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. Never theless, similar to many other phosphorites, Cd is either not correlat ed to P2O5, or has a slightly negative correlation with it. There is a clear correlation between the Cd content, the textural and the compos itional characteristics of the phosphorites, and the structural positi on of the phosphate deposits. Well developed 'synclinal' sections, com prised of many thin strata of laminated (non-disturbed) pelletal phosp horites, have much higher Cd concentrations than the condensed 'anticl inal' sections which consist of a few amalgamated thick beds of massiv e (reworked) phosphorites. Cd is in general poorly related to the pres ent organic matter content, but has a good correlation with organic ma tter in unoxidized beds of the 'synclinal' sections. Cd is strongly co rrelated to Zn in the relatively undisturbed beds of the 'synclinal' s ections but shows no correlation in the highly reworked beds of 'antic linal' sections. The source of Cd is upwelled seawater that is enriche d in both P and Cd. This is the basis of the relationship between phos phogenesis and Cd concentration. Reducing conditions during decomposit ion of organic matter immobilize Cd in the pore fluids of the bottom s ediments. Precipitation of Cd-rich sphalerite and adsorption on organi c matter appear to be the main processes for its initial speciation an d concentration in the Negev phosphorites. Cd is concentrated mainly i n laminated (non-disturbed) microbially generated phosphorites where a uthigenic apatites are also found. Cd does not as a rule replace Ca in the structure of authigenic (primary) sedimentary apatites. This find ing 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 mobilizati on of the sulfide-bound Cd. Scouring activity, bioturbation, and recyc ling and transportation of stirred-up phosphate corpuscles in an oxidi zing environment appear to be effectual for removing Cd. Oxidation alo ne, 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 apa tites. The cadmium concentration and the Cd/Zn ratio of a phosphorite may help to determine whether a phosphorite is pristine or not and to unravel its diagenetic history. (C) 1997 Elsevier Science B.V.