Uranium geochemistry and fission-track mapping of phosphorites, Zefa field, Israel.

Uranium concentrations and oxidation states were determined for 45 phosphorite samples from the Zefa deposit in the Negev Desert of S Israel. Analysis of samples was performed by NAA and fission- track methods. Bone fragments were found to contain the lowest U concentrations (80-150 ppm), while replated bone fragments are richer (120-260 ppm) as are ovulites (140-330 ppm). Non-apatitic grains, calcitic matrix and cement are U-poor. The U/P₂O₅ ratio varies greatly between samples (3 X 10^-4 to 6 X 10¹⁴), depending on the U concentration of all grains rather than on the abundance of various grain types. Tetravalent U varies in the range 15-54% of the total U. These results are compatible with the accepted models of phosphorite formation, according to which organic matter serves both as a phosphorus-contributing and U-concentrating phase.-J.E.S.