Gamma-ray spectrometric data: Modelling to map primary lithology

During the last 10 years gamma-ray spectrometric surveys by governments and mineral companies have generated increasingly accurate and detailed data on K, Th and U surface concentrations of much of the mineral rich land currently being explored. Interpretations of these data commonly involve no more than subjective visual classification of colour images, either images of the concentrations of the individual elements, or images of the three elements combined in a RGB composite. This interpretation method does not extract the maximum information from the data.

In areas of exposed and consolidated rocks, potassium, thorium and uranium contents generally correlate within each major rock type. For display and interpretation purposes there are advantages in separating the major rock types using K, Th and U values, then, for each major rock type, calculating the data in terms of components. One component measures the distance along the average geochemical variation path, and other components measure deviations of Th and U from this path. Maps of these components allow the more detailed quantification of original lithology, mapping of any sharp breaks in lithology, and mapping of later chemical modification. Compared with using principal components, the use of deviations of Th and U from the mean path as the second and third components, allows for the big difference of the three elements in fractional errors, and makes it easier to relate the components to geology.