Origin of ooidal ironstones in relation to warming events: Cretaceous-Eocene Bakchar deposit, south-east Western Siberia

Abstract

Abstract An integrated study involving petrographical, mineralogical and geochemical investigations proposes the origin of the Bakchar ironstone deposit by upward migration of a mixture of Fe-rich brine and hydrothermal fluid through marine sediments. Trace element concentrations of the ironstone deposits rule out continental derivation of Fe, while high REE concentrations indicate mixed hydrogeneous and hydrothermal source. Three main types of ironstone within the Bakchar deposit are characterized by distinct mineralogical assemblage of authigenic minerals corresponding to oxic, methanic and weakly sulfidic dysoxic conditions. Changing redox conditions of the depositional environment, methane seepage in warm seawater and availability of iron influx possibly determined the final mineralogy of three varieties of ironstones. The association of siderite with ferrimagnetic sulfides like greigite and pyrrhotite in type-1 ore indicate anaerobic oxidation of methane with limited bacterial sulfate reduction. The ironstone deposits of Narym, Kolpashevo and Bakchar are contemporaneous with geological events like ocean anoxic event 3 (OAE-3), Cretaceous-Paleogene (K-Pg) boundary and Palaeocene-Eocene Thermal Maximum (PETM), respectively, all corresponding to warm seawater conditions.