Surface Indicators of Geothermal Activity at Salt Wells, Nevada, USA, Including Warm Ground, Borate Deposits, and Siliceous Alteration

Abstract

Surface indicators of geothermal activity are often present above blind or concealed geothermal systems in the Great Basin, but their expressions are sometimes subtle. When mapped in detail, these indicators yield valuable information on the location, structural controls, and potential subsurface reservoir temperatures of geothermal fluids. An example is provided by the Salt Wells geothermal system in Churchill County, Nevada, USA, where surface features define a 9- km-long area that matches the drill-defined extent of a large subsurface thermal anomaly. At higher elevations above the water table, these features include opalized sediments, siliceous sinter, and argillic alteration, and at lower elevations where water tables reach the surface in the playa environment, they include warm (geothermally heated) ground, ephemeral hot and warm springs/seeps, actively forming zones of opal and calcium carbonate-cemented sediments, and borate evaporite crusts. These features often mark the surface traces of active hydrothermal conduits (faults). Where the water table is shallow at Salt Wells, warm ground can be mapped by measuring temperatures at a depth of 30 cm. This is best accomplished in the winter when background temperatures are low. Similarly, the sampling of ephemeral hot springs and seeps at Salt Wells is most easily accomplished during the wetter, cooler conditions of winter, because the springs dry up during the summer. Large areas of playa at Salt Wells are fed by geothermal groundwater. By identifying individual upwelling zones, it is possible to obtain samples of relatively pristine geothermal fluids close to the surface that yield geothermometer estimates of reservoir temperatures similar to those from nearby geothermal wells. Because of a lack of associated springs and seeps, similar geothermal playas could easily exist elsewhere in the Great Basin and escape notice. Such playas might be identified in some cases by a distinctive salt crust composition that includes higher-than-normal concentrations of borate minerals.