Gamma radiation, an aid to geologic mapping on the Arabian shield, Kingdom of Saudi Arabia

Aerial gamma-radiation measurements in the Jabal Al Qarah quadrangle, Saudi Arabia, correlate with and complement magnetic data in distinguishing common Precambrian rock types. Areas of lower total-count gamma radiation correlate with areas of more intense magnetic patterns, which in turn correlate with areas of mafic rocks, as suggested from the study of the geophysical and geologic data of the Jabal Ishmas and Jabal Yafikh quadrangles adjacent to the north. In contrast, areas that reflect a lower magnetic intensity tend to show considerable variation in the radiation intensity and can be interpreted as being underlain by granitic rocks. On the basis of extrapolation of geophysical-geologic relationships established previously, selected radiation levels may be used to identify mappable rock units. Thus, radioactivity levels of 2,000 to 4,000 cpM suggest mafic rocks, levels of 4,000 to 6,000 cpM represent metavolcanic and metasedimentary rocks, levels of 7,000 to 10,000 cpM are representative of granodiorite gneiss, and levels of more than 11,000 cpM typify granitic rocks. The spectral gamma-radiation data are used to evaluate total-count anomalies and indicators of geologic processes of enrichment, and estimating the amount of isotopes of U, Th, and K within each of the lithologic units. (U.S.)