Geophysical setting of the blue Mountain Geothermal Area, North-central Nevada and its relationship to a crustal-scale Fracture Associated with the Inception of the Yellowstone Hotspot

The Blue Mountain geothermal field, located about 35 km northwest of Winnemucca, Nevada, is situated along a prominent crustal-scale fracture interpreted from totalintensity aeromagnetic and gravity data. Aeromagnetic data indicate that this feature is related to the intrusion of mafic dikes, similar to the Northern Nevada Rift (Zoback et al., 1994), and may be associated with the inception of the ~16 Ma Yellowstone Hotspot. This pre-existing large-scale crustal feature may have influenced the location of the geothermal prospect and the spatially associated epithermal gold deposit on the western flank of Blue Mountain. Other epithermal gold deposits in north-central Nevada are also strongly correlated with this and other similar crustal-scale fractures associated with the Yellowstone Hotspot (Ponce and Glen, 2002). Combined geologic and geophysical studies suggest this crustal feature may influence the migration of hydrothermal fluids and that other crustal-scale features in northern Nevada may be associated with geothermal resources. We investigate mafic dikes exposed along the western flank of Blue Mountain, and encountered in drill-holes DB-1 and DB-2 at depths of about 500 and 750 meters, respectively. The dikes are composed of gabbro to diorite and physical-property measurements indicate they have an average saturated-bulk density of 2,852 kg/m 3 and a moderately magnetic susceptibility of about 18.0 x 10 -3 SI. Gravity and magnetic modeling reveal that the dikes are much more extensive in the subsurface than previously thought. Geologic investigations by Wyld (2002) indicate the dikes are approximately 10 Ma and paleomagnetic investigations yield directions consistent with a mid-Miocene age, suggesting they may be related to the emergence of the Yellowstone Hotspot.