PHOSPHORITE GEOCHEMISTRY - STATE-OF-THE-ART AND ENVIRONMENTAL CONCERNS

Major advances in our understanding of the genesis and diagenesis of m arine phosphate deposits (phosphorites) have been made by recent geoch emical studies. This work is reviewed and considered in relation to en vironmental issues arising from the exploitation of phosphorite deposi ts. Unaltered phosphorites contain francolite, a highly substituted ca rbonate fluorapatite, as the only phosphate mineral. It is the mineral ogy of this phase, together with kinetic and thermodynamic factors, wh ich govern many of the primary geochemical characteristics of phosphor ites, although metamorphism or weathering may fundamentally alter the composition of a deposit. Francolite precipitation is inhibited by Mg in seawater, but occurs nonetheless in PO4-rich porewaters via the for mation of a metastable precursor. Phosphate originates largely from th e breakdown of organic matter in surficial sediments, apatite precipit ating within cm-dm of the sediment/water interface. Coupled redox-cycl ing of Fe and P provides one means of maintaining high-PO4 levels in s ediments. Incorporation of trace elements, including Cd, U, Y, Zn and the rare-earth elements (REEs), occurs in association with the bacteri ally and redox-mediated degradation of particulate phases in the sedim ent, during or soon after francolite precipitation. REE distributions commonly reflect those in seawater. Uranium is incorporated largely as U(IV), but may be oxidised to U (VI) by seafloor weathering. Oxygen ( deltaO-18), carbon (deltaC-13) and sulphur (deltaS-34) stable-isotope studies of francolites provide a wealth of palaeoenvironmental informa tion, and confirm that precipitation generally occurs under oxic to su boxic conditions in surficial sediments. Uranium-and C-14-dating toget her with Sr-isotope ratios (Sr-87/Sr-86) may be used to measure nodule and grain growth rates and precisely date phosphogenic episodes. Nodu les accumulate slowly at < 0.1-1 cm kyr-1 while phosphate grains may f orm in < 10 years. Variations in epsilonNd values of francolite provid e a basis for palaeoceanographic studies. Environmental issues associa ted with phosphate fertiliser production and use are discussed. Toxic metal, particularly Cd, and radionuclide contamination of the environm ent from phosphogypsum and fertilisers are considered. It is concluded that insufficient data are currently available for an accurate assess ment to be made of the environmental impact and consequent human healt h implications of such contamination. Future advances in phosphorite g eology will necessitate further experimental work, well-integrated fie ld and laboratory studies using established geological and geochemical methods, and the application of new high-sensitivity spatially resolv ed analytical geochemical techniques.