Interpretation of Thorium to Uranium Ratios in Granitic Rocks and Implications for Uranium Exploration: ABSTRACT

Abstract

Ratios of thorium to uranium for granitic rocks within the range of 3 to 5 are generally thought to be normal. Possible economic significance has been attached to granitic rocks that have thorium to uranium ratios either higher or lower than the normal range. Interpretations are commonly based on measurements of surface or shallow bore-hole samples that may have been affected by near-surface processes, such as uranium loss by leaching. The thorium to uranium ratios that existed prior to changes produced by near-surface processes can be calculated from lead isotope data, provided that certain limitations and boundary conditions are met, such as closed-system behavior and large ratios of thorium and uranium to lead relative to the age of the system. Lead isotope analyses for several suites of Precambrian peraluminous granites and granite gneisses show that the calculated thorium to uranium ratios for most of the suites are in the range of 1 to 3. The range of calculated thorium to uranium ratios within each suite is generally smaller than the range of measured values. The mean thorium to uranium value for the measured ratios is generally larger, in some places by more than an order of magnitude. The difference between measured and calculated thorium to uranium ratios tends to increase with increasing thorium. Finally, the difference between calculated and measured thorium to uranium ratios is smallest in the granite gneisses. The difference between calculated and measured thorium to uranium ratios is interpreted to be the result of recent and variable uranium loss in response to incipient weathering. By analogy, other peraluminous granite suites with large and variable thorium to uranium ratios are possible sources for low-temperature uranium deposits. Peraluminous suites with small and uniform thorium to uranium ratios are unlikely sources for low-temperature End_Page 789------------------------------ deposits. Present data do not preclude association of these suites with high-temperature or mechanically concentrated uranium deposits. End_of_Article - Last_Page 790------------