Prospecting for a Blind Geothermal System Utilizing Geologic and Geophysical Data, Seven Troughs Range, Northwestern Nevada

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1 GRC Transactions, Vol. 38, 2014 Prospecting for a Blind Geothermal System Utilizing Geologic and Geophysical Data, Seven Troughs Range, Northwestern Nevada Corina Forson 1, James E. Faulds 1, and Phil Wannamaker 2 1 Nevada Bureau of Mines and Geology, University of Nevada, Reno NV 2 Energy and Geoscience Institute, University of Utah, Salt Lake City UT Keywords Nevada, Basin and Range, structural controls, Seven Troughs Range, blind geothermal system, exploration, geologic mapping, Magnetotelluric, two-meter survey Abstract The Seven Troughs Range in Pershing County, Nevada, has been identified as an area with high potential for hosting a blind geothermal system. A reconnaissance 2D magnetotelluric (MT) survey across the western U.S. has shown a possible correlation between broad low resistivity anomalies, extending from the upper to deep crust, and loci of high enthalpy geothermal systems. Where MT surveys have been conducted across the Basin and Range, anomalous zones of high conductivity strongly correlate with known high temperature geothermal systems, such as Dixie Valley, Coso, and McGinness Hills; as well as anomalies in 3 He R/ Ra. Along the regional MT transect only a few areas of pronounced upwelling are not connected with any known geothermal systems (e.g., Kumiva Valley region). These locations are hypothesized to have high potential for hosting blind geothermal systems. Thus, this project focuses on identifying favorable structural settings for geothermal activity in the Kumiva Valley area and more specifically the Seven Troughs Range, where the low resistivity anomaly reaches the near surface. Detailed geologic and structural analyses were conducted to identify favorable structural settings for geothermal activity that are likely to host a blind geothermal system. An initial two-meter temperature survey was also conducted to further assess two favorable structural settings in the area. Introduction A majority of the operating geothermal power plants and prospects have utilized the presence of surface manifestations (i.e., geysers, boiling mud pots, hot springs, sinter terraces etc.) to locate areas of high geothermal potential (e.g., Bradys geothermal system; Faulds et al., 2010). However, many resources have little or no surface manifestations and are considered hidden or blind resources. Researchers estimate that only ~20% of the accessible geothermal resources in the Great Basin have been discovered, suggesting that opportunities abound for discovering blind or hidden geothermal systems (Brook et al., 1979; Richards and Blackwell, 2002). If prospecting for a truly blind geothermal system, it becomes important to distinguish between a blind, hidden, and conventional geothermal resource. All geothermal systems have three main elements: heat source (magmatic or high geothermal gradient in amagmatic settings), reservoir (to hold and sustain the hot water), and fluid, which is the medium that transfers the heat. Conventional geothermal systems exhibit fluid surface manifestations, such as fumaroles or hot springs. Hidden geothermal systems lack fluid features on the surface but contain more subtle features (or paleo manifestations), such as siliceous sinter, travertine, tufa deposits, and/or hydrothermally altered rocks. Vegetation can also serve as an indicator, as it can be concentrated in lineaments along faults that leak fluids or be absent near faults leaking toxic gasses. Blind geothermal systems are confined to the subsurface; lacking any obvious fluid features and/or paleo-manifestations associated with conventional or hidden systems. Within both magmatic and amagmatic environments faults and fractures within the brittle crust play an important role in generating and maintaining permeability and transporting fluids to the surface. In amagmatic systems, and more specifically the extensional setting of the Basin and Range, moderate to steeply dipping faults and fractures promote the transmission of meteoric fluids to depth (Blackwell, 1983). The depth of circulation required to attain high temperatures depends on the permeability of the rocks and the geothermal gradient. Areas with known recent faulting (i.e., Quaternary) are ideal for prospecting for geothermal systems, because the most substantial fluid migration in a fault system is likely during and after rupture events (Micklethwaite and Cox, 2004). It comes as no surprise that a majority of the known geothermal systems in the Great Basin region of the Basin and Range province are spatially associated with faults, and many are temporally associated with Quaternary faults. In the Great Basin many geothermal systems are found proximal to normal and oblique-slip faults, suggesting that such faults control or chan- 369

