Evidence for a Magmatic Source of Heat for the Steamboat Springs Geothermal System Using Trace Elements and Gas Geochemistry

Transcription

1 Geothermal Resources Council Transactions, Vol. 27, October , 23 Evidence for a Magmatic Source of Heat for the Steamboat Springs Geothermal System Using Trace Elements and Gas Geochemistry Greg B. Arehart, Mark F. Coolbaugh and Simon R. Poulson Great Basin Center for ~eothermal Energy, ~nivers~ty of Nevada, Reno, NV. USA Keywords with active magmatic provinces. In the Great Basin, they are ~eot~~rma ~team~o~t Springs, ~~gma~ic, geoc~e~is~r~ closely associated with very youthful silicic volcanism at the trace element margins of the extending area (squares, Figure 1). The second type of geothermal system is the extensional type, which is nearly unique to the Great Basin, and which is not associat~d with young ABSTRACT silicic volcanism, but instead is associated with regions of high heat flow and recent faulting in areas of thinned and extending crust (Wisian et. al., 1999). Generally, these systems are located within the interior of the Great Basin. It has been shown empirically in the Great Basin and elsewhere The objective of this project is the development of a representative geochemical database for a comprehensive suite of elemental and isotopic parameters (i.e., beyond the typical data suite) for geothermal fluids in the Great Basin. Preliminary assessment suggests that there are significant differences between magmatic-driven and extensional geothermal systems. In particular, fluids in magmatic systems have higher concentrations of arsenic and higher ratios of Li/Cl, B/Cl, and Ce/Cl than fluids from extensional systems. Once understood in the context of Great Basin geothermal systems, such differences can be utilized to more effectively explore for, assess the potential of, and develop exploitation strategies for these two types of systems. Magmatic-driven systems are, in general, more favorable targets for power production than extensional systems because of a higher geothermal temperature gradient. Steamboat Springs, NV provides a test case for this approach. There has been debate over the nature of the Steamboat Springs geothermal system, but the trace element and gas geochemical data are most consistent with magmatism as a driving heat mechanism. Similar trace element investigations, coupled with other geological parameters, may help to identify magmatic-driven systems elsewhere within the Great Basin. (Figure 2, overleaf) that ~agmatic-~pe systems have a higher temperature gradient with depth and are potentially more productive targets. Therefore, any indicators that suggest a system is driven by magmatic heat can be very important in exploration. Recent research, reported here, suggests that there may be distinctive geochemical differences in the fluids between extensional and Most geothermal systems are the result of penetration of meteoric fluids into the crust, heating of those fluids, and consequent buoyant upflow of the fluids. Two types of geothermal systems in the Great Basin have been recognized as being able to support geothermal power plants. The first type has been termed magmatic geothermal, not because of magmatic fluids, but because of a magmatic heat source. The vast majority of power-producing geothermal systems around the world are of this type, associated Figure 1. Location map for geothermal systems (circles), power plants (triangles), and young silicic volcanic rocks (squares) in the Great 8asin. 2 69

