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'Value per mil.

2Data from Mariner and others (1975).

face réservoir control may be related to events or structures not expressed in either the drill holes or at the surface. Nevertheless, some inferences can be made which have given a first approximation of the relationship between geology and geothermal resource localization.

Information from the drill holes, supported by mapping, suggests that the Desert Peak geothermal field may be situated at the intersection of two structural trends, an east-northeast fault system and a north-northeast fault pattern so characteristic of the Basin and Range physiographic province (fig. 2). There is no surface evidence to suggest that the northwest-trending Walker Lane (fig. 2) is involved, but gravity data discussed later suggest that structures of similar trend do exist in the area. The stratigraphy, as deduced from lithologie logs and from mapping, is complex. To date detailed correlations from well to well and outcrop to outcrop have been generally unsuccessful, owing to a lack of any areally persistent marker beds, discontinuous exposures, and a remarkably monotonous volcanic section. Also, the stratigraphy was studied only to the extent necessary to complete mapping. The correlations that have been made in the wells are of a general nature (with one exception), but even the generalities have proven helpful. The rock names used herein are based on hand-specimen examination, geophysical log response (particularly the gamma ray), microscopic examination of cuttings, and in a few cases, thin sections. Because the majority of volcanic rocks are porphyritic with aphanitic groundmasses, the assigned names may not correctly reflect the chemical composition of the rocks.

Five major rock types are exposed in the northern portion of the Hot Springs Mountains: 1) a Tertiary(?) hornblende-quartz diorite intrusive near the Desert Queen Mine (pi. 13); 2) a basal Tertiary sequence of silicic volcanic rocks ranging from rhyolitic to andesitic ash flows, lava flows, and tuff breccias; 3) Tertiary intermediate to mafic volcanic rocks, principally andesite and basalt; 4) lacustrine and fluviatile Tertiary sedimentary rocks; 5) Quaternary Lake Lahontan sediments, alluvium, playa deposits, and sheet and dune sands.

The northern Hot Springs Mountains are the type locality for the Truckee, Desert Peak, and Chloropagus Formations, all Tertiary. The Truckee Formation was named by King (1878) for exposures in the northeastern part of the Hot Springs Mountains. Axelrod (1956) defined the Desert Peak and Chloropagus Formations as part of an analysis of the fossil flora of the area, and at the same time he clarified the stratigraphy of the Truckee Formation, which in its type locality does not match King's description (Axelrod, 1956). The Tertiary and Quaternary rocks in the area rest unconformably upon Mesozoic(?) metavolcanic, metasedimentary, and intrusive rocks. The Mesozoic rocks are not exposed in the Hot Springs Mountains but are important as they contain the geothermal reservoir.

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