2 nel hydrothermal fluids. Faults oriented orthogonal to the least principal stress direction are most likely to host high temperature geothermal systems (Faulds et al., 2004, 2006, 2011). Within the northwestern Great Basin, the regional extension direction trends west-northwest. Accordingly, normal faults striking north-northeast host most of the geothermal systems (Coolbaugh et al., 2005; Faulds et al., 2005). However, many north-northeast-striking faults in this region show no signs of geothermal activity. Therefore, it is necessary to recognize common structural settings of known geothermal systems to aid in the discovery of blind systems with no surface manifestations. Several fault patterns have been recognized as particularly favorable structural settings for geothermal activity (Curewitz and Karson, 1997; Faulds et al., 2006, 2011), including: 1) overlapping normal faults (step-overs) with hard linkage, 2) terminations of major normal faults (horse-tailing), 3) intersections of normal, strike-slip, and/or oblique-slip faults, 4) accommodation zones where belts of oppositely dipping normal faults intermesh, and 5) transtensional pull apart zones. These structural controls can be identified via topographical manifestations, such as 1) major steps in rage fronts, 2) interbasinal highs, 3) mountain ranges consisting of relatively low, discontinuous ridges, and 4) lateral terminations of mountain ranges (Faulds et al., 2006, 2011). Motivation and Purpose of This Study The purpose of this study is 1) to determine the structural setting and geothermal potential of the central Seven Troughs Range Figure 1. The great Basin physiographic province outlined in red. The Four geothermal/structural domains are highlighted in green (from Faulds et al., 2004). From north to south these are: The Surprise Valley belt (SV), the Black rock Desert belt (BRD), the Humboldt Structural Zone (HSZ), and the Walker Lane Geothermal belt (WLG). The Walker lane and Eastern California Shear Zone boundary is denoted with the hachured polygon on the map. The black star denotes the location of the Seven Troughs field area. The inset map is a close up of the Seven Troughs field area and the surrounding mountain ranges, valleys, wells and springs. 370 in Pershing County, northwestern Nevada, and 2) to estimate the possible locations of blind geothermal resources in that area. The field area is located ~50 km northwest of Lovelock, Nevada (Figure 1). This specific field area was chosen based on the location of a regional magnetotelluric (MT) anomaly (Wannamaker et al., 2011; Figure 2A). In addition, an ongoing study funded by the Department of Energy aims to integrate MT data, soil gas flux and geochemistry, and structural analysis to recognize heretofore undiscovered blind geothermal resources. The reconnaissance 2-D MT survey has shown a possible correlation between low resistivity in the deep crust and magmatic underplating in areas with high rates of recent extension (e.g., Dixie Valley region). The deep crustal, low resistivity anomalies are linked to tapering zones of lower resistivity in the upper crust, suggesting that deep crustal fluids can upwell to remarkably shallow depths (Wannamaker et al., 2011). The presumed lower crustal magmatic underplating may be spatially correlated to large crustal breaks that may accommodate the efficient transfer of high temperature fluids from deeper levels to the near surface. Where reconnaissance MT surveys have been conducted across the Basin and Range, anomalous zones of high conductivity strongly correlate with known high temperature geothermal systems, such as Dixie Valley, Coso, and McGinness Hills (Wannamaker et al., 2011) (Figure 2B). Commonly, these systems exhibit significant 3He R/Ra anomalies, a further indication of magmatic input (Kennedy and van Soest, 2007). Along the MT transect conducted by Wannamaker et al. (2011), only a few areas of pronounced upwelling associated with deep crustal, low resistivity anomalies are not connected with any known geothermal systems (e.g., Kumiva Valley region, Figure 2B&C). These locations are hypothesized to have high geothermal potential. Thus, this project focuses on identifying favorable structural settings for geothermal activity in the Kumiva Valley area and more specifically the Seven Troughs Range, where the low resistivity anomaly reaches the near surface (Figure 2C). The field area lies in the central Seven Troughs Range (Figures1and 3), where the Kumiva Valley MT anomaly is nearest to the surface (Wannamaker et al., 2011). This area may be analogous to Dixie Valley or McGinness Hills (Figure 2B), where enhanced geothermal activity and possible deep crustal breaks correlate with areas of elevated conductivity. These low-resistivity upwelling zones likely mark areas in which geothermal fluids circulate from depth to the surface. However, there is no known geothermal system in the Seven Troughs Range. Nonetheless, the 2-D MT anomaly in this area (Figure 2C) and a preliminary analysis for favorable structural settings suggest that this location has relatively high potential to host a blind geothermal system. The Kumiva Valley area is therefore a greenfield exploration area without current recognized systems. It lies in a complex structural setting currently undergoing oblique extension (Rhodes et al., 2010; Hammond et al., 2011). To aid in the discovery and evaluation of blind resources, it is important to utilize geologic, geo-

