Abstract: Lithology and Alteration Mineralogy of Reservoir Rocks at Coso Geothermal Area, California

Susan J. Lutz, Joseph N. Moore, John F. Copp

Coso is one of several high-temperature geothermal systems associated with recent volcanic activity in the Basin and Range proving Chemical and fluid inclusion data demonstrate that production is from a narrow, asymmetric plume of thermal water that originates from a deep reservoir to the south and then flows laterally to the north. Geologic controls on the geometry of the upwelling plume were investigated using petrographic and analytical analyses of reservoir rock and vein material The nature of the low-angle outflow zone and the overlying cap that prevents a surface expression of the geothermal system appears to be related to a combination of lithologic, structural and mineralogic factors.

The position of the outflow plume is partially controlled by the distribution of fractured crystalline intrusives within foliated metamorphic rocks. Intrusive-metamorphic lithologic contacts are characterized by sericite-pyrite alteration and correlate with fluid entries in the wells. The base of a thick intrusive unit in several wells coincides with the 250°C isotherm based on fluid inclusion data.

A smectite clay zone developed in the overlying metamorphic rock acts as a cap to the productive zone and inhibits vertical movement of the geothermal fluids above the main upwelling zone. The upwelling zone lies within a epidote-quartz veined, coarse-grained granite at depth in the southern portion of the field.

The mineralogy of the clays varies systematically with.depth and temperature. The distribution of fine-grained clay minerals with depth indicates that the smectite cap thickens dramatically from the north to the south, and overlies a strongly sericitized zone in the upwelling portion of the reservoir. Wairakite-chlorite-epidote-calcite-quartz veins at the contact with the deep granite rec;l deposition from the hottest geothermal fluids (342°C) in the Coso field.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada