Phase equilibria in rocks of the paleoproterozoic banded iron formation (BIF) of the Lebedinskoe deposit, Kursk Magnetic Anomaly, and the petrogenesis of BIF with alkali amphiboles

BIF with alkali amphibole at the Lebedinskoe iron deposits, the largest in Russia, were metamorphosed at 550°C and 2–3 kbar and contain ferriwinchite, riebeckite, actinolite, grunerite, and aegirine-augite. All reaction textures observed in the rocks were produced during the prograde metamorphic stage and represent the following succession of mineral replacements: Gru → Rbk, Act → Win → Rbk. Data obtained on the textural relations and compositional variations of Ca, Ca-Na, and Na Al-free amphiboles point to the complete miscibility in the actinolite-ferriwinchite and ferriwinchite-riebeckite isomorphic series. Riebeckite is formed in BIF during the prograde metamorphic stage, with the participation of a fluid insignificantly enriched in Na⁺ and at increasing oxygen fugacity. The critical factors controlling the development of alkali amphiboles and Ca-Na pyroxenes in carbonate-bearing BIF is the oxygen activity and the presence of at least low concentrations of Na⁺ ions in the fluid. The minerals contain Fe³⁺, and all reactions producing them are oxidation reactions. The origin of riebeckite late in the course of the mineral-forming process is caused by the Ca²⁺Mg²⁺ → Na⁺Fe³⁺ heterovalent isomorphic replacement in calcic and calcic-sodic amphiboles and by the oxidation of grunerite in the presence of a fluid enriched in Na ions.