Abstract: Alteration History of Jurassic Reservoir Rocks in the Dixie Valley Geothermal Field, Nevada

LUTZ, SUSAN J. and DICK BENOIT

The geothermal reservoir at Dixie Valley is hosted in fractured Jurassic rocks contained within the hanging wall of the Stillwater fault. Within the Jurassic sequence, there are four major stages of alteration related to the fault system. The paragenetic sequence from oldest to youngest consists of: Stage I) biotite-potassium feldspar veins, and epidote-chlorite calcite veins, Stage II) sericitization and associated quartz-calcite veins, Stage III) chalcedonic quartz-dolomite-chlorite/smectite-barite veins, and Stage IV) wairakite-epidote-quartz calcite veins. Stage IV appears to have been produced by the modern geothermal system along the still-active normal fault, whereas, Stages I through III are related to older hydrothermal alteration along the fault.

Alteration in the geothermal reservoir is similar to that described from outcrops along the footwall of the Stillwater fault. The Jurassic rocks share a common alteration history which is recorded by the superposition of younger Stage II and III assemblages containing sericite and chalcedony, upon older (Stage I) assemblages containing biotite and epidote. This paragenesis is interpreted to represent upward displacement of the footwall to shallower, cooler depths.

Stage II sericite has been dated at 21-25 Ma and occurs in northwest-striking fractures. A change in extension direction at about 10-13 Ma is recorded by Stage III veins that strike northeast. Stage III assemblages appear to indicate the presence of cool basinal fluids in the fault prior to the development of the modern geothermal system. Northwest extension and descent of the present hanging wall occurred after the deposition of relatively flat-lying, Miocene basalt flows. Stage IV minerals (wairakite and epidote) were deposited by the modern geothermal system (T= 248 degrees C) after the underlying Jurassic rocks descended to their present depths (8,000 to 10,000 ft).

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado