Geothermal Potential of the Pyramid Lake Paiute Reservation

Transcription

1 Geothermal Potential of the Pyramid Lake Paiute Reservation, Nevada, USA: Evidence of Previously Unrecognized Moderate-Temperature ( C) Geothermal Systems Mark F. Coolbaugh1, James E. Faulds', Chris Kratt', Gary L. Oppliger1, Lisa SheveneIl 1 ", Wendy Calvin 1, William J. Ehni', and Richard E. Zehner 1 1Great Basin Center for Geothermal Energy, University of Nevada, Reno, NY, USA 'Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV, USA 'Desert Research Institute, Reno, NV, USA 4Ehni Enterprises, Inc., Carson City, NV, USA Keywords Pyramid Lake, geothermal, tufa, Pyramid, Smoke Creek, Needle Rocks, exploration ABSTRACT At least two and possibly tbree previously unrecognized moderate-temperature (I C) geothermal areas have been identified during a geothermal exploration program on the Pyramid Lake Paiute Reservation (PLPR) in west-central Nevada, USA. At each of these new areas -- Pyramid Rock, the southwestern Smoke Creek Desert (SWSMD), and east Astor Pass, -- as well as at the known Needle Rocks geothermal area, hot springs andlor near-surface upwellings of hot water are associated with calcium carbonate (tufa) towers forming today in Pyramid Lake or formed in late Pleistocene Lake Lahontan. The structural controls on geothermal fluid flow are still being investigated, but the intersection of Quaternary faults appears to have localized the shallow upwelling of thermal fluids. Near-surface thermal waters, where identified, have temperatures ranging up to C, whereas quartz and Na-K -Ca-Mg geothermometers suggest reservoir temperatures on the order of -150 to 165 C. Exploration work began with compilation of existing data into a geothermal geographic information system (GIS), and acquisition of an airborne hyperspectral remote-sensing survey of the entire reservation. These efforts were followed by a reconnaissance field survey that prioritized the investigation of anomalies identified in the initial GIS and hyperspectral work. The field survey, which included temperature measurements of springs and existing wells; reconnaissance structural and geological assessments; and geochemical analyses of selected spring and well waters, identified the current areas of interest. Two promising areas have been investigated in detail with magnetic and gravity surveys; shallow temperature measurements from 2-meter-deep auger holes; detailed geologic mapping; and temperature-gradient drilling. Temperature-gradient drilling is in progress, and early results include the intersection of near-boiling waters within 60 m (200 ft) of the surface just east of Astor Pass, -6 km northwest of Needle Rocks in an area with no springs. Introduction A favorable overview of the renewable energy potential on the PLPR by the Department of Energy's (DOE) GeoPowering the West (GPW) program in 2001 led to funding of a detailed geothermal resource assessment (Clutter, 2005). To conduct the assessment, the tribe chose High Desert GeoCulture, LLC, to provide overall technical management, and selected the Great Basin Center for Geothermal Energy (GBCGE) at the University of Nevada, Reno (UNR) to help design and implement a geothermal exploration program. Exploration activities began in the fan of 2004, with data acquisition and a remote-sensing survey, and continued through 2005 with a variety of field geological, geophysical, and geochemical surveys. Temperature-gradient drilling began in late 2005 before being temporarily suspended by wet winter weather. This paper presents exploration program results as of late 2005., A Figure 1. Location ofthe Pyramid Lake Paiute Reservation (blue outline) in Nevada. Colors represent maximum estimated temperatures of known geothermal systems. A trend of higher temperatures follows the northeastern margin of the Walker lane (heavy black line; taken from Faulds et al., 2004) and includes the reservation. Grey circles are geothermal systems with estimated temperatures;:: 150 C. MAY I.!toE

2 I, Geothermal Potential of the Pyramid Lake Paiute Reservation Regional Structure Pyramid Lake, which forms the heart of the 1,800 km' PLPR in west-central Nevada (Figure I), is the largest body of water remaining from late Pleistocene Lake Lahontan. The bottom of Pyramid Lake, elevation of 3,450 ft, is second only to Dixie Valley in terms of low elevation in the northern half of the Great Basin. This low elevation helps explain why the lake is the final discharge point of the Truckee River, whose waters originate from the Sierra Nevada Mountains and Lake Tahoe to the west. The low elevation also provides a clue to the area's high geothermal potential, because the same tectonic forces and faults responsible for forming deep valleys in northern Nevada can also provide deep conduits for geothermal ftuids. A complex pattern of Quaternary faulting on the PLPR provides a particularly favorable setting for the formation of geothermal systems. The reservation lies along the northeastern boundary of the Walker Lane, a zone of northwest-striking dextral faults related to the San Andreas fault (Stewart, 1988). In west-central and northwestern Nevada, evidence suggests that a portion of this strike-slip motion transfers to northerly