Uranium and thorium in Late Proterozoic volcanic rocks from northwestern Africa

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The abundances of U and Th in the Late Proterozoic calc-alkaline volcanic rocks of northwest Africa which have undergone low-grade metamorphism, are highly variable. Dacites and rhyolites have higher contents of these elements than basic rocks although there is no obvious correlation of U and Th with SiO 2. In the acidic rocks, U and Th are closely related to K, whereas in the basic and most intermediate lavas, the two elements correlate with immobile incompatible elements such as Zr, Nb, La and Ce. In comparison with equivalent fresh Recent rocks, the acidic volcanic rocks have lower contents of U and Th. As expressed by the low U/K and Th/K ratios, U and Th in these rocks are also depleted relative to K. The depletion of U and Th, which is probably related to secondary processes, suggests that the acidic volcanic rocks from northwestern Africa may have been an important source of radioactive elements for concentration in deposits.