Alberta Geological Survey staff discovered the Oldman radioactive occurrence during its 2008 field program. The program was part of a project to evaluate uranium potential of southern Alberta.
The siltstones of the Willow Creek Formation host the radioactive zone. On the outcrop, measurements of radioactivity showed values in excess of 1000 cps. Chemical analyses of the rocks from the outcrop showed values of up to 169 ppm uranium. To understand the nature of the occurrence of the uranium minerals in the siltstones, we created thin sections, performed X-ray diffractometry and scanned electron microscope and microprobe studies from radioactive samples.
Unexpectedly, the uranium was not in the matrix or on the grainsï¿½ surfaces, but rather as inclusions in the grains of quartz and, possibly, feldspar or chert. This mode of uranium occurrence was unexpected for our target model of sandstone-hosted uranium mineralization. To the best of our knowledge, this has not been reported anywhere in the world. To check this result, we performed selective analysis of the samples using two different chemical analyses. These chemical analyses, however, showed that most of the uranium is not confined in the quartz. That may mean that most of the uranium occurs in the matrix, but the concentrations are too low to see on the microprobe images.
The origin of the uranium inclusions in the quartz grains remains unexplained. Additional field and regional geological studies could explain the distribution of anomalously uraniferous rocks in the Upper Cretaceous and Tertiary formations of southern Alberta, as well as evaluate their economic potential.