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1 NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material. The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement. This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law.

2 Geothermal Resources Council ~ r ~ s ~ Vol. ~ i 21, o ~ Septem~r/O~o~r s, 1997 Sulfate-Water isotope Geothermom~t~ and lead Isotope Data for the Regional Geothermal System in the Twin Falls Area, South-Central Idaho R.H. Mariner', H.W. Young*, T.D. Bullen', and C.J. Janikl W.S. Geological Survey, Menlo Park, California 2-US. Geological Survey, Boise, Idaho ABSTRACT Sulfate-water isotope geothexmometry for the geothermal system at Twin Falls, Idaho indicates aquifer-tempera~res of 90" to 106 C; most sites are between 90O and 93 C. 206Pb/2wPb and 208Pb/204Pb of individual thermal waters are principally a function of how much lead has been dissolved &om the carbonate and silicate fractions of the Paleozoic li~estone collected west of Grand View Peak. Although most thermal waters are recovered from Tertiary rhyolite, very little ofthe dissolved lead is from the rhyolite. Recharge to this system occurs in northern Nevada and the fluid moves northward in the Paleozoic limestones. The occurrence ofthermal fluid in the Idavada Volcanics near and south of Twin Falls, Idaho is the result of upward movement of this fluid from the Paleozoic limestone. Introduction Thermtil fluids occur at many locations along virtually the entire length of the Snake River Plain in southern Idaho so itch ell and others, 1980). In south-central Idaho, thermal waters occur principally between the Snake River and the mountains of northern Nevada. Topographically the area consists of a plain which slopes gently upward from the Snake River to the m o ~ ~ of i n ~ o s ~ Nevada. e ~ Numero~ wells scattered over the plain and near the Snake River (Figure I) draw on thermal water in a regional geothermal system. Several perennial streams cross the plain in deeply incised canyons. A low range of hills, ~ clud~g Grand View Peak, separates the Salmon Falls Creek and Goose Creek drainages. Quaternary and Tertiary volcanics and Paleozoic marine sediments underlie the area (Lewis and Young, 1989). Basalts of Quate~~ age occur in the northern part of the area. Slightly older basalts and detrital units of the Glenns Ferry Formation i 5'00' I,' I 1 I I, I I R5 a; : 07/ "'.; / N 4 R7 1 I \ a ~umboldt Fo~at~on Grand View 1 14"OO' Figure 1. Locations of wells samples and rocks collected. Numbersfl, correshnd to ID numbers used in Table 1 (water samples) and Table 2\'" frock samples). 7 97

