Micromineralogical Evidence for High-Temperature Mineralized Fluid Interaction with Dolomites Hosting the Poperechny Fe–Mn Deposit (Russian Far East)

Microminerals and microspherules were studied in altered dolomites of the Murandavsky suite in the vicinity of their contact with iron–manganese ores of the Poperechny deposit (Russian Far East). It is shown that dolomite alteration was caused by a fluctuating influence of high-temperature fluid in the liquid and gaseous state. The liquid phase of this fluid is responsible for dolomite recrystallization and silicification, as well as the formation of metasomatic minerals, while the gaseous fluid triggered various explosive phenomena accompanied by localized melting of host rock lithologies, an influx of components uncommon to dolomites, and the formation of noble metal, iron–oxide, and carbonate–silicate microspehrules. Chemical analysis of the newly formed minerals and microspherules suggests presence of K, P, Fe, Ti, F, S, and rare earth elements in the fluid. The results support the general model for the magmatic–hydrothermal formation of iron–manganese deposits in the Lesser Khingan ore district, which includes hydrothermal alteration of host dolomites with iron and manganese inputs accompanied by various fluid-explosive phenomena and the formation of the mineralized pyroclastic rocks.