Gamma-ray spectrometer studies of hydro-thermally altered rocks

The uranium, thorium, and potassium content of hydrothermally altered rocks in the vicinity of several copper and copper-lead-zinc deposits in Arizona was determined by chemical analysis. Potassium in the more intensely altered zones is about twice that in unaltered areas. There is no corresponding increase in thorium, so a higher potassium/thorium ratio also results from alteration; abnormally high uranium, locally in secular disequilibrium, was observed at the Bagdad porphyry copper deposit. A truck-mounted gammarray spectrometer was used to obtain a spectrum of gamma emission at each sample location. Equating the spectrometric with the chemical data gave standard errors of 0.6 percent potassium, 3 ppm thorium, and 10 ppm uranium for the outcrop analyses with the spectrometer. The amount of potassium introduced by hydrothermal alteration is thought sufficient in quantity to be detected by either surface or aerial spectrometry; spectrometric detection of anomalous uranium is probably marginal, owing to the disequilibrium. The potassium/thorium ratio may provide a means of distinguishing, in such surveys, between hydrothermally altered high-potassium zones and unaltered extrinsically high-potassium intrusives.