Geochemistry of cadmium in the Negev phosphorites

Cadmium (Cd) in the Negev phosphorites, as in many other phosphorites, is the most enriched trace element relative to ‘average’ shale. Nevertheless, similar to many other phosphorites, Cd is either not correlated to P2O5, or has a slightly negative correlation with it. There is a clear correlation between the Cd content, the textural and the compositional characteristics of the phosphorites, and the structural position of the phosphate deposits. Well developed ‘synclinal’ sections, comprised of many thin strata of laminated (non-disturbed) pelletal phosphorites, have much higher Cd concentrations than the condensed ‘anticlinal’ sections which consist of a few amalgamated thick beds of massive (reworked) phosphorites. Cd is in general poorly related to the present organic matter content, but has a good correlation with organic matter in unoxidized beds of the ‘synclinal’ sections. Cd is strongly correlated to Zn in the relatively undisturbed beds of the ‘synclinal’ sections but shows no correlation in the highly reworked beds of ‘anticlinal’ sections. The source of Cd is upwelled seawater that is enriched in both P and Cd. This is the basis of the relationship between phosphogenesis and Cd concentration. Reducing conditions during decomposition of organic matter immobilize Cd in the pore fluids of the bottom sediments. Precipitation of Cd-rich sphalerite and adsorption on organic matter appear to be the main processes for its initial speciation and concentration in the Negev phosphorites. Cd is concentrated mainly in laminated (non-disturbed) microbially generated phosphorites where authigenic apatites are also found. Cd does not as a rule replace Ca in the structure of authigenic (primary) sedimentary apatites. This finding has practical applications since it makes possible the production of a phosphate concentrate with a much lower Cd concentration than the original phosphorite. Exposure to oxic conditions leads to mobilization of the sulfide-bound Cd. Scouring activity, bioturbation, and recycling and transportation of stirred-up phosphate corpuscles in an oxidizing environment appear to be effectual for removing Cd. Oxidation alone, without fabric disruption and mobilization of the phosphate grains does not significantly lower Cd concentrations, since the released Cd may be almost immediately readsorbed on the various components of the phosphorite such as iron oxyhydroxides, organo-clay fractions and apatites. The cadmium concentration and the CdZn ratio of a phosphorite may help to determine whether a phosphorite is pristine or not and to unravel its diagenetic history.