High-resolution gamma-ray spectrometry in uranium exploration

Sedimentary-type uranium deposits accumulate at favorable sites along a migration path, which may be kilometres in length. Their source is a large volume of rock from which the uranium has been leached. The geochemical mobilities and half lives of uranium and its daughters vary widely so that they are transported at different rates, from the source rocks, along the migration path, to their ultimate site. The radioactive disequilibrium resulting from this process has been well documented in the immediate vicinity of ore deposits, and disequilibrium is commonly recorded on gamma-ray logs up the hydraulic gradient iron: uranium ore. Little is known about the state of secular equilibrium in the leached host rocks, which often represent the only part of the migration path that is at or near the surface and is thus most accessible to the exploration geophysicist. 

High-resolution gamma-ray spectrometry provides a means of investigating the disequilibrium associated with uranium leaching and migration. Direct measurement of uranium can be made by this method and the equivalent weight percents determined for each of the six daughter product decay groups that characterize the state of radioactive equilibrium. The technique has been used quantitatively in laboratory studies where the results compare favorably with radiochemical analyses, and field experiments suggest that the method is applicable at the outcrop.

High-resolution gamma-ray spectrometry, as an integral part of an exploration program, may expedite the search for uranium by 1) confining preliminary aerial surveys to a widely-spaced grid, to search for anomalies of large areal extent but moderate amplitude, 2) surface measurements in such anomalous areas to determine whether there is a uranium deficiency accompanied by an excess of daughter products, and 3) defining the subsurface migration path by borehole spectrometry.