2 Arehart, et. ai. 5 1 n e 15 B E2 1 Y U Temperature (deg C) e Steamboat Springs T u ~ e y ~ n s i ~ n ~ x Great Basin extensional Great Basin prospects X Boiling Curve IModfied torn Flvnn and Xhochol. XI1 Temperature Gradient, World Geothermal Systems Figure 2. Plot of depth vs. reservoir temperature for a variety of geothermal systems. There is an empirical separation between magmaticdriven systems and extensional systems as shown by the solid line. This appears to hold true for not only the Great Basin geothermal systems, but also for other systems around the world. Of particular note is the position of the geothermal systems in western Turkey, which are in an extensional environment and are the closest analogues elsewhere to the Great Basin geothermal systems. The single point at the upper right of the diagram represents superheated steam at Coso. magmatic systems. If this is indeed the case, then geochemical analyses might provide an impo~antool for distinguis~ng extensional from magmatic systems, and exploration strategies could be predicated on these differences. The Steamboat Springs geotherm~ system provides a test case where we can examine a number of geochemical characteristics that may assist in making a distinction. Several 1.I million year-old rhyolite domes occur in the Steamboat Springs area but these are considered too old to be viable heat sources for the current geothermal system. So the question arises: Are there younger unexposed rhyolites at depth that are providing the heat source, so that Steamboat Springs is a magmatic-type system, or is Steamboat really an extensional-type system, since it is located close to a well-known active rangefront fault? Results of Geochemical Analysis In the past 12 months, we have assembled and augmented the available representative geochemical and geological data on fluid compositions for a wide variety of geothe~al systems in the Great Basin region. These data have been integrated into a geographical information system (GIS) to more fully explore relationships between the chemistry of fluids and other features known to correlate with geothermal systems (Garside and Schilling, 1979; Garside, 1994; Arehart et al., 22; Coolbaugh et al., 22; Shevenell et al., 22). In spite of limited data for some elements, prelimin~y assessment reveals intriguing patterns, some of which apply to the classification of the Steamboat Springs system. Major Element C ~e~istry Preli~inary results indicate that in terms of major fluid constituents, it is difficult to tell the two types of systems apart. That a 17 A Japan o West US ~ a ~ ~ a ~ ~ - cos o Mammoth e Steamboat + Dixie Valley 2 i Q? Si2 (ppm) Figure 3. Plot of chloride vs. Si2 for the Great Basin and selected other geothermal systems. includes chloride, ph, and most other major componen~. Figure 3 illustrates the example of chloride, which is shown on the y-axis, and silica, which is shown on the x-axis, where it serves as a proxy for temperature, because for many undeveloped systems we do not have reservoir temperatures. From these data, it is clear that it is not possible to distinguish the two types of fluids based solely on major element ge~hemis~y, Many of the extensional type systems can be classified as sodium-chloride wgters, similar to many geothermal systems elsewhere in the world. In contrast to the major element geochemist^, trace elements appear to provide some discriminators for the two types of systems, at least in the context of the Great Basin. A preliminary assess~ent of these patterns is presented here, along with some speculations on potential causes for the observed differences. Arsenic ~ eoc~e~istry High-temperature geothermal systems appear to have elevated As concentrations compared to lower-tempera~re systems (Figure 4). This suggests the possible utility of As concentrations in fluids as an indicator of a higher-temperature (and therefore more energy-productive) system at depth, a potenti~lly impo~nt addition to the list of geothermometers. The slopes of the As-temperature relationships between the magmatic and ~xtensional systems are clearly different, with magmatic systems having considerably more As than extensional systems at all temperatures. This suggests that As may be useful in discriminating between the two types of systems. The position of Steamboat Springs within the magmatic group on the plot of 2 7

3 A Japan West US Magmatic - coso o Mammoth a Steamboat + Dixie Wallev IO 8 n EQ p 6 Y u c Q).I $ 4 A Japan - coso o Mammoth Steamboat x CB Extenslonal + Dixie Vallev 2 5 IO Reservoir Rock As, ppm (based on global averages) Figure 4. Plot of arsenic content of geothermal fluids vs. pprn Si2 (as a proxy for temperature) for a variety of geothermal systems. All systems except those included as GB extensional and DixieValley are magmatic. The trend line for magmatic systems is shown by the dashed line and the trend line for extensional systems is shown by the solid line. Figure 4 suggests that in terms of As content, the deeper fluids at Steamboat may be driven by magmatic heat. Several possibilities are suggested to explain the correlation between As and temperature: 1) high-temperature systems are associated in space with As-rich host rocks or magmas, 2) As content of the fluids is a function of the age of the system, or 3) high-temperature systems are more aggressive and leach the available As from host rocks more efficiently. To assess the first possibility, Figure 5 is a plot ofas content of geothermal fluids (for the same set of geothermal systems shown in Figure 4) as a function of the As content of the major reservoir rock. In this assessment, the average As content for various rock types is taken from Turekian and Wedepohl (1961) because we have no rock As data. From Figure 5, it is inferred that there is no correlation between reservoir rock type and As in geothermal fluids. This does not rule out the possible role of magmatic contributions to the As budget. Closer scrutiny of the data allows some additional insights into the second and third possibilities. Early in the development of a geothermal system, the reservoir rocks are more likely to be relatively fresh and less amenable to leaching by geothermal fluids. As the system matures, higher-temperature fluids that have interacted with the reservoir rock for longer periods of time may alter the rocks significantly; this mineral transformation increases the availability of As (depending on the residence of As in the rock, which is unknown for all of the systems studied). Thus, intermediate-age fluids may contain more As than young fluids. As the system continues past maturity and temperatures decreases, Figure 5. Plot of arsenic in fluids vs. arsenic content of reservoir rock for geothermal systems. most of the As in the original reservoir rock may already have been removed, resulting in a decreased As content of old-age geothermal fluids. If this life-cycle is at all representative of what happens in geothermal systems, the implications are important for development of geothermal systems. Young, expanding systems should have fairly fresh reservoir rock and relatively low As content in fluids; intermediate-age systems should have more-altered rock and relatively higher As content in fluids; and declining systems may have similar As content to young systems, but more altered reservoir rocks. Therefore, careful attention to alteration features, coupled with fluid geochemistry, may allow assessment of the stage of life of a geothermal system. Such assessment must be done in the context of the setting of the geothermal system. Chloride Geochemistry A second interesting spatial correlation exists in the Great Basin between areas of higher-temperature geothermal systems and chloride content of geothermal fluids (Arehart et al., 22). Geothermal systems developed in magmatic terrains (i.e., dominated by igneous host rocks) generally have chloride contents in the range of lo3-14 ppm (Henley, 1984) whereas those developed in other types of host rocks can have highly variable chloride contents (e.g. Salton Sea, - 19, ppm C1; Henley, 1984). Many of the sub-basins of the Great Basin contain evaporite minerals, which could be contributing to the relatively elevated CI signature of these systems in the western Great Basin. However, inspection of a plot of C1 vs. Si2 for the Great Basin and other global geothermal systems (Figure 3) indicates 271

