Petrographic and Fluid Inclusion Evidence for Past Boiling, Brecciation, and Associated Hydrothermal Alteration above The Northwest Geysers Steam Field, California
- Key words
- Exploration; USA; California; Sonoma; The Geysers; Petrographic Studies; Fluid Inclusions; Boiling; Brecciation; Greywache; Hydrothermal Fluids; Salinities
- Location
- The Geysers, California
- Conference
- Geothermal Resources Council Transactions
- Year
- 1989
- Session
- Exploration; Geological surveys; Petrology/petrography; Flui
- Language
- English
Abstract
Many of the wells drilled in the Northwest Geysers have encountered with thin, scattered zones of hydrothermally brecciated graywacke above the steam reservoir. The breccias are characterized by open space fillings of quartz, calcite, adularia, albite, pyrite, and pyrrhotite. Secondary sericite occurs within the clasts. Fluid inclusion measurements indicate that he quartz and calcite formed from boiling fluids with temperatures between 313° and 223°C. These fluids had estimated salinites ranging from .35 to 1.7 equivalent weight percent NaC1 and variable but significant gas contents. Variations in the homogenization temperatures of these inclusions suggest that they record the downward movement of the water table with time. As the water table declined, slightly acidic condensate reacted with the breccia to produce sericite.
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