3 Mariner, et ai. (lower Pleistocene and upper Pliocene) are exposed in the northern part of the area. The Miocene Banbury Basalts are the thickest basalt units and may be up to 1,000 feet thick in this area (Malde and Powers, 1972). Although locally significant, the basalts are not as ubiquitous as the Idavada Volcanics (Miocene and Pliocene), which are comprised of silicic welded tuffs and occasional basalt flows. The Idavada Volcanics are at least 2,000 feet thick in part of the area and were originally considered to be the major aquifer for the thermal water (Lewis and Young, 1989). The thickness and extent of Paleozoic rocks (principally limestone) beneath the Tertiary cover is unknown. Up to 5,000 feet of Paleozoic rock has been reported in the mountains of northern Nevada (Schroeder, 1912). The hydrothermal system in the area was not extensively utilized until the mid- 1970s. Most development has been near the Snake River. Water temperatures in most wells range from 25" to 70 C (Mariner and others, 1991). The fluid is used for space heating, greenhouse operations, irrigation-, and aquaculture. Well depths range from 350 to 2,200 feet (Lewis and Young, 1989). As a result of increased utilization, well-head pressures have declined as much as 25 pounds per Square inch (-60 ft. of water head), and some previously flowing wells must now be pumped. The original conceptual model for the system, proposed by Lewis and Young (1989), consisted of a single 2,000 foot thick aquifer in the Idavada Volcanics with recharge near the Idaho- Nevada boundary. Regional systems, such as this, obtain heat from the geothermal gradient not from a local heat source; heat is swept up as the fluid moves over large distances. Near the' Idaho-Nevada boundary, thermal water is often recovered from Paleozoic limestones. Lead isotopic values for thermal waters of the Twin Falls area (Mariner and Young, 1995) show that thermal fluids from Paleozoic limestones at depth are leaking upward into the overlying Tertiary volcanic rocks. Although water-rock reaction has overprinted the normal chemical characteristics of a limestone related water (low silica and fluoride) with the chemical characteristics of a rhyolite related water (high silica and fluoride),-it has. not been complete enough.to. obliterate the characteristic lead isotope compositions derived from the Paleozoic limestone. Water from thermal wells in the Twin Falls area (Mariner and others, 1991) is slightly to moderately alkaline in ph (7.3 to 9.6). Water from wells in limestone (limestone-related water) is low in silica (20 to 30 ma), sodium (10 to 24 mg/l), and fluoride (generally less than 2 mg/l). Water from wells in rhyolite (rhyolite-related water) is variable in composition but the most clearly rhyolite-related waters are relatively high in silica (>70 mg/l), sodium (100 ma), and fluoride (>2 mg/l). Mariner and Young (1995) used lead isotope values for rock samples from areas as far away as Yellowstone National Park to estimate isotopic ratios in the aquifer-rock for the Twin Falls system. To improve on the possible range of lead isotopic ratios of the rock in the area, eleven rock samples from potential aquifer-rocks of Paleozoic to Tertiary age were analyzed for 206Pb/204Pb and 208Pb/204Pb. These values will be compared with ratios for waters in Mariner and Young (1 995) to better determine the aquifer-rock type. Cation and silica geothermometers provide information about temperatures in the producing aquifer, however, where a deeper system is providing water to the producing aquifer, temperatures in the deeper aquifer are unknown. All wells in the northern part of the area near Twin Falls and the Snake River are completed in the Idavada Volcanics. The sulfate-water isotope geothermometer reacts more slowly to changes in temperature than does the silica or the various cation ratio geothermometers. Based on the rate equation of Lloyd (1 968), at 92 C isotopic equilibrium between sulfate and water would be 98% achieved in 300 years at a ph of 7 or in 8,700 years at a ph of 9. The increase in ph of $e fluid as it moves from the limestone to the rhyolite helps preserve evidence of temperatures in the deeper limestone. Water samples from eight thermal wells (locations shown on Figure 1) in south-central Idaho and the adjacent part of northeastern Nevada, were collected for sulfate-water isotope analysis. This data will show how' temperatures change in the deep system over a large area and establish the maximum temperatures present in the system. Data Major element chemical and isotopic data for the eight wells sampled for 6'80,u~~atc analysis are shown on Table I. Values of 6180,u~t,l, in the thermal waters show a very narrow range (-1.1 to /o0). Results of lead isotope analysis on eleven rock samples from possible aquifer-rock in the Twin Falls area are given in Table 2. Five samples from the Miocene Idavada Volcanics and one from the Miocene Humboldt Formation have a small range in 206Pb/204Pb (18.6 to 18.9) and 208Pb/204Pb (39.3 to 39.8). One sample of Jurassic granite (Contact Pluton) was much more enriched in 206Pb/204Pb (20.3) and 208Pb/204Pb (40.4). Paleozoic limestones are very variable in their lead isotope ratios. Values for a carbonate fraction and a silicate fraction are given for each limestone. The carbonate fraction is the part of the rock which is soluble in weak acid and the silicate fraction is the remainder of the rock which is soluble only in strong acid. Lead isotope ratios of the carbonate fraction of the four Paleozoic limestones collected in the area range from 19.0 to 23.9 in 2"Pb/204Pb and from 38.1 to 39.0 in 208Pb/204Pb. Lead isotope ratios of the silicate fraction of the Paleozoic limestones in the area range from 21.3 to 27.8 in 206Pb/2"Pb and from 38.7 to 40.1 in 208Pb/204Pb. Discussion Sulfate-water isotope relations indicate that isotopic equilibrium between dissolved sulfate and water was last achieved at temperatures of 90" to 106 C (Table 3). Five of the seven sites are very uniform in estimated sulfate-water isotopic equilibrium temperatures (90' to 93 C). Temperatures of 93 C for the part of the system in northern Nevada require that recharge is at least several tens of miles and perhaps more to the south. The two "hottest" sites (sulfate-water isotope geothennometer temperatures of 105" and 106 C) are in the northern part of the area I 198

