Comparison Between Lead Isotopes 234U-238U Activity Ratio And Saturation Index In Hydrogeochemical Exploration For Concealed Uranium Deposits

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Journal Article: Comparison Between Lead Isotopes 234U-238U Activity Ratio And Saturation Index In Hydrogeochemical Exploration For Concealed Uranium Deposits

At several sites where uranium mineralization was known - West Morvan test area and Lodeve basin (France), and Cigar Lake (Saskatchewan, Canada) - we have applied both isotopic and conventional geochemical methods based on major and trace elements concentrations for the detection of concealed uranium deposits. Isotopic methods comprise: (1) the analysis of lead isotopes in groundwater, in order to find any contribution of radiogenic lead from the supposed surrounding or overlying mineralizations: (2) the measurement of 234U/238U activity ratios, variations of which have been proven to be correlated with the presence of uranium accumulatiions. The conventional geochemical methods consist simply in looking for uranium or accompanying elements in groundwaters. A more elaborate method uses the notion of saturation indices of uranium-bearing minerals, thus requiring accurate and complete determinations of major ions and other parameters of groundwaters. The data show that determination of lead isotopes is efficient in many contexts, and gives results that are rather independent of the geochemical characteristics (i.e., reducing or oxidizing) of the waters. The information is global and expresses the presence of uraiium accumulation in the vicinity of the sampling point. No information about grades and distance to the mineralization can be obtained. The anomalous lead can be transported either in solution or on particles, as lead is readily adsorbed on many substrates such as clays or oxides. The analysis of 234/238U activity ratios in groundwaters provide information which is more difficult to decipher. In general, oxidized waters encountering uranium accumulations show high uranium concentrations and low 234U/238U activity ratios, but groundwaters which are more reducing or close to uraninite saturation will produce either high 234U/238U activity ratios if the exchange surface between uranium-bearing phases and groundwater is large enough to promote preferential 234U enrichment in the fluid phase (selective leaching or alpha recoil) or low 234U/238U activity ratios if the exchange surface is smaller. High 234U/238U are observed above dispersed uranium accumulations, frequently situated in the vicinity of massive uranium deposits. This pattern is observed at all studies sites, where contrasting signatures between massive ore zones and surrounding secondary mineralizations are found. In this case, the 234U/238U activity ratio may give information on the succession of the relative ore grades. The use of uranium concentration anomalies has often been questioned, because: (1) non-significant anomalies may appear in oxidizing waters; (2) reduced waters may not be able to record significant U anomalies. The concept of saturation indices of uranium minerals generally allows a more accurate detection of the mineralized areas, but it requires a careful determination of redox potentials, in some cases by in situ measurements, but more generally by calculating the redox potential from dissolved iron, assuming equilibrium with iron oxides. Waters flowing through mineralized areas are generally saturated with respect to uranium minerals, without any clear U concentration peak (this is the case around the Cigar Lake U deposit). If they are not saturated, they are generally strongly oxidized and thus show very high uranium concentrations (this is the case of the Nord-Treviels part of the Lodeve uraniferous basin). If no uranium accumulation is present, saturation indices are generally strongly negative, but in strongly reducing waters, even far from any uranium deposit, one can obtain uraninite saturation. This is the case in Lodeve sulphide-bearing waters, or in alkaline geothermal waters from the Pyrenees. If we analyze both U concentration and U saturation index, we can see that waters flowing through mineralized zones have either high U concentrations or are very close to U saturation. One can say that, with little doubt, any water that does not possess one of these two characteristics is of no interest for uranium detection.

Pierre Toulhoat and Catherine Beaucaire

Published Journal 
Journal of Geochemical Exploration, 1991



Pierre Toulhoat,Catherine Beaucaire. 1991. Comparison Between Lead Isotopes 234U-238U Activity Ratio And Saturation Index In Hydrogeochemical Exploration For Concealed Uranium Deposits. Journal of Geochemical Exploration. (!) .