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203%/ Summary of the geology and isotope geochemistry of Steamboat Springs, Neuada DONALD E. WHITE 1, HARMON CRAIG 2 1 U.S. Geological Survey, Menlo Park, California 2 Department of Earth Sciences - University of California - La loila, California and FRED BEGEMANN 3 3 Max-Planc -Institat jar Chemie (Otto-Hahn-Institut), Mainz INTRODUCTION Some geothermal areas have had very complex geologic histories, and present-day.transfer of heat and mineral substances from depth to the surface may also be exceedingly complex in detail. The transferring medium, at least in the upper part of the systems, is predominantly water of surface origin ( CRAIG and others, ]:956), and the channels of migration are geologically controlled. Within a single spring system, multiple areas of recharge- having differing travel times, chemical and isotopic compositions, and other characteristics - are possible and even probable. A clear understanding of these complex relationship cannot be gained by any single approach. Geologic study, geophysical surveys, classical geochemistry of solid and fluid phases, and last but by no means least, isotope geochemistry of the pertinent stable and radioactive species each can contribute meaningful data, and integration of all the data can solve most of the outstanding problems. Many approaches have been applied, with varying degrees of success, at Steamboat Springs, Nevada. Publication authorized by the Director U.S. Geological Survey. 9
Donald E. White, Harmon Craig and Fred Begemann TOPOGRAPHIC AND HYDROLOGIC RELATIONSHIPS The Steamboat Springs area is in the western part of the Great Basin. The Carson Range, a few miles west of the springs, is a northward offshoot of the northwestward-trending Sierra Nevada Range. A chain of basins, including Washoe, Pleasant, and Steamboat Valleys and Truckee Meadows, separates the Carson Range from the adjacent range to the east, the semiarid Virginia Range. Altitudes range from :[0,778 feet at the summit of the Carson Range to 4,400 feet in Truckee Meadow; the thermal area is close to 4,600 feet. Annual precipitation is 50 to Ioo inches near the crest of the Carson Range, 20 to 40 inches in the Virginia Range, and Close to IO inches in the intervening basins and Steamboat Springs. Tributary drainage is eastward or westward from the bordering ranges toward the basins. The master stream of the area, Steamboat Creek, heads in Washoe Basin and flows northward along the axis of the chain of basins and the base of the hot spring terraces to the Truckee River, which in turn flows into Pyramid Lake, the undrained remnant of Pleistocene Lake Lahontan. The main streams from the Carson Range flow throughout the year; two of these, Galena Creek and Whites Creek, are of particular interest to the hydrology of Steamboat Springs. The principal streams of the Virginia Range, on the other hand, dry up or flow only feebly in their lower parts during the long dry summers, with no surface discharge reaching Steamboat Creek except during stages of unusually high runoff; normally the water sinks underground in permeable gravels near the western margin of the range. Summer temperatures and rates of evaporation are relatively high in the chain of basins traversed by Steamboat Creek, but are low in the adjacent ranges, particularly in the high Carson Range with its general cover of conifer forests. SUMMARY OF GEOLOGIC RELATIONSHIPS The rocks of the region consist of a deeply eroded crystalline basement overlain by thick Cenozoic volcanic rocks and lake and stream deposits (THOMPSON and WHITE, in press). The basement 10
Geology & isotope gechemistry of Steamboat Springs rocks are regionally and contact-metamorphosed volcanic and sedimentary rocks of probable Mesozoic age that are intruded by granitic rocks related to the Sierra Nevada batholithic complex of Cretaceous age. After deep erosion that exposed the granitic rocks, sporadic Cenozoic volcanism produced thick volcanic accumulations in the Virginia Range and generally lesser quantities elsewhere. During and after these eruptions, sedimentary rocks accumulated in structural basins that were forming between the volcanic mountains. In late Pliocene or early Pleistocene time, flows of basaltic andesite and domes of pumiceous rhyolite were extruded. Steamboat Hills, with Steamboat Springs at its northeastern end, is a relatively small structural and topographic high situated near the axis of the line of northward-trending structural basins between the two major ranges of the area. Pre-Tertiary basement rocks lie at or near the surface throughout most of the hills; Cenozoic volcanic and sedimentary rocks either did not accumulate to great thicknesses or were eroded away about as fast as they were deposited. A basaltic andesite volcano erupted lava flows that now" cap ridges in the northeastern part of Steamboat Hills near the springs. A pumiceous rhyolite dome essentially contemporaneous with the basaltic andesite was extruded near the southwestern end of the hills. Experimental geochemistry on the «granite» system and the proportions of normative feldspars to quartz in the obsidian of the domes indicate that the rhyolite magma evolved in an environment where the water-vapor pressure was between 2,000 and 3,000 bars (WHITE, THOMPSON, and SANDBERG, in press). The indicated water content was 6 to 8 percent, and the temperature immediately prior to eruption was probably close to 675 C. A minimum depth of burial of 6 to g km for the magma chamber is indicated by these data. Hydrothermal activity that probably was related to the rhyolitic magma chamber antedates the local basaltic andesite and rhyolite extrusions, and has been practically continuous up to the present time. Siliceous sinter of several different ages was deposited during each period of vigorous discharge of thermal springs. At other times, when topographic and ground water relations were such that springs did not discharge from the main sinter terraces, convection was probably still active in the thermal systems, but 11
