Origin of giant stratabonnd deposits of magnesite and siderite in Riphean carbonate rocks of the Bashkir mega-anticline, western Urals

In the Bashkir mega-anticline (western Urals) stratabound magnesite, sideri te, fluorite and base-metal deposits are hosted by a sequence of Riphean se diments with a thickness of more than 12 km. The giant deposits of siderite (Bakal) and sparry magnesite (Satka) belong to the largest known mineral d eposits of this type on Earth but are still disputed with respect to their origin. Both the Fe- and Mg-carbonate ores are clearly characterized by mim etic preservation of sedimentary and diagenetic textures of the host carbon ate sediments, giving evidence of epigenetic metasomatic replacement. In th e stratiform magnesite deposits of Satka, O- and C-isotopes, REE pattern an d the lithostratigraphic position of the ore, point to the reflux of early diagenetic Mg-rich brines being responsible for the selective replacement o f brecciated dolomite. The Bakal siderite deposits are hosted by Lower Riph ean carbonate rocks and are controlled by a Lower to Middle Riphean unconfo rmity marked by deep erosion and subsequent transgression-related sedimenta tion of coarse clastics. Their independence of carbonate lithofacies and th eir trace element distribution are indicative of metasomatic processes. Fe- bearing fluids have probably been generated by low-grade metamorphic (catag enetic) devolatization from underlying argillites, causing the metasomatic formation of large siderite ore bodies in the Bakal carbonates due to the f ocusing of the fluid flow by the overlying Mid-Riphean coarse clastics.