothermal system is located on the flank of the active Galunggung volcano in West Java. This study will focus on the epidote-bearing mineral assemblages in the system and the chemical reactions that produce epidote. Compositional variability in hydrothermal epidote has been attributed to a number of possible factors including bulk rock composition, fluid chemistry, water to rock ratios, and variations in temperature and pressure. This study aims to evaluate whether the chemical composition of epidote is influenced by: (1) the composition of associated plagioclase, (2) distance from veins, and (3) depth. This study will use wavelength dispersive spectroscopy analysis and X-ray diffraction analysis to facilitate the identification of influential variables in determining the variation in the composition of epidote in the Karaha-Telaga Bodas geothermal system. Initial findings suggest that the plagioclase composition and the distance from veins and depth do not play a role in epidote composition. Nevertheless, chlorite composition does appear to change with depth. Adjustments to the initial hypotheses and new hypotheses have evolved as a result of the initial data. Introduction and Background Karaha-Telaga Bodas Geothermal Field and Galunggung Volcano Karaha-Telaga Bodas is a vapor-dominated geothermal system located on the southeast flank of the Galunggung volcano in West Java, Indonesia. The Karaha-Telaga Bodas geothermal system located near a subduction zone, and has characteristics that make it distinct from other geothermal systems in extensional basins, such as those found in the western United States, and caldera systems, which can be found in Yellowstone. Galunggung volcano is one of the island arc volcanos formed by the subduction of the Australian plate under the Eurasian plate. According to Moore and others (2008) and Bronto (ms, 1989) the Galunggung volcano is comprised of the Galunggung Group that encompasses the Old Galunggung Formation, the Tasikmalaya Formation, and the Cibanjaran Formation overlying a granodiorite basement. The Galunggung Group is comprised mainly of basaltic pyroclastic flows, lahars, and lava flows that are dated between 50,000 to 100,000 years (Bronto, ms, 1989) (see Table 1). The Karaha-Telaga Bodas geothermal system has evolved over time from an over-pressurized liquid-dominated system to a current vapor-dominated system. This change is presumably caused by the collapse of the southeast flank and the depressurization of the liquid-dominated geothermal system at approximately 4,200 ± 150 years BP (Moore and others, 2008) and (Bronto, ms, 1989). Fluid flow through the system is dominantly controlled by fractures that are the consequence of the present day strike-slip stress regime (Nemčok and others, 2007). Moore and others (2008, 2004a) have studied the present temperatures, pressures and fluid composition in the Karaha-Telaga Bodas geothermal field, and have categorized four distinct mineral assemblages that characterize the evolu- 465

2 Table 1. Composition of representative Galunggung volcanic rocks. Old Galunggung Caldera Formation tion of the geothermal system. The early stage mineral assemblage reflects the mineralogy of the liquid-dominated system and is identified by the presence of epidote, illite, actinolite, biotite and tourmaline. The early stage mineral assemblage includes plagioclase grains with compositions ranging roughly from 50 to 92 percent anorthite (Bronto, ms, 1989). The following stage reflects a mineral assemblage that is characterized by amorphous silica, quartz, and chalcedony from the boiling of fluids. The third stage mineral assemblage reflects the inflow of steam-heated water and the deposition of calcite, anhydrite and wairakite. The fourth stage represents a mineral assemblage of salts (NaCl, KCl, and FeCl x ) on the surface of the rocks. Figure 4 indicates the depth of the first appearance of a mineral. This study will focus primarily on the mineral epidote in the first mineral assemblage Sample L VB30A AK Major Elements (wt %) SiO TiO Al 2 O Fe 2 O 3 * MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total Fe 2 O 3 * all iron is considered ferric. Data from (Bronto, ms, 1989). Table 2. Minerals identified in samples using Energy Dispersive Spectroscopy. Sample Minerals 396 m Adularia*, chlorite, chloroapatite, epidote, hematite or magnetite, ilmentite, plagioclase, pyrite, quartz, titanite 961 m Adularia*, calcite, chalchopyrite, chlorite, epidote, fluorine, hematite or magnetite, ilmenite, plagioclase, pyrite, quartz, rutile*, titanite 1,193 m Amphibole, barite, chalcopyrite, epidote, hematite or magnetite, plagioclase, pyrite, quartz 1,249 m Adularia, amphibole, chlorite, hematite/magnetite, ilmenite, plagioclase, pyrite, titanite * Wavelength Dispersive Spectroscopy used to identify phases. Figure 1. Map of Java, Indonesia. The square indicates the location of Galunggung Volcano where the Karaha-Telaga Bodas geothermal system is located. (from Moore and others, 2008). Figure 2. Topographic map of Galunggung Volcano. Drill cores, crater, fumaroles, thermal springs, and lakes are labeled. (from Moore and others, 2008). 466

3 T-2 McConville, et al. Elevation (m) S Kawah Saat Telaga Bodas volcanic conduit granodiorite (dashed where inferred) system (Moore and others, 2008). meters Figure 3. North-south cross section of a region in Figure 2. (modified from Moore and others, 2008) of a conceptual model of the Karaha-Telaga Bodas geothermal field with labeled drill cores. Solid horizontal lines in the subsurface represent the temperature measured, the dashed lines are inferred temperatures. Kawah Karaha N - Hydrothermal Minerals of Interest Understanding the factors that affect the chemical composition of epidote provides insight into the complex dynamics of the hydrothermal system over time, and has implications for developing geothermal energy production. Epidote is an important mineral to study because it is one of the few minerals that contains ferric iron as an essential structural constituent. The chemical variation of epidote can provide insight into the history of the temperature, pressure, permeability and fluid chemistry of the geothermal system, if properly interpreted (Browne, 1978; Giggenbach, 1981; Henley and Ellis, 1983; Bird and others, 1984; Reyes, 1990; Absar, 1991; Reed, 1994; Muramatsu and Doi, 2000; and Bird and Spieler, 2004). Epidote has been found in hydrothermal systems at temperatures greater than 200 C, but it is most often found in volcanic rocks between 230 C and 260 C (Bird and Spieler, 2004). The range of temperatures, pressures, and thermodynamic chemical potentials have been determined through experimental data, fluid inclusion analysis, isotopic composition analysis of coexisting phases, stratigraphic reconstruction, and the thermodynamics of mineral assemblages (Bird and others, 1984). Hydrothermal epidote forms in veins and vugs and can replace carbonates, iron-oxides, and silicates (Bird and Spieler, 2004). Epidote can also form as a product of volcanic glass alteration, leaving behind an altered groundmass with relatively unaltered mineral grains (Figure 4) (Moore and others, 2008). Variability of Fe 3+ /Al 3+ in hydrothermal epidote has been attributed to a number of possible factors, including hydrothermal oxidation and carbonation reactions, bulk rock composition, fluid chemistry, fluid to rock ratios, and variations in temperature and pressure. However, the principle factors thought to affect the chemical composition of epidote are bulk rock and fluid composition (Bird and others, 1984, 2004) and Figure 4. North to south cross section of Karaha-Telaga Bodas geothermal system. Black solid lines indicate the first appearance of minerals of interest. (A) First appearance of smectite, smectite-illite, and illite. (B) First appearance of epidote, actinolite, biotite, and tourmaline. The top of the propylitic zone is defined by the first appearance of epidote. In sample 396 m epidote is present above the stated epidote surface Moore et al. (2008) defines. (C) First appearance of anhydrite and wairakite (from Moore and others, 2008). 467

4 Shikazono (1984). Geothermal systems are often highly altered and it is often difficult to identify the original host rock composition. It is important to study the chemical variations of mineral phases found with epidote that exchange iron and aluminum to determine their ability to limit or enhance the production and chemical composition of epidote.. For example, epidote in the presence of hematite has been found to contain more iron and less aluminum compared to epidote found with prehnite, pyrite, and pyrrhotite (Shikazono, 1984). Additionally, chemical reactions involving epidote, chlorite, prehnite, titanite, and rutile can be used to understand the complexities of alteration. The J.B. Thompson Jr. method can be used to identify continuous chemical reactions that form and vary the chemical composition of epidote and associated mineral phases (see Appendix A) and (Thompson, 1982). The composition of the mineral phases and the presence of certain mineral assemblages will provide insight into the chemical reactions that have occurred in the Karaha-Telaga Bodas geothermal system. Chemical Reactions Involving Epidote The chemical composition of epidote is a reflection of the mineral assemblage at a given temperature and pressure. In this section I will decipher some of the continuous and discontinuous reactions that involve epidote based on the phases identified using Energy Dispersive Spectroscopy (EDS) (see Table 2) and phases Moore and others (2008) identified in the Karaha Telaga Bodas geothermal system. These phases include: actinolite, calcite, chlorite, clinopyroxene, epidote, plagioclase, prehnite, quartz, and titanite. Only a few of the aforementioned phases are found together in each sample. Identified phases are used to algebraically derive balanced chemical reactions using the J.B. Thompson Jr. (1982) methodology (see Appendix A). These reactions do not occur in isolation, but serve as a guide to understand the reactions that may occur with phases present. Exchange components will be used instead of explicit phases to best generalize and not limit the scope of the reaction. By identifying mineral assemblages in each of the samples (see Table 2) and measuring the chemical composition of each phase, I plan on identifying the chemical reactions that contribute to the chemical composition of epidote in in the Karah-Telaga Bodas geothermal system. In particular, I am interested in the changes in Fe/Al ratio in epidote and associated phases and the Fe/Mg ratio changes in phases associated with epidote. Below are some of the chemical reactions that could be taking place in the Karaha-Telaga Bodas geothermal system. Ca 2 Al 2 Si 3 O 10 (OH) Al 2Si -1 Mg SiO FeAl -1 = 1 2 H 2O + FeMg O 2 + Ca 2 Al 3 Si 3 O 12 (OH) (Prh ) (Qtz) (Cz) This continuous reaction has an additive component of oxygen and exchange components. The Al 2 Si -1 Mg -1 and FeMg -1 exchange components operate in chlorite, whereas the FeAl -1 exchange component operates in chlorite, epidote, and prehnite. If the reaction proceeds to the right, as a reduction reaction, the iron content in chlorite, epidote, and prehnite decreases (it is exchanged for aluminum). The volume of prehnite and quartz decreases and oxygen and water are produced. The chlorite becomes less aluminous and its iron increases with respect to magnesium. The effect of oxygen as a product of this reaction is the reduction of ferric iron in epidote and to a lesser extent in prehnite and chlorite to ferrous iron in chlorite. The production of water in this reaction indicates a net mass transfer from prehnite to epidote. If this reaction were viewed in isolation one would see a decrease in the volume of prehnite and an increase in the volume of epidote. Additionally the epidote would become a paler green and the chlorite would not appreciable change in volume, but it would become darker in green color. Mg 5 AlAlSi 3 O 10 (OH) 8 + 6FeMg O 2 = 4H 2 O + 4SiO 2 + 6FeAl -1 + Al 2 Si -1 Mg -1 (Clc ) (Qtz) Clinochlore is the magnesium endmember of the chlorite group. In this reaction the Al 2 Si -1 Mg -1 and FeMg -1 exchange only occurs in the chlinochlore phase and the FeAl -1 exchange occurs in epidote (and prehnite if present). If the reaction proceeds to the right as an oxidation reaction, clinochore will react and become more magnesium-rich relative to iron. Additionally, the magnesium and silica in the chlorite will exchange for two aluminums. Consequently the chlorite will be more aluminous and lighter in color and the Fe/Al ratio will decrease in chlorite. The iron reacted with the magnesium in the FeMg -1 exchange component will increase the iron in the preexisting epidote. Epidote will be greener in color and have a higher Fe/Al ratio. 3Ca 2 Al 2 Si 3 O 10 (OH) 2 + 2CO 2 = 2Ca 2 Al 3 Si 3 O 12 (OH) + 3SiO 2 + 2H 2 O + 2CaCO 3 (Prh ) (Cz) (Qtz) (Cal) If the reaction proceeds to the right, prehnite reacts with carbon dioxide to produce clinozoisite, quartz, water, and calcite. Although it is not explicitly expressed in this reaction there is an exchange between aluminum and iron in prehnite and in epidote (clinozoisite). During this reaction prehnite decreases in volume, while clinozoisite and calcium carbonate increase in volume. The production of clinozoisite (epidote) is dependent on the ratio of moles of prehnite to moles 468