3 Figure 2. A) Reconnaissance MT transect coverage of the Great Basin and neighboring provinces by University of Utah. Major physiographic provinces are southern Modoc Plateau (SMP), western, central and eastern Great Basin (WGB, CGB, EGB), Wasatch Transition Zone (TZ) and Colorado Plateau interior (CPI). Geothermal systems or districts pertinent to this proposal are Kumiva Valley area (KV), Dixie Valley (DV) and McGinness Hills (MG) (Wannamaker et al., 2011). B) 2-D MT resistivity inversion model across the Great Basin showing localized shallowing of high-temperature, lower crustal conductors in warmer colors. The Seven Troughs Range is denoted by 7T and has the shallowest part of the large conductive anomaly under the Black Rock-Kumiva Valley area (BR-KV). The lower crustal conductor under Buena Vista and Dixie Valley and its crustal-scale connection to the geothermal system is marked by BV-DV (blue) (Wannamaker et al., 2011). C) The central Seven Troughs Range field area (pink star) with 2-D MT model overlaid on top to show where the shallowest upwelling occurs in relation to the Seven Troughs Range; warmer colors indicate areas of low resistivity. The MT survey stations are denoted with white dots. All MT data from Wannamaker et al., physical, and geochemical techniques to find the required elements (e.g., heat source, fluid to transport the heat, and permeability in a reservoir) for geothermal energy production. We have conducted detailed geologic mapping, structural analysis, and a 2 m temperature survey to delineate the most likely areas in the central Seven Troughs Range for blind geothermal activity. Detailed geologic mapping was conducted at a scale of 1:24,000 on color stereographic air photos over ~80 km 2 of the central Seven Troughs Range. Mesozoic through Quaternary stratigraphy was defined 371 in the field area. A two-meter temperature survey was conducted to determine if shallow temperature anomalies were present in areas with favorable structural settings. Upon completion of geologic and structural analysis a more detailed MT survey and soil gas study will be conducted. Stratigraphic Framework The Seven Troughs Range is composed of Mesozoic metasedimentary rocks, Cretaceous plutonic rocks, Tertiary volcanic and sedimentary rocks, and Quaternary sediments, alluvium, landslide, and spring deposits (Figure 3). Basement rocks in the Seven Troughs Range consist of upper Triassic to Jurassic, low-grade metasedimentary rocks, primarily interfingering argillite, mudstone, phyllite, quartzite and carbonates (Burke and Silberling, 1973; Hudson et al., 2006) (Figure 4). The mudstone, argillite, and quartzite locally contain abundant quartz veins and quartz breccias. The metasedimentary rocks are part of the Auld Lang Syne group, a very thick (up to ~7.5 km) package of argillaceous and sandy strata deposited in a shallow marine, deltaic environment (Burke and Silberling, 1973). In the middle or late Jurassic and early Cretaceous the Auld Lang Syne sequence was deformed in a back-arc fold and thrust belt (the Luning-Fencemaker thrust system) (Oldow, 1983, 1984; Wallace, 1987). The deformation associated with thrusting likely caused much of the low-grade metamorphism in this sequence. The metasedimentary rocks of the Auld Lang Syne group are intruded by Cretaceous granodiorite (Johnson, 1977) associated with emplacement of the Sierra Nevada batholith (Wernicke et al., 1987). Small tungsten deposits are locally found in scheelite-bearing tactite along the margins of the granodiorite intrusions (Wallace, 1987). The Mesozoic basement rocks are locally nonconformably overlain by Oligocene (?) non-welded to welded ash-flow tuffs and tuffaceous sedimentary rocks. The tuffs crop out extensively in the western portion of the field area but are scarce in the eastern part. Overlying the Oligocene (?) tuffs and Mesozoic basement in the Seven Troughs Range is a suite of middle Miocene bimodal volcanic rocks (Figures 3 and 4). These interfingering volcanic rocks can be broken out into five main sequences: (1) older basalt flows; (2) middle rhyolite flows, domes, dikes, and interbedded epiclastic sedimentary rocks, (3) younger basalt flows, cinders, and necks, (4) capping rhyolite flows and domes, and (5) alternating flows of upper rhyolite and basaltic andesite that locally overlies the basalt flows, cinders, and necks. The upper rhyolite locally includes lenses of both vitrophyre and tuffaceous sedimentary rocks. The Seven Troughs mining district resides on the east side of the range within a bimodal eruptive center and contains both basaltic andesite and rhyolite lava flows, felsic domes, and interbedded clastic sedimentary rocks from sequences 1-5 above. Basalt