striking normal faults of the Great Basin interior (Faulds et ai., 2004, 2005a). This zone of transfer, which parallels the northeastern margin of the Walker Lane, is associated with a relatively large number of geothermal systems (Figure I), perhaps because the transfer from strike-slip to extensional faulting promotes a more complex pattern of deep faulting and greater dilation on the normal faults (Faulds et ai., 2004; Coolbaugh et ai., 2005). In any case, the PLPR appears to lie within a major zone of strain transfer, as the right-lateral Pyramid Lake fault terminates northward and merges with a complex system of normal faults (Faulds et ai., 2005b). Previous Geothermal Work Previous geothermal exploration on the PLPR was largely limited to the Needle Rocks area on the north edge of Pyramid Lake, where a spectacular series of 90 m (300 ft) tall calcium carbonate towers form two northwest-trending subparallel ridges more than 2 km long (Figure 2). Hot springs at the base of the Needle Rocks near lake level have temperatures as high as boiling (Garside and Schilling, 1979) and yield geothermometer reservoir temperature estimates ranging from 143 C for the quartz geothermometer to 213 C for the Na-K Ca-Mg geothermometer (Table I). Western Geothermal, Inc. drilled three wells at the Needles in the 1960s. The deepest well was 5,930 feet (1,807 m) deep and encountered a flow of 400 gpm at a temperature of 242 F (117 C), and boiling water continues to geyser from the well today. The potential for producing geothermal power at Needle Rocks is considered good (Geothermal Development Associates, 1988), but important cultural features in the area preclude development. Reconnaissance Exploration The possibility of finding geothermal targets outside of Needle Rocks was considered good from the beginning of this 26 Figure 2. Physiographic features in the Pyramid Lake geothermal study area. Shaded bottom of Pyramid lake shown in blue. Black lines are Quaternary faults from Machette et al. (2003). North is up. Width of map is approximately 52 km. study because of the large size of the reservation (1,800 km 2 ), its location in a very favorable tectonic setting, and the fact that very little exploration had been conducted outside the Needle Rocks. Consequently the reconnaissance phase of exploration was designed to be as thorough as possible given the time and budgetary constraints. Reconnaissance exploration began with the compilation of relevant data into a computerized geothermal GIS, and acquisition and analysis of hyperspectral remote-sensing imagery of the entire reservation. These efforts were followed by a field reconnaissance survey and additional regional gravity measurements to supplement the limited existing data. A description of each step is given below. GRC BULLETIN I,. I

3 Geothermal Potential of the Pyramid lake Paiute Reservation Table 1. Chemical analyses for springs and wells from the Pyramid lake region, Washoe County! Nevada. Needle sample 39 is from Mariner et al. (1974, 1975); Needle samples 404 and 408 are from Grose and Keller (1975). The remainder of the samples were analyzed at Nevada Bureau of Mines and Geolog)1 as part of this study. References for geothermometer equations are: quartz (no steam loss) and chalcedony (Fournier, 1977; Fournier, 1981); and Na K-Ca-Mg (Fournier and Truesdell, 1973; Fournier and Potter, 1979). Ave Temp = average of quartz and Na-K-Ca-Mg geothermometers. Sample Key 39 Key 404 Key 408 PYR l SCD-2 I\rea Needles Needles N~edles Pyram Rk Smoke Ck De.. ~riplion spring spring well spring. Aries. well. Temp C " BC03- mgil B, mgil [ F- mwl CJ mgll N03-2 mwl 3.6 i mg/l C, mgil F, mgil K rug/l Li mg/l 0.<)4 1.6() Mg mgil i Mil mg/l N, mg/l [ Si02 I mgil Charge Sum Cat < Charge SumAn Charge Balance <) l i Geothermom Quartz Geothermom Chalced Gcothermom T _ N " KCaMg Geothennom T_SiO-' Qtz-NaKCaMg A~'e Tt.'01p Geothermal GIS The GIS served as a focal point for gathering data relevant to geothermal exploration into a digital format, allowing relationships between data types to be easily viewed and assessed in a continually evolving flexible map format. Ultimately, the GIS permitted the team to become aware of information gaps, so that the field reconnaissance program could be appropriately targeted. Also, maps could be rapidly updated as more detailed information became available to facilitate planning of temperature-gradient holes. ESRI's ArcGIS 9.0 and ArcView" 3.3 software platforms were used. To date, the GIS indudes 22 Gb of files, including: I) well and spring data from state and federal governments and the PLPR; 2) temperature-gradient holes, geothermal wells, and water geochemistry from multiple sources; 3) magnetic and gravity data from several sources; 4) hyperspeetral imagery and mineral maps derived from the hyperspeetral imagery; 5) nighttime Landsat thermal imagery; 6) shallow temperature drilling data; 7) earthquake locations; 8) Quaternary fault databases; and 9) a variety of topographic and aerial photographic base maps. Hyperspectral Remote Sensing Because it was physically impossible within the scope of this project for geologists to map every side canyon and hillside for clues to geothermal activity, hyperspectral imagery was used as a reconnaissance tool An airborne HyMapTN scanner was flown by HyVista Corporation over the entire SCO-6 SeD-7 SmokeCk SmokeCk Artes...'ell Aries. well <) PYR-3 East Ast well WSTR Numana Spring reservation in the fall of 2004 at a resolution of approximately 5 meters. This imagery incl udes 128 different image bands, each with a spectral resolution of nm, covering visible through mid-infrared wavelengths. The imagery was processed at the Arthur Brant Laboratory for Exploration Geophysics at UNR, using expert systems to reduce dimensionality in the data and provide abundance maps of geothermally related minerals including calcium carbonate. gypsum, silica, alunite, kaolinite, and other clay minerals. The results were output into a series of digital maps and entered into the geothermal GIS. Kratt et ai. (2005) discussed the methodology and results in detail. Field Reconnaissance Survey The field reconnaissance p"rogram was systematic and multi-disciplinary, with several objectives: I) field check areas of interest identified by the GIS compilation and hyperspectral surveys; 2) measure the temperatures of as many springs and wells as possible throughout the reservation; 3) geochemically analyze selected waters and calculate geothermometer temperatures; and 4) identify geological and structural (fault) environments favorable for geothermal systems. Field equipment included global positioning system (GPS) units, digital thermometers. a 550-m-long (1800-ft) wire-line temperature probe for measuring well temperatures, and an Analytical Spectral Devices, Inc. FieldspeC spectroradiometer for identifying minerals mapped by the hyperspectral survey. Water samples were collected and analyzed at the Nevada Bureau of Mines and Geology (NBMG), using standard procedures adapted from Amorsson (2000). Temperatures were measured from 200 springs and wells, and 20 water samples were taken for fuji chemical analysis at the NBMG. Reconnaissance Gravity Measurements Although adequate in many areas, pre-existing gravitystation coverage was not sufficient to identify and delineate many potentially important structural features. To complete this structural picture, 740 new regional-scale gravity stations were acquired through the contract services of Magee Geophysical Services LLC (440 stations) and UNR graduate students at UNR (300 stations), using state-of-the-art gravity and GPS survey methods and full termin corrections. Most of these stations were spaced at in tervals of 1 to 4 kilometers across the PLPR. MAY I JUNE 2007

4 Detailed Exploration The Astor Pass and southwestern Smoke Creek Desert (SWSCD) areas are being investigated with detailed gravity and magnetic surveys, shallow temperature surveys, detailed geological mapping, geochemical analyses, and temperaturegradient drilling. Procedures and methodologies of the geophysical work and shallow temperature surveys are discussed below. Detailed Gravity and Magnetics Surveys At the east Astor Pass and SWSCD target areas, approximately 1,000 new gravity stations were collected on 400 by 400 meter grids by Magee Geophysical Services LLC. Each detailed survey covered about 75 km'. As with all new gravity data collected for this project. the measurements were acquired using state-of-the-art gravity and GPS survey methods and processed with full terrain correction& UNR students also collected 50 additional high detail (100m spacing) stations over the Aster Pass target. The detailed-scale gravity-station data were combined with the regional-scale data and a series of derivative products were generated, including the complete Bouguer anomaly, isostatic residual, total horizontal gradient, first derivative of the upward-continued isostatic residual, and Euler de-convolutions to estimate the depth to basement. Gravity reductions for the complete Bouguer data were evaluated using several density estimates for the surface terrain. including 2.3, 2.4, and 2.5 g/cm 3 Detailed-scale aeromagnetic surveys using ultralight aircraft were flown by Pearson DeRidder and Johnson Inc. over the same areas as the detailed-scale gravity surveys. Each aeromagnetic survey consisted of about 1,280 line-kilometers flown at a typical sensor height of 100m above ground, with 100-meter line spacings and 300-meter-spaced cross lines. Operating at a 70 to 100 km per hour air speed, this system employed a 10 Hz sample-rate cesium-vapor magnetometer, differential GPS navigation and positioning, and a radar altimeter. Shallow Temperature Surveys The discovery of a very high temperature gradient (8S0F/lOO ft or 950 C/km) in the first temperature-gradient hole east of Astor Pass suggested the possibility of augmenting temperature-gradient drilling with much shallower temperature measurements. An initial attempt at measuring temperatures at a 36 inch (0.9 m) depth did not yield good anomaly definition. but 84 inch (2.2 m) auger holes were much more successful. Because of time constraints and equipment availability. the normal methods of measuring temperatures in shallow holes (Olmsted, 1977; LeSchackand Lewis, 1983; Trexleret ai., 1982) were modified for the Pyramid Lake survey& The procedure involved drilling holes with Eijkelkamp hand-auger equipment. Immediately after hole