Formation Conditions of Ferromagnesian Metasomatic Carbonates in the Lower Riphean Terrigenous–Carbonate Rocks of the Southern Urals

Abstract

Ferromagnesian carbonate metasomatites in limestones of the Lower Riphean Suran Formation in the Avzyan ore region (Bashkir meganticlinorium) are represented by large Fe-magnesite deposits (e.g., Ismakaevo deposit) and breunnerite stocks (Bogryashka ore occurrence). Metasomatic zonation is manifested as alterations in the series: limestone → dolomite → Fe-magnesite (breunnerite). Fe-magnesite contains up to 8 mol % FeCO₃; breunnerite, from 10 to 40 mol % FeCO₃. Metasomatic fluid represented the Ca–Na–Mg chloride brine with Fe admixture and was linked with processes of the remobilization of evaporite brines buried in Lower Riphean rocks. Salinity and homogenization temperature of fluid inclusions are: 20–26 wt % equiv NaCl and 200–240°C, respectively, in magnesites; 10–15 wt % equiv NaCl and 140–190°C, respectively, in breunnerites. Interaction of the fluid with terrigenous rocks in a tectonically active zone of the Mashak riftogenic graben ensured the enrichment of brine with different ligands and the accumulation of MREE and HREE in metasomatic products. Upon the migration of fluid across the limestone sequence and its cooling, Fe-magnesite was formed in the frontal zone (Ismakaevo deposit); breunnerite, in the rear zone adjacent to the terrigenous shales (Bogryashka ore occurrence).