4 dikes, epithermal veins, and coeval hydrothermal alteration are abundant in the mining district. The dikes intrude the lower basalt and middle rhyolite sequences (sequences 1-3) but are truncated by sequences 4 and 5 above (Hudson et al., 2006). Epithermal gold and silver veins lace the mining district and formed during the emplacement of sequences 1 and 2. Hydrothermal alteration (mainly argillic with some local propylitic) of the host rocks was coeval with the vein emplacement and is commonly spatially associated with basalt dikes and faulting. Hudson et al. (2006) used 40 Ar/ 39 Ar dating to constrain the timing of vein deposition and hydrothermal alteration. The vein host rocks (primarily sequence 2) range in age from 14.43±0.09 to 13.80±0.68 Ma; adularia from one of the veins yielded an age of 13.83±0.02 Ma. In addition, a post-ore capping rhyolite is 13.79±0.06 Ma (Hudson et al., 2006). Because the capping rhyolite truncates veins and dikes and is only roughly 600 ka younger than some of the vein host rocks, ore-forming processes and hydrothermal alteration were probably short lived. Structural Framework The Seven Troughs field area is characterized by a series of mainly north- to northeast-trending, west-tilted fault blocks bounded by moderately to steeply east-dipping normal faults. In the southern portion of the field area, a large horst block composed of Mesozoic basement is bound on the east and the west by rangebounding faults marked by Quaternary scarps (Adams and Sawyer, 1999; Figure 3). To the north, the range-bounding fault on the eastern flank of the Seven Troughs Range steps to the right, whereas the rangebounding fault on the west side apparently has a smaller left step (Figure 3). In the central part of the study area the northern portion of the western range-bounding fault arcs to the east and strikes east-northeast. This segment exhibits a sinistral component of slip, which is common for faults of this orientation under the current extension direction (Faulds et al., 2005). The Seven Troughs mining district and associated middle Miocene bimodal eruptive center apparently fills a northnortheast-trending synvolcanic graben (Hudson et al., 2006). Numerous normal and oblique-slip faults cut through the mining district and accommodate both offset of the middle Miocene volcanic rocks and relative uplift of the Mesozoic basement. Pervasive argillic alteration accompanies the mineralization and less abundant propylitic alteration associated with dike emplacement. Although abundant hydrothermal alteration correlates with the mineralization, no known active geothermal system is associated with the mineralization. Figure 3. Simplified Geologic map of the central Seven Troughs Range, Pershing County, NV. Inset map shows location of the Seven Troughs field area (black box), springs and wells in the area are denoted with a black triangle and the USGS Quaternary faults from the USGS Quaternary fault and fold database are categorized by age of faulting (Adams and Sawyer, 1999). Slip and Dilation Tendency Analysis Faults that are oriented orthogonal to the least principal stress are more likely to slip and therefore generate permeability and channel geothermal fluids from depth to the near surface (Morris et al., 1996; Curewitz and Karson, 1997; Ferrill and Morris, 2003). The likelihood of slip (slip tendency) on a given fault surface is contingent upon the rock type, the ratio of shear to normal stresses, the orienta- 372

5 Figure 4. Schematic Stratigraphic column of the Seven Troughs field area. Black lines point to the units where the corresponding 40Ar/39Ar age dates were sampled from. The age dates were obtained from Don Hudson (2006). tion of the fault plane, and the in situ stress field (Morris et al., 1996). Dilation tendency of a fault is controlled by the resolved normal stress, which relies on fluid pressure and the lithostatic and tectonic stresses (Moeck et al., 2009). Calculating which faults in the study area have the highest slip and dilation tendency is valuable for deciding where to conduct more detailed geophysical and geochemical studies. A preliminary analysis of the faults most favorably oriented for slip and dilation has been conducted using 3-D stress software and then plotted in ArcGIS 10.1 (Figure 5). This analysis is a best-case scenario calculation, as it assumes that the faults dip 60 in the slip tendency calculation