Integrated Alteration Mineralogy and Fluid-inclusion Study at the Dixie Valley Geothermal Field. Nevada

Cuttings from geothermal wells in Dixie Valley, Nevada document the complex history of the Jurassic reservoir rocks now contained within the hanging wall of the Stillwater normal fault. Six stages of postOligocene alteration can be recognized on the basis of mineral paragenesis, rock associations, fluid-inclusion temperatures and apparent salinities. The paragenetic sequence from oldest to youngest consists of: Stage I) epidote-chlorite-calcite veins; Stage 11) illite; Stage 111) wairakite-quartz-calcite-potassium feldspar-epidote veins; Stage IV) mixed-layer illitesmectite and quartz-calcite veins; Stage V) chalcedonic quartz-dolomite-calcite-chlorite/smectitebarite-hematite veins; and Stage VI) quartz-calcite veins. Fluid-inclusion data yield the following temperatures and apparent salinities in weight percent NaCl equivalent: Stage I) 270" to 325°C and 1.4 to 1.6%; Stage 11) 220" to 270°C and 1.6 to 1.9%; Stage 111) 230" to 240°C and 1.2%; Stage IV) 180" to 220°C and 0.4 to 0.5%; Stage V) <160"C and 0.0%; and Stage VI) 200" to 270°C and 0.0 to 0.5%. Presentday fluid inclusions deposited in wellbore scale yield homogenization temperatures of about 200°C and salinities of 0.0 to 0.4 weight percent NaCl equivalent. Moderately saline fluids of the Stage I through 111 assemblages appear to be associated with hydrothermal systems that developed during initial Basin and Range extension. Low salinity and low temperature Stage JV and V inclusions suggest the presence of cool fluids at shallow depths along the Stillwater fault. Stage VI quartz and calcite veins were deposited by the modern geothermal system (Tmax = 248"C, salinity = 0.1-0.2 wt %) along this still-active fault. INTRODUCTION Dixie Valley is a fracture-dominated geothermal system located on the east side of the northern Stillwater Range in west-central Nevada. With a 62 megawatt power plant (operated by Oxbow Power Corp.), Dixie Valley is the largest non-volcanic geothermal system in the Basin and Range province. Although the circulation of geothermal fluids within the field is dominantly controlled by fracture permeability along the still-active rangefront fault (the Stillwater normal fault), most wells produce from a zone of fractured Jurassic-age rock located above the trace of the main fault. These Jurassic rocks have undergone a long and complicated structural history that includes both Mesozoic compressional and Cenozoic extensional events (Plank, 1996). Our studies have focussed on the post-Oligocene alteration of these rocks, and the relationship and cumulative effect of these alteration events upon the permeability of the current geothermal reservoir (Lutz et al., 1996; 1997). From our petrographic and fluid inclusion work, we now recognize six major mineral assemblages that represent alteration events associated with the early stages of Basin and Range extension and the evolution of the Dixie Valley geothermal system. I