The Valhalla uranium-vanadium deposit, 40 km north of Mount Isa in Queensland, Australia, was discovered by prospectors in 1954.
The uranium mineralisation is refractory and there is a high proportion of calcite in the gangue; hence the uranium cannot be recovered by normal acid leach processing. The Valhalla deposit occurs within a series of brecciated metasediments and altered basalts which are part of the Palaeoproterozoic Eastern Creek Volcanics. The 1670 Ma Sybella Granite crops out 10 km SW of the deposit. The deposit is located along the projected trend of the Mount Isa Fault zone and the brecciation of the host rocks may be related to this fault.
Mineralisation forms two tabular, vertically-dipping zones striking N-S. The bulk of the mineralisation is in the main zone (Valhalla) which extends for 675 m at the surface and plunges 50°S. A much smaller zone, known as Valhalla South, occurs 1200 m to the south. Uranium-vanadium mineralisation is confined to zones of red-coloured hematite-magnetite-carbonate alteration within the brecciated shales and tuffaceous sediments. Sodic metasomatism and albitites are ubiquitous. Higher-grade uranium mineralisation is coincident with the presence of sodic pyroxenes (aegerine) and sodic amphiboles (arfvedsonite). The uranium is in brannerite and to a lesser extent in uraniferous zircon - both these minerals are refractory. Vanadium and minor copper mineralisation are present. Secondary uranium minerals in the weathered zone are metatorbernite, uranophane and carnotite.
Valhalla has an identified mineral resource of 11.48 million tonnes containing 36.5 million pounds of U3O8 (uranium oxide) at an average grade of 3.17lbs/t (1.44kg/t) U3O8. Vanadium credits amount to 2.81 lbs/t (1.30kg/t) V2O5. (Valhalla Uranium Ltd, 2006).
Source: Geoscience Australia, 2002.
