Oolitic Ironstones and Carbonate Mn Ores of the Marsyaty Deposit (Northern Urals, Russia): A Key Study of Mineralogy and Geochemistry

This article describes the mineralogical and geochemical features of the Marsyaty Mn–Fe sedimentary deposit in the Northern Urals (Russia). Oolitic ironstones of the deposit are localized in the Cenomanian coastal sandstones. Manganese ores (carbonate and carbonate–oxide types) lies within the Lower Paleocene siliciclastic sediments and are separated from the oolitic ironstone (iron oxide and iron carbonate types) by a polymictic gravelite bed. Authigenic Fe3+ oxyhydroxides (goethite and ferrihydrite), chamosite/berthierine and late siderite predominate in the ironstones; kaolinite, apatite, perhamite, calcite and dolomite are minor constituents. Rhodochrosite and rancieite are the major minerals of the manganese ore; Mn-dominated phyllosilicates (parsettensite? and caryopilite?) are rare. Both ore types contain authigenic glauconite, montmorillonite, sulfides (sphalerite/wurtzite, galena, and pyrite), gibbsite/boehmite and REE phosphates. Both ore types contain detrital quartz, ilmenite, zircon, monazite, epidote, titanite, muscovite and feldspars. The δ13Ccarb value (VPDB) varies from −18.5 to −23.3 in both ironstone types and from −10.0 to −41.0 ‰ in the manganese ore. The negative C isotopic composition and numerous organic remains indicate the involvement of microbial processes in the formation of both types of carbonate ores. The Fe and Mn ores belong to one transgression–regression sedimentation cycle and formed consecutively during the evolution of the West Siberian basin. A unique feature of the Marsyaty deposit includes two ore formation stages within a limited area and over a relatively short geological period: the accumulation of (i) oolitic ironstones enriched in Mn first and then (ii) manganese ores only.