Preliminary hydrogeologic appraisal of selected hydrothermal systems in northern and central Nevada

Abstract

Several hydrothermal systems in northern and central Nevada were explored in a hydrogeologic reconnaissance. The systems studied comprise those at Stillwater and Soda Lakes-Upsal Hogback in the Carson Desert, Gerlach, Fly Ranch-Granite Range, and Double Hot Springs in the Black Rock Desert, Brady's Hot Springs, Leach Hot Springs in Grass Valley, Buffalo Valley Hot Springs, and Sulphur Hot Springs in Ruby Valley. The investigation focused on (1) delineating of areas of high heat flow associated with rising thermal ground water, (2) determining the nature of the discharge parts of the hydrothermal systems, (3) estimating heat discharge from the systems, (4) estimating water discharge from the systems, (5) obtaining rough estimates of, conductive heat flow outside areas of hydrothermal discharge, and (6) evaluating several investigative techniques that would yield the required information quickly and at relatively low cost. The most useful techniques were shallow test drilling to obtain geologic, hydraulic, and thermal data and hydrogeologic mapping of the discharge areas. The systems studied are in the north-central part of the Basin and Range province. Exposed volcanic rocks of latest Tertiary and Quaternary age are chiefly basaltic. Basaltic terranes are generally regarded as less favorable for geothermal resources than terranes that contain large volumes of young volcanic mocks of felsic to intermediate composition. Most of the known hydrothermal systems are associated with Basin and Range faults which are caused by crustal extension across the province. An area of high heat flow centered at Battle Mountain and possibly other areas of high heat flow may be related to crustal heat sources. However, some of the hydrothermal systems studied appear to be related to deep circulation of meteoric water in areas of 'normal' regional heat flow rather than to shallow-crustal heat sources. Discharge temperatures of thermal springs in the region range from slightly above mean annual air temperature (8?-12?C at most places) to boiling or slightly hotter. Geochemical data indicate that, in the major systems, subsurface temperatures at which thermal waters equilibrate with reservoir rocks range from 150? to more than 200?C. These data also indicate that the major systems are of the hot-water type rather than the vapor-dominated type. Depths of thermal-water circulation probably range from 2 to 6 kilometres in areas of 'normal' regional heat flow (~2 heatflow units) and from 1 to 3 kilometres in areas of high heat flow (~3-4 heat-flow units) such as near Battle Mountain. Most of the heat is discharged from the hydrothermal systems studied by (1) conduction through near-surface materials heated as a consequence of thermal-water convection, (2) convection as springflow, and (3) convection as steam discharge from spring pools, vents, fumaroles, and cracks. The mate of heat discharge by radiation from warm ground and by convection as lateral ground-water outflow is believed to be small in most systems and is not estimated. Estimates of net heat discharge from the systems studied range from about 0.8 x 106 calories per second at Buffalo Valley Hot Springs to about 14 x 106 calories per second at Stillwater. These estimates represent the approximate magnitude of the excess heat discharge from the thermal areas that results from the upward convection of hot water from deep sources. Water discharges from the hydrothermal systems by springflow, evapotranspiration, steam discharge, and lateral ground-water outflow. Estimated discharges range from about 0.2 x 106cubic metres per year from the Buffalo Valley Hot Springs system to about 3 x 106 cubic metres per year from the Stillwater system. In most of the hydrothermal systems studied and, by inference, in other similar systems in northern and central Nevada, the scale for potential .commercial development for production of electricity or for other uses may be constrained by the