4 ~ ~ ~ Mariner, et al. Table 1. Chemical and isotope data for selected thermal waters in the Twin Falls area [Chemical concentrations are in mgll. Isotope values are in per mil] ID# Location t C ph SO2 Ca Mg Na K HC0,'S04 Cl F 6Dt,20 6'80H20 St8OSo4 1 8s 14E 30acd2 2 8s 14E 33ccal 3 9s 15E f2ccal 4 9s 17E 29accl 5 9s 17E 33bbcl 6 10s 17E 4cdal 7 14s 15E 16ddcl 8 47N 65E 17cbcl Total alkalinity as HCOl IO e Table 2. Lead isotope ratios of possible aqujf~r-rock in the Twin Falls area ID# Rocktype Location 2osPb/204Pb 'U7Pb/209Pb 208Pb/204Pb R1 R2 R3 R4 RS R6 R7 R8 R9 ldavada Volcanics Idavada Volcanics Idavada Volcanics. idavada Volcanics Idavada Volcanics Humboldt Formation Jurassic granite Paleozoic Is. -carbonatel - silicate2 Paleozoic Is. carbonate' - silicate2 10s 13E 24 47E 66E 30 45N 64E 19 16s 15E E 19 45N 63E 24 44N 66E 30 47N 66E 28 46N 64E R10 Paleozoic 1s. - carbonate' 46N 64E silicate Rl 1 Paleozoic Is. -carbonatel 13s 17E silicate I Fraction of limestone soluble in weak acid. 2 Fraction of limestone soluble only in strong acid. near the Snake River. The sulfate-water isotope geothermometer does not indicate tempera-tures appreciably hotter than estimated fiom the Na-K-Ca geothe~ometer (Table 3). The similarity in estimated temperatures between the sulfate-water isotope and Na-K-Ca geothexmometers requires that the water has been in the rhyolite long enough for chemical equilibrium to be established. Temperatures of 90 C must occur, at least locally, in the Tertiary rhyolite. Temperatures in the Paleozoic limestone are apparently 90" to 93 C over a very large area and these values are maintained locally in the Tertiary rhyolite by fluid upflow. Mariner and Young (1995) used lead isotope and dissolved silica data to demonstrate that the thermal waters had reacted with Paleozoic limestone and Tertiary rhyolite. Waters clearly related to limestone are higher in 206Pb/204Pb and 208 Pb/*04Pb than waters from most rhyolites. Lead isotope ratios for rhyolite-related waters were much more variable than for limestone-related waters. Rhyolite-related waters have lead isotope values that overlap the values of limestone-related waters. Lead isotope ratios show that fluids with chemical compositions typical of a rhyolite source have spent most of their circulation time in a Paleozoic limestone at greater depth ( ~ ~ nand e Young, r 1995). As the fluids move upward from the Paleozoic limestone into the Tertiary rhyolite both chemical and isotopic compositions of the waters should change toward a more "rhyolite-like" composition. The rock data referenced in Mariner and Young (1995) were from the Yellowstone area (Doe, 1976; Doe and others, 1982; and Leeman and others, 1977). Lead isotope data were not available for rocks in the Twin Falls area. Values for 206Pb/204Pb and 20*Pb/204Pb in the rock samples from the area (Table 2) and of the thermal waters reported in Mariner and Young (1995) are shown on Figure 2. Many of the thermal waters have lower 20aPb/204Pb and 208Pb/204Pb 199

$Mariner, et al. 40.5 40,A Jurassic granite /+ 39.5 39 a i 6 38.5 8 38 18 19 20 21 22 204 206Pb/ Pb 23 24 A Jurassic granite V carbonate fraction silicate fraction Figure 2.$

5 Mariner, et al ,A Jurassic granite / a i Pb/ Pb A Jurassic granite V carbonate fraction silicate fraction Figure 2. *mpb/lwpb versus ZmPbPPb for rocks and waters in the Twin Falls area. Data for thermal waters are from Mariner and Young (1 995). than any of the rock samples. Although this could indicate the presence of an unsampled rock unit of low 206Pb/2"Pb and 208Pb/204Pb, it is more likely that the lead isotope ratios of the thermal waters are a function of how much interaction the waters have had with the carbonate and silicate fractions of the Paleozoic limestone. The silicate fraction of the limestone is usually much more enriched in 206Pb/2"Pb and 208Pb/204Pb than the carbonate fraction (Table 2; Figure 2). Both fractions were prepared for analysis by dissolution in acid. Although a weak acid was used to dissolve the carbonate fraction it undoubtedly also dissolved some silicate. Thermal waters are much less corrosive than the acid used in the preparation of the carbonate fraction for analysis and would dissolve even less silicate. The limestone at 13s 17E 7 has a carbonate fraction 206Pb/204Pb and 208Pb/204Pb indistinguishable fiom some of the thermal waters. A line through the silicate and carbonate fractions of this sample extends through the lead isotope compositions of most thermal waters of the Twin Falls system (Figure 2). The linear distribution of lead isotope ratios for the thermal waters is therefore a function of the relative amounts of carbonate and silicate that the specific thermal water dissolved from this Paleozoic limestone. Waters with very low 2"Pb/204Pb and 208Pb/204Pb have reacted primarily with carbonates; waters with 206Pb/204Pb between 18.5 and 19.2 and 208Pb/204Pb between 38 and 39 have reacted with more of the silicate fraction. The water with 206Pb/2@'Pb of 20.0 and 208Pb/204Pb of 39.2 is not anomalous, it has just reacted with more silicate in the limestone than the other thermal waters. The few thermal waters which plot off the trend have higher 208Pb/204Pb and have changed toward the ratios of the Tertiary volcanics. 4 Table 3. Geothermometry for selected thermal waters in the Twin Falls area [Temperature are in OC] 9s 17E 29accl

