CHANGES IN CRUSTAL FLUID CIRCULATION RELATED TO THE END OF DETACHMENT FAULTING, SILVER PEAK/FISH LAKE VALLEY, NEVADA

Extensive thin sheets of siliceous sinter and calcite travertine in western Nevada appear to mark the onset of normal faulting and the end of detachment faulting. Infrared (IR) spectral data in these areas suggests extensive, relatively high temperature subsurface hydrothermal circulation was prevalent during detachment faulting, fed by older normal faults and capped by the detachment upper plate. Latest Miocene structural reorganization related to the Mina Deflection produced normal faults that cut the upper detachment plate, allowing somewhat lower temperature fluids to reach the surface and deposit the sinter/travertine sheets.

In the Silver Peak-Fish Lake Valley area (SP-FLV), combining field geologic mapping and IR spectral measurements indicates a progression of hydrothermal alteration similar to the sequence observed in major turtleback structures of Death Valley. At SP-FLV, after the initiation of detachment faulting, deep hot upwelling fluids rose along pre-existing lower-plate normal faults, producing chloritic alteration or dolomitization, spreading laterally upon reaching the detachment. This style of circulation formed the southwestern Nevada chlorite-talc mining district. In Death Valley, fluid circulation reverses as the detachment shoals, and low temperature downwelling fluids produce minor low-temperature alteration. In SP-FLV latest Miocene normal faulting cuts the detachments, allowing relatively cooler fluids to emerge at the surface, depositing extensive sinter and travertine. In a nearby geothermal borehole, IR signatures of downwelling cool fluids are observed along the detachment. The structural transition from detachment to normal faulting is marked by a change in crustal fluid circulation, and a change in rock alteration and hydrothermal deposition. The early, high temperature, relatively closed-system circulation yielded Mg-metasomatism and economic deposits of talc, later lower temperature open system circulation deposited extensive sinter/travertine, and still-cooler modern circulation yields moderate-quality geothermal resources.