5 McConville of carbon dioxide, 3 to 2, respectively. The amount of clinozoisite (epidote) produced will be dependent on the relative amount of prehnite and carbon dioxide present in the system. Ca 2 Fe 3 Si 3 O 12 (OH) + SiO H 2O + 2 TiO 2 = 2 CaTiSiO O Fe 6Si 4 O 10 (OH) 8 (Ep ) (Qtz) (Rt) (Tnt) (Fe-chl endmember) If the reaction proceeds to the right, the epidote endmember pistacite reacts with quartz, water, and rutile to produce titanite, oxygen and a theoretical iron endmember of chlorite. The oxygen is an external variable in this reaction. Proceeding from left to right, this is a redox reaction, wherein oxygen reacts with hydrogen to form water. If the reaction proceeds from right to left, it is an oxidation reaction, and the ferrous iron is oxidized to form ferric iron. During oxidation, the volume of pistacite increases at the expense of the volume of the iron endmember of chlorite. In this reaction titanite serves as a calcium buffer that can consume or supply the system with calcium. In drill core T-2, titanite may have formed from TiO 2 in volcanic glass or from rutile in the lava flow, but it is not an original igneous phase in the parent Old Galunggung Formation. Fe 5 Al 2 Si 3 O 10 (OH) 8 + SiO CaTiSiO 5 = 1 3 H 2O Ca 2Al 3 Si 3 O 12 (OH) TiO Fe 6Si 4 O 10 (OH) 8 (Chm ) (Qtz) (Tnt) (Cz) (Rt) (Fe-chl endmember) If this reaction proceeds to the right, chamosite reacts with quartz and titanite to produce water, clinozoisite, rutile, and a less aluminum rich iron endmember of chlorite. The more aluminous chlorite will be lighter than the less aluminous chlorite on the left. As identified in the previous equation, titanite serves as a calcium buffer. Free water on the right side of the reaction indicates a dehydration reaction brought on by increasing temperature. Ca 2 Fe 3 Si 3 O 12 (OH) Fe 6Si 4 O 10 (OH) SiO 2 = Ca 2 Fe 5 Si 8 O 22 (OH) H 2O O 2 (Ep ) (Fe-chl endmember) (Qtz) (Fe2-Act) If the reaction proceeds to the right as a reduction reaction, pistacite will react with a theoretical iron endmember chlorite and quartz, to produce ferroactinolite, water, and oxygen. The ferric iron in pistacite will be reduced to produce ferrous iron in ferro-actinolite. Ferro-actinolite will increase in volume at the expense of the pistacite, the iron endmember of chlorite, and quartz. If the reaction were to proceed to the left as an oxidation reaction, the volume of ferro-actinolite would decrease and the volume of pistacite, iron endmember chlorite, and quartz would increase. As the reaction is written there appears to be no change in the Fe/Al ratio in epidote, chlorite, or actinolite. If the reaction proceeds to the right there is an increase in temperature and the pistacite and chlorite will be dehydrated. In geothermal systems, actinolite first appears between 280 to 300 C. The bottom hole temperature of drill core T-2 is 321 C (Moore and others, 2008). Although the exchange FeMg -1 is not explicitly stated in the reaction, actinolite and chlorite can participate in the exchange. With increasing metamorphic grade, chlorite becomes more magnesium rich. Ca 2 Al 3 Si 3 O 12 (OH) Fe 6Si 4 O 10 (OH) SiO 2 = Ca 2 Fe 5 Si 8 O 22 (OH) H 2O Al 2Si -1 Fe -1 (Cz ) (Fe-chl endmember) (Qtz) (Fe2-Act) If the reaction proceeds to the right, clinozoisite, the theoretical iron endmember of chlorite and quartz will react to produce ferroactinolite, water, and an exchange component Al 2 Si -1 Fe -1. The exchange component Al 2 Si -1 Fe -1 influences the chemical composition of chlorite and actinolite. On the right side of the reaction the exchange component Al 2 Si -1 Fe -1 influences the chemical composition of actinolite and chlorite by making them more aluminum-rich. Note again that the exchange between iron and magnesium is not explicitly stated, however, it is present in this reaction. The volume of clinozoisite, iron endmember chlorite, and quartz decrease and the volume of actinolite increases. If the reaction proceeded to the left, the ferrous iron in the ferroactinolite would not be oxidized and consequently there would be no exchange reaction between aluminum and iron in the clinozoisite. However, it is possible that other ongoing reactions could facilitate an increase in Fe/Al in the epidote. 1 4 O H 2O + CaFeSi 2 O 6 + CaAl 2 Si 2 O 8 à Ca 2 Al 2 FeSi 3 O 12 (OH) + SiO 2 (Hd) (An) (Ep) (Qtz) Hedenbergite, anorthite, water and oxygen react to produce epidote and quartz. This reaction is a discontinuous reaction. This reaction describes one way that epidote can be produced from minerals in a basalt by hydrothermal alteration. Additionally, oxides in basaltic glass can react with one another to produce epidote and other hydrothermal minerals. Objectives of Research and Broader Implications In this study, I will evaluate the hypotheses that the variation in the composition of epidote (principally along the FeAl -1 exchange vector) in samples from drill core (designated as T-2) can be attributed to (1) the chemical composition 469