and are vertical in the dilation tendency analysis. In both slip and dilation tendency A B Figure 5. Simplified fault map of the Seven Troughs Range showing some of the major faults in the field area. Color on faults indicates which faults are more likely to slip (A) and dilate (B) under the inferred current stress regime; warmer colors are most favorably oriented for slip/dilation. This analysis assumes a normal faulting regime and that the minimum principal stress direction trends The minimum principal stress direction for this area was gleaned from fault inversion data from the nearby Brady s geothermal field, as well as Yucca Mountain, Hawthorne, and Dixie valley (Siler, personal communication, 2013). The slip and dilation tendency analyses assume the best-case scenario, with all faults dipping 60 for the slip tendency analysis and 90 for the dilation tendency analysis. 373

6 calculations, the minimum principal stress direction is orientated 116.5, which is the average extension direction for this region based on available borehole breakout data and fault inversion data from several geothermal systems in the Basin and Range (Siler, personal communication; Figure 5). Two-Meter Temperature Survey A two-meter temperature survey was conducted on the west and east sides of the central Seven Troughs Range to assess whether any shallow temperature anomalies were associated with the favorable structural settings (Figure 6). The highest measured temperature was 19.3 C just north of Porter Spring on the west side of the field area (red dots in Figure 6). The temperature for Porter Spring is 20 C, so this temperature is on par with the upwelling of the spring fluids. The background temperature at twometer depth in the area is 13.5 C, and most of the temperatures recorded were near background (yellow dots in Figure 6). A few above background temperatures (~15-16 C) were recoded along the east side of the range (orange dots in Figure 6) in the vicinity of the fault intersection and right step. Rocky ground made it difficult to insert the probes to take measurements in all of the desired locations. We hope to return for a follow-up survey with a rock drill to obtain a more uniform grid around the areas of interest. Based on the current data, no high temperature shallow anomaly has been found in the field area. However, the thick alluvium could be acting as a thermal insulator. Alternatively, an impermeable clay cap could prevent fluids from reaching near surface levels. Discussion and Conclusions A reconnaissance 2D MT transect across the Great Basin conducted by Wannamaker et al. (2011) shed light on a possible relationship between zones of low resistivity in the shallow crust that taper up to the near surface proximal to high temperature geothermal systems. The low resistivity anomaly situated under Kumiva Valley shallows under the Seven Troughs Range but is not associated with any known geothermal resource. Detailed geologic mapping and structural analysis were conducted in the central Seven Troughs Range to determine the potential for a blind geothermal system in this area. Two areas have a favorable structural setting for generating permeability and channeling geothermal fluids to the near surface: 1) On the east side of the southern Seven Troughs Range the range-bounding fault steps to the right and intersects a major east-northeast-striking fault. Slightly elevated two-meter temperatures have been found in this vicinity. 2) On the west side of the field area in the vicinity of Porter Spring, the range steps to the left near the northern termination of an east-dipping normal. This area has the highest recorded two-meter temperatures. Although the two-meter temperature survey does not reflect the presence of geothermal fluids within two-meters of the surface at these locations, the 2D low resistivity MT anomaly and favorable structural settings warrants further analysis for blind geothermal systems in the area. More detailed 3D MT coverage in the area and soil gas surveys in the area will help to constrain the potential for blind geothermal systems in the Seven Troughs Range. Acknowledgments This work was funded by a Department of Energy grant (DE-EE ). I would like to acknowledge: Nick Hinz for all of his help, patience and guidance, Don Hudson for thoughtful discussions on the Seven Troughs mining district, Chris Sladek and Connor Newman for help with the two-meter temperature survey, and my field assistant Mitch Allen for all of his hard work. Figure 6. Fault map of the Seven Troughs field area. Colored dots show locations of twometer temperature survey points; warmer colors indicate warmer temperatures. References Adams, K., and Sawyer, T.L., compilers, 1999, Fault number 1627, Seven Troughs Range fault zone, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, 374

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