SIDERITE OF MARINE OOIDAL IRONSTONES OF BAKCHAR DEPOSIT AS PROXY OF SPECIFIC LITHOGENESIS

The relevance of the research. This article presents the study of formation conditions of siderite in the environment of the marine ironstones precipitation of the Bakchar deposit. The origin of marine ironstone deposits including relationships with other mineral deposits keeps discussed scientific issue long time. The formation of siderite in the environment of an ooidal ironstone deposition is poorly understood regarding the carbon and metals sources, mobilization and transportation, as well as the conditions and kinetics of carbonate precipitation. The goal of the work was to the study the origin and evolution of siderite as the key cement mineral of ooidal ironstones to assess the factors for the geological processes of deposit formation in the example of the Cretaceous-Paleogene Bakchar deposit in Western Siberia. The methods: optical microscopy, petrographic analysis, scanning electron microscopy (SEM) with local energy dispersive analysis (EDS), microthermometric analysis, Raman spectroscopy and isotope mass spectrometry. Results. Siderite in ooidal ironstones can be of two main variations that differ in morphology, chemical and isotopic composition, fluid inclusions due to different carbon sources and precipitation conditions. The first variety of siderite is formed due to the mobilization of hydrothermal methane fluids. The second one reflects the carbonate formation on the seabed. The authigenic mineral associations of cement in ironstones at thin intervals (from 2 to 6 m in thickness) mark the change of geochemical zones from methane through sulfide (sulphate-methane transition zone) to ferruginous (iron reduction zone). Layers with similar vertical zonation of in situ minerals, represented by the co-occurrence of siderite, phyllosilicates, hydrogoethite, and rare sulfides, are proximal zones that indicate periods of an intense exhalative of metal-bearing fluids.