4 o - cos o ~ ~rnoth Steamboat + Dixie Valley h E 1?. n v A E P Q v c - ~ - cos ~~m~~ rn Steamboat 8 /* /* - /- 1 lo i 9* 5 m Chloride (pprn) Figure 6a. Plot of lithium vs. chloride for various geothermal systems. The trend line for magmatic systems is shown by the dashed line and the trend line for extensional systems is shown by the solid line. s japan o Steamboat x C8 Extensional + Dilxle Valley?. i 5 I 4- X I / a a*/* / X x x I I I I I Chloride (ppm) Figure 6b. Plot of cesium vs. chloride for various geothermal systems. The trend line for mag~atic systems is shown by the dashed line and the trend line for extensional systems is shown by the solid line. 2 4, 6 8 Chloride (ppm) Figure 6c. Plot of boron vs. chloride for various geothermal systems. The trend line for magmatic systems is shown by the dashed line and the trend line for extensional systems is shown by the solid line. that there is no difference between C1 content of most Great Basin systems and C1 of other global systems. Therefore, it is suggested that we are not observing any input of C1 to Great Basin systems from the near surface, but that the C1 content of these systems is a function of the generally higher temperature of these systems, and the consequent increased interaction with reservoir rocks. Other Trace Elements Several other trace elements appear to show important differences between magmatic and extensional systems. Plots of Li vs. Cl, Cs vs. C1 and B vs. C1 (Figure 6) all delineate the two system types. In each of these cases, the Steamboat fluids fall into the magmatic group. Cu, Pb, Zn, V, Sn, Be, and Re have similar dist~butions in both magmatic and extensional systems and do not appear to be useful as discriminators. A plot of Sc vs. Si2 (not shown) suggests that Sc correlates very well with Si2 and could be utilized as a geo~ermom~ter; a similar relation is observed for Ge, though it is less clearly developed. Several other elements, including TI, Sb, and Se do not show clear patterns. However, at present, we do not have high-temperature data for all trace elements of the extensional systems, so more complete data analysis must await the collection of these additional data. Non-Condensable Gas Data Data for the non-co~d~nsable gas fract~on of the geothe~al fluids from the Great Basin are shown in Figure 7. There are two 2 72

