Late Jurassic/Early Cretaceous phosphates of Nordvik, North Siberian Basin
Late Jurassic and Early Cretaceous calcite-rich phosphate (fluor/hydroxy apatite) concretions in the North Siberian basin (Nordvik) have been analysed. Their mineralogical and geochemical (major, trace and rare earth element) characteristics are presented in a sedimentological context. Low oxygen conditions prevailed along large parts of the seafloor of this wide epicontinental sea. In the Nordvik region this, in combination with slow and very fine-grained clastic sedimentation, and high algal production, resulted in the formation of phosphatic concretions. The concretions mainly consist of microcrystalline to ultracrystalline apatite and calcite, with traces of sphalerite, pyrite, kaolinite, quartz and albite. The clastic composition of the concretions is comparable with the surrounding Nordvik shales. Dysoxic–anoxic conditions in the last precipitating pore-water phases (early diagenetic) are reflected in enrichments in, for example, vanadium, uranium and the rare earth elements in the concretions.