Geology, alteration, and magmatic-hydrothermal history of The Geysers felsite -- potential applications for exploration and development

Conference · Fri Jan 01 04:00:00 EST 1993

OSTI ID:7369647

Hulen, J B; Nielson, D L ^[1]

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1. Univ. of Utah Research Institute, Salt Lake City, UT (United States)

The [open quotes]felsite[close quotes] is a shallow, young, granitic intrusive body centrally located within and beneath. The Geysers steam field. The field and the felsite are coaxial, and hydrothermal alteration effected by hot-water dominated geothermal systems antedating the modern steam reservoir shows systematic vertical zonation with respect to the pluton. The research summarized in this communication was undertaken both to clarify the role of the pluton in reservoir evolution, and to characterize critical felsite-specific controls on the fields's deep porosity and permeability. The felsite comprises at least three major intrusive phases. Two are high-silica granites probably older than 1.3 Ma. The third is granodiorite (1 Ma), temporally and chemically equivalent to overlying extrusive dacites of the Clear Lake volcanic field. All three intrusive phases are too old to be heat sources for the modern steam field, but probably were the heat engines for the prior liquid-dominant systems. Younger, deeper magmatic heat sources are strongly implied for the current vapor-dominated regime. Porosity in the felsite is provided by: (1) Extensively mineralized fractures and breccias, probably of both tectonic and high-temperature hydrothermal origin; and (2) miarolitic cavities in the upper levels of the pluton. The latter could be analogous to calcite-dissolution cavities in overlying metagraywacke -- they could serve as storage sites for the fields's liquid water reserves. Porosity in these fractures, breccias, and vugs in partially occluded by hydrothermal vein minerals deposited in prior hotwater-dominated systems --tourmaline, ferroaxinite, quartz, potassium feldspar, epidote, actinolite, prehnite, and many others. Such secondary mineralization conceptually could serve as an excellent exploration guide to potentially productive portions of the felsite beyond the field's present boundaries.

• DOE Contract Number: AC07-90ID12929
• OSTI ID: 7369647
• Report Number(s): CONF-930484--; ON: DE94001714
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
150200* -- Geology & Hydrology of Geothermal Systems
150300 -- Geothermal Exploration & Exploration Technology
152000 -- Geothermal Data & Theory
EXPLORATION
FAILURES
FRACTURES
GEOLOGY
GEOTHERMAL FIELDS
GEOTHERMAL SYSTEMS
GEYSERS GEOTHERMAL FIELD
GRANITES
Geothermal Legacy
HYDROTHERMAL ALTERATION
IGNEOUS ROCKS
MINERALIZATION
PERMEABILITY
PLUTONIC ROCKS
POROSITY
ROCKS