6 Mariner, et al. Summary Variability in lead isotope compositions of the thermal waters is principal~y a ~nction of differences in the lead isotope ratios of the carbonate and silicate fractions of the Paleozoic limestone. Small shifts inlead isotopic compositions toward the ratios of the Tertiary rhyolites are observed in only a few thermal waters. Although most thermal fluids are recovered fiom a Tertiary rhyolite and have the major element chemical characteristics of rhyolite-related waters, no thermal fluid circulates solely in the Tertiary volcanics. Reaction with the rhyolites produces fluids rich in silica, sodium, and fluoride, but the lead isotope compositions are not as easily changed, preserving evidence of fluid mo~ement in Paleozoic limestones at greater depth. Limestone-related waters which have not reacted with rhyolite occur only in the southern part of the system. Sulfate-water isotope relations indicate maximum aquifer-tempera~res of about 105OC locally but temperatures of 90" to 93OC are more common. The Na-K-Ca, chalcedony, and sulfate-water isotope. geothermom-eters produce similar temperature estimates in many rhyolite-related waters. Although temperatures of more than 7OoC have not been encountered in wells drilled in the rhyolite, chemical geothermometry indicates that temperatures of 90 C occur, at least locally, in the Tertiary volc~ics and are prevalent in the Paleozoic limestone at greater depth. A sulfatewater isotope geothermometer temperature of 92 C near Jackpot, Nevada requires that recharge to this regional system occurs at least a few tens of miles and perhaps more to the south. Acknowledgments This work was supported by the National Water-Quali~ Assessment program for the Upper Snake River Basin and by the U.S. Department of Energy. Their support does not constitute endorsement of the views expressed in this publication. References Cited Doe, B.R., 1976, Lead isotope data bank: 2624 samples and analyses cited: U.S. Geological Survey Open-File Report ,104 p. Doe, B.R., Leeman, W.P., Christiansen, R.L., and Hedge, C.E., 1982, Lead and strontium isotopes and related trace elements as genetic tracers in the Upper Cenozoic rhyolite-b~alt ~ oci~jon of the Yellowstone Plateau Volcanic Field: Journal of Geophysical Research, v. 87, p Leeman, W.P., Doe, B.R., and Whelm, J., 1977, Radiogenic and stable isotope studies of hot spring deposits in Yel~ows~ne National Park and their genetic implications: Geochemical Journal, v. 11, p Lewis, R.E., and Young, H.W., 1989, The hydrothermal system in central Twin Falls County, Idaho: U.S. Geological Survey Water-Resources Investigations Report ,44 p. Lloyd, R.M., 1968, Oxygen isotope behavior in the sulfate-water system: Journal of Geophysical Research, v. 73, p Malde, H.E., and Powers, H.A., 1972, Geologic map of the Gfenns Ferry- Hagerman area, west-central Snake River Plain, Idaho: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-696, scale 1 :48,000,2 sheets. M~ner, R.H., and Young, H.W., 1995, Lead and s ~onti~ isotope data for thermal waters of the regional geothermal system in the Twin-Falls and Oakley areas, south-central Idaho: Transactions of the Annual Meeting of the Geothermal Resources Council, v. 19, p Mariner,R.H., Young, H.W., Evans, W.C., and Parliman, D.J.,1991, Chemical, isotopic, and dissolved gas compositions of the hydrothermal system in Twin Falls and Jerome counties, Idaho: Transactions of the Annual Meeting of the Geothermal Resources Council, v. 15, p Mitchell, J.C., Johnson, L.L, and Anderson, J.E., 1980, Geothermal resources of Idaho, Plate 1 of Geothermal ~ vestig~io~ in Idaho, Part 9, Potential for direct heat application of geothermal resources: Idaho Department of Water Resources Water Info~ation Bull~tin 30, scale 1 :500,000. Schroeder, F.C., 1912, A ~connaissan~ of the Jarbidge, Contact, and Elk Mountain mining districts, Elko County, Nevada: U.S. Geological Survey Bulletin 497,36 p. 201

NOTICE CONCERNING COPYRIGHT RESTRICTIONS

NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials These materials have been made available for use in research, teaching, and private study, but may not be used for