Geothermal fluids produced from two of the largest production geothermal fields in the Great Basin have helium isotope ratios that are anomalously high relative to basin-wide trends. These data indicate that the geothermal systems, Dixie Valley, Nevada and McGinness Hills, Nevada have an anomalously high fraction of mantle derived fluid. These connections to deeply derived fluid and heat may supplement crustal heat production and be responsible, in part, for the anomalously high production capacity, relative to other Great Basin geothermal fields, that Dixie Valley and McGinness Hills support. Deep-seated crustal structures across the Great Basin and around the world are known to be associated with structural reactivation, can have relatively high permeability, and can act as fluid flow conduits. These deep seated structures across the Great Basin control upflow of deeply derived heat and fluids into the shallow geothermal systems at Dixie Valley and McGinness Hills, contributing to their productivity. (C) 2015 Published by Elsevier Ltd.