Calcretes, uranium deposits

Exploration for economic uranium deposits has expanded to many geologic environments that have been generally overlooked in the past. Most notable among these are (1) granitic uranium deposits, (2) alkalic igneous-hydrothermal uranium deposits, (3) altered acidic or alkalic volcanic ash, ash flow or volcaniclastic uranium deposits, (4) unconformity-related uranium deposits and (5) calcrete uranium deposits in desert groundwater environments. 

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). 

Because of the high temperatures involved in magmatic- and metasomatic-related uranium deposits, variations in their 238y 23Sy pg jgg should be minimal, unless later alteration has resulted in mobilization of uranium. The calcrete deposits measured so far (Fig. 1) are known to have uranium derived from a near-by igneous source, and they have... 

Geochemical controls on uranium precipitation in calcrete palaeochannel deposits of Namibia... 

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 ... 

The correlation to calcrete-gypcrete deposits is that saline groundwater may mobilize uranium both laterally and vertically through cyclic diffusion and pumping in a similar, abet on a smaller scale. This may occur in response to... 

Characterization of carnotite uranium deposit in calcrete channels, Trekkopje, Namibia. In Proceedings of Africa Uncovered Mineral Resources for the future. SEG-GSSA 2008 Conference, 7 -1 o" July 2008, 114-121. Boyle, D.R. 1984. The genesis of surficial uranium deposits. In Toen, P.D. (ed) Surficial Uranium Deposits. lAEA-Tecdoc-322. Vienna, 45-52. 

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... 

Langford F. F. A supergene origin for vein-type uranium ores in the light of the Western Austrahan calcrete-carnotite deposits. Econ. Geol, 69, 1974, 516-26. 

Geochemical Controls The speciation of uranium and vanadium has been determined for groundwaters from the calcrete-hosted carnotite deposits. These were completed in Geochemists Workbench v.7 using in-... 

Field measurements in sedimentary fluvial-type calcrete deposits also suggest that present-day groundwater in these areas may also display potential to both dissolve and precipitate uranium in the near surface. Chemical dilatancy and evaporation-driven diffusion that promote de-complexing, diffusion, and reprecipitation mechanisms are seen to play integral parts in the continued chemical reworking and modification of these calcrete-hosted carnotite deposits. 

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... 

Cameron, E.M., Leybourne, M.I., Kelley, D.L. 2002. Exploring for deeply covered mineral deposits formation of geochemical anomalies at the Spence copper porphyry deposit, Chile. Geology, 30, 1007-1010. Carlisle, D. 1978. The distribution of calcretes and gypcretes in the southwestern United states and their uranium favourability. Grand Junction, Dept. Energy Report, GJBX-29-78, 274 p. 

Carlisle, D. (1983) Concentration of uranium and vanadium in calcretes and gyp-cretes. In Wilson, R.C.L. (Ed.) Residual Deposits Surface Related Weathering Processes and Materials. Special Publication 11. London Geological Society, pp. 185-195. 

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. 

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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