Donald E. White, Harmon Craig and Fred Begemann the saline water escaped below the surface and was discharged directly into Steamboat Creek. At the present time, for example, less than IO percent of the thermal water of the system is discharged in measurable springs, and about 90 percent escapes underground directly into Steamboat Creek at the eastern bases of the spring terraces (WHITE, I957) Three well-defined systems of faults have been recognized in Steamboat Hills. An east-northeast system is parallel to the axis of the hills and is largely restricted to the basaltic andesite and older rocks. A set of northwest-striking faults is approximately contemporaneous with the more prominent east-northeast system. The most numerous faults in and near the thermal area strike northward; some of these are relatively old, bzit others displace sinter and alluvium of probable middle Pleistocene age and are the youngest faults in the area. The faults show no evidence of late Pleistocene and Recent displacement, although local earthquakes are relatively frequent. The Steamboat Springs fault system of the north-striking group is largely concealed by younger alluvium and spring deposits but it is evident from drill-hole data in and east of the Low and Main Terraces. Some evidence supports west-dipping reverse or thrust movement for the system, but the preponderant evidence favors east-dipping normal faults ( HITE, THOMPSON, and SANDBERG, in press). Structural control for the High Terrace is parallel to and west of the Steamboat Springs fault system; control for the older structurally deformed spring deposits southwest of the High Terrace is obscure. In summary, the regional geology indicates clearly that crystalline Pre-Tertiary igneous and metamorphic rocks occur at or near the surface throughout many square miles adjacent to Steamboat Springs. All deep circulation within these basement rocks must be controlled by permeable channels localized in faults and fractures. Artesian circulation of thetype that characterizes many areas of sedimentary rocks cannot exist in the spring system, except 10- cally in the shallow sedimentary cover. 12
Geology & isotope gechemistry of Steamboat Springs ISOTOPE GEOCHEMISTRY The isotopic composition of Steamboat Creek water shows only slight variations except from evaporational effects during the summer months. The heavy isotopes, D and 018, start to increase markedly in June, attain maximum values early in August, and are again nearly «normal» during and after October. The summer maximum is clearly related to high summer temperatures and extensive evaporation, particularly in the Washoe Lakes at the head of the creek (CRAIG and others, :[956). Evaporational effects are of the nonequilibrium kind that has been emphasized by CRAIG..aD of average creek water is about -90 per mil (SMOW) and 8018 is near-ii per mil, which is on the high -Ols side of the trend line of most surface waters. Galena Creek in the Carson Range, on the other hand, is isotopically very near this trend line (OD = 8 8018 + IO; CRAIG, :[96:[), with 8D near -I]:3 per mil and 8018 «[5.3 per mil. Streams from the Virginia Range, like Steamboat Creek, are on the high -018 side of the trend line, probably because of evaporational effects; 6D is about -I]:5 per mil and 6018 is near -]:4.5 per mil. The deeper water of South Steamboat well near the southern limit of the thermal area consists, from chemical and physical evidence, of meteoric water migrating into the upper part of the hot Spring System (VVHITE and BRANNOCK, I950) Isotopically, water from this well is intermediate between the runoff of the Carson and the Virginia Ranges and is very unlike that of Steamboat Creek; average dd is -I]:5 and 8018 is -I5.0 per mil. The hot springs are virtually identical in deuterium content to that of South Steamboat well but range from 2.0 to 3.5 per mil higher in O18. This major shift in 018 content is best explained by exchange of oxygen between circulating meteoric water and silicate minerals that are being hydrothermally altered. Other small variations have several different explanations: direct near-surface dilution of the dominant meteoric water of deep circulation by water similar to that of South Steamboat well ( HITE and BRANNOCK, ]:950' and unpublished data) ; equilibrium boiling below the water table as hot water deep in the system and originally near :[70 C rises into lower hydrostatic pressures near the surface (WHITE, 13
