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

Abstract

In the Bashkir mega-anticline (western Urals) stratabound magnesite, siderite, fluorite and base-metal deposits are hosted by a sequence of Riphean sediments with a thickness of more than 12 km. The giant deposits of siderite (Bakal) and sparry magnesite (Satka) belong to the largest known mineral deposits of this type on Earth but are still disputed with respect to their origin. Both the Fe-and Mgcarbonate ores are clearly characterized by mimetic preservation of sedimentary and diagenetic textures of the host carbonate sediments, giving evidence of epigenetic metasomatic replacement. In the stratiform magnesite deposits of Satka, O-and C-isotopes, REE pattern and the lithostratigraphic position of the ore, point to the reflux of early diagenetic Mg-rich brines being responsible for the selective replacement of brecciated dolomite. The Bakal siderite deposits are hosted by Lower Riphean carbonate rocks and are controlled by a Lower to Middle Riphean unconformity marked by deep erosion and subsequent transgression-related sedimentation of coarse clastics. Their independence of carbonate lithofacies and their trace element distribution are indicative of metasomatic processes. Fe-bearing fluids have probably been generated by lowgrade metamorphic (catagenetic) devolatization from underlying argillites, causing the metasomatic formation of large siderite ore bodies in the Bakal carbonates due to the focusing of the fluid flow by the overlying Mid-Riphean coarse clastics. R. Ellmies (u) · G.

Formation of Magnesium Carbonates on Earth and Implications for Mars

“…Importantly for geochronological considerations, this implies that Sm will be more easily incorporated in to magnesite than Nd motivating the use of a 147 Sm-143 Nd isotope chronometer (see Section 6). REE abundances in sedimentary magnesite may signal formation via diagenetic alteration of dolomite rather than direct precipitation prior to burial (e.g., Ellmies et al, 1999;Fernández-Nieto et al, 2003;Lugli et al, 2002;Morteani et al, 1982). For example, Fernández-Nieto et al (2003) demonstrated that magnesites intermixed with dolomites have lower LREE but similar HREE when compared with the dolomite.…”