Comparative Analysis of Fluid Chemistry from Cove Fort, Roosevelt and Thermo: Implications for Geothermal Resources and Hydrot

Home , Anderson (crater), Blackett (crater)

GRC Transactions, Vol. 39, 2015

Comparative Analysis of Fluid Chemistry From Cove Fort, Roosevelt and Thermo: Implications for Geothermal Resources and Hydrothermal Systems on the East Edge of the Great Basin

Stuart Simmons1, Stefan Kirby2, Joe Moore1, Phil Wannamaker1, and Rick Allis2 1EGI, University of Utah, Salt Lake City, Utah 2Utah Geological Survey, Salt Lake City [email protected]

Keywords Geothermal resources, hydrothermal geochemistry, exploration, geology, Cove Fort, Roosevelt, Thermo, Great Basin, Sevier thermal anomaly

Abstract

We assessed the thermal water compositions from Cove Fort, Roosevelt, and Thermo, and scattered hot springs to evaluate the controls on thermal water compositions within the Sevier thermal anomaly on the eastern edge of the Great Basin. The reservoir temperatures range from 150 to 250° C, and the reservoir rocks are diverse, including granite-gneiss, marine carbonates, and siliciclastic sequences. On the basis of major anions, the thermal waters are classified as chloride, sulfate, and hybrid chloride-sulfate type that contain variable but lower concentrations of bicarbonate. Low Mg and relatively low Cl/B further distinguish the reservoir waters at Cove Fort and Roosevelt compared to high-Mg waters with high Cl/B ratios. The oxygen and hydrogen isotopic compositions reflect the influence of local meteoric water, and the range of compositions reflect geography, recharge elevation, degree of water-rock interaction, and possibly age. Positive correlation between helium isotope R/Ra values and reservoir temperatures indicate mantle helium and magmatic heat are associated with convective fluid flow. Application of chemical geothermometers shows that aqueous silica concentrations are the most reliable geochemical indicator of a minimum resource temperature; in low-Mg thermal waters, hot Na/K temperatures possibly reflect deep equilibration with basement crystalline rocks. Locally, heat transfer is focused along range front faults in the form of convective geothermal systems with relatively small diameter upflow zones. The large regional endowment of thermal energy associated with hot basement rocks suggests there is considerable potential for finding a spectrum of blind resources including those occurring in deep sedimentary aquifers and EGS reservoirs.

Figure 1. Map of the Sevier thermal anomaly, showing the locations of springs and thermal areas with respect to alluvium filled basins (Qal), Quaternary volcanic centers (Qv), and uplifted highlands (TMP), which expose variably deformed sequences made of igneous, metamorphic and sedimentary rocks of Tertiary through to Precambrian age (Hintze, 1980; Cole, 1983; Mabey and Budding, 1987; Ross et al., 1993; Blackett and Wakefield, 2002). Small black circles represent the thermal springs. The hatched area represents the area of the Pavant Butte prospect (Hardwick and Chapman, 2012; Gwynn et al., 2013; Allis et al., 2015b).

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Introduction

Cove Fort, Roosevelt, and Thermo are convective geothermal systems that are part of the Sevier thermal anomaly (Fig. 1), which lies on the eastern edge of the Great Basin (Mabey and Budding, 1987; Blackett, 2007). The regional thermal anomaly extends across parts of the eastern Basin and Range Physiographic Province and the western part of the Basin and Range-Colorado Plateau transition zones (e.g. Wannamaker et al., 2001), and it is characterized by elevated heat flow, active seismicity, and Quaternary basalt-rhyolite magmatism (Mabey and Budding, 1987; Blackett, 2007). As is typical of many of the geothermal systems in the Great Basin, fluid flow at reservoir depths is strongly influenced by basin-bounding faults (N-S trending), which are a relatively young geologic feature resulting from Basin and Range extension. Within our study area, fluid flow is also affected by geological complexity arising from low-angle thrust/ detachment faults, a mix of metamorphic, igneous (intrusive and extrusive), and sedimentary rocks, plus the development of structures inherited from underlying Precambrian basement. The proximity of three closely spaced geothermal systems, within a regional thermal anomaly, combined with scattered hot springs, provides an ideal setting in which to evaluate the controls on thermal water compositions. In this report we describe and compare the chemical and isotopic compositions of thermal waters from Cove Fort, Roosevelt, Thermo, and hot springs in the region. This is part of a larger investigation directed at understanding a range of geothermal resource types in southwestern Utah.