5 i Steam boat shallow Steamboat deep GB Extensional A""' (thermogeni~ crustal gas) - -.I' Steamboat shallow -\ Steamboat deep@\ o GB Extensional -tp \ 1 1 He 2 Ar Figure 7a. Triangular plot for geothermal fluids from the Great Basin. Open circles are geothermal and volcanic gas data from Giggenbach (1 992). sets of data from the Steamboat system, data from the shallow fluids (e5 m depth) and data from deep fluids (>lo m depth). On the triangular plots of Figure 7, the data clearly fall into two different fields, with the shallow fluids having ch~acteristics of air or air-saturated water, and the deep fluids having a significantly more magmatic component. When compared to the other available noncondensable gas data for extensional systems in the Great Basin, the Steamboat system has a significantly more magmatic character. Isotopic Data Isotopic data for fossil geothermal environments in the Great Basin also may provide insights into fluid sources and pathways. The extant limited stable isotope data (H, C,, S) do not show any simple trends. However, He isotope data from a traverse across the Great Basin suggest that Steamboat Springs has a mantle He signature (Kennedy et al., 22). Significance and Future Work The geochemical indicators that have been developed for geothermal systems across the Great Basin, coupled with the position of Steamboat Springs in Figure 2, provide compelling evidence that the deep Steamboat system is driven by magmatism that is not evident at the surface. The data set being generated through this research has provided the first comparison of the geochemistry of magmatic-related geothermal systems to extensional geothermal systems. Additional data collection, currently underway, will continue to clarify and refine these differences. These data will ultimately allow better assessment of the potential resources of a given thermal system and help determine whether a geothermal system is high-temperature or low-temperature, in the context of magmatic vs. extensional systems. The data being developed will provide the basis for related research that will have important implications for the understand- Figure 7b. Triangular plot for geothermal fluids from the Great Basin. Open circles are ~eothermal and volcanic gas data from Giggenbach (1992). ing of the two (or more?) types of geothe~al systems in the Great Basin. This will include addressing questions about the sizes, lifetimes, and stage of life of geothermal systems, management of systems to optimize energy ex~action, and potential environmental issues associated with exploitation. Ultimately, the data will contribute to our understanding of where, how, and why these systems form. Summary The goal of the research reported here has been, and continues to be, compilation and augmentation of a comprehensive geochemical database for a wide range of elemental and isotopic parameters as one of the first steps in understanding the variety and nature of geothermal systems in the Great Basin. Preliminary analysis of the data suggests it is possible to dis~ri~nate between higher-temperature and lower-temperature systems, and between magmatically-driven and extensional geothermal systems. Such data, and the models derived from them, will be useful in developing exploration models and exploitation strategies for geothermal energy in the Great Basin. Acknowledgments We are very grateful for the discussions and insights of Ted DeRocher of Caithness Operating Company. Ted's detailed knowledge of the geochemistry, sampling methods, and physical behavior of geothermal systems helped guide the project. Caithness Operating Company and Ormat provided access to geothermal production wells. The support and assistance of the Great Basin Center for Geothermal Energy at the University of Nevada, Reno is much appreciated. This research is based upon work supported by the U.S. Department of Energy under instrument number DE-FG7-2ID

6 References Arehart, G.B., Coolbaugh, M.F. and Poulson, S.R., 22, Geochemical characterization of geothermal systems in the Great Basin: Implications for exploration, exploitation and environmental issues: Proceedings, Annual Meeting, Reno, NV., Sept ,22, Geothermal Resources Council Transactions, v. 26, p I. Coolbaugh, M.F., Taranik, J.V., Raines, G.L., Shevenell, L.A., Sawatzky, D.L., Bedell, R. and Minor, T.B., 22, A geothermal GIS for Nevada: Defining regional controls and favorable exploration terrains for extensional geothermal systems; Proceedings, Annual Meeting, Reno, NV., Sept , 22, Geothermal Resources Council Transactions, v. 26, p Flynn, T. and Schocket, D.N., 21, Commercial development of enhanced geothermal systems using the combined EGS-hydrothermal technologies approach: Geothermal Resources Council Transactions, v. 25, p Garside, L.J., 1994, Nevada Low-Temperature Geothermal Resource Assessment, Nevada Bureau of Mines and Geology Open-File Report 94-2, Garside, L.J. and Schilling, J.H., 1979, Thermal waters of Nevada: Nevada Bureau of Mines and Geology Bulletin 91, 163 p. Giggenbach, W., 1992, The composition of gases in geothermal and volcanic systems as a function of tectonic setting; Water-Rock Interaction-7, p Henley, R.W., 1984, Chemical structure of geothermal systems: Reviews in Economic Geology, v. 1, p Kennedy, B.M., Johnson, S., Benoit, D., Shuster, D.L., Janik, C., Goff, E, and van Soest, M., 22, Noble gas isotope geochemistry at the Dixie Valley geothermal field: Dixie Valley Workshop (DOE-funded research), June 12-13, 22, Great Basin Center for Geothermal Energy, University of Nevada, Reno, Nevada. Shevenell, L.A., Garside, L., Arehart, G.B., van Soest, M. And Kennedy, B.M., 22, Geochemical sampling of thermal and nonthermal waters in Nevada to evaluate the potential for resource utilization; GRC Annual meeting, 22. Turekian, K.K. and Wedepohl, K.H., 1961, Distribution of the elements in some major units of the earth's crust; GSA Bulletin, v. 72, p Wisian, K.W., Blackwell, D.D., and Richards, M., 1999, Heat flow in the western United States and extensional geothermal systems: Proceedings, 24'" Workshop on Geothermal Reservoir Engineering, Stanford, CA., p