Gypcrete

At the prevailing pH in the Namibian groundwaters, the predicted solubility of carnotite is low and close to saturation. From one hole in the Tubas deposit, carnotite saturation is close to 0 and predicted to be over saturated around the water-table zone and in the near-surface upper 2m of the gypcrete. Where Eh is positive carnotite is predicted to be nearsaturation. This indicates that carnotite accumulation at or above the regional water-table can occur by upward diffusion of uranyl carbonate species with possible precipitation due to nucleation on clay minerals or gypsum, as evidenced in the Tubas River. 

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... 

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. 

Bao H., Thiemens M. H., and Heine K. (2001) Oxygen-17 excesses of the Central Namib gypcretes spatial distribution. Earth Planet. Sci. Lett. 192, 125-135. 

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. 

Carlisle, D. (1980) Possible Variations in the Calcrete-Gypcrete Uranium Model. US Department of Energy, Open File Report GJBX-53 (80), 38 pp. 

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

A similar marine-aerosol origin (cf. Calhoun and Bates, 1989 Calhoun et al., 1991) is offered for the surficial sulphate deposits of Namibia (Eckardt and Spiro, 1999), albeit over a smaller area. Furthermore, 8170 data from both Namibian gypcretes (whose sulphate has deflated from playas Eckardt et al., 2001) and some Atacama sulphates also clearly demonstrate an atmospheric component (Bao et al., 2000, 2001, 2004 Thiemens, 2006). Accordingly, these data strongly suggest that some playas, and on regional scales, obtain a large portion of their solutes from marine sources. 

Chen, X.Y. (1997) Pedogenic gypcrete formation in arid central Australia. 

Jacobson, G., Arakel, A.V. Chen, Y. (1988) The central Australian groundwater discharge zone evolution of associated calcrete and gypcrete deposits. Australian Journal of Earth Sciences 35, 549-565. 

Hartley, A.J. May, G. (1998) Miocene gypcretes from the Calama Basin, northern Chile. Sedimentology 45, 351-364. 

Gypcrete A conglomeratic surficial sedimentary crust cemented by secondary gypsum or anhydrite. Often formed under strongly evaporative conditions, it is common in playa lake beachrock (see also caliche, calcrete, silcrete, ferricrete). 

A common feature of arid regions is the cementation of sediments by the precipitation of mineral matter from the groundwater. The species of salt held in solution, and also those precipitated, depends on the source of the water, as well as the prevailing temperature and humidity conditions. The process may lead to the development of various crusts or cretes in which unconsolidated deposits are cemented. The most commonly precipitated material is calcium carbonate (Netterburg, 1994). As the carbonate content increases in these soils, it first occurs as scattered concentrations of flaky habit, then as hard concretions. Once it exceeds 60%. the concentration becomes continuous. These deposits are referred to as calcrete (Fig. 5.9). The calcium carbonate in calcrete profiles decreases from top to base, as generally does the hardness. The development of calcrete is inhibited beyond a certain aridity since the low precipitation is unable to dissolve and drain calcium carbonate towards the water table. Consequently, in very arid climates, gypcrete may take the place of calcrete. 

Carlisle D. Possible variations on the calcrete-gypcrete uranium model. Open File Rep. U.S. Dep. Energy GJBX-53-80, 1980, 38 p. 

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