completion. temperatures were measured at two locations: I) at the bottom of the open hole; and 2) in the middle of the soil held by the core barrel. This technique is subject to a variety of temperature-measurement errors, including those caused by core-barrel friction. variations in pre-drilling temperature of the cdring device, collapse of shallow soil into the bottom of the hole, changes in air temperature that affect instrument temperature compensation, and changes in ground temperature due to changes in weather. The primary advantage of the technique is obtaining a temperature in real-time, which can be used to optimize the location of the remaining auger holes. A shallow temperature survey at Astor Pass (25 holes) was completed under relatively constant weather conditions over a 10 day period in late November, 200S, using a digital K-type thermocouple meter and probe with a precision of approximately +1-1 to2 C. The survey at SWSCD (29 holes) was completed in II days in the first half of December, 2005 under deteriorating weather conditions which eventually forced a suspension of auger driijingwhen duplicate holes revealed an appreciable change in subsurface temperatures. A 3-wire Pt RTD (platinum resistance temperature device) with a precision of C was used at SWSCD, which is relatively unaffected by surface temperature variations. Both surveys were success- Figure 3. a) Pyramid Rock viewed looking north from Anaho Island. b) Boiling spring on west side of Pyramid Rock at arrow in a). 28 GRC BUllETIN

5 ful in outlining pronounced subsurface temperature anomalies; temperatures (at a depth of 2.18 m) ranged from 14 to 26"C at Astor Pass and 14 to 24"C at SWSCD. Results: Geothermal Prospects Three areas outside the immediate Needle Rocks geothermal area were identified as having potential for moderate-temperature geothermal fluids, defined here as containing temperatures approaching or exceeding 150"C. They are discussed below. Pyramid Rock Pyramid Rock, the namesake of Pyramid Lake, is the tallest and most massive single calcium carbonate (tufa) tower in Nevada (Figure 3). It projects 110 meters above an meter-elevation lake level, but most of the mass is below the water surface. Lake-bottom bathymetry suggests that the base of the spire is 85 m below the surface (and is 400 m long by 200 m wide), which, if correct, implies a total tower height of nearly 200 m. The Paiute Tribe has undoubtedly known since antiquity of the existence of a hot spring at Pyramid Rock. r n the English literature, however, reports are conflicting about the nature of springs here and on nearby Anaho Island. Garside and Schilling (1979) described them as "warm", while an NBMG database listed temperatures of 59"C. In contrast, Benson et a1. (1995) ascribed an 89"C temperature to the Pyramid Rock spring. No previous chemical analyses are known. With help from a Pyramid Lake scarch and rescue boat, the Pyramid Rock hot spring was visited in the spring of 2005, and at that time it was vigorously spouting from a vertical rock face a few centimeters above lake level (Figure 3). A temperature of 97"C was measured (boiling) and steam was observed issuing from a crack 4 m above the spring. However, the reported spring on Anaho Island could not be located during a several-hour search during which many pelicans, geese, and rattlesnakes were sighted, but no thermal springs. The only original published reference to springs on Anaho Island, by Waring (1965), is sufficiently vague that the reference could actually apply to Pyramid Rock. Although the existence of thermal springs on Anaho Island remains uncertain. other features indicate thermal activity on the east side of Pyramid Lake in addition to the Pyramid Rock spring. Garside and Schilling (1979) reported divers finding underwater hot springs near Pyramid Rock. John Jackson, Water Resources director, Pyramid Lake Paiute Tribe (PLPT; verbal communication, 2005) reported observations of bubbling waters at an unspecific location on the lake surface north of Pyramid Rock near Red Bay (along what would be the Lake Range fault zone), although the authors did not find thesc phenomena during a brief 15-minute boat search in The tectonic setting of the Pyramid Rock area appears complicated. The island is situated at a point where the geomorphic expression of the north-striking Lake Range fault zone (on west side of Lake Range; Figure 2) dies out to the south as it meets what appears to be a northeast-striking fault defined by the subaqueous northern base of Anaho Island. MAY I JUNE 2007 Pyramid Island itself appears to lie near the western end of an almost l-km-long line of tufa towers, which strikes an unusual (for Quaternary structures in western Nevada) N80"W. More regionally, Pyramid Rock lies within a complex transfer zone of fault motion between the dextral Pyramid Lake fault to the west and the Lake Range normal fault to the east (Faulds et aj" 2005b; Drakos et ai., 2005). A hot spring water sample from Pyramid Rock yielded a 153"C estimate of reservoir temperature based on both the quartz and K-Na-Ca-Mg geothermometers (Table I), suggesting the potential for generating electricity. For religious reasons, the PLPT has excluded this area from development. Southwestern