Leach Geology

Leach Hot Springs are located in Grass Valley approximately 25 miles south of Winnemucca, Nevada. Over the past years the Department of Energy and companies have shown considerable interest and spent substantial sums of money to acquire data related to the geothermal resource in this area. The public domain has access to a vast amount of this geological information. This data is in the process of being collected for review and interpretation. Information obtained from the SMU Geothermal Database indicates a potential thermal anomaly that could reach C/KM of 250 degrees C. This anomaly is leased or under application to lease by Great Basin Geothermal.

EXTRACTED FROM THERMAL WATERS OF NEVADA
LARRY J. GARSIDE
JOHN SCHILLING
1979
Leach Hot Springs are located near the south end of Grass Valley in S36, T32N, R38E, slightly more than 1 mile 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 204¾ F at that elevation. Temperatures as high as 212¾ F are reached within 100 feet of the surface (Olmsted and others, 1975). The estimated thermal aquifer temperature is between 311¾ and 349¾ F for the various chemical geothermometers (Mariner and others, 1974). The spring flow is used for stock watering and irrigation at a ranch just west of the springs.
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 several hundred feet (Olmsted, 1974; Olmsted and others, 1975).
Leach Hot Springs lie on a prominent 20- to 30-foot-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. This major fault can be traced for several miles to the north of the springs, and probably continues to the south through Mud Spring and somewhat to the west of the Goldbanks Mining District on the eastern edge of the East Range. In the Goldbanks Mining District, 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) report that the Goldbank mineralized rocks are intruded and overlain by 12 to 15-m.y.-old basalts and rhyolites, indicating a Miocene age for the Goldbanks deposits. Therefore, it seem likely that the Goldbanks mineralization is a shallow manifestation of the hydrothermal systems which produced the many 14 to 16-m.y.-old 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 half-mile-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 to 15 m.y. old (Noble and others, 1975; Wollenberg and others, 1975). Many of these faults cut some of the alluvial deposits in the valley and act as ground-water 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).
The estimated thermal reservoir temperature based on silica and alkali-element geothermometers, is 311¾ to 349¾ F (Mariner and others, 1974). The total heat discharge of the geothermal system is about 1.8 x 106 cal/sec. (Olmsted, 1974b; Sass and others, 1976.)