Geochemical controls on uranium precipitation in calcrete palaeochannel deposits of Namibia

Author(s): 
Rob J.Bowell, A.Barnes, J.Grogan, Matthew Dey
Date: 
Monday, June 1, 2009
First presented: 
24th International Applied Geochemistry Symposium 2009
Type: 
Published paper
Category: 
Geochemistry

Palaeo-river channels containing calcrete are important potential hosts for economic uranium mineralization in many parts of southern Africa particularly Namibia. The main feature of these deposits are the dominance of the mineral Carnotite [K2(UO2)2(VO4)2.3(H2O)] as the main uranium host in these channels. However other phases such as andersonite (Na2K3UO3(CO3)3(H2O)6), liebigite (Ca2UO2(CO3)3(H2O)10), rutherfordine (UO2CO3), swartzite (CaMgUO2(CO3)3(H2O)12), tyuyamunite (Ca(UO2)2(VO4)2.5-8H2O), and urancalcarite [Ca(UO2)3(CO3)(OH)6.6H2O] are also present in minor amounts. These minerals are associated with epigenetic calcite, dolomite, gypsum, palygorskite, and strontianite. Typically a complex stratigraphy of calcite as transported and epigenetic calcite can be observed in the deposits. Uranium precipitation can occur more than once in the paragenesis indicating carnotite can be dissolved and re-precipitated in the palaeochannel possibly in response to fresh alkaline groundwater. Clear evidence exists that carbonates, gypsum, and uranium minerals are precipitated interstitially in the granite dominated detrital that fills the channel. Grade in these channels is typically higher at shallow depth with often the highest grade being at surface indicating that evaporation may play a role in precipitating uranium. Mineral equilibrium calculations, based on known mineralogy and groundwater chemistry have been used to construct geochemical models to understand ore genesis and assist exploration for carnotite hosted calcrete deposits. Multiple phases of calcite and carnotite formation are indicated in mineral paragenesis and an explanation for this may be that there has been frequent re-working of the deposit by inflowing groundwater. In addition, high evaporation in the near surface environment generate conditions for another mechanism for carnotite formation. Species activities and mineral saturation have been determined. These demonstrate that for many of the deposits carnotite is close to saturation or is only weakly undersaturated demonstrating that recent or even seasonal dissolution and re-mobilization of uranium may occur in these deposits leading to wide hydrogeochemical halos for trace levels of uranium in groundwater around these deposits. However inherent difficulties in the analysis of trace levels of metals in high salinity groundwater typical of palaeochannels may limit application in exploration.

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