The Gerlach thermal area is at the south end of the Granite Range in the southern Black Rock Desert ( figure). It includes two major groups of springs, Great Boiling Springs in Secs. 10, 15, T32N, R23E about 1.3 km northwest of Gerlach and Mud Springs in Sec. 16, T32N, R23E about 1.8 km west of Gerlach.

The springs were first described by Fremont (1845) who reported them as "The most extraordinary locality of hot springs we had met during the journey." He mentioned that one large, circular pool was entirely occupied by boiling water, which boiled up at irregular intervals with great noise. Presumably this was at the Great Boiling Springs area. Fremont measured temperatures up to 97.7°C.

It has been reported that a borax works operated for a short time at Gerlach Hot Springs, but Papke (1976) believed that this information is probably not true. There is not a large amount of boron in the spring water, and no borates can be found at the site.

Great Boiling Springs were used extensively for bathing for many years ( figure). Some pools are too hot for swimming; a 19-year-old woman was scalded to death in one of these in 1973, an indication of the danger inherent in geothermal areas. The Gerlach General Improvement District built a bath house using geothermal fluids in 1989. The facility was planned for use by tourists and local residents. The facility has been unable to obtain a permit from the health department because of plugging of water filters by sediment from the well. No bathing facilities are available at the present (2002). Geothermal groundwater apparently extends under at least part of the town, as at least two Gerlach homes use geothermal wells for space heating. The water in one well is reported to be 35-36°C (unpub. data, Nevada Division of Minerals). Mud Springs ( figure) have mainly been used for stock watering and irrigation.

The hot springs issue from unconsolidated lacustrine and alluvial deposits, and hydrothermally altered granodiorite crops out nearby ( figure). Both the unconsolidated deposits and the granodiorite are hydrothermally altered along a fault west of Great Boiling Springs and in places are difficult to distinguish from each other. To the west of the thermal areas, the southern end of the Granite Range consists of relatively uniform medium-crystalline granodiorite that contains several scattered, somewhat elongate inclusions of diorite or gabbro. The thermal water has probably been in contact with granodiorite and related plutonic rocks of the Granite Range throughout most of its path from probable recharge areas high in the range to where it rises into the unconsolidated deposits beneath the springs (Olmsted and others, 1975).

The hot-spring clusters are associated with northeast-striking basin-and-range faults along the east side of the Granite Range ( figure). Fault scarps that are inches to several feet high appear to control the location of the spring clusters. The west side is usually the upthrown side on these faults (Grose and Keller, 1975b), and some offset deposits are as young as Holocene. Some faults in lacustrine and alluvial fan deposits near the hot springs may represent rupture of incompetent materials in response to movement along a single fault zone in the underlying granodiorite (Olmsted and others, 1975). An upfaulted block of altered granodiorite between the Great Boiling Springs and the Granite Range is believed to represent an exposed part of an ancestral Gerlach Hot Spring system. Several geophysical studies (Grose and Keller, 1974a, 1975b; Christopherson and others, 1977; Long and others, 1977c) also provide data that may be useful in structural and geologic interpretations. Sperandio and Grose (1976) suggested that the Gerlach thermal area may be along a deep-seated, north-south fault zone which extends from Winnemucca Lake to High Rock Lake (see Fly Ranch).

The spring deposits of the Gerlach thermal springs are predominantly siliceous sinter, and the concentration of dissolved solids in the waters is high in comparison with most other hot-spring waters in northern and central Nevada (Mariner and others, 1974). Some spring deposits are reported to be anomalously radioactive (60 to 65 µR/hr), according to Wollenberg (1974b). The Great Boiling Spring area is also well known for its mud volcanoes and other mud vent activity (Russell, 1885, p. 52; White, 1955b). The mud volcanoes have been reported to erupt clots of mud to heights of at least 30.5 m. They are characterized by sporadic and apparently unpredictable intervals of activity separated by very much longer intervals of quiescence (White, 1955b). Active deposition of native sulfur is reported from the hottest spring at Great Boiling Spring (Matlick and Ehni, 1995).

The temperatures measured in springs and pools range up to a maximum of 97.7°C (Grose and Keller, 1975b) and shallow subsurface measurements are over 120°C ( figure). In addition to shallow temperature-gradient holes drilled by the U.S. Geological Survey in 1973, Cordero (now Sunoco Energy Development Co.) drilled several gradient holes to depths of 91 to 183 m in 1972. ESI/San Emidio Resources, Inc. drilled three temperature gradient holes to depths of 243 to 457 m and one observation/production well to 914 m a short distance north of the Great Boiling Spring, a continuation of exploration begun in 1993 (Benoit and others, 1995, p. 156). The maximum temperature encountered in these wells was 129.4ºC at about 150-175 m. This well was drilled about 185 m northwest of an earlier Sunoco gradient hole that recorded 124.4ºC at 38 m (Matlick and Ehni, 1995). Mariner and others (1974) have estimated the reservoir temperature at 167°C using the silica-quartz geothermometer and 175°C using the sodium-potassium geothermometer.

Photos
Airphoto
Bathhouse and swimming pool at Gerlach Hot Springs (Great Boiling Springs, Washoe County.
Building, Gerlach Hot Springs.
Boiling pool, Gerlach Hot Springs.
Nevada Swimming Holes, Gerlach Hot Springs.