Smoke Creek Desert Two warm springs (Rotten Egg Spring and Round Hole Spring) and several warm artesian wells with temperatures up to 37"C were previously documented along the southwest margin of the Smoke Creek Desert (Garside and Schilling, Figure 4. Springs, wells, surface mineralization and surface traces of faults in the southwestern portion of the Smoke Creek Desert. Black lines are surface faults as indicated by either r<lnge front break in topography, scarps, slumps, evaporite minerals, carbonate tufa, and springs. Purple shapes = gypsum mapped with HyMap, yellow shapes = tufa maped with HyMap. Orange ellipse = area of tufa towers in range front ramp structure. Star = spring or well: green < 20 (, yellow C, orange c C, roo <': 37"C. SquarE's = shallow auger hole temperature measurement {maximum reading}: dark blue < 16 C, medium blue "'6-1 we, light blue (, green 19-20"(, yellow "(, orange (, red.;:: 22 C. Background is a visible light and near infrared hyperspeclral image, in which shades of red indicate vegetation and white are(ls have salt minerdls. North is up. 29

6 ' 'j,., I 1979; Figure 2, 4). The reconnaissance team was attracted to this area because no geothcrmometer temperature estimates were available. and because the area contained the largest gypsum anomaly detected by the hyperspectral survey. Field investigations rapidly confirmed a large sulfate-rich soil area on the western margin of Smoke Creek Desert above the elevation of the playa. A 49 C artesian well, not in any known database. was soon located. In addition, a conspicuous area of tufa towers was noted along an apparent 600-meter stepover in the range-front fault to the west (Figure 4,5). Geochemical analyses of artesian well water revealed a correlation between water temperatures, chloride contents, and discrepancies between the Na-K-Ca-Mg and quartz geothermometers (Table I) suggesting mixing of near-surface cold groundwaters with deeper geothermal fluids. In spite of the apparent mixing, geothermometer temperatures of wel1 waters reach and exceed 150 C. Thus, this area was selected for further study. The detailed gravity survey subsequently revealed that displacement along the western range-front fault of the Smoke Creek Desert was at a maximum immediately north of the aforementioned range-front stepover (Figure 6), and that this range-front fault splays southward into several subsidiary southwest to southeast-striking faults (Figure 6). The westernmost of these fault-splays forms the step-over in the range-front fault, which corresponds to a change in dip of volcanic rocks in the range, suggesting a possible cbange in dip of normal faults along the range front (i.e. more structural complexity). In the piedmont east of the range front, several Quaternary fault scarps are present, accompanied by shallow slump failures and karst-type topography in water-soluble sulfate-rich soils. The complete Bouguer gravity anomaly and its derivative anomaly maps further suggest a buried mini-graben adjacent to the western edge of the valley (Figure 6), similar to that documented for Dixie Valley (McKenna et ai., 2005). Tbe detailed aeromagnetic anomaly shows two distinct magnetic lows, one near the exposed stepover along the western range front and a second one further east in the eastern part of the splayed fault zone (Figure 6). Figure 5. a) Calcium carbonate tufa towers in Smoke Creek Desert viewed from 37 C artesian well. Ramp structure between two en echelon range front faults is highlighted. b) Tufa to\vers along range-front fault on margin of Smoke Creek Desert, with ramp structure <It the left middle-ground of photo. Figure 6. Horizontal gravity gradient derived from detailed geophysical survey of a portion oi the Smoke Creek Desert.' Warmer background colors represent higher gradients. Red lines denote spla)'ed pattern of faults south\vard from major range front. Blue polygons outline two magnetic lows that may correspond to zones of hydrothermal alteration. Black lines are faults, purple polygons are gypsum, and stars are springs and wells using same symbology as Figure 4. North is up. Background is a visible light and near infrared hyperspectral image, in which shades of red indicate vegetation. 30 GRC BULLETIN

7 To help locate future temperature-gradient holes, a shallow auger-hole temperature survey was completed. The survey (Figure 4) encountered shallow cold groundwater in the eastern half of the survey area (in the area of artesian wells), underneath which the shallow temperature survey cannot see. In the western half of the survey area, a temperature anomaly was detected near the calcium carbonate tufa towers. An anomalous 35 C temperature measured in a l6-ft (5 m)-deep abandoned dry well (in December, 2005) confirms this temperature anomaly. One possible exploration model for the SWSCD involves thermal waters rising within the stepover in the range-front fault. At the time of Lake Lahontan, these fluids would have reached the surface to form hot springs and tufa towers. Radiometric dating of tufa towers in the vicinity of Pyramid Lake using "c (Benson et ai., 1995) and uranium-series (Szabo et ai., 1996) methods indicates that the towers are up to 60,000 years in age. More recently, when