| Abstract |
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 post-Oligocene 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 illite-smectite and quartz-calcite veins; Stage V) chal-cedonic quartz-dolomite-calcite-chlorite/smectite-barite-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) 270o to 325oC and 1.4 to 1.6%; Stage 11) 220o to 270oC and 1.6 to 1.9%; Stage 111) 230o to 240oC and 1.2%; Stage IV) 180o to 220oC and 0.4 to 0.5%; Stage V) <160oC and 0.0%; and Stage VI) 200o to 270oC and 0.0 to 0.5%. Present day fluid inclusions deposited in wellbore scale yield homogenization temperatures of about 200oC and salinities of 0.0 to 0.4 weight percent NaCl equiva-ent. 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 = 248oC, salinity = 0.1-0.2 wt %) along this still-active fault. |