Geothermal Systems and Geological Context Located along the west flank of the Mineral mountains, the Roosevelt geothermal resource is the largest and hottest known in the study area. Power production began in 1984, and the current installed capacity is ~30 MWe. The reservoir is hosted by granitic and metamorphic basement rocks that have been heated by recent intrusions related to 0.5-0.8 Ma rhyolite domes (e.g., Moore and Nielson, 1994). It has a temperature of 240-250° C, which is con- tained within a relatively small area <3 km2 (Allis and Larsen, 2012). The intersection of high angle north-south trending normal faults with somewhat older east-west trending structures contribute significant fracture perme- ability (Mabey and Budding, 1987; Fig. 2). The thermal gradient in the Acord-1 well (~10 km west of Roosevelt; Fig. 1) has a temperature gradient >65° C/km, suggesting Roosevelt lies inside an extensive zone of anomalous con- ductive heat flow (~100 km2) with EGS resource potential (e.g. Allis et al., 2015a). The Cove Fort geothermal system occurs inside a set of north-south trending range-front faults on west side of the northern Tushar mountains, just a few kilometers west of a large Pleistocene basaltic-andesite flow (Rowley et al., 2013). Surface expression of thermal activity extends over a large area ~50 km2, including fumaroles and sulfur deposits (Ross and Moore, 1994). A shallow steam zone (<500 m depth) formed the reservoir for the first phase of power production, which began in 1985. The current generation facility comprises a 26 MWe binary plant, commissioned in 2013 by ENEL Green Power (Sacerdoti, 2015). New production wells drilled to 2300 m depth tap Figure 2. Sketch cross sections through the Cove Fort, Roosevelt, and a liquid-dominated reservoir (150-170° C) hosted by a Thermo geothermal reservoirs (same scale; vertical=horizontal), showing sequence of Paleozoic-Mesozoic sedimentary strata made the distribution of Pre-Cambrian crystalline rocks (PC), Paleozoic and up of marine carbonates and siliciclastic units (Fig. 2). Mesozoic sedimentary rocks (PMs, Ps, Ms), Tertiary intrusions (Ti), and Tertiary volcanic rocks (Tv), based on published maps and sections The Thermo geothermal system is located in the (Nielson et al., 1986; Anderson et al., 2012; Rowley et al., 2013). northeast part of the Escalante Desert (Blackett and Vertical, bold black lines represent wells, and thin black lines represent Wakefield, 2002). Surface activity lies to the north of the faults (normal, reverse, thrust).

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production area and thermal springs are associated with northeast trending steeply dipping normal faults. Quaternary volcanic and intrusive rocks crop out within about 10 kilometers of the thermal area (Rowley, 1978). A steeply dipping east-west trending fault separates the hot springs area from the main production zone to the south, and the intersection of these two fault systems may localize the geothermal system similar to Roosevelt and Cove Fort (Anderson et al., 2012). Republic Geothermal first drilled the area and obtained a maximum temperature of 177° C at 2000 m depth (57-29). Raser Technologies (now Cyrq Energy) drilled a series of new wells (30 years later) to the east and commissioned a 10 MW binary plant in 2009. The reservoir covers an area ~ 5 km2, and it is hosted in a Paleozoic-Mesozoic sequence comprising marine carbonates and siliciclastic units. Most production comes from the Redwall limestone, which occurs at the base. The underlying basement is made of crystalline rocks (gneiss and granite), and the contact between them is separated by what appears to be a low angle detachment fault (Anderson et al., 2012; Fig. 2). The Sevier and Black Rock deserts (Fig. 1) encompass several hot springs and Quaternary volcanic centers (Blackett and Wakefield, 2002; Blackett, 2007), which includes Pavant Butte, an apparently large geothermal resource hosted Me- sozoic and Paleozoic strata (Hardwick and Chapman, 2012; Gwynn et al, 2013; Allis et al., 2015b). The deserts occupy structurally complex basins that are filled with Quaternary alluvium and form low-lying areas in between north-south trending ranges. Quaternary volcanic centers are aligned roughly north-south, and they are composed of basalt flows and cinder cones, although minor amounts of rhyolite and rhyodacite erupted too (Blackett and Wakefield, 2002). Crater (also known as Abraham or Baker) is a large thermal area located on the east edge of the Crater Bench basalt lava flows; the area comprises numerous springs having the hottest temperature (87° C) and an aggregate flow rate of 90-140 L/sec (Rush, 1983), equivalent to a power output of ~20-45 MWth. The Meadow-Hatton thermal springs have cooler temperatures (40-60° C) and lower flows (<5 L/s), but they are located within and around a thick travertine mound that covers ~1 km2. Joseph, Monroe, and Red Hill hot springs (60-77° C) occur in the southern part of the Sevier valley, and they are associated with Quaternary faults. The combined discharge of the Monroe and Red Hill hot springs is 20 L/s, and the discharge of Joseph hot spring is 2 L/s (Blackett and Wakefield, 2002), equivalent to power outputs of 5.0 and 0.4 MWth, respectively.

