Gamma-ray spectrometry has been used to measure potassium (K), uranium (U), thorium (Th), heat generation (A), Th/U, and (A – Ak)/K (Ak = heat from potassium) for over 600 samples in the Idaho batholith. Values of heat generation are remarkably consistent within a given pluton, but vary markedly and predictably between plutons of different composition and(or) different levels of emplacement. Values of (A – Ak)/K are similar for suites of rocks that were emplaced at approximately the same depth during the same intrusive event, but are considerably different between suites belonging to different intrusive events emplaced at different depths. Thus, on the basis of their (A – Ak)/K ratio and various geologic parameters, the rocks of the Idaho batholith can be divided into four intrusive groups. Table 1 lists these groups with their characteristics, weighted according to areal abundances of the constituent plutons.
The remarkable uniformity of heat generation for a given igneous unit, even over large geographical areas, and the uniformity of (A – Ak)/K. for rocks intruded during the same intrusive event suggest that these parameters are diagnostic properties of igneous rocks, and may be used by the geologist to map individual units, to correlate different rock types intruded during the same intrusive event, and to separate similar rock types intruded during different intrusive events.