Radioelement concentrations in the deep profile through Precambrian basement of the Vredefort structure.

Profiles across the Vredefort basement provide an opportunity for studying the distribution of U, Th, and K with depth in the upper and middle crust. Detailed radioelement concentration profiles across the granitic basement showed that 1) the central part of the basement core is depleted in Th and U relative to the perimeter, and the concentration of Th and U falls off regularly from the basement margin inward, 2) the distribution of the radioelements and the distribution of heat production with depth over the outer 8km are consistent with an exponential depth function, 3) the felsic rocks of the central basement are characterized by high K/U and Th/U ratios and the ratios K/U, K/Th, and Th/U increase from the margin inward, and 4) mafic granulites, which occur in the central regions of the dome, have 'normal' K/U, K/Th, and Th/U ratios. High-grade metamorphic environment resulted in the depletion of both U and Th relative to K in the greater proportion of the felsic rocks from the central regions of the Vredefort basement. Depletion of U was far greater than that of Th. The normal Th/U, K/U, and K/Th ratios in mafic rocks, in close proximity to the felsic rocks, suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the Vredefort basement.-Authors