Chemical and Isotopic Compositions of Fluids The chemical and isotopic data are summarized in Table 1. Using a classification scheme based on major anions, most of thermal waters can be classified as chloride, sulfate or hybrid chloride-sulfate waters (Fig. 3). The chloride concentrations range from 160 to 4,240 ppm, the sulfate concentrations range 65 to 1500 ppm, and the bicarbonate concentrations range from 100 to 450 ppm. At Cove Fort and Roosevelt the reservoir water compositions have a similar range of chloride and sulfate concentrations, despite having different reservoir host rocks made of Paleozoic marine strata (Cove Fort) and crystalline Precambrian gneiss/Tertiary granite (Roosevelt). At Thermo, two distinct water compositions exist, chloride- sulfate (57-29) and sulfate-chloride (21a-34 and hot spring), which are both elevated in bicarbonate compared to Cove Fort and Roosevelt (Table 1, next page). Except for Monroe and Red Hill, the other thermal spring waters are predominantly chloride-sulfate composition. Monroe and Red Hill are sulfate-chloride (-bicarbonate) composition.

Figure 3. Ternary plots showing relative concentrations of Cl-HCO3-SO4 and B-Cl-Mg in thermal waters from Table 1. Labels refer to Thermo waters.

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Table 1. Compilation of thermal water compositions and isotope data.

Location T°C pH Li Na K Ca Mg Cl F SO4 HCO3 B SiO2 Roosevelt 14-2 265 5.90 28.00 2150 390.0 9.2 0.60 3650 5.2 78 29.0 490 54-3 260 25.30 2320 461.0 8.0 3860 6.8 72 232 29.9 562 Hot Spring 85 2080 472.0 19.0 3.30 3810 7.1 65 158 404 Hot Spring 55 7.90 0.27 2500 488.0 22.0 0.00 4240 7.5 73 156 38.0 312 Fumarole 94 Thermo Thermo 21a-34 121 8.46 260 34 64.0 12.00 160 4.5 480 237 0.7 62 Thermo 57-29 177 6.40 2.1 961 75 36.0 0.70 1014 10 500 330 1.7 440 Hot Spring 89.5 7.98 1.30 380 52 71 10 225 6.6 480 360 0.9 113 Cove Fort 42-7 (1982) 178 5.00 1241 254 51.0 5.00 1639 6 332 100 10.0 237 P-91-4 (1996) 163 6.00 5.00 1143 220 96.0 9.00 1691 6 393 201 10.0 165 34-7B Thermal Springs Crater/Baker 84 6.50 1.00 830 57 340 52 1500 2.5 1500 156 0.9 69 Hatton 63 7.10 3.00 1041 137 438 86 1790 3.8 1018 425 3.5 48 Meadow 41 6.70 3.60 1058 148 468 93 1803 9.6 1090 416 5.5 57 Cudahy 32 7.70 220 8 75 17 370 1.2 100 125 0.2 60 Twin Peaks 28 7.60 1200 14 135 48 2400 1.7 400 190 0.4 60 Red Hill 76.5 6.30 0.72 590 60 290 34 660 2.8 890 416 2.8 58 Monroe 70 6.20 0.63 530 55 300 36 620 2.7 880 447 3.0 59 Joseph 63 6.50 1.90 1450 50 260 44 1700 3 1200 408 4.9 90

Table 1. Compilation of thermal water compositions and isotope data (continued). 18 Location δ O ‰ δ D ‰ R/Ra T Na/K T qtz-SiO2 References Roosevelt 14-2 -13.7 -116 285 251 Bowman & Rohrs, 1981; Capuano & Cole, 1982 54-3 294 263 Capuano & Cole, 1982 Hot Spring 308 234 Capuano & Cole, 1982 Hot Spring 292 213 Mundorff, 1970; Capuano & Cole, 1982 Fumarole 2.25 Kennedy & van Soest, 2007 Thermo Thermo 21a-34 255 112 J. Moore, unpublished data Thermo 57-29 213 241 J. Moore, unpublished data Hot Spring -14.3 -118 0.9 259 144 Cole, 1983; Kennedy & van Soest, 2007 Cove Fort 42-7 (1982) -15.7 -121 297 192 Bowman & Rohrs, 1981; Moore et al., 2000 P-91-4 (1996) -15.1 -123 291 167 Moore et al., 2000 34-7B 0.62 Tonani et al., 1998 Utah Thermal Spgs Crater/Baker -16.1 -126 0.29 204 118 Cole, 1983; Kennedy & van Soest, 2007 Hatton -16.6 -124 255 100 Mabey & Budding, 1987 Meadow -17.2 -124 261 108 Ross et al., 1993 Cudahy -14.6 -114 163 111 Cole, 1983 Twin Peaks -14.6 -112 104 111 Cole, 1983 Red Hill -16.95 -127 0.21 234 109 Cole, 1983; Kennedy & van Soest, 2007 Monroe -16.95 -128 0.2 235 110 Cole, 1983; Kennedy & van Soest, 2007 Joseph -17.3 -133 0.15 160 132 Cole, 1983; Kennedy & van Soest, 2007

Further subdivision of water types is evident from inspection of the relative proportions of Cl, B, and Mg (Fig. 3). Cove Fort and Roosevelt are low Mg waters with distinct and coherent Cl/B ratios (170-200 and 100-125, respectively). Magnesium is removed from geothermal waters via incorporation into clays during heating and water-rock action (e.g., Giggenbach, 1988), whereas Cl/B ratios are uniform for thermal waters having a common deep source (e.g., Ellis and Mahon, 1977). Thermo 57-29 is also a low-Mg water with a Cl/B ratio ~600, in contrast to Thermo 21a-34 and the hot spring, which are high-Mg waters with Cl/B ~230. All the other thermal spring waters are high-Mg waters with Cl/B ratios that range from 200 to >1000. The oxygen and hydrogen isotope ratios plot close to the global meteoric water line (Fig. 4). Cove Fort, Roosevelt, and Thermo, however, show a slight enrichment in δ18O, which is usually attributed to high temperature water-rock isotope exchange (e.g., Bowman and Rohrs, 1981). The compositions of local meteoric waters are known only for Roosevelt

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and Cove Fort, and these values are isotopically heavier compared to the thermal waters. This suggests that the reservoir thermal waters come from nearby highlands (Kirby, 2012). The thermal spring waters have a range of isotopic compositions that generally reflect local meteoric water sources: Joseph is distinct from Monroe and Red Hill; Meadow and Hatton are similar; Crater and Twin Peaks are very different (Fig. 4). The highest helium isotope ratios are found at Roosevelt (2.25 R/Ra), Thermo (0.9 R/Ra), and Cove Fort (0.6 R/Ra), and all three show evidence of mantle helium, with Roosevelt having the largest component. Presum- ably, mantle helium is transported to mid-crustal depths by intrusions of magma, where it can then be transferred to the surface by convective geothermal fluid flow (e.g., Kennedy and van Soest, 2007). This model is consistent Figure 4. Stable isotope compositions of thermal waters and meteoric water (MW) that occurs in the vicinity of Roosevelt and Cove Fort with the positive correlation between R/Ra and reservoir (Table 1). temperatures in these three systems. Helium isotope data are available for some of the thermal spring waters with R/Ra values ranging from 0.15 to 0.3. These values are elevated compared to radiogenic crust, and indicate the presence of mantle helium too, albeit a smaller component compared to Roosevelt, Thermo, and Cove Fort.

Quartz-SiO2 and Na/K Equilibration Temperatures Comparisons of Na/K and quartz-silica equilibration temperatures using equations supplied by Fournier (1991) and Giggenbach (1991) are shown in Table 1 and Figure 5. At Roosevelt, both aqueous geothermometers correlate reasonably well with measured temperature. For the other areas, however, Na/K ratios give equilibration temperatures that are substantially hotter than the reservoirs in Cove Fort and Thermo. While subject to the effects of cooling, we tentatively conclude that aqueous silica concentrations are the most reliable geochemical indicator of minimum resource temperature. For high-Cl and low-Mg thermal waters at Cove Fort and Thermo, the Na/K ratios may reflect true equilibration temperatures in the deeper (>3 km depth) hotter parts of the geothermal systems. This seems plausible given the strong indication that thermal fluid compositions are mainly shaped by Figure 5. Comparison of aqueous silica concentrations and temperatures water-rock interaction involving feldspar-bearing, crystal- for thermal waters, the highest silica values correspond to production line basement rocks. waters from Roosevelt. Solubilities of quartz, chalcedony and cristobalite (Fournier, 1991) are shown for comparison. Provisional Interpretation The compositions of thermal waters at Cove Fort, Roosevelt, and Thermo reflect a similar history, involving deep circulation of meteoric waters subsequently modified by hot water-rock interaction (>250° C). Geological evidence suggests that the main deep lithology is made up of crystalline basement rocks (i.e., gneiss, granite). The precise source of aqueous Cl, SO4, and HCO3 are unclear, but the occurrence of young volcanic centers along with anomalous helium isotope ratios open the possibility that some portion comes from intruding magmas similar to geothermal fluids in volcanic regions (e.g., Giggenbach, 1997). If gneiss and granite are the only deep lithology, a rock source seems unlikely, unless solutions acquire anionic constituents upon descent and heating through dissolution of salts, carbonates, and sulfates. At reservoir depths, there is some indication that thermal waters are slightly modified by interaction with Paleozoic-Mesozoic

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sedimentary rocks. During continued ascent to the surface, steam-loss through boiling effects the concentrations of aqueous and gaseous constituents. Where fluids reach shallow aquifers, they will be affected by dilution and outflow under the influence of hydraulic gradients. The best example of this occurs at Roosevelt (Fig 6); by contrast a shallow outflow appears to be absent at Cove Fort. The compositions of thermal spring waters (Table 1) suggest they are fed by separate, isolated geothermal systems, albeit with modest resource temperature (<135° C), judging from aqueous silica concentrations. The high Mg and high Cl/B ratios coupled with enrichments in SO4, and to a lesser extent HCO3, suggest these waters are strongly modified by interaction with salts and clays in alluvial basin fill. Alternatively, they may have interacted with brine pore fluids in subjacent Mesozoic-Paleozoic sedimentary rocks, inferred to exist from resistivity data (Hardwick and Chap- man, 2012; Allis et al., 2015b). For either scenario, the Na/K ratios are unlikely to give reliable equilibration temperatures unless the fluid had a long residence time at stable temperature in contact with sodium and potassium feldspars. The widespread occurrence of weakly anomalous helium isotope ratios in thermal spring waters, with evidence of a minor component of mantle helium, suggests that the regional Sevier thermal anomaly owes its origin to deep intrusion(s) of magma. Locally, heat transfer is focused along range front faults in the form of convective geothermal systems with relatively small diameter upflow zones as seen at Cove Fort, Roos- evelt, and Thermo. The large regional endowment of thermal energy associated with hot basement Figure 6. Groundwater compositions in vicinity of the Roosevelt Hot Springs rocks suggests there is considerable potential for (RHS), showing lateral dispersion and dilution in shallow aquifers that form an finding blind resources including those occurring outflow zone. in deep sedimentary aquifers and EGS reservoirs (Allis et al., 2015a and b).

Acknowledgments Funding for this report was provided by DoE grants DE-EE0005128 (Moore and Allis PIs) and DE-FOA-0000841 (Wannamaker PI).

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