GRC Transactions, Vol. 39, 2015

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Tourmaline in Geothermal Systems: An Example From Darajat, Indonesia

Thomas M. Etzel1,2, Joseph N. Moore2, John R. Bowman1, Clay G. Jones2, Rindu Grahabhakti Intani3, Glenn Golla3, and Greg Nash2

1Dept. of Geology and Geophysics, University of Utah, Salt Lake City UT 2Energy & Geoscience Institute, University of Utah, Salt Lake City UT

3Chevron Geothermal Indonesia, Ltd

KeywordsTourmaline, water-rock interaction, vapor-dominated, Indonesia, Darajat, geochemistry

ABSTRACT

The chemical compositions of two separate tourmaline populations (Stage 1 and Stage 3) from well DRJ-S1 at Darajat have been determined. This data, alongside petrologic observations, is used to improve our understanding of the evolution of the Darajat vapor-dominated geothermal system. Stage 1 tourmalines (replacing feldspar) have distinctly higher Fe/(Fe + Mg) and Na/(Na + Ca) ratios than Stage 3 tourmalines (formed in anhydrite veins). Mineral paragenesis and the high Fe content of the tourmalines suggest that Stage 1 formed in a higher temperature, fluid-dominated environment following the emplacement of subvolcanic intrusives. Ca-abundant Stage 3 tourmalines formed after descending steam condensates caus-ing advanced argillic alteration began to neutralize. Continued anhydrite and calcite deposition (due to the heating of descend-ing steam condensates) at shallow levels reduced porosity and permeability, impeding reservoir recharge, resulting in the current vapor-dominated system.

Introduction

Tourmaline is a common accessory mineral in a vari-ety of geologic settings (Hawthorne and Henry, 1999; Henry and Dutrow, 2012), but it is rarely reported from geothermal systems. In this paper, we describe the mineral paragenesis and composition of tourmaline from the vapor-dominated geothermal system at Darajat, Indonesia. These compositions are then compared to tourmalines from a variety of geothermal and magmatic systems.

Tourmaline is a borosilicate mineral that can incorporate a broad range of major and trace elements in its cation and anion sites. Its general formula is given by the expression XY3Z6(T6O18)(BO3)3V3W where: X = Na, Ca, K; Y = Li, Mg, Fe2+, Mn2+, Al, Ti4+; Z = Mg, Al, Fe3+, Cr3+, V3+; T = Si, Al, B; B = B, V = OH,

Figure 1. Simplified well and location maps of Darajat. Wells used in the study are highlighted in red circles and red well traces. Darajat and Salak geothermal fields, highlighted in yellow on inset map, are owned and operated by Chevron Geothermal Indonesia. Karaha-Bodas is circled in red.

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O; and W = OH, O, F (Hawthorne and Henry, 1999; Henry et al., 2011). This compositional spectrum has led to the rec-ognition of seventeen ideal end-members (Henry et al., 2011).

Geologic Setting

Darajat is located in West Java, approximately 50 km southeast of Bandung and 25 km west of Garut (Figure 1). The field is owned and operated by Chevron Geothermal Indonesia, Ltd and currently has a capacity of 271 MWe. The Darajat reservoir is developed primarily in intrusive rocks that are informally referred to as the Andesite Complex, a feature recognized in many wells by early Amoseas Indonesia Inc. geologists, who first explored the field. The intrusive rocks consist primarily of diorite and biotite hornfels, representing their contact metamorphic equivalents. Lava flows are mainly andesitic in composition, although basalts are also present (Bogie and MacKenzie, 1998; Herdianita, 2006; Rejeki et al., 2010). The intrusive rocks are interpreted to be the remnants of a subvolcanic complex that was altered and eroded prior to the formation of the modern vapor-dominated geothermal system. The Andesite Complex is overlain by a thick sequence of pyroclastic and volcaniclastic rocks that cap the system.

Hydrothermal Alteration and Occurrence of Tourmaline

Hydrothermal altera-tion has been characterized in core and well cuttings from four wells—DRJ-S1, DRJ-29, DRJ-21, and DRJ-18ST1—in the Darajat field (Moore unpub., 2007); the locations of these wells are shown in Figure 1. Figures 2-3 show the distribution of hydrothermal alteration minerals (including tourmaline) in wells DRJ-S1 and DRJ-29 based on petrographic and X-ray diffraction analyses. The hydrothermal minerals can be divided into four distinct stages based on textural relationships and cross-cutting veins first described by Moore (unpub., 2007) (Table 1).