Donald E. White, Harmon Craig and Fred Begemann :[96I) ; nonequilibrium evaporation at the surface of small pools of low discharge (CRAIG and others, :[956); and concentration of light isotopes in the upper part of water and vapor columns in capped thermal wells. The carbon of all CO Species in water of Galena Creek within the Carson Range is isotopically very similar to atmospheric CO (5( about --6.4 per mil). Away from the range front, the CO, carbon increases in concentration but changes to 8-13 per mil, probably owing to influence of organic activity. Virginia Range samples are similarly low in C13 but are higher in total (02. The c 3 data from South Steamboat well support the D and 018 data in suggesting subsurface mixing of water that is recharged very near the flanks of the Virginia and Carson Ranges. The thermal chloride waters are very high in CO, with BC13 about -8.5 per mil. Separation of a vapor phase as water rises near the surface lowers the content of dissolved CO2 in the remaining water but increases the relative 8C13 content. Calcium carbonate precipitated in one erupting thermal well of normal high chloride content is completely lacking in detectable c 4, probably because of the considerable age of any atmospheric carbon that may be dissolved in the dominant meteoric water component, and probably even more to its very great dilution by «dead» volcanic carbon. Calcite from an erupting well at the Steamboat Resort, on the other hand, has 4.2 percent of the (14 content of modern carbonate, consistent with the known nearsurface entry of young meteoric water from the same aquifer that was tapped in the South Steamboat well. The prebomb tritium content of Steamboat Creek water is not known, but was probably between 4' and 8 T units. Water samples collected from South Steamboat well and two hot springs each prove the entry of some meteoric water of very short subsurface travel time, one month or less, but the greatly dominant meteoric component of almost constant stable isotope composition must be at least 50 years old. Chemical, isotopic, and physical evidence favors recharge of young meteoric water from at least three specific sources: (I) direct precipitation of rain and snow on the spring terraces; (2) precipitation in small drainage basins immediately west of the spring ter- 14
Geology & isotope gechemistry of Steamboat Springs races; and (3) at least in the Low Terrace, direct shallow inflow from Steamboat Creek. Most of the total water is also of meteoric origin, but has an age of at least 50 years. The isotopic evidence does not prove the existence of any water of direct magmatic origin (CRAIG and others, I956, P. 35-36) ; an upper limit of the quantity that could be present below the limit of detection is probably on the order of 5 percent, but the actual amount present is prob.ably closer to I percent. The latter also satisfies chemical relationships that are very difficult to reconcile in a system with no magmatic water. BANWELL (]:963), WHITE ( I957), and others have considered the serious heat-flow problems of thermal spring systems that demand either very large contributions of volcanic steam or very high thermal gradients to transmit the required heat through rocks of low conductivity, or some combination of these two modes of heat transfer. BANWELL (:[963) has suggested a very deep circulation and absorption of meteoric water into the magma, followed by regurgitation of this new magmatic water in high proportions Go percent or more of total) in the discharging springs. BANWELL'S mechanism neatly satisfies the heat-flow problem and also the observed similarities in deuterium content between spring waters and associated meteoric waters of many thermal areas (CRAIG and others, 2[956), but it does not explain how meteoric water under its own hydrostatic pressure can penetrate into or through a broad zone of very hot incompetent rocks characterized by lithostatic pressures that must normally surround a body of molten magma. Moreover, the Ols of the new magmatic water will have equilibrated with the very large reservoir of oxygen high in O18 in the magma body, (TAYLOR and EPSTEIN ]:962), imposing upper limits to the content of new magmatic (originally meteoric) water that can be present in any system. At Steamboat Springs, for example, this upper limit is about I5 percent, which is still far too low to solve the heatflow problem. REFERENCES BANWELL C. J. I963. Thermal energy from the earth's crust. Introduction and Pt. I. New Zealand Jour. Geology and Geophysics. 6:52. CRAIG H. I96I. Isotopic variations in meteoric waters. Science. I33, 3465 I702. 15
Donald E. White, Harmon Craig and Fred Begemann CRAIG H., BoATO G., WHITE E. I956. The isotopic geochemistry of thermal waters [Chap.] 5 of Nuclear processes in geologic settings. Natl. Research Council Comm. Nuclear Sci., Nuclear Sci. Ser. Rept. no. I9 : 29. TAYLOR P. jr., EPSTEIN S. I962. Relationships between 018/016 ratios in coexisting minerals of igneous and metamorphic rocks. Pt. I. Principles and experimental results. Ged. Soc. America Bull. 73, 4 :46I. THOMPSON A., WHITE D. E. In press. Geology of the Mount Rose Quadrangle and the regional setting of Steamboat Springs, Washoe County, Nevada. U.S. Geol. Survey P,of. Paper 458-A. WHITE D. E. I957 Thermal waters of volcanic origin. Geol. Soo. America Butt. 68, Ii:I637 WHITE D. E. Ig6I. Preliminary evaluation of geothermal areas by geochemistry, geology, and shallow drilling. United Nations Conf. on New Sources of Energy, Rome, Italy, I96]: (preprint), I2. WHITE D. E., BRANNOCK W. W. I950. The sources of heat and water supply of thermal springs, with particular reference to Steamboat Springs, Nevada. Am. Geophys. Union Trans. 3I, 4: 566. WHITE D. E., T OMPSON A., SANDBERG C. A. In press. The rocks, structure, and geologic history of the Steamboat Springs thermal area, Washoe County, Nevada. U.S. Ged. Survey prof Paper 458-B. 16