Lead isotopic compositions of galenas from the Witwatersrand and Orange free State, and their relation to the Witwatersrand and Dominion Reef uraninites
New Pb isotopic analyses are presented for forty-one galenas from various mines in the Witwatersrand and the Orange Free State. Certain of the galenas have large proportions of radiogenic lead, and one sample from the St. Helena mine contains a spectacular variation in the proportions of the lead isotopes within a hand specimen. The addition of radiogenic lead to these galenas is probably complementary to the loss of radiogenic lead from the Witwatersrand uraninites, and the implications of this hypothesis on the uraninite age measurements are pursued. The possible times of abstraction of radiogenic load from the uraninites, have been obtained through graphical analysis of the galena data. The hypothesis implies the former existence of "parent uraninite" grains, which have undergone chemical alterations; in this view the present Witwatersrand uraninites have maintained textural identity with the parent uraninites, but have been "rejuvenated" chemically through successive alterations and losses of radiogenic lead. The most probable age of crystallization of the parent uraninites is ~3000 million years. This is in agreement with an independent investigation of uraninites from the Dominion Reef, which are probably representative of the "parent uraninites". The effect of mineral separation procedures in concentrating the uraninites for age measurement is also discussed.