NSS 75th Anniversary Convention

Leach Hot Springs

Coordinates:   40.6037, -117.6457
Water temperature:   197.6°F
PDF report:   Leach_Hot_Springs.pdf


Leach Hot Springs are located near the south end of Grass Valley in Sec. 36, T32N, R38E, slightly more than 1.6 km west of the major frontal fault on the west side of the Sonoma Range. The springs have several other names, including Pleasant Valley, Nelson's, and Guthrie Hot Springs. The spring temperatures reported at Leach are up to boiling, which would be 95.5°C at that elevation. Temperatures as high as 100°C are reached within 30 m of the surface (Olmsted and others, 1975) as seen in the figure. According to Richards and Blackwell (2002, p. 71), Sorey and Olmsted (1994) reported a temperature of 112°C at 17 m in 1994. The spring flow is used for stock watering and irrigation at a ranch just west of the springs. Hot groundwater is also reported in the Panther Canyon area about 10 km southeast of the springs (Welch and others, 1981).

The springs issue from steeply inclined, fault-controlled conduits in Quaternary alluvium and Tertiary sedimentary rocks. Late Paleozoic and early Mesozoic sedimentary and volcanic rocks are exposed east of the springs and probably underlie the spring area at depths of hundreds of meters (Olmsted, 1974; Olmsted and others, 1975).

Leach Hot Springs lie on a prominent 7- to 10-m-high fault scarp in the alluvium. This scarp is part of a system of faults related to the major north-trending boundary fault along the Sonoma Range to the east. The fault at the springs can be traced for about 5 km to the northeast and 2.5 km southwest. The major north-south fault that bounds the Sonoma Range in this area continues to the south through Mud Spring. In the Goldbanks mining district about 10 km southwest of Leach Hot Springs, a mercury-bearing silica "apron" of chalcedony and minor opaline silica forms a north-trending, linear mantle over Miocene fanglomerates and silicic tuffs (Dreyer, 1940; Noble and others, 1975). The silica deposition here is of hot springs origin (Dreyer, 1940) and appears very similar to the sinter deposits in the Leach Hot Springs area. However, Noble and others (1975) reported that the Goldbanks mineralized rocks are intruded and overlain by 12-15 Ma basalts and rhyolites, indicating a Miocene age for the Goldbanks deposits. Therefore, it seems likely that the Goldbanks mineralization is a shallow manifestation of the hydrothermal systems that produced the many 14- to 16-Ma precious deposits in northern Nevada (Noble and others, 1975), and not directly related to the present hot-springs activity in the area.

More than 30 separate springs are present at Leach Hot Springs, and the material depositing presently and in the past is predominately silica. Old sinter, mainly chalcedony, is exposed along a kilometer-long zone to the east of the springs. This sinter is believed to be considerably older than the opaline sinter now being deposited to the east of the springs. This fragmental sinter is composed of pebble- to sand-sized fragments of white to light-gray opaline silica down gradient from the springs. The fragments have been distributed by spring runoff (Olmsted and others, 1975).

The fault system at Leach Hot Springs was apparently established in early Miocene, as a basalt dike along one of the faults in the spring area is 14-15 Ma (Noble and others, 1975; Wollenberg and others, 1975). Many of these faults cut some of the alluvial deposits in the valley and act as groundwater barriers. A zone of intersecting lineaments southwest of the springs corresponds to an area of appreciable microearthquake activity, suggesting that active faulting may be associated with hydrothermal activity (Majer and others, 1976).Heat flow in the Northern Great Basin and Range Province is extensively discussed by Blackwell (1983).

Subsurface temperatures at the base of the valley fill at Leach Hot Springs (>150°C) and vicinity are centered on the springs and an area about 1.7 km to the southeast, where temperatures of 92°C are recorded in well G105 located in SW¼ NW¼ Sec. 6 T31N, R39E (Welch and others, 1981). Temperatures at the base of the valley fill southwest of the mouth of Panther Canyon (approximately Sec. 27, T31N, R39E) about 10 km southeast of Leach Hot Springs are reported to be higher than 100°C (Welch and others, 1981). Sunoco Energy Development Co. U.S.A. No. 11-36, located in NW¼ NW¼ NW¼ Sec. 36 T32N, R38E, was drilled to a depth of 2610.6 m in 1979 (Barton and Purkey, 1993, p. 16). A bottom-hole temperature of 125°C was reported (GeothermEx, 2004). A deep geothermal test (Aminoil USA 11-36) was drilled about 1.2 km northwest of the hot springs in 1980. It encountered only 125°C at 2,600 m (Sorely and Olmsted, 1994). Wilt and others (1980) describe geophysical work on the Panther Canyon thermal area.

The estimated thermal reservoir temperature, based on silica and alkali-element geothermometers, is 155-176°C (Mariner and others, 1974), although Sorey and Olmsted estimate reservoir temperatures as ranging from 150 to 180°C (1994). The total heat discharge of the geothermal system is about 1.8 x 106 cal/sec. (Olmsted, 1974b; Sass and others, 1976). Welch and others (1981) suggested discharge at the Leach Hot Springs is either from modern-day precipitation at an elevation higher than that in the immediate vicinity, or it was recharged during an earlier, colder, climatic period. This conclusion is based on the fact that deuterium values of the thermal waters are more negative than that of the surrounding cold waters (Yeamans, 1983, p. 161). The hydrothermal system of Leach Hot Springs is extensively discussed by Sorey and Olmsted (1994).

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Additional Resources

Data for this listing was provided by the Nevada Bureau of Mines and Geology.

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