GEOLOGICAL SETTING OF PHOSPHORITES IN THE DZABKHAN SUBBASIN (WESTERN MONGOLIA)

A phosphorite-bearing series of deposits in the Vendian Tsaganolom Formation has been described. It is a thick carbonate formation, a stratigraphic counterpart of phosphorite-bearing deposits in the Khubsugul basin of carbonate accumulation. The Dzabkhan phosphorite-bearing region is far apart from the main zone of the Khubsugul Basin, but is its part (subbasin) in paleogeographic terms. The structure and composition of the Dzabkhan phosphorite series are shown to be essentially similar to the phosphorite series of the main part of the Khubsugul and Karatau phosphorite-bearing basins. In lithology and geochemistry, the Tsaganolom Carbonate Formation is distinctly subdivided into underphosphoritic, phosphoritic and overphosphoritic members. The phosphoritic member itself is subdivided in the same way. The phosphorites are confined to an interval in the carbonate formation, which is intermediate between regressive and transgressive parts of a large carbonate cycle and is specified by contrast composition of sediments. The Dzabkhan phosphorites are distinguished by its sublittoral setting, which led to weak productivity of their occurrences in the localities known up to date.