Calcrete channel

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. 

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 ... 

Data has been obtained from published sources and listed data held in the library of the Geological Survey of Namibia. This data has been used to assess the geochemical environment in the vicinity of channel fill and pedogenic uraniumbearing calcrete deposits. The data has been analysed by various methods and laboratories so direct comparison of the data has to be treated with caution. A summary table of the geochemistry of waters from each of the deposits is given in Table 1. 

Figure 2.1 Settings for calcrete development. In fluvial settings pedogenic calcretes can develop on floodplains and terraces, whereas groundwater calcretes may form in channel deposits or around the capillary fringe and upper part of the phreatic zone in more permeable parts of the floodplain. In alluvial fans paired calcretes may develop on the fans, with hydromorphic calcretes near discharge zones.

Reeves, C.C. (1983) Pliocene channel calcrete suspenparallel drainage in West Texas. In Wilson, R.C.L. (Ed.) Residual Deposits. Special Publication 11, Geological Society of London, pp. 179-183. 

The Upper Lunde Member ( 850 m thick) marks another change in the depositional facies and style of these Upper Triassic continental beds. The lower boundary of this member is usually assigned to the base of the first marked fluvial-channel sandstone. The lower part of the upper member consists of braided stream channel sandstones and units of reddish-brown floodplain mudstones characterized by palaeosols with calcrete concretions. The upper part of the member also comprises middle-sinuous stream deposits, interchanging with rather mature reddish-brown, calcrete-rich palaeosols (Nystuen Fait, 1995). 

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