6 of associated plagioclase, (2) proximity to veins, and/or (3) depth. Hydrothermal alteration is the principal mechanism for producing epidote in this geological setting. The measurement of iron to aluminum will be calculated by measuring the mole fraction of pistacite, X ps = Fe 3+ /(Fe 3+ +Al 3+ ). Plagioclase is a calcium bearing phase and can be a reactant in the production of epidote. Plagioclase grains are relatively unaltered compared to the groundmass in each sample. In samples with calcic plagioclase there should be more epidote, holding all other variables constant. The mole fraction of anorthite will be measured and will be compared to the mole fraction of pistacite in each sample. Veins are the principal mechanism of fluid transport in the Karaha-Telaga Bodas geothermal system (Nemčok and others, 2007). The water/rock ratio will contribute to the alteration of the host rock and consequently influence the production of epidote. Variability in the composition of the epidote with respect to distance from vein may provide insight into the water/rock ratio and fluid chemistry of the geothermal system. Depth is a proxy for temperature, and according to, Aranson and others (1993) epidote becomes more aluminum-rich with increasing temperatures. There are many factors that can influence the composition of epidote in geothermal systems; this study aims to measure whether the variables mole fraction anorthite, distance from vein, and depth, influence the composition of epidote. Simultaneously, the composition of iron bearing phases like actinolite, chlorite, and prehnite (if present) will be analyzed in order to examine the relationship in each assemblage. Specifically, I will measure the Fe/Al and Fe/Mg ratio of actinolite and chlorite. Hypothesis: The mole fraction of pistacite (X ps ) in epidote is a function of: (1) The chemical composition of associated plagioclase (2) Proximity to veins (3) Depth Experiment Design Ten thin sections from nine different depths in the T-2 drill core (396 m, 889 m, 961 m, 980 m, 1,044 m, 1,177 m, 1,193 m, 1,249 m and 1,378 m (2 thin sections)) will be examined by optical microscopy, Energy Dispersive Spectroscopy (EDS) and Wavelength Dispersive Spectroscopy (WDS). The JXA-8900 SuperProbe at the University of Maryland will be used, with the assistance of Dr. Philip Piccoli, to determine the phases in each sample and the chemical composition of epidote, plagioclase, chlorite and amphibole. X-Ray Diffraction (XRD) analysis will be done on selected samples in order to identify phases that could not be identified using optical microscopy or EDS. Identifying all phases in the sample is essential to understanding the reactants and products of geothermal reactions. Phases of interest that may be present in small quantities are the following: prehnite, magnetite, hematite, diopside, biotite and wairakite (Moore and others 2008). XRD analysis will be done by at the X-Ray Crystallography Center at the University of Maryland with Peter Zavalij. Demonstration of Feasibility (1) Chemical Composition of Plagioclase With Respect to Epidote Plagioclase and epidote grains from samples 396 m and 961 m were analyzed by EPMA. The mole fractions of anorthite in plag (n=1) from sample 396 m is 0.53 to 0.85 and in plagioclase (n=5) in sample 961 m is 0.72 to Figure 5. Chemical composition of plagioclase in terms of mole fraction of anorthite with respect to depth (m) of sample. The error bars represent a 2σ uncertainty, the error bars are contained within the point. 49 measurements were taken for sample 396 m and 7 were taken for sample 961 m. Figure 6. Mole fraction of pistacite with respect to depth (m) of sample. The error bars represent a 2σ uncertainty. 30 measurements were taken for sample 396 m and 10 measurements were taken for 961 m. 470

