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2005 year, number 4

Fe-Mg CARBONATES AND VARIATIONS IN ISOTOPIC COMPOSITIONS OF CARBON AND OXYGEN AT THE OZERNOE HYDROTHERMAL-SEDIMENTARY PYRITE-POLYMETALLIC DEPOSIT (Transbaikalia)

K.R. Kovalev, G.S. Ripp*, E.G. Distanov, and M.V. Baulina
Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Geological Institute, Siberian Branch of the RAS, 6a ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
Keywords: Hydrothermal-sedimentary sulfide-polymetallic ores, Fe-Mg-carbonates, carbon and oxygen isotopes
Pages: 379-394

Abstract

The Ozernoe hydrothermal-sedimentary pyrite-polymetallic deposit in western Transbaikalia abounds in Fe-Mg-carbonates (predominantly siderites), which make up two major genetic groups of siderite ores: hydrothermal-sedimentary ores and their varieties recrystallized during hydrothermal-metasomatic processes. These groups of ores differ in structure, morphology, mineralogy, and geochemistry. Hydrothermal-sedimentary siderite ores form sheet bodies, make up ore sulfide-siderite rhythmites, have a fine-grained texture, and are polymineral siliceous-sulfide-carbonate formations. Siderites of these ores are Zn-enriched. Recrystallized ore varieties form deposits of complex morphology along the southeastern limb of the Ozernoe syncline on the flanks of silicic-alkaline metasomatites. These ores occur at different stratigraphic levels and coexist with stratified bodies of sulfide and sulfide-siderite ores. Exo- and endocontact zones of siderite deposits abound in coarse-crystalline siderite-barite-sulfide ores.
The isotopic compositions of carbon and oxygen of siderites, ankerites, and dolomites of hydrothermal-sedimentary ores and chemogenic and reef limestones of the ore-bearing section are generally the same as the isotopic compositions of coeval marine carbonates: δ13C = -1.6...+2.0<194> and δ18O = +15...+22<194>. Carbon plays a minor role in the formation of hydrothermal-sedimentary ores. Siderite varieties produced through recrystallization of sedimentary siderites during superposed metasomatic processes inherit the isotopic composition of their predecessors. Great variations in isotopic compositions of carbon and oxygen are observed within the large Ozernoe-Vasil'evskoe ore-bearing structure enclosing the Ozernoe deposit. The isotopic compositions of carbon and oxygen in carbonates of ores and ore