Measurements of the hard gamma radiation from the pre-Cambrian rhyolite porphyry of the Bourbon, Missouri, well core show a direct relationship between mineralization and radioactive content. Low activities are, in general, associated with high mineral density, whereas the mineral density is a function of the magnetite content. The low activities are most probably due to a leaching out of the potassium content. This indicates that gamma-ray well logs should be capable of accurately measuring the location of mineral deposits in potassium rich rock. Potassium analysis of the core is planned by the Missouri Geological Survey.