CONTROLLING FACTORS IN THE GEOCHEMISTRY OF THE NEGEV (SOUTHERN ISRAEL) PHOSPHORITES: CLARIFYING THE RULES OF THE GAME

Research on the geochemistry of Mishash (late Campanian) Negev phosphorites during the last years, combining electron probe measurements, FTIR spectroscopy, wet chemistry and mineralogical analyses, enabled us to better understand the factors constraining the chemistry of these rocks and its variations in space. Two main geochemical phosphorite facies featuring two separated stages in phosphogenesis are differentiated in the Mishash sequence: 1) a pristine, microbially generated, P-poor phosphorite facies, more preserved in basinal sections, and 2) a recycled (reworked), amalgamated phosphorite facies, predominant in condensed sections near the Campanian highs and forming most of the economic phosphorites. The distribution of the U(IV) fraction, Cd, Zn, F/P₂O₅, CO₂/F, Y, as well as REEs including Ce (Ce/Ce*) and Eu (Eu/Eu*) anomalies and HREE enrichment, is closely regulated by these two facies. The F/P₂O₅ ratio is much lower (0.090-0.107) in the pristine than in the recycled facies (0.107-0.120), and F deficient francolites (CFA) with F/P₂O₅ ratios as low as 0.80 (lower than the stoichiometric ratio in fluorapatite) are commonly found in the pristine facies. In addition, the lower F/P₂O₅ in the pristine facies is coupled with a 1) higher Cd and Zn concentration, 2) a considerably reduced U(IV) fraction, 3) lower REE/P₂O₅ and Y/P₂O₅ ratios, 4) less negative Ce and Eu anomalies, 5) a lower HREE (Dy-Er) enrichment, 6) an increase in Fe-rich smectites in the clay fraction, and 7) a presence of structural OH in CFA. These geochemical trends in the two Negev phosphorite facies may be explained by varying time-spans of CFA growth, changes in sediment burial rates, and principally, by different residence times of the phosphate phase in the suboxic, diagenetic zone near the sediment – water interface where CFA precipitates. Transit of the CFA phase from the pristine state to the recycled state is associated with entry of more F, CO₂, and U(IV) in the CFA lattice, oxidation and removal of sulfide bound Cd and Zn, and uptake of more REE and Y. Considering the response of the Mishash section to the Campanian structure, these results enable us to make some predictions as to the chemical composition of the Mishash phosphorites across the Negev depositional space.