Stage 1 mineralization represents the develop-ment of a liquid-dominated system. This stage includes argillic, phyllic and propylitic alteration that is interpret-ed to postdate the high-temperature alteration occurring in the Andesite Complex. As the system waned, zeolites (wairakite and laumontite), calcite and anhydrite were deposited. Stage 2 is represented by the formation of chalcedony after quartz. In places, the chalcedony forms overgrowths on earlier formed quartz as well as on calcite, anhydrite and wairakite. The formation of chalcedony is interpreted to reflect the boiling of an evolving liquid-dominated geothermal system and the initial formation of the vapor-dominated regime in a manner similar to that described by Moore et al.

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Figure 2. Overview of hydrothermal alteration mineralogy in well DRJ-S1 based on petrographic observations and XRD analyses. Alteration styles are as follows: 1 = argillic, 2 = silicification, 3 = phyllic, 4 = propylitic and 5 = advanced argillic. Black lines indicate the mineral has been observed at that interval; thick-ness of the line indicates abundance. Depth is in meters from the surface. I/S = interlayered illite-smectite; C/S = interlayered chlorite-smectite.

Table 1. Stages of alteration in the Darajat system. Modified from Moore (unpub., 2007).

Stage Process Alteration Type Mineralogy1 Upwelling of high temperature

NaCl fluids; liquid-dominatedShallow: argillic-phyllic alterationDeep: propylitic

Shallow: smectite, I/S, illiteDeep: epidote, chlorite, actinolite, biotite, tourmaline

2 Boiling; conversion to vapor-dominated conditions

Silicification Chalcedony, quartz

3 Descent and neutralization of steam condensates

Shallow: advanced argillic; sealing of marginal fractures

Shallow: pyrophyllite, diaspore, anhydrite, calcite, tourmalineDeep: Wairakite

4 Surficial oxidation of H2S Advanced argillic alteration Alunite, kaolinite

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(2008) for the formation of the nearby Karaha-Telaga Bodas system. Stage 3 mineralization is represented by the continued deposition of calcite and anhydrite. Stage 4 is represented by continued acid leaching and further silicification; the timing of this event is poorly understood.

Tourmaline is widely distributed through-out both wells in trace to minor amounts (Figs 2-3). This study focuses on tourmaline in well DRJ-S1. At least two petrographically distinct tourmaline populations can be identified and are associated with Stages 1 and 3. Stage 1 tourmaline partially replaces plagioclase and clinopyroxene in the diorites. It occurs in shades of blue, green and brown pleochroic crystals, often in weakly altered intrusive rocks (Figure 4). In contrast,

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WEll: DRJ-29Figure 3. Overview of hydrothermal alteration mineralogy in well DRJ-29OH based on petrographic observations and

XRD analyses. Alteration styles are as follows: 1 = argil-lic, 2 = silicification, 3 = phyllic, 4 = propylitic and 5 =

advanced argillic. Black lines indicate the mineral has been observed at that interval; thickness of the line indicates

abundance. Depth is in meters from the surface. I/S = inter-layered illite-smectite; C/S = interlayered chlorite-smectite.

↓ Figure 4. Tourmaline (Tur), epidote (Ep) and pyrite in the altered diorite. Photomicrographs of core collected from well DRJ-29 at 944.9 m. A) Plane polarized light. B) Crossed nicols. Plane of view is 0.7mm.

Figure 5. Open space in tuff filled first by acicular tourmaline (Tur) before wairakite (Wrk) and anhydrite (Anh). Photomi-crograph of core collected from well DRJ-S1 at 1807 m. A) Plane polarized light. B) Crossed nicols. Field of view is 1.4 mm. Anh = anhydrite; Tur = tourmaline; Wrk = wairakite.

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Stage 3 tourmaline forms radiating aggregates of crystals that are commonly distributed on the margins of anhydrite veins above the intrusive rocks (Figure 5). These tourmaline crystals typically lack the strong pleochroism and color characteristic of the deeper crystals. Pleochroism, if present at all, is characteristically weak in shades of light green.

Table 2. Calculated tourmaline mineral formulas. Ions are grouped according to site location. Ion values are averages of multiple spot locations within a grain. “n =” indicates the number of spot analyses contributing to the averaged value. “☐” indicates X-site vacancy. Boron is assumed to be stoichiometric (B = 3 apfu).

