The Cahill Formation - host to uranium deposits in the Alligator Rivers Uranium Field, Australia

All the uranium deposits of the Alligator Rivers Uranium Field of Australia, which between them contain a quarter of the worlds reasonably assured uranium resources, are strata-bound within the Cahill Formation. Three of the four economic deposits are more or less conformable within a lower member near the base of the formation, which contains carbonate and carbonaceous rocks. The formation forms a poorly exposed but continuous folded belt, 5 km or more wide, over an area of about 15 000 square kilometres. Exposure is sparse, the rocks are generally deeply weathered, and knowledge of the stratigraphy of the unit is based largely on the results of shallow stratigraphic drilling. Quartzo-feldspathic and micaceous metasediments are the dominant rock types in the Cahill Formation as a whole, and they have been metamorphosed to the amphibolite grade (staurolite-almandine subfacies). The unit appears to unconformably overlie the Archaean-Lower Proterozoic granite-gneiss-migmatite Nanambu Complex and the Lower Proterozoic psammo-pelitic Mount Partridge Formation, and is overlain, in places unconformably, by the Fisher Creek Siltstone. It grades into the migmatite-gneiss-granite terrain of the Nimbuwah Complex in the north-east. The formation is a facies equivalent of the Koolpin Formation (host to uranium mineralization in the South Alligator Valley uranium field) to the south, and, in places, was separated from it during deposition by a basement high of Mount Partridge Formation. The carbonate-carbonaceous sequence was probably deposited under shallow near-shore shelf conditions, along with considerable admixed terrigenous material. The upper part of the formation represents a period of transgression over the shelf. Uranium was concentrated under reducing conditions which developed locally in many places within the carbonate shelf. Subsequent concentration and reconstitution took place a number of times, the main period being during 1800 m.y. regional metamorphism and deformation. Mobilization of uranium away from the carbonate-carbonaceous sequence took place only under high temperature and pressure conditions such as those attending formation of the Nimbuwah Complex, where the metal was relocated in favourable low pressure structures.