7 0.93. The limited range of values in sample 961 m could be attributed to fewer measurements as well as less zoning. The data suggests the composition of plagioclase in sample 961 m is more anorthite rich than in sample 396 m. A possible explanation for the different plagioclase compositions could be the depths at which each grain is found. Plagioclase found at greater depths may by more anorthite rich due to a different magma composition. The rims of the plagioclase grains may be anorthite poor due to hydrothermal alteration. Hydrothermal fluids alter the plagioclase rim and make it albite-rich. The epidote compositions measured in samples 396 m and 961 m ranges from X ps = 0.15 to 0.32 and Figure 7. Mole fraction of pistacite with respect to distance from vein (mm). The error bars represent a 2σ uncertainty. Each epidote grained was analyzed twice, a total of 32 measurements were taken for epidote grains near the vein. Figure 8. Mole fraction of Fe/Mg in chlorite with respect to distance from vein (mm). The error bars represent a 2σ uncertainty. Each chlorite grain was analyzed twice, a total of 14 measurements were taken. Figure 9. Mole fraction of Fe/Al in chlorite with respect to distance from vein (mm). The error bars represent a 2σ uncertainty. Each chlorite grain was analyzed twice, a total of 14 measurements were taken. Figure 10. Mole fraction of pistacite with respect to depth (m) of sample. The error bars represent a 2σ uncertainty. 30 measurements were taken for sample 396 m and 10 measurements were taken for 961 m. Figure 11. Mole fraction of Fe/Mg in chlorite with respect to distance from vein (mm). The error bars represent a 2σ uncertainty. 10 measurements were taken for sample 396 m and 14 measurements were taken for 961 m. Figure 12. Mole fraction of Fe/Al in chlorite with respect to distance from vein (mm). The error bars represent a 2σ uncertainty. 10 measurements were taken for sample 396 m and 14 measurements were taken for 961 m. 471

8 from X ps = 0.16 to 0.32, respectively. There is a wide spread of chemical composition for epidote that does not appear to be correlated with the chemical composition of anorthite. The broad range of chemical composition in epidote indicates that the minerals present are not in equilibrium and changes in factors that influence epidote composition have changed over the time of epidote production. More samples need to be analyzed to determine whether a trend exists between the mole fraction of anorthite in plagioclase and the composition of epidote. The abundance of plagioclase may contribute to the abundance of epidote; this hypothesis will be tested by mapping two thin sections with notably different amounts of plagioclase and measuring the areas of the plagioclase and the epidote in each thin section. (2) Proximity to Veins One vein intersecting at an angle greater than 45 relative to the cut of the thin section was identified in sample 961 m. Two WDS measurements were taken in each epidote and chlorite grain and their corresponding coordinates were documented. The X ps of epidote does not appear to have a trend with respect to the distance from the vein. Measurements were taken from 0.4 mm to 3.10 mm from the vein and may not have been sufficiently far away to observe a positive or negative correlation. In future measurements greater distances will be covered with respect to the vein. The mole fraction of Fe/Mg and Fe/Al in chlorite are between 0.73 and 1.23 and 0.8 to 1.11, respectively. This further indicates the possibility that the measurements were not taken far enough away from the vein. The ratio of Fe/ Mg and Fe/Al in chlorite and the X ps do not appear to indicate significant compositional change in either phase. At this time there is not enough data to confidently identify a trend. As in the aforementioned case of epidote, the composition of chlorite grains will be measured at greater distances from veins. (3) Depth Preliminary analysis suggests that chemical composition of epidote does not vary with depth. Thirty measurements were taken in sample 396 m and all but two measurements are clustered between X ps and Sample 961 m does not have enough measurements to make a similar observation. In the future, at least 30 measurements should be taken for epidotes in the groundmass. Chlorite appears to have a greater Fe/Mg and Fe/Al ratio in sample 961 m than at 396 m. Chlorite grains appear to become more iron-rich with respect to magnesium and aluminum with increasing depth. The difference in Fe/Mg and Fe/Al in chlorite may be caused by different original bulk rock composition. However, if both samples have the same original bulk rock composition, fluids and permeability are likely the factors controlling the chemical composition. More importantly, within the same thin section there is a wide variety of composition for epidote and chlorite, further confirming disequilibrium between phases on a micro scale. The fluid appears to be the driving factor in the variation of composition in epidote and chlorite. Compositions and textures will be compared with oxidation and reduction assemblages in order to identify areas of high and low permeability. At this time there appears to be no direct relationship between the X ps with the composition of associated plagioclase, distance from vein, and depth. However, new insights were made based on the data collected. Based on the data collected, the plagioclase composition may not influence epidote composition, but the abundance of epidote may be correlated to the abundance of plagioclase. Likewise, the epidote and chlorite compositions did not appear to be correlated with distance from the vein, but it is possible that the range of distance was not sufficient to identify a positive or negative correlation. Lastly, the epidote composition did not appear to change with respect to depth, however, the Fe/Al and Fe/Mg ratios of chlorite did. Further research is needed to understand the variables and chemical reactions in the geothermal system that may be influencing variations in phase composition in these preliminary results. Eight other samples must be analyzed in order to better understand whether the variations of epidote composition are correlated with the working hypotheses. Analysis from XRD may provide additional insight into the chemical composition of epidote and associated phases discussed in the introduction and background. Summary Epidote composition is influenced by an array of factors; primarily bulk rock composition, fluid chemistry, temperature, permeability and pressure (Browne, 1978; Giggenbach, 1981; Henley and Ellis, 1983; Bird and others, 1984; Shikazono, 1984; Reyes, 1990; Absar, 1991; Aranson and others, 1993; Reed, 1994; Muramatsu and Doi, 2000; and Bird and Spieler, 2004). Bronto (ms, 1989) characterizes the primary bulk rock as basalt (see Table 1). The characterization of epidote and associated mineral assemblages is crucial in understanding the chemical reactions that produced or influenced variations in chemical composition. Epidote grains are one of the best records of the history and evolution of the geothermal system. This investigation will measure both the chemical composition of epidote at nine depths in the Karaha-Telaga Bodas geothermal field drill core T-2 and the composition of minerals present in epidote assemblages. This study will characterize mineral reactants that produce epidote, like plagioclase, and minerals that buffer epidote production, like titanite. In par- 472