Sample = 975.4-1 975.4-7 975.4-5 821.4-1 821.4-2 670.6-2 670.6-1 670.6-4 670.6-3 807.7-1 807.7-2n = 3 2 5 4 3 5 5 4 3 3 2

Stage = 1 1 1 3 3 3 3 3 3 1 1X-Site

Na 0.521 0.618 0.490 0.436 0.383 0.318 0.355 0.312 0.384 0.304 0.431Ca 0.770 0.224 0.170 0.333 0.514 0.306 0.230 0.195 0.622 0.110 0.163

K 0.004 0.000 0.008 0.000 0.058 0.086 0.169 0.099 0.099 0.255 0.007☐ 0.194 0.155 0.332 0.229 0.186 0.320 0.280 0.398 0.128 0.332 0.399

Y-SiteFe 1.412 1.303 1.446 0.276 0.554 0.716 0.491 0.657 0.633 1.075 0.868

Mg 1.317 1.497 1.253 1.598 1.736 1.973 1.598 1.674 1.648 1.609 1.528Mn 0.015 0.014 0.022 0.004 0.021 0.031 0.019 0.020 0.032 0.040 0.012Al 0.057 0.123 0.179 0.507 0.523 0.225 0.384 0.108 0.483 0.372 0.581Ti 0.434 0.029 0.031 0.038 0.007 0.025 0.017 0.300 0.011 0.021 0.000

Z-SiteAl 5.697 5.993 6.000 6.000 6.000 5.892 6.000 5.725 6.000 5.674 6.000

T-SiteSi 6.060 5.982 6.068 6.577 6.159 6.137 6.492 6.517 6.193 6.210 5.986Al 0.009 0.060 0.000 0.000 0.000 0.000 0.000 0.106 0.000 0.000 0.036

B-SiteB 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000

W-SiteO 0.669 0.670 0.666 0.608 0.634 0.497 0.536 0.558 0.557 0.635 0.707F 0.000 0.005 0.000 0.022 0.033 0.247 0.177 0.149 0.144 0.000 0.000

Cl 0.015 0.000 0.013 0.067 0.016 0.008 0.019 0.014 0.021 0.055 0.017OH 3.335 3.335 3.333 3.304 3.317 3.248 3.268 3.278 3.317 3.354 3.354

Chemical Characteristics of the Tourmaline

Electron microprobe analyses were conducted at the University of Utah using a Cameca BX50 automated electron microprobe (EMP) with four wavelength disper-sive spectrometers, anaccelerating voltage of 15 kV, 20 nA sample current and a 1-micron beam diameter. Standards included albite (Na), sanidine (Al, K), diopside (Mg, Ca, Si), rutile (Ti), rhodonite (Mn), hematite (Fe), fluorite (F), and tugtupite (Cl). Structural formulae were calculated on the basis of the general formula XY3Z6(T6O18)(BO3)3V3W by normalizing the sum of T+Y+Z to 15 (15 cations). Light elements (B, H, O) could not be analytically determined; boron was assumed to be stoichiometric (i.e., B = 3) follow-ing the justification provided by Henry and Dutrow (1996); oxygen and hydrogen were determined following the pro-cedure originally described by Grice and Erict (1993). Li, assumed to be a negligible component in these tourmaline samples, has not been accounted for. All iron is assumed to be Fe2+. Tourmaline was analyzed in core samples from four depth intervals in well DRJ-S1. Tourmalines associ-ated with Stage 1 were analyzed from intervals 975.4 m and 807.7 m while Stage 3 tourmalines were analyzed from intervals 821.4 m and 670.1 m; calculated mineral chemistry is presented in Table 1.

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Larderello, ItalyKaraha - Telaga Bodas, IndonesiaSan Jorge porphyry, ArgentinaHnilec granite, SlovakiaThe Geysers, Califronia

Figure 6. General chemical composition diagram based on alkali con-tent of tourmaline (Hawthorne and Henry, 1999). Filled circles represent individual analyses of a sample from the Darajat system. Open symbols represent published data from other sites.

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Tourmaline compositions are plotted in figures 6 and 7. For comparison, compositions of tourmaline from Karaha-Telaga Bodas (Moore et al., 2004), The Geysers (Jones et al., in prep), Larderello (Cavarretta and Puxeddu, 1990) and two intrusive systems (Dill et al., 2012; Jiang et al., 2008) are also shown. The tourmalines from Karaha and The Geysers occur in association with mineral assemblages characteristic of Stage 3 at Darajat, and the tourmalines from Larderello and the two intrusive systems occur in mineral associations characteristic of Stage 1 at Darajat. Chemical variation in Darajat tourmalines is most noticeable in Al, Ca, Na, Mg, and Fetot while K, Ti, Mn, F and Cl concentrations are minimal and show little variation with depth or between stages. Compositionally, most tourmaline samples plot within the alkali group (Figure 6). Three analyses with no computed X-site vacancies plot within the calcic group on the right arm of the ternary diagram. These analyzed grains are quite small, so this high Ca concentration could, in part, be the result Ca-bearing minerals such as anhydrite and wairakite inadvertently contributing to those tourmaline analyses.

