Evaluation of lead isotopic methods for uranium exploration, Koongarra Area, Northern Territory, Australia

Abstract

Analysis of radioactive ( 210Pb) and stable lead isotopes in near-surface samples has been tested as a method of uranium exploration in the Pine Creek Geosyncline, Northern Territory, Australia. The lead isotopes were extracted from the samples by a mild leaching agent and were measured by alpha spectrometry for 210Pb and by mass spectrometry for stable lead isotopes. The results are compared with those obtained by conventional methods utilizing measurements of radioactivity and radon (Track Etch) in situ and 226Ra, 228Ra and U contents of soils. The major problems addressed were whether the lead isotopic methods are more sensitive than the conventional methods and whether they can discriminate “real” anomalies from the common barren anomalies found in black soils and swamps which contain radium in excess of the uranium present. Four test areas, representing a range of exploration problems, were chosen in the vicinity of the Koongarra uranium deposits and 25 samples from each area were analyzed. Most samples have more 226Ra than uranium. Radium analyses of several water samples show the source of this radium to be non-uraniferous rocks within the Kombolgie sandstone. The results for soil 226Ra, radon, scintillometry and 219Pb were generally closely correlated, and as a result, the 210Pb method was not considered to have any advantages over the conventional methods. At the Koongarra X prospect, which has a weak surface expression, the 206 Pb 294 Pb ratio gave the strongest indication of the underlying uranium mineralization with an anomaly to background ratio of 12.5. However, this ratio is correlated with uranium content and does not offer any particular advantages over uranium analyses alone. More subtle indications of uranium mineralization were found by relating the radiogenic lead ( 206Pb) and the thorium-derived lead ( 208Pb) to the common lead content ( 204Pb). A plot of 208 Pb 204 Pb versus 296 Pb 284 Pb (horizontal axis) is linear for country rock samples, irrespective of the amount of more recently introduced 226Ra. Samples above uranium mineralization lie off this trend, along a line of near-zero slope. By the use of this plot, indications were found of the Koongarra No. 2 orebody, which is concealed by about 40 m of barren overburden; none of the other techniques detected this mineralization.