Problems of phosphorite genesis

Modern notions of phosphorite genesis are reviewed, with emphasis on the role of A. L. Yanshin's studies. We have recognized three stages in the development of the theory of phosphorite genesis: biogenic; orthochemogenic (according to A. V. Kazakov's theory); and modern, biochemogenic diagenetic. We considered the features of phosphorites belonging to major genetic types, analyzed the global distribution of the resources of marine phosphorites with time, and revealed two main phosphorite accumulation epochs - Vendian-Cambrian and Cretaceous-Paleogene. The structure-tectonic conditions of phosphorite deposition proceeding in global tectonomagmatic epochs are considered, It is shown that the largest deposits are confined to shelves (land margins). The phosphorite deposition zones are connected with the ocean, partially isolated, or occur in the interior continental areas. These are mainly shallow-water zones with enhanced bioproductivity, peneplain provenance of the terrigenous sediment component, and arid climate. Understanding of phosphorite genesis requires considering the peculiarities of various types of phosphorites and changes of the process of phosphate mineralization with evolution of geologic processes. The most probable mechanisms and conditions of phosphorite genesis are outlined.