The endowments of geothermal energy at Roosevelt Hot Spring and the Utah FORGE site have been associated with magmatic heat, mainly on the basis of 0.5 to 0.9 ka rhyolites occuring in the Mineral Mountains and from mantle helium isotope signatures in produced geothermal fluids. Here we report another major manifestation of magmatic activity in the form of a large mantle helium anomaly covering greater than 200 km^2 that occurs in cold groundwaters sampled from shallow geothermal wells in the center of the North Milford Valley. There are three interesting aspects worth highlighting. First, the extent of the mantle helium anomaly is exceptional and greatly exceeds the dimensions of the known geothermal resources based on what can be deduced from surface geology, heat flow and deep drilling. Second, the mantle helium anomaly is offset from the center of maximum heat flow, which encircles the region encompassing the Utah FORGE site and Roosevelt Hot Springs. Third, the uniformity of the isotopic ratios between 2- and 2.5-times Ra is the same as that measured at Roosevelt Hot Springs, and this suggests a common magmatic source in the form of a felsic melt body similar in composition to the rhyolite that erupted over 500,000 years ago. These aspects might lead one to infer that the mantle helium groundwater anomaly in North Milford Valley is the product of subsurface outflow originally sourced from Roosevelt Hot Springs; however, the pattern and area of the helium anomaly extends well beyond the edges of the outflow plume, requiring separate subvertical flow paths of deep sourced helium. Along with verifying the dimensions of magmatic intrusion(s) that supply heat for Roosevelt Hot Springs and Utah FORGE, the findings suggest the potential for discovering additional geothermal resources in the North Milford Valley.

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