9 ticular, I will investigate whether the mole fraction pistacite (X ps ) is correlated to: (1) mole fraction anorthite in associated plagioclase, (2) distance from veins and (3) depth. Preliminary results indicate mole fraction anorthite does not influence X ps, nevertheless, a new working hypothesis has emerged as to whether the amount of plagioclase present in the sample influences the amount of epidote in the sample. Data gathered from measuring the composition of epidote and chlorite with respect to distance from vein did not appear to show a trend, however, in future analysis a greater range of distances will be measured to resolve the question of whether scaling is the issue. Lastly, the composition of epidote does not appear to be influenced by increasing depth, as first hypothesized. However, since the composition of associated chlorite did vary, there is a question as to what variables are causing this shift from higher to lower Fe/Mg and Fe/Al ratios from greater to lower depths. Further analysis and research is needed to reevaluate the original working hypotheses and test new hypotheses to better understand the production and evolution of epidote in the Karaha-Telaga Bodas geothermal system. References Absar A., Hydrothermal epidote - an indicator of temperature and fluid composition. Journal of Geological Society of India, v. 38, p Aranson J.G., Bird D.K., Liou J.G., Variables Controlling Epidote Composition in Hydrothermal and Low-Pressure Regional Metamorphic Rocks. 125 Jahre Knappenwand, v. 49, p Bird D.K., Helgeson, H.C, Chemical Interactions of Aqueous Solutions with Epidote-Feldspar Mineral Assemblages in Geologic Systems. II. Equilibrium Constraints in Metamorphic/Geothermal Processes. American Journal of Science, v. 281, p Bird D.K., Schiffman P., Elders W.A., Williams A.E., McDowell S.D., Calc-silicate mineralization in active geothermal systems. Economic Geology, v. 79, p Bird D.K., Spieler A.R., Epidote in Geothermal Systems. Reviews in Mineralogy and Geochemistry, v. 56(1), p Bronto, S., ms, Volcanic geology of Galunggung, West Java, Indonesia. New Zealand, University of Canterbury, Ph. D. thesis. Browne P.R.L., Hydrothermal alteration in active geothermal systems. Annual Review of Earth and Planetary Science, v. 6, p Giggenbach W.F., Geothermal mineral equilibria. Geochimica Cosmochimica Acta, v. 45, p Gustafson, L.B., Hunt, J.P., The porphyry copper deposit at El Salvador, Chile. Economic Geology, v. 70(5), p Harvard University, Fall Physical Sciences 2: A Summary of Error Propagation. Henley R.W., Ellis A.J., Geothermal systems ancient and modern. A geochemical review. Earth-Science Reviews, v. 19, p Lagat, J., Hydrothermal alteration mineralogy in geothermal fields with case examples from Olkaria Domes geothermal field, Kenya. Short Course IV on Exploration for Geothermal Resources, p Moore, J. N., Allis, R. G., Nemcok, M., Powell, T. S., Bruton, C. J., Wannamaker, P. E., Norman, D. I., The evolution of volcano-hosted geothermal systems based on deep wells from Karah Telaga Bodas, Indonesia. American Journal of Science, v. 308, p Moore, J. N., Christenson, B., Browne, P. R. L., and Lutz, S. J., 2004a. The mineralogic consequences and behavior of descending acid-sulfate waters: an example from the Karaha - Telaga Bodas geothermal system, Indonesia. Canadian Mineralogist, v. 42, p Muramatsu Y., Doi N., Prehnite as an indicator of productive fractures in the shallow reservoir, Kakkonda geothermal system, northeast Japan. Journal of Mineralogical and Petrological Sciences, v. 95, p Nemčok, M., Moore, J. N., Christensen, C., Allis, R., Powell, T., Murray, B., Nash, G., Controls on the Karaha Telaga Bodas geothermal reservoir, Indonesia. Geothermics, v. 36(1), p Reed M.H., Hydrothermal alteration in active continental hydrothermal systems. Geological Association of Canada, Short Course Notes, v. 11, p Reyes A.G., Petrology of Philippine geothermal systems and the application of alteration mineralogy to their assessment. Journal of Volcanology and Geothermal Research, v. 43, p Shikazono, N., Compositional Variations in Epidote from Geothermal Areas. Geochemical Journal, v. 18, p Thompson J., Composition Space: An Algebraic and Geometric Approach. Reviews in Mineralogy and Geochemistry, v. 10(2), p Appendix A J.B. Thompson Jr. Method The J.B. Thompson Jr. Method (1982) uses Gauss-Jordan elimination to add and subtract components of linear equations in order to solve a system of equations algebraically. The J.B. Thompson Jr. Method can be applied to chemical formulas and balanced chemical reactions can algebraically derived. In addition to chemical formulas, exchange components can be used to represent an element exchanges with in the phases, an example of this is the Tschermak exchange Al2Si- 473

