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

Abstract

Uranium concentrations and oxidation states were determined on 45 samples from the Campanian Zefa phosphorite field in southern Israel. Optically anisotropic bone fragments have the lowest uranium concentration of all apatitic grains in the rock (80-150 ppm). Replaced isotropic bones are richer in U (120-260 ppm), as are isotropic ovulites (140-300 ppm). Nonapatitic grains and calcitic matrix and cement are U poor (usually less than 5 ppm). The ratio between U concentration in isotropic and anisotropic components in any rock is nearly constant around 1.6. The U/P/sub 2/O/sub 5/ ratio in a sample depends on the uranium concentration in all grains rather than on the abundance of various grain types. U(IV)/U varies in the range of 15 to 54 percent of the total U. The ratio of U(IV)/U for the Zefa deposit falls in the field of onland phosphorites. The dispersion of the U/P/sub 2/O/sub 5/ ratio among samples within each columnar section of the Zefa field is similar to the dispersion of the average U/P/sub 2/O/sub 5/ ratios among the six columnar sections and is not different from the dispersion of that ratio between different phosphate deposits in Israel. The above results are compatible with accepted models of phosphoritemore » formation, according to which organic matter serves both as a phosphorus-contributing and uranium-concentrating phase.« less