the lake dried up, the hot waters may have begun infiltrating nearsurface sediments and flowing eastward down the hydrologic gradient toward the playa, where they could develop artesian pressures beneath clay-rich sediment layers in the playa. Some of these thermal waters could mix with shallower colder groundwaters before reaching the surface in wells. Preliminary chemical modeling of the artesian well waters predicts that the discrepancy between cation and quartz geothermometers can be resolved if cold waters with a chemistry similar to water in the coldest artesian well (20 C) mix with a 65 C thermal water. Required mixing ratios do not exceed roughly 1:1 (in the 37 C artesian well), and in this mixing scenario, the cation and quartz geothermometers both predict reservoir temperatures of roughly 165 C in the parent thermal fluid. The structural complexity of this area suggests several alternative exploration models in which potential geothermal reservoirs could lie significantly east of the range front fault. Evidence for such models include: I) Quaternary fault scarps in the piedmont, 2) a possible buried mini-graben and fault splays east of the range front as indicated by the gravity survey, and 3) a magnetic low east of the range front, which may indicate subsurface hydrothermal alteration. fast Astor Pass A 75-km 2 -area northwest of Needle Rocks, which includes much of the Terraced Hills (Figure 2, 7), is also being explored in greater detail. This area attracted initial interest because large areas of argillic alteratiod were found in Miocene volcanic rocks of the Terraced Hills, and because a series of young northwest-striking faults cut the volcanic rocks. These faults could potentially provide fluid conduits between the clay-altered areas and hot springs at Needle Rocks farther to the south, as first hypothesized by Bonham and Papke (I 969). The hyperspectral remote-sensing survey identified a number of previously unmapped kaolinite/halloysite outcrops in the Terraced Hills, significantly expanding the distribution of documented occurrences beyond an active hauoysite mine Figure 7. Springs, wells, surface mineralization and horizontal gmvity gradients in the east Astor Pass area north of Pyramid Lake. Northwest-striking fault fabric can be discerned from the trends in higher horizontal gravity gradients, as indicated by the more closely spaced black lines (black lines are contours of 1 km upward continued residual gravity). Blue shapes = kaolinite!halloysite mapped with HyMap, yellow shapes =- tufa maped with HyMap. Star :; spring or wei[: green < 20 (, yellow 20-26"C orange C, red ~ 3rc. Squares = shallow auger hole temperature measurement (average reading): dark blue < 18 C, medium blue (, light blue 20-2 PC, green c C, yellow (, orange 23-24QC, red 2: 24 C. NTC 1 and NTC 2 are the first two temperature-gradient holes drilled in Figure 8. A tufa tower liocated next to NTG 2 in Figure 7) projects 30 m above the floor of a 1 km wide northwest-trending graben east of Astor Pass. View is towards the southeast. The southwestern margin ofterraced Hills is visible in the background. No springs are present in this valley. MAY I JUNE

8 " Geothermal Potential of the Pyramid Lake Paiute Reservation I i, I, i I on the western edge of the hills (Bonham and Papke, 1969). Other clues to the presence of thermal activity beneath or near the Terraced Hills include a large tufa tower 6 km northwest of Needle Rocks (Figure 8) and two warm cattle wells, one of which is 26 C and yielded a 143 C K-Na-Ca-Mg geothermometer temperature (Table I). After reconnaissance survey completion and detailed gravity and magnetic geophysical work, two temperature-gradient wells were drilled east of Astor Pass at the southwestern edge of the Terraced Hills (Figure 7). The southern hole tested a hypothesized intersection of a northeast-trending topographic lineament with a northwest-striking fault defined by the detailed gravity and magnetic surveys. The northern hole tested the same northwest-striking fault where it passes near a 30-mtall tufa tower (Figure 8). Both holes encountered high temperature gradients at shallow depths, with peak temperatures at - 60 m (200 ft), and became isothermal from 60 to 140 m (200 to 450 ft). Temperatures in the northern hole were hottest, peaking at 86 C at a 60 m depth near bedrock. The shallow auger-hole temperature survey (Figure 7) indicates the first two temperature-gradient holes were drilled along the axis of a northwest-trending zone of maximum shallow temperatures. The fact that the shallow survey found the highest temperatures and highest horizontal temperature gradients in the immediate tufa tower vicinity suggests that the tufa tower represents the surface expression of a subsurface conduit for rising thermal groundwater. At some time in the past, but likely within the last 60,000 years (Benson et ai., 1995; Szabo et ai., 1996) when lake levels were higher, thermal groundwater reached the surface to form the towers, but now the rising thermal groundwater appears to bleed off into unconsolidated sediments near the top of bedrock before reaching the surface. The location of the tufa tower at a change in the trend of the Astor Pass valley, and the fact