10 1Mg-1, a silica and magnesium atom are substituted for two aluminum atoms. The following matrices were calculated in order to first address the potential chemical reactions occurring among iron bearing minerals in drill core T-2. The bolded numbers in the smaller matrices indicate equations used for calculating balanced reaction. Phases = 9 Components = 7 Reactions = 2 Phase SiO 2 Al 2O 3 CaO MgO FeO O 2 H 2O Ca 2Al 2Si 3O 10(OH) Ca 2Al 3Si 3O 12(OH) Mg 5AlAlSi 3O 10(OH) SiO O H 2O FeAl FeMg Al 2Si -1Mg Phase Al 2O 3 CaO MgO Ca 2Al 2Si 3O 10(OH) 2 H 2O SiO 2 [Al 2Si -1Mg -1 + SiO 2 (FeMg -1 - (FeAl O 2))] Ca 2Al 3Si 3O 12(OH) 0.5H 2O SiO 2 1.5[Al 2Si -1Mg -1 + SiO 2 (FeMg -1 - (FeAl O 2))] Mg 5AlAlSi 3O 10(OH) 8 4H 2O SiO 2 + 5[FeMg -1 - (FeAl O 2)] - [Al 2Si -1Mg -1 + SiO 2 (FeMg -1 - (FeAl O 2))] [(FeMg -1 - (FeAl O 2)] [(FeMg -1 - (FeAl O 2)] [Al 2Si -1Mg -1 + SiO 2 (FeMg -1 - (FeAl O 2))]- [Al 2Si -1Mg -1 + SiO 2 (FeMg -1 - (FeAl O 2))] Reactions: 1) Ca 2Al 2Si 3O 10(OH) 2 H 2O 3SiO 2 [Al 2Si -1Mg -1 + SiO 2 (FeMg -1 - (FeAl O 2))] = Ca 2Al 3Si 3O 12(OH) 0.5H 2O 3SiO 2 1.5[Al 2Si -1Mg -1 + SiO 2 (FeMg -1 - (FeAl O 2))] Ca 2Al 2Si 3O 10(OH) 2 + Al 2Si -1Mg -1 + SiO 2 + FeAl -1 = H 2O + FeMg -1 + O 2 + Ca 2Al 3Si 3O 12(OH) 2) Mg 5AlAlSi 3O 10(OH) 8 4H 2O 3SiO 2 + 5[FeMg -1 - (FeAl O 2)] - [Al 2Si -1Mg -1 + SiO 2 (FeMg -1 - (FeAl O 2))] = 0 Mg 5AlAlSi 3O 10(OH) 8 + 6FeMg -1 + O 2 = 4H 2O + 4SiO 2 + 6FeAl -1 + Al2Si -1Mg

11 Phases = 9 Components = 7 Reactions = 2 Phase TiO 2 CaO SiO 2 Fe 2O 3 FeO Al 2O 3 H 2O Ca 2Fe 3Si 3O 12(OH) Ca 2Al 3Si 3O 12(OH) Fe 6Si 4O 10(OH) Fe 5Al 2Si 3O 10(OH) CaTiSiO TiO SiO O H 2O Phase Fe 2O 3 FeO Al 2O 3 Ca 2Fe 3Si 3O 12(OH) 3SiO H 2O 2(CaTiSiO 5 - TiO 2 - SiO 2) [Ca 2Al 3Si 3O 12(OH) 3SiO H 2O 2(CaTiSiO 5 - TiO 2 - SiO 2)] - [Ca 2Al 3Si 3O 12(OH) 3SiO 2 0.5H 2O 2(CaTiSiO 5 - TiO 2 - SiO 2)] [Fe 6Si 4O 10(OH) 8 4SiO H 2O] - [Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O] Fe 5Al 2Si 3O 10(OH) 8 3SiO H 20 (2/3)[Ca 2Al 3Si 3O 12(OH) 3SiO 2 0.5H 2O 2(CaTiSiO 5 - TiO 2 - SiO 2)] (5/6)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O) O 2 + (2/3)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O) Reactions: 1) Ca 2Fe 3Si 3O 12(OH) 3SiO 2 0.5H 2O 2(CaTiSiO 5 - TiO 2 - SiO 2) (3/4)[ O 2 + (2/3)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] = 0 Ca 2Fe 3Si 3O 12(OH) + Fe 6Si 4O 10(OH) 8 + SiO 2 = Ca 2Fe 5Si 8O 22(OH) 2 + H 2O + O 2 2) Fe 5Al 2Si 3O 10(OH) 8 3SiO 2-4H 20 (2/3)[Ca 2Al 3Si 3O 12(OH) 3SiO 2 0.5H 2O 2(CaTiSiO 5 - TiO 2 - SiO 2)] (5/6)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O) = 0 Fe 5Al 2Si 3O 10(OH) 8 + SiO 2 + CaTiSiO 5 = H 2O + Ca 2Al 3Si 3O 12(OH) + TiO 2 + Fe 6Si 4O 10(OH) 8 475

