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Proceedings 15th Geothermal Workshop 1993 119 HYDROGEOCHEMICAL CHARACTERIZATION OF THE ALVORD BASIN GEOTHERMAL AREA, HARNEY COUNTY, OREGON, USA M.L. A.M. JOHN' N.C. StateUniversity, Portland, Oregon, USA National Laboratory, Argonne, Illinois,USA SUMMARY - The geothermal system within the Alvord basin, Harney County, Oregon discharges bicarbonate-rich water at three hot-spring areas. System recharge by meteoric water occurs the Steens Mountain-Pueblo Mountain fault block. Water circulates to at least 2.5 km before rising along range-front and faults. Minimum reservoir temperature estimated near 200' C and residence time the system may be over 10,000 years. Flow pathways, volume, and boiling account for compositional in spring discharges. 1. INTRODUCTION wide. The basin lies between the Steens Mountain and Pueblo Three hot-spring areas discharge Mountains to the west and from a geothermal system within the topographically lower ranges to the Alvord basin southeastern southeast and east. Oregon: Borax Lake and associated springs, Alvord Hot Springs and Suggested displacements on the Mickey Springs (Figure 1). Steens Mountain-Pueblo Mountains range-front faults vary from 2,500 In 1989 , Anadarko Petroleum m to 3000 m. This displacement Corporation drilled a geothermal suggests a long-term slip rate of exploration well to a depth of 451 0.33 for the last 9.3 Ma. m approximately 2.2 km southwest of Analysis of fault scarps along the Borax Lake. The well was flow- Alvord segment of the Steens Moun- tested for 22 hours yielding a flow tain fault zone indicates the most rate of 400 gpm with a measured recent movement occurred during the temperature of Listing of late Holocene (Hemphill-Haley et the Borax Lake Chub, Gila al.,1989). as an endangered species on May 28, 1980 under the Voluminous Tertiary volcanic and Endangered Species Act of 1973 minor volcaniclastic rocks are designates critical habitat at exposed the Steens Mountain Borax Lake. The proximity of Borax range front. Mesozoic and Lake to the exploration well Paleozoic metamorphic and granitic drilled by Anadarko Petroleum intrusive rocks, including Permian Corporation and the or Triassic metasedimentary and critical habitat the lake provides metavolcanic rocks, occur in the for the Borax Lake Chub has focused Pueblo Mountains, but are not the greatest attention on this exposed north of the Oregon-Nevada area. The issue of the effects of lineament that separates the Pueblo future geothermal operations on the Mountains from the Steens Mountain. chub habitat was one of the motivating factors for this study. The lowest units exposed on Steens Mountain are the this paper, pertinent Alvord Creek, Pike Creek, and information from previous studies Steens Mountain and geologic, isotopic and clastic sediments, rhyolitic ash- geochemical data from fluids, gases flow tuf and and rocks obtained during this basalt and andesite, respectively. study are used to estimate the These early to middle Miocene temperature, lithology and location volcanic rocks are overlain by the of geothermal reservoirs, and to middle Miocene Steens Basalt. The define recharge areas, residence Steens Basalt exceeds 1,000 m and times and circulation pathways. is composed of tholeiite to high alumina basalt (Hart and Carlson, 2. REGIONAL GEOLOGY 1985). Welded ash-flow tuffs erupted from the volcanic The Alvord basin the northern field overlie the Steens Basalt Basin and Range province of the (Walker and Rytuba southwestern United States. The and McKee,1984). basin a complex N-NE striking graben 112 km long and up to 13 km The Alvord basin filled with 120 volcaniclastic sediments of late Miocene to Holocene age. physical studies indicate the sediments are up to 0.5 km thick (Griscom and Cleary, 1976). The basin was occupied by Pleistocene Lake Alvord which dried approximately 10,000 Glacial flour, diatomite, and grained shoreline facies are ex- posed within the basin. The northern Basin and Range prov- ince characterized by abnormally high heat flow and anomalous thin- ning of the Earth's crust (Sass et al., 1981). Estimates of regional heat flow the vicinity of the Alvord basin range from 60-100 (Blackwell et al., 1978). Heat flow measured drill holes the Alvord basin ranges from 52- 268 (Brown and Peterson,1980) . The dominant hydrologic regime in the Basin and Range province is 0 range-to-valley flow flan, 1988). The shallow ground water system is recharged by stream flow supplied by mountain precipitation that infiltrates alluvial fans. The deep ground water and thermal Figure 1-Location map for the water systems are fed by fault- Alvord basin, southeastern Oregon. controlled deep circulation of 1-2. cold springs; 3. Mickey meteoric water. Springs 4. Alvord Ranch; 5. Alvord Hot Springs; 6. Thomas place; 7. 