Tourmalines have been further subdivided into specific end-member categories (figure 7) following an accepted computational procedure of Henry et al. (2011). Computed tourmaline compositions range from schorl-dravite (alkali abundant subcategories) to uvite-feruvite compositions (calcic abundant subcategories). Stage 1 tourmalines are inter-mediate schorl-dravite in composition. Tourmalines from interval 975.4 m are classified as schorl, save one Ca-rich analysis that is classified as uvite. Tourmalines from interval 807.7 are dravitic. Stage 3 tourmalines range from dravite to uvite; tourmalines from interval 670.1 m are predominately dravitic, with a small number of calcic-rich specimens plotting in the uvite field, while tourmalines from interval 821.4 m range from intermediate dravite-uvite to dravite in composition. Stage 1 tourmalines have distinctly higher and more variable Fe/(Mg + Fe) ratios, with interval 975.4 m having the overall highest values (Figure 7a), than Assemblage 3 tourmalines. Henry and Dutrow (1996) pointed out that metagranitoid tourmalines are typically enriched in Fe. For example, main stage metagranitoid tourmalines in Hnilec granites (Jiang et al., 2008) have very Fe-rich compositions (Fig. 7a). Stage 1 tourmalines at Darajat formed during the early stages of hydrothermal alteration shortly after pluton emplacement, and are therefore related to metamorphism in the contact aureole. Tourmalines documented from similar occurrences at Karaha-Telaga Bodas, Indonesia (Moore et al., 2008), Larderello, Italy (Cavarretta and Puxeddu, 1990) and at the Geysers, California have similar Fe/(Mg + Fe) ratios (Figure 7), attesting to the metamorphic nature of Stage 1 tourmalines at Darajat.

With one exception, Stage 1 tourmalines have higher Na/(Na + Ca) ratios than Stage 3 tourmalines (Figure 7a), but Stage 3 tourmalines have more variable Na/(Na + Ca) ratios than Stage 1 tourmalines. Both groups of tourmaline exhibit significant variations in Na and Al contents (Fig. 7b). Na is negatively correlated with Al in Stage 1 tourmalines, with two exceptions, and Na is positively correlated with Al in Stage 3 tourmalines, primarily within the tourmalines from interval 670.1 m. The NaAl(CaMg)-1 exchange vector can explain in general the positive correlation of Na and Al, and variable Na/(Ca + Na) (Na-Ca substitution) on the X-site in Stage 3 tourmalines (Figure 7b). However because Na/(Ca + Na) ratios are relatively constant in Stage 1 tourmalines, the variations in Na content and negative correlation of Na with Al in Stage 1 tourmalines are more likely the result of the NaMg(☐Al)-1 exchange vector (Fig. 7b).

Natural and laboratory-generated data have revealed a correlation between increasing Na content of tourmaline and increasing temperature in a metamorphic environment (von Goerne et al., 2001). The Na-enriched Stage 1 tourmalines likely formed at higher reservoir temperatures than did Stage 3 tourmalines, although fluid inclusion data are needed for validation. However, reservoir rock chemistry will also have an important impact on tourmaline composition. Stage 1

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tourmalines formed at the expense of magmatic minerals abundant in Na, while Stage 3 tourmalines formed in anhydrite veins sourcing cations from a Ca-enriched hydrothermal fluid.

Despite the occurrence of fluorite and zunyite in Stage 3, fluorine is a relatively insignificant component of tour-malines except for interval 670.1 m. Although Henry and Dutrow (2011) found that as X-site charge decreases F content also decreases, our limited data set fails to produce a similar observation. The significance of F- and B-bearing minerals however is their suggestion of a magmatic contribution to hydrothermal fluids during the formation of assemblage 3.

Summary and Conclusions

A significant feature of the mineral assemblages in the tourmaline-bearing Darajat wells is the presence of miner-als characteristic of advanced argillic alteration (Figure 8). These minerals include diaspore, pyrophyllite, zunyite and kaolinite. In places, alunite and fluorite are also present. Calcite and anhydrite are common, as is wide-spread silicification and intense cation leaching. Pyrophyllite and diaspore are diagnostic of temperatures exceeding ~2500C and acidic conditions (pH <2-3). Tourma-line and fluorite are temporally associated with the advanced argillic alteration assemblages but persist to greater depths.