12 Phases = 8 Components = 6 Reactions = 2 Phase CaO Al 2O 3 Fe 2O 3 FeO SiO 2 H 2O Ca 2Fe 3Si 3O 12(OH) Ca 2Al 3Si 3O 12(OH) Fe 6Si 4O 10(OH) Ca 2Fe 5Si 8O 22(OH) Al 2Si -1Fe SiO O H 2O Phase CaO Al 2O 3 Fe 2O 3 Ca 2Fe 3Si 3O 12(OH) 3SiO H 2O [Ca 2Fe 5Si 8O 22(OH) 2 8SiO 2 H 2O (5/6)( Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] Ca 2Al 3Si 3O 12(OH) 3SiO H 2O [Ca 2Fe 5Si 8O 22(OH) 2 8SiO 2 H 2O (5/6)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] 1.5[Al 2Si -1Fe -1 + SiO 2 + (1/6)( Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] [Ca 2Fe 5Si 8O 22(OH) 2 8SiO H 2O (5/6)( Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] - [Ca 2Fe 5Si 8O 22(OH) 2 8SiO 2 H 2O (5/6)( Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] [Al 2Si -1Fe -1 + SiO (1/6)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] - [Al 2Si -1Fe -1 + SiO 2 + (1/6)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] O 2 + (2/3)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O) Reactions: 1) Ca 2Fe 3Si 3O 12(OH) 3SiO 2 0.5H 2O [Ca 2Fe 5Si 8O 22(OH) 2 8SiO 2 H 2O (5/6)( Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] (3/4)[O 2 + (2/3)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] = 0 Ca 2Fe 3Si 3O 12(OH) + Fe 6Si 4O 10(OH) 8 + SiO 2 = Ca 2Fe 5Si 8O 22(OH) 2 + H 2O + O 2 2) Ca 2Al 3Si 3O 12(OH) 3SiO 2 0.5H 2O [Ca 2Fe 5Si 8O 22(OH) 2 8SiO 2 H 2O (5/6)(Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] 1.5[Al 2Si -1Fe -1 + SiO 2 + (1/6)( Fe 6Si 4O 10(OH) 8 4SiO 2 4H 2O)] = 0 Ca 2Al 3Si 3O 12(OH) + Fe 6Si 4O 10(OH) 8 + SiO 2 = Ca 2Fe 5Si 8O 22(OH) 2 + H 2O + Al 2Si -1Fe

13 McConville, et al. Appendix B Thin Section Scans (optical) Sample 396 m Sample 889 m Sample 1,193 m Sample 1,249 m Sample 961 m Sample 1,378 m (A) Sample 980 m Sample 1,044 m Sample 1,117 m Sample 1,378 m (B) All sample numbers are based on the depth from the surface of the drill core. Thin sections are 27 x 46 mm. 477

14 Appendix C Representative Energy Dispersive Spectroscopy Images Barite Chlorite Calcite Chloroapatite Chalcopyrite Epidote 478

15 Hematite/Magnetite Pyrite Ilmenite Titanite Plagioclase Quartz Note: there is no EDS image for amphibole. 479

16 Appendix D Wavelength Dispersive Spectroscopy Data for Chlorite, Epidote, and Plagioclase BD = below detection limit Chlorite Detection limit for chlorite (ppm) Na2O MnO K2O CaO SiO2 MgO FeO TiO2 Al2O3 Chlorite Sample 396 m Wavelength Dispersive Spectroscopy Composition (wt %) No. Description Na2O MnO K2O CaO SiO2 MgO FeO TiO2 Al2O3 Total 1 traverse, points manually selected traverse, points manually selected traverse, points manually selected DB Sample 396 m Wavelength Dispersive Spectroscopy Relative Uncertainty due to Counting Statistics No. Na2O MnO K2O CaO SiO2 MgO FeO TiO2 Al2O Sample 961 m Distance from Vein Wavelength Dispersive Spectroscopy Composition (wt %) No. Distance Na2O MnO K2O CaO SiO2 MgO FeO TiO2 Al2O3 Total (mm) C C C C

17 C C C C Sample 961 m Wavelength Dispersive Spectroscopy Relative Uncertainty due to Counting Statistics No. Na2O MnO K2O CaO SiO2 MgO FeO TiO2 Al2O3 C C C C C C C C Epidote Detection limit for epidote (ppm) Oxide Na2O MnO K2O CaO SiO2 MgO FeO TiO2 Al2O3 Epidote Sample 396 m Wavelength Dispersive Spectroscopy Composition (wt %) Grain Description Na2O MnO K2O CaO SiO2 MgO FeO TiO2 Al2O3 Na2O 1a elongate BD traverse, points auto selected, each µm apart BD BD BD b short traverse, BD points auto selected each 6.87 BD µm apart

18 traverse across epidote grain towards plagioclase grain points taken 9.94 µm apart core core BD rim rim core BD core rim core BD rim BD BD rim rim rim Sample 396 m Wavelength Dispersive Spectroscopy Relative Uncertainty due to Counting Statistics Grain Na2O MnO K2O CaO SiO2 MgO FeO TiO2 Al2O3 1a b