ABSTRACT
Stratabound to stratiform bodies of sparry magnesite are widespread in only a few clearly defined dolomite horizons of the Lower and Middle Riphean era of the Southern Urals. The dolomitic host rocks are of shallow marine facies with sedimentary unconformities in the upper part and underlyed by clayey sediments. The magnesite shows discordant lateral contacts to the host dolomites. Metasomatic magnesite bodies where formed after lithification of the carbonates but the genesis is bound to the sedimentary basin. 3-5 times increased FeO and MnO contents in magnesites and Mg-undersaturated host dolomites are geochemical features of the metasomatic nature of the magnesites. 18O-depletion of magnesite reflects the influence of meteoric water on the formation of the Mg-bearing fluids. The formation of the magnesites took place by a laterally or descending fluid flow of Mg-enriched groundwater, formed on the greenstone belts surrounding the Riphean sedimentary basin. The fluid flow followed brecciated lithologies of the Riphean carbonate platforms during the emerging periods.
