The geothermal reservoirs at Roosevelt Hot Springs and Utah FORGE EGS share common host rocks comprising Oligocene-Miocene granitoids and Precambrian gneiss representing end-member conditions of convective and conductive heat flow that formed within a large-scale magmatic geothermal system. Native state convective heat transfer at Roosevelt Hot Springs of 60-70 MW, equating to heat loss of 3-7 W/m^2, is restricted east of the Opal Mound fault over ~10 km^2, whereas to the west conductive heat flow ranges 100-200 mW/m^2 over greater than 50 km^2 as expressed by linear temperature gradients measured in Utah FORGE wells. The magnetotelluric resistivity structure shows that the crystalline basement, which hosts geothermal reservoirs, is thick and likely extends to 15 km depth, covering an area greater than 400 km^2 and a volume greater than 8000 km^3. Judging from the modeled very high resistivity, it is also impermeable except for a narrow N-S trending corridor (1-2 km wide) of elevated conductivity extending to greater than 10 km depth directly beneath Roosevelt Hot Springs. A separate subvertical pipe-like zone of enhanced electrical conductivity extends continuously to greater than 20 km depth to the northwest of Utah FORGE that supplies mantle helium into the shallow groundwater regime in the center of the Milford valley. The endowment of anomalous thermal energy within conductively heated basement rock is estimated to be 2.5E+17 kJ which is equivalent to greater than 400 km^3 of just solidified felsic magma. This is consistent with the large mantle helium anomaly (1.9 to 2.6 R/Ra) which includes both Roosevelt Hot Springs and the Milford valley groundwaters and covers greater than 200 km^2. That a zone of partial melt currently exist beneath 10 km depth is indicated by reduced seismic shear velocities at mid crustal depths, otherwise the geometry and distribution of any sort of magma body is difficult to resolve. A felsic magmatic composition is also consistent with the 0.9 to 0.5 Ma eruption of rhyolite in the Mineral Mountains, and this suggests that silicic magmatism has been semicontinuous for almost 106 years. Overall, the thick impermeable nature of the crystalline basement rock beneath the east side of the Milford valley and forming the core of the Mineral Mountains impedes ascent of magma and bifurcates the flow of deep-sourced helium to the surface. As a result, convective hydrothermal heat transfer is strongly localized within a narrow corridor of highly fractured rock, whereas a much larger volume of rock is heated directly and conductively by magmatic intrusion stimulated by mafic underplating.

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