Significance of radioelements distribution in the Precambrian rocks of Jabel Sayid, western Saudi Arabia, using spectrometric and geochemical data

In situ gamma-ray spectrometry measurements were carried out using RS-230 BGO spectrometer over an area of 9.4 km(2). One hundred and seventy eight spectrometric points were collected along parallel profiles with spacing 325 m in the Easting X direction and Northing Y direction. The results are treated statistically to determine the minimum, maximum, arithmetic mean (X), SD (S), and coefficient of variability (CV %). The spectrometric survey revealed the presence of radioactive zones in the aplite-pegmatite with maximum eU content of 1,550 ppm and eTh of 7,974 ppm, whereas alkali granite has an average eU content of 12 ppm and eTh of 34 ppm. Felsite has an average eU and eTh close to that of alkali granite, 11 and 32 ppm, respectively. Pegmatite veins cutting alkali granite have high values of eU and eTh, an average of 34 and 101 ppm, respectively. The metamorphosed volcanic rocks of the Mahd Group demonstrate the lowest radioactivity with an average eU content of 0.8 ppm and eTh of 1.6 ppm. These rocks partly show evidences of post-magmatic alteration processes, for example, silicification, sericitization, and oxidation. Excluding metavolcanics, the strong positive correlation between eU and eTh for the different rock varieties reflects their geochemical coherence during magma crystallization and indicates that both elements are largely accommodated into accessory minerals and were not disturbed by alteration. The CV values of the different rock units show normal distribution, where all the percentages of CV values are less than 100, with the exception of eU of felsite that shows CV % value slightly higher than 100. The aplite-pegmatite, alkali granite, and the associated pegmatite veins show negative value of uranium outward migration, but are still the most favorable host rocks for uranium and thorium mineralization. A tentative interpretation of such contradiction suggests that uranium is largely accommodated into accessory minerals and only limited labile uranium redistribution has occurred and uranium was precipitated in association with alteration products. This interpretation is supported by the presence of kasolite Pb(UO2)SiO4.(H2O), along the fractured zones and quartz veins in the aplite-pegmatite. The limited uranium migration in the study area was also revealed by the low weathering rate estimated by the calculations of chemical indices of alteration.