Stage 3 tourmalines ex-ist as a fracture-filling phase alongside late-stage calcite and anhydrite overprinting earlier propylitic assemblages containing quartz, epidote and actinolite. Due to the retro-grade solubility of both calcite and anhydrite, tourmaline-bearing anhydrite veins were deposited by downward perco-lating acid-sulfate waters that developed as steam condensate percolated downward. Water-rock interactions acted to neutralize the descending acid-condensates by introducing Ca ions, leached from the plagioclase abundant wall-rock, into solution. This resulted in generally higher Ca/(Ca+Na) ratios for stage 3 tourmalines.

Moore et al. (2008) described similar relationships at Karaha-Telaga Bodas. They observed tourmaline in well T-2, the well drilled closest to the acid Telaga Bodas Lake. This lake is interpreted to overlie a magmatic vapor chimney whose composition is strongly influenced by magmatic gases, particularly HCl and SO2. Advanced argillic alteration occurs in the upper part of T-2, and in addition to tourmaline, fluorite and native sulfur are locally present. These minerals were not found in other wells at Karaha-Telaga Bodas. Fluid inclusions trapped in anhydrite indicate that tourmaline deposition occurred at temperatures of about 235°C. Based on these relationships, it was concluded that the B and F were magmatic in origin.

The origin of the B and F at Darajat is less certain, although the close association with diaspore, pyrophyllite, and the F-bearing minerals zunyite and fluorite, is also strongly suggestive of a magmatic origin.

Darajat is currently a vapor-dominated system. Hydrothermal mineral assemblages identified in this work and pre-vious studies (Moore unpub., 2007; Herdianita, 2006) suggest the system was once liquid dominated. Using paragenetic relationships, the evolution of this geothermal system is summarized in figure 9 and below.

Stage 1 alteration minerals document an early liquid dominated system. Shallow argillic-phyllic alteration and deep propylitic alteration indicates temperatures increased with depth. The presence of tourmaline in proximity to the contact near the diorite intrusions suggests: 1) temperatures exceeded 300 oC (Corbett and Leach, 1998) and 2) that magmatic fluids introduced B into the system.

Widespread chalcedony deposition, associated with Stage 2, signifies a change in the physio-chemical conditions of the reservoir. Chalcedony in geothermal systems indicates temperatures below 180 °C (Fournier, 1985) and is therefore rarely observed in the propylitically altered high temperature portions of a system. Fluid inclusion data from this genera-tion of chalcedony is needed to constrain temperatures during this time. However, vapor rich inclusions trapped in quartz

Figure 8. Correlation of tourmaline occurrences and advanced argillic alteration assemblages at Darajat.

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crystals observed by Moore (2007) suggest a rapid reduction in reservoir pressures, allowing fluids to flash, thus creat-ing vapor to be trapped in inclusions during chalcedony deposition. Rapid depressurization resulting in the deposition of chalcedony also occurred in Karaha – Telaga Bodas; this was interpreted to have resulted from a flank collapse event (Moore et al., 2008). It is possible this too has occurred at Darajat. Alternatively, movement along the Kendang fault could have led to rapid depressurization. In either case, Stage 2 marks the transition from liquid-dominated to vapor-dominated.

During stage 3 alteration ascending magmatic volatiles condensed, forming acidic fluids that migrated laterally and downward through fractures and faults (fig. 9). These migrating acidic fluids enriched in H2S, SO4 and CO2 interacted with the wall-rock, resulting in advanced argillic alteration at shallow depths. This interaction introduced Ca ions into solution, thus gradually increasing the pH. As these slightly acid-to-near neutral waters continued to descend below the advanced argillic horizon, increasing temperatures resulted in calcite and anhydrite deposition. Physiochemical condi-tions were such that boric acid in solution also reacted with Ca ions and the surrounding wall-rock to crystallize stage 3 tourmalines. Vapors enriched in SO2, CO2, B and F present in the steam reservoir resulted in stage 3 mineral assemblages. Continued fracture filling decreased the porosity and permeability, impeding reservoir recharge, therefore sustaining vapor dominated conditions that still persist.

Acknowledgements

The authors thank the staff and management of Chevron Geothermal Indonesia, Ltd for providing the samples and well log data for this study. We would also like to thank Wil Mace for his help with the electron microprobe analyses.

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Figure 9. North to south cross-section of the Darajat field. The reservoir consists of diorite intrusions and Stage 1 propylitic alteration (green). The tectonic erosional surface (red line) resulted following a flank collapse. Younger tuffs, lahars and subvolcanic intrusions were then deposited. Ascending acid-rich vapors condensed (white arrows) and began to percolate downward through faults and fractures; these condensates formed advanced argillic alteration halos (dark green) and Stage 3 anhydrite-tourmaline veins.

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