3. HOT SPRINGS OF THE ALVORD BASIN Kueny Ranch; 8. well; 9. artesian well; 10. Borax Lake and The hot-springs areas discharge springs to the north. Depressions from faults along which late are shown by a dot pattern. Pleistocene to late Holocene displacement has been documented km. An artesian well is located (Hemphill-Haley, 1989; Hook, 1981; 0.8 km northeast of Borax Lake. Cummings and John, 1993). opaline silica sinter terraces Mickey Springs and Alvord Hot occur at each spring, but sinter is Springs discharge from the range- not actively depositing. Sinter front faults of Mickey Butte and deposition occurred at each spring Steens Mountain, respectively. Mud at different times relative to the pots, an intermittently active Holocene geomorphic evolution of geyser, steam vents, boiling and the Alvord basin. warm pools and silica sinter fossil vents and mounds occur at Mickey Borax Lake and associated springs Springs. At Alvord Hot Springs discharge from an intra-basinal small springs occur along a NE- fault within Pueblo striking fault. Borax Lake a 0.04 km , shallow lake (less than 1 m) whose bottom At each hot-spring area, and shore are lined by silica temperature and were measured sinter. Hot water enters the lake and fluid samples were collected from seeps located at the base of a from the springs with the highest funnel-shaped vent that is greater temperature and flow rate. than 30 m deep. The lake surface Discharge temperatures at Mickey 8 m above the surrounding valley Springs and springs north of floor and occurs at the crest of a Lake are 94 to C and over 100 shield-shaped mound. This mound is the bottom of Borax Lake (30 an erosional feature protected by m). At Alvord Hot Springs, dis- the silica sinter deposits that rim charge temperatures are between 72 the lake. The silica sinter and C (Table 1). directly overlies diatomite that was deposited Pleistocene Lake 4. HYDROGEOCHEMISTRY Alvord. North of Borax Lake are hot springs that discharge along a The fluids are dilute Na-HCO, with fault over a distance of 0.9 significant amounts of and (Table 1). The highest concentra 121 1 - of thermal and from - and Alvord for Alvord with a t 110 C. north Borax Lakm. C. a t Alvord Hot Springs. spring a t north Borax Lakm. Artesian well north- A: Lakm, I: Springs north Borax Lakm, C: Alvord Hot Springs, and of Borax D. Alvord Hot E. D: Springs at Springs. quartz no assuming maximum o in of total dissolved solids (TDS) occur at Alvord Hot Springs. Constant ratios and relatively con- stant concentrations of Li, Cl, B, compositions of thermal waters and and F are noted among springs a suite of volcanic rocks were within each area. The ratio determined. The Steens Basalt and ranged from 0.03 to 0.07 age-equivalent Pueblo Basalt are Sorey et (1991) note that ratios the most voluminous volcanic units near 0.05 are indicative of in the vicinity of the Alvord interaction with metasedimentary basin. Strontium isotope ratios rocks, while Goff et (1987) note range from 0.70346 to that ratios between 0.01 and 0.1 0.70392 (Hart et Carlson commonly indicate interaction with and Hart,1987). volcanic rocks. The variation in below these basalts have Sr/ Sr concentrations between quarter- ratios between 0.70398 and 0.70430. ly sampling during 1.5 years was A rhyolite ash-flow tuff less than 8% (an exception occurred above the Steens at springs' north of Borax Lake Basalt has an isotopic value of between March and September, 1992 0.70723. The strontium isotopic when variations were less than compositions of metavolcanic and 15%). metasedimentary rocks was not determined. Thermal waters at the The 6D values for cold springs, ee hot-spr areas have wells, and streams located the ratios between 0.70424 and basin (-107 to -131, overlap 0.70478 (Table 4). The strontium those of thermal waters (Table 2). isotope ratios of the thermal The values for thermal fluids, waters are consistent with those of are heavier than the world volcanic rocks below the Steens meteoric water line. Basalt. Table 4 - Strontium isotope concentrations and ratlos stratigraphic units collected Cron mast of Mountain. The Alvord basin. 12/91 Borax Lakm -117 -13.7 6/92 - 14.7, -14.0 9/92 -120 -13.9 11/91 Spring. north -124 -14.9 6/92 of Borax Lakm -119 -15.4, 9/92 -121 -14.3 Pueblo Basalt 6/92 well -121 -16.2 9/92 -12s - 15.1 12/91 Alvord Hot -119 -13.6 6/92 Spring - 14.1 9/92 -119 -13.2, 12/91 -124 - 14.6 6/92 spring. 9/92 -121 -14.0 Chemical and isotopic geothermo- meters were used to calculate res- 5. DISCUSSION ervoir temperatures (Table 3). Results for both the and In this discussion, we examine the geothermometers indicate chemical characteristics that were minimum reservoir temperatures imposed during recharge, residency between 170 and silica the reservoir and upflow.

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