U-Th-Pb systematics of some granitoids from the northeastern Yilgarn Block, Western Australia and implications for uranium source rock potential

Abstract

The Mount Boreas-type granite and spatially associated syenitic granitoid of Western Australia yield Pb-Pb ages of 2370 ± 100 Ma and 2760 ±210 Ma, respectively. Th-Pb ages, although less precise, are concordant with these ages, and therefore the apparent ages are interpreted to be the crystallisation ages for these two units. U-Pb ages are variable and for the most part anomalously old, which suggests a Cainozoic uranium loss. However, this loss is generally small (3µg/g); therefore, neither granitoid in its fresh state provides a good source for nearby calcrete-hosted uranium deposits. The possibility remains that the Mount Boreas-type granite that has been completely weathered during the Tertiary could have been a source for the calcrete-type uranium deposits in W.A. Although the Mount Boreas-type granite is highly fractionated, it does not bear a strong geochemical imprint of a sedimentary precursor. This feature contrasts it with apparently fresh granitoids from other parts of the world that have lost large amounts of uranium (20µg/g) and are associated with large roll-type and other low temperature-type uranium deposits. © 1981 Taylor & Francis Group, LLC.