Genesis principles for the precipitation of carnotite in calcrete drainages in Western Australia

Abstract

Carnotite, K 2 (UO 2 ) 2 V 2 O 8 3H 2 O, is a commonly occurring mineral in the calcreted drainage systems of arid and semiarid Western Australia. Weathering of granitoid rock comprising the Archean Yilgarn Block is the likely source of uranium, and ground-water analyses from a representative catchment near Wiluna confirm high (10 to 442 ppb) uranium concentrations in present-day ground waters. Two types of carnotite deposit within the representative catchment can be identified. The first occurs within the margins of the chemical delta formed as the drainage system enters Lake Way and owes its existence primarily to remobilization of carnotite and to decomplexing of uranyl carbonates in waters of higher salinity. Vanadium analyses and solubility indices suggest that the carnotite upstream is at present redissolving and that a different mechanism was primarily responsible for its original precipitation. A redox process, the oxidation of vanadium (IV) to vanadium (V) is suggested as an important genesis feature, first by the observed color range in natural carnotites and by the fact that color changes can be induced in both natural and synthetic carnotite by the application of oxidizing or reducing agents. Second, field measurements also suggest that present-day ground waters of the Yilgarn Block display a natural Eh range which spans the experimentally observed vanadium (IV)-vanadium (V) redox curve. The decomplexing and redox mechanisms are seen to play important parts in carnotite genesis irrespective of the postulated source for vanadium.