Further assessment of stable lead isotope measurements for uranium exploration, Pine Creek Geosyncline, Northern Territory, Australia

Soil samples from the vicinity of Mount Brockman in the Pine Creek Geosyncline, Northern Territory, Australia, have been analyzed for stable Pb isotopes and two methods for displaying the data to reveal subtle indications of U mineralization have been investigated. The first, a plot of ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb, relates the U-derived lead (²⁰⁶Pb) and the Th-derived lead (²⁰⁸Pb) to the common lead content (²⁰⁴Pb). For country rocks, the two ratios are linearly related, irrespective of the amount of recently introduced ²²⁶Ra, whereas samples from near U mineralization lie off this trend along a line of near-zero slope. The second, a plot of ²⁰⁷Pb/²⁰⁶Pb vs. ²⁰⁴Pb/²⁰⁶Pb, gives a linear data array that may be interpreted either as indicating an apparent age of 1445 ± 20 Ma for the group of samples or as a mixing of the lead from mineralized and barren (country) rocks. In the latter interpretation, a threshold can be selected, with values below the threshold indicating U mineralization. Whereas the first method can be used in searching for any U deposit with a high U/Th ratio, the second plot is specific for a particular area. The value of both plotting methods is illustrated for the Mount Brockman area of the Pine Creek Geosyncline. Not only has the stable Pb isotope method been able to show that known radiometric anomalies are unrelated to U mineralization in this area, it has also been successful in identifying a previously unknown surface indicator of the Koongarra No. 1 deposit.