Egg-shaped uraninite nodules were discovered near Yambla creek in central Australia during work carried out on the Arunta uranium exploration project by Power Reactor and Nuclear Fuel Development Corporation.
The Yambla area is geologically situated in the eastern part of the Arunta Inlier which consists of several lower Prot-erozoic metamorphic sequences. The distribution and mode of occurrence of the uraninite nodules has been investigated by detailed surface mapping, intensive mapping and sampling in trenches. It was found that the uraninite nodules are distributed over a 1 km north-south trend on the surface and in the trenches are intimately associated with plagioclase-scapolite phase (white phase) and amphibole-rich phase (dark phase) in an amphibolite host rock. The surface trend parallels the strike of the host sequence.
From chondrite-normalized REE pattern, radiometric age dating and high thorium content, the uraninite is estimated to be produced by high temperature mineralizing fluids at some 360 million years age which corresponds to the Carboniferous Alice Springs Orogeny. The white and dark phases originated by metasomatic replacement during injection of a high temperature (above 400°C) fluid along fractures and metamorphic foliation planes.