that the radial arms of the tufa tower are subparallel to each of the valley trends, suggests that the location of the tower is controlled by the intersection of west-northwest and northwest -striking faults. More structuralsetting knowledge of this geothermal area will be gained as detailed mapping and temperature-gradient drilling continue. At the moment, the location of a possible deeper geothermal reservoir is unknown. butit could exist either nearby or farther to the north in the Terraced Hills. Other Areas A number of other areas with varying geothermal potential have been identified on the reservation. A warm spring (27"C) and warm well (28'C) were found east of the Truckee River and east of the Numana Fish Hatchery (Figure 2) with hyperspectral imagery help that detected sulfate crusts in the area. The warm spring is relatively saline (Table 1) and would have generated a high cation geothermometer (170 C) temperature were it not for its high Mg content. Possible mixing of shallow high-mg groundwater with deeper thermal waters could produce the observed chemistry. The Numana area lies near the intersection of the right-lateral Pyramid Lake fault and the left-lateral Olinghouse fault, and topographic lineaments suggest that several north-northeast-striking normal faults may splay from the Pyramid Lake fault in this area. Several warm springs with temperatures up to 24 C occur along the east side of the Lake Range (Figure 2). Topographic lineaments suggest these springs are controlled by a small-displacement range front fault with at least one en echelon stepover near the north end of the reservation. Geothermometer temperature estimates are low, however. Several warm wells are known in the Nixon area; geochemical analyses of these waters are needed to help assess the potential for district heating or electrical energy production. Gravity surveys indicate the area between Nixon and Pyramid Rock is characterized by large magnitude variations in the depth to bedrock, in tum suggesting a complex structural pattern associated with fault motion transfer from the strike-slip Pyramid Lake fault to more northerly striking normal faults along the Lake Range and Winnemucca Lake (Drakos. 2006). Other potential environments for geothermal activity that warrant further investigation include high-displacement normal faults along the western margins of the Lake and Fox Ranges, a potentially large step-over in normal faulting in the Fox Canyon area, and northeast-striking normal faults at the extreme southern end of the San Emidio Desert. Discussion and Conclusions The discovery of additional geothermal resource areas on the PLPR indicates significant potential for geothermal energy development on the reservation. It also illustrates that it is possible to use modern grass-roots-style reconnaissance exploration techniques to find previously unknown geothermal systems in the Great Basin that have the potential to produce electricity. Futwe exploration on specific areas of interest discussed herein should help clarify the geological and structural controls on geothermal fluid flow, and hopefully lead to identification of deeper geothermal reservoirs. It is noteworthy that two of the geothermal areas of interest (SWSCD and Numana) have surface sulfate crusts detectable with remote sensing, and that in four areas (Needle Rocks, Pyramid Rock, SWSCD, and East Astor Pass), near-surface thermal waters are associated with calcium-carbonate tufa towers. In two of these areas (SWSCD and East Astor Pass), hot waters that formerly reached the surface to form tufa towers no longer reach the surface today. Such tufa towers abound in the Great Basin. Further exploration of tufa towers and additional searches for sulfate anomalies, both on the reservation and elsewhere, is likely to lead to the discovery of additional geothermal system& Acknowledgements We foremost wish to express our appreciation to John Jackson, Donna Noel, and the Pyramid Lake Paiute Tribe for allowing this research to happen, and for helping in many ways to facilitate the field exploration work. We are also very appreciative to Shuman Moore and Mack Shelor of High Desert GeoCulture LLC, whose vision of geothermal exploration and development has made it possible to meld research with exploration to produce optimal results. 32 GRC BULLETIN

9 Geothermal Potential of the Pyramid lake Paiute Reservation References Amorsson, S., 2000, Isotopic and Chemical Techniques in Geothermal Exploration, Development, and Use: sampling methods, data handling, interpretation: International Atomic Energy Agency, Vienna, Austria, 351 p. Benson, L.. Kashgarian, M., and Rubin, M., Carbonate deposition, Pyramid Lake subbasin, Nevada: 2. Lake levels and polar jet stream positions reconstructed from radiocarbon ages and elevations of carbonates (tufas) deposited. in the Lahontan basin: Palaeogeography, Palaeoclimatology. Palaeoecology, v. 117., p. 1~30. T.J., 2005, Pyramid Lake geothermal: Native American tribe seeks development potential at the Pyramid Lake Paiute Reservation in Nevada: Geothermal Resources Council Bulletin, v. 34, n. 2, p Clu~ter, Coolbaugh, M.F., Arehart, G.B., Faulds, IE., and Garside, L.J., 2005, Geothermal systems in the Great Basin, western United States: Modern analogues to the roles of magmatism, structure, and regional tectonics in the formation of gold deposits, in Rhoden, H.N., Steininger, R.c., and Vikre, P.G., eds., Geological Society of Nevada Symposium 2005: Window to the World, Reno, Nevada, May 2005, p Drako~ P.S., Fauld~ 1.E., and Henry, C.D., 2005, Tertiary StratigraphY and Structure of the southern Lake Range northwest Nevada: Assessment of kinematic links between strike-slip and nonnal faults in the northern Walker Lane: Geological Society of America Penrose Conference. Drakos, P.S., Tertiary Stratigraphy and Structure of the southern Lake Range northwest Nevada: Assessment of kinematic links between strike~slip and normal faults in the northern Walker Lane: M.S. thesis, University of Nevada, Reno. Faulds, J.E, Coolbaugh, M., Blewitt, 0., and Henry, c.d., 2004, Why is Nevada in hot water? Structural controls and tectonic model of geothermal systems in the northwestern Great Basin: Proceedings, Annual Meeting, Palm Spring~ CA, Aug. 29-Sep. 1,2004, Geothennal Resources Council Transactions, v. 28, p Faulds. IE., Henry, CD., and Hinz, N.H., 2oo5a, Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific - North American plate boundary: Geology, v. 33, no. 6, p Fauld~ J.E., Henry, C.D., Hinz, N.H., Drakos, P.S., and Delwiche, B., 2oo5b, Transect Across the Northern Walker Lane, Northwest Nevada and Northeast California: An Incipient Transform Fault Along the Pacific-North American Plate Boundary, in Pederson, 1., and Dehler, CM., eds., Interior western United States: Geological Society of Americd Field Guide 6, p , doi: 1O.1130/2005.fld006(06). Fournier. R.o., 1977, Chemical geothermometers and mixing models for geothermal systems: Geothermics 5: Fournier, R.n, 1981, Application of Water Geochemistry to Geothermal Exploration and Reservoir Engineering, in Rybach, L. and Muffler, L.IP., eds., Geothermal Systems; Principals and Case Histories, Wiley and Sons, Chichester, p Fournier, R.o. and Potter, R.W. II, 1979, Magnesium correction to the Na-K-Ca chemical geothermometer: Geochimica et Cosmochimica Acta, v. 43, p Fournier, R.O. and Truesdell, A.H., 1973, An empirical Na-K-Ca geothermometer for natural waters: Geochimjca et CosmocbirnicaActa. v. 37, P Garside, L.l and Schilling, J.H., 1979, Thermal Waters of Nevada: Nevada Bureau of Mines and Geology Bulletin 91, 163 p. Geothermal Development Associates, 1988, Needles Geothermal Power Plant, Phase I Feasibility Study: Internal Report prepared for the Pyramid Lake Paiute Tribe by Geothennal Development Associates. Grose, L.T., and Keller, G.v" 1975, Colorado School of Mines Nevada Geothermal Study Progress Report no.4-for Period February 1, 1975 to October 31,1975: Colorado School of Mines Report, National Science Foundation Grant GI Kratt, c., Calvin, C, and Coolbaugh, M., Hyperspectral mineral mapping for geoihermaj exploration on the Pyramid Lake Paiute Reservation, Nevada: Geothermal Resources Council Transactions, v. 29, p LeSchack, L.A. and Lewis, IE., 1983, Geothermal prospcciing with Shallo-Temp surveys: Geophysics, v. 48, n. 7, p Machette, M.N., Haller, K.M., Dart, R.L, and Rhea, S.8., 2003, Quaternary fault and fold database of the United States: US. Geological Survey Open-File Report , hup:llqfaults,cr.usgs.gqv/fau1tsl. Mariner, R.H., Presser, T.S., Rapp, IB., and WiUey, L,-M., 1975, Minor and Trace Elements, Gas, and Isotope Compositions of the Principal Hot Springs of Nevada and Oregon: U.S. Geological Survey Open File Report, 27 p. Mariner, R.H., Rapp, J.B., Willey, L.M., and Presser, T.S., 1974, Chemical Composition and Estimated Minimum Thermal Reservoir Temperatures of the Principal Hot Springs of Northern and Central Nevada: U.S. Geological Survey Open-File Report , 32 p. McKenna, J.R., Blackwell, D.D., Richards, M.C. and Swenson, n.v., 2005, Natural state modeling, structure, preliminary temperature and chemical synthesis of the Dixie VaJIey, Nevada geothermal field: Proceedings, thirtieth workshop on geothermal reservoir engineering, Stanford University, Stanford, CA., Jan. 31-Feb. 2, 2005, p Olm~ted, EH., 1977, Use of temperature surveys at a depth of 1 meter in geothermal exploration in Nevada: United States Geological Survey Professional Paper I044-B, 25 p. Stewart, I H., 1988, Tectonics of the Walker Lane beh, western Great Basin: Mesozoic and Cenozoic deformation in a zone of shear, in Ernst, W. G., ed., The Geotectonic development of California: Prentice Hall, Englewood Cliffs, New Jersey, p Szabo, B.l, BUsh. CA., and Benson, L.v., 1996, Uranium-scriesdatingof carbonate (tufa) deposits associated with Quaternary fluctuations of Pyramid Lake, Nevada: Quaternary Research, v. 45, p l. Trexler, D.T., Koenig, B.A., Ghusn, G. Jr.. Flynn, T., and Bell, E1., 1982, Low-to~moderate-temperature geothermal resource assessment for Nevada: area specific studies. Pumpernickel Valley, Carlin and Moana: United States Department of Energy Geothennal Energy Report DOElNVl (DE ). Waring, G.A., 1965, Thermal Springs of the United States and Other Countries of the World: U.S. Geological Survey Professional P<d.per 492,383 p. (Ed. Note: To see the figures in color, readers are referred to a CD of the GRC Transactions, 2006, VoL 30, p. 59.) MAY I JUNE