Conditions for the formation of Fe-Mg metasomatic carbonates in the Lower Riphean terrigenous-carbonate sediments of the Southern Urals
Fe-Mg carbonate metasomatites in the limestones of the Suran suite of the Lower Riphean in the Avzyansky ore district of the Bashkir meganticlinorium are represented by large deposits of Fe-magnesite (Ismakaevo deposit) and breinerite stocks (Bogryashka deposit). The metasomatic zonality is represented by a series of limestone - dolomite - Fe-magnesite (breinerite). Ferrous magnesite contains up to 8 mol. % FeСO3. In breinerite iron saturation varies from 10 to 45 mol. % FeСO3. The metasomatic fluid was a brine of Ca, Na, Mg chlorides with an impurity of Fe, and was connected with the remobilization processes of evaporite brines buried in sediments of the Lower Riphean. The salinity and homogenization temperature of fluid inclusions in magnesites are in the range of 20-26% eq. NaCl and 200-240°C, and in breinerites - 10-15% eq. NaCl and 140-190°C, respectively. The interaction of fluid with terrigenous rocks in the tectonically active zone of the Mashak riftogenic graben led to the enrichment of various ligands and more active hydrothermal redeposition in the metasomatic products of medium and heavy lanthanides. During the migration of fluid through the limestone unite in the cooling process, Fe-magnesite of the Ismakaevo deposit was formed in the frontal zone, and the breinerite of Bogryashka deposit - in the rear zone adjacent to the shale source of brine.