Geologic Framework of Jurassic Reservoir Rocks in the Dixie Valley Geothermal Field, Nevada: Implications from Hydrothermal Alteration and Stratigraphy

The main reservoir in the Dixie Valley geothermal field of Nevada is hosted in fractured Jurassic rocks within the hanging wall of the Stillwater fault. Within the Jurassic sequence, at least four major stages of alteration related to the fault system can be identified. The paragenetic sequence from oldest to youngest consists of Stage I) biotite-potassium feldspar veins, and epidote-chlorite-calcite veins; Stage 11) sericitization and associated quartz-calcite veins; Stage 111) chalcedonic quartz-dolomite-chlorite/smectite-barite veins; and Stage IV) wairakite-epidote-quartz-calcite veins. Stages 1-111 predate geothermal activity and are regional in extent, whereas, Stage IV veins are associated with fluids of the modern thermal system. Jurassic igneous rocks and Miocene basalt in the subsurface are in the same stratigraphic position as in outcrop. This suggests that the Jurassic rocks were already unroofed and near the surface when the Miocene basalt flowed over them. Descent of the present hanging wall started some time after deposition of the basalt flows. The presence of Stage 111 minerals (chalcedony and dolomite) in both the Jurassic rocks and in the Miocene basalt probably records hydrothermal alteration at shallow depths along the fault. Stage IV alteration (wairakite and epidote) minerals were deposited by fluids of the present geothermal system after these rocks descended to their present depth.