The Geysers felsite
- Univ. of Utah, Salt Lake City, UT (United States)
The {open_quotes}felsite{close_quotes} is a northwest-trending pluton of batholithic dimensions which underlies and partially hosts The Geysers vapor-dominated geothermal system. It is a composite igneous body with three readily mappable rock types - granite, microgranite porphyry, and late granodiorite. The pluton is affiliated, compositionally and almost certainly in part temporally, with the overlying 1.1 Ma Cobb Mountain volcanic center at the southern margin of the Clear Lake volcanic field, although portions of the intrusive may be at least as old as 1.3-1.4 Ma. Intrusion of the felsite, at the crustal levels explored by drilling, is interpreted to have taken place along periodically reactivated, northwest-trending Cenozoic strike-slip faults. The upper part of the felsite in the central and northwestern Geysers is intensely mineralized with borosilicate (tourmaline plus ferroaxinite), commonly accompanied by potassium metasomatism of the granitic host rocks. These anomalies as well as the geometrically, geochemically, and thermally distinct southeastern and northwestern portions of the steam field are separated by the downward projection of a major northeast-trending regional lineament (the Cobb Creek lineament) which may have formed initially as an antithetic shear in the regional strike-slip fault regime. Steam entries in the felsite are apparently concentrated along the top of and above the granodiorite, and (with notable exceptions) in portions of the pluton relatively impoverished in secondary borosilicates.
- DOE Contract Number:
- AC07-95ID13274
- OSTI ID:
- 494371
- Report Number(s):
- CONF-960913--
- Country of Publication:
- United States
- Language:
- English
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