Calcrete, 239 deposits Australia

Mann, A.W. Horwitz, R.C. (1979) Groundwater calcrete deposits in Australia some observations from Western Australia. Journal of the Geological Society of Australia 26, 293-303. 

Uranium U(VI) minerals are most often products of the oxidation and weathering of nearby primary U(IV) ore minerals such as uraninite [U02(c)I and coffinite [USi04(c)l (cf. Pearcy et al. 1994). They also form by evaporative concentration of dissolved U(VI), particulary under arid conditions. Schoepite (/J-UOj 2H2O) is fairly soluble and, therefore, is a rare mineral, whereas carnotite K2(U02)2(V04)2j and tyuyamunite (Ca(U02)2(V04)2j, which have lower solubilities (particularly above pH 5) are the chief oxidized ore minerals of uranium. The plots in Figs. 13.5 and 13.6 indicate that uranyl minerals are least soluble in I0W-CO2 waters, and, therefore, are most likely to precipitate from such waters. This is con.sistent with the occurrence of carnotite and tyuyamunite in oxidized arid environments with poor. soil development (Chap. 7), such as in the calcrete deposits in Western Australia (cf. Mann 1974 Dall Aglio et al. 1974), and in the sandstone-hosted uranium deposits of the arid southwestern United States (cf. Hostetler and Carrels 1962 Nash et al. 1981). The... 

Sur cia.1 Deposits. Uraniferous surficial deposits maybe broadly defined as uraniferous sediments, usually of Tertiary to recent age which have not been subjected to deep burial and may or may not have been calcified to some degree. The uranium deposits associated with calcrete, which occur in Australia, Namibia, and Somaha in semiarid areas where water movement is chiefly subterranean, are included in this type. Additional environments for uranium deposition include peat and bog, karst caverns, as well as pedogenic and stmctural fills (15). 

Superficial Deposits associated with calcrete occurring in Australia, Namibia, and Somalia. 

The discovery of the calcrete hosted surfical uranium deposits of Namibia demonstrated the presence of widespread uranium in calcrete filled palaeochannels (Hambleton-Jones 1984) and similar mineral deposits have been observed elsewhere in Southern Africa, USA and Australia (Carlisle 1978 Hambleton-Jones 1978 Mann Deutscher 1978). The host rocks are typically lenticular bodies of alluvium, soil or detritus material cemented by calcite, gypsum, palygorskite, and other mineral phases. Uranium mineralogy is dominated by the mineral Carnotite [K2(U02)2(V04)2.3(H20)] as the main mineral in these channels. However other phases such as andersonite (Na2K3U03(C03)3(H20)6), liebigite (Ca2U02(C03)3(H20)io ... 

Where uranium and vanadium concentrations are in pg/L and potassium and bi-carbonate are in mg/L. Where the CSI is equal to zero then groundwater chemistry and carnotite saturation are in equilibrium and the mineral has the potential to be present. The assessment of groundwater chemistry in the vicinity of calcrete-hosted carnotite deposits indicates that a wide geochemical halo exists and that this halo can be identified during exploration. In Australia direct analysis of groundwater and the CSI have been demonstrated as suitable methods for exploration. In Namibia, in reality although the approach may be useful for... 

Carlisle, D., Merifield, P.M., Orme, A.R. Kolker, O. (1978) The distribution of calcretes and gypcretes in southwestern United States and their uranium favorability. Based on a study of deposits in Western Australia and South West Africa (Namibia). Open File Report 76-002-E. Los Angeles University of California. 

Figure 2.4 (A) Stage V calcrete from the Upper La Mesa surface, Las Cruces area. New Mexico. This surface is over 400 ka. (B) Thick pisolitic calcrete horizon in Quaternary deposits of Fisherman s Bay, South Australia. Lens cap for scale.

Because of their ephemeral nature there are no known uraniferous calcretes older than about 300000 years.The known deposits contain, however, about 2.4% of the Western world s reasonably assured 30 uranium resources. The largest and best-known are those at Yeelirrie, Western Australia (believed to contain about 50000 ton UjOs), and Langer Heinrich, Namibia. Other occurrences are in these countries as well as in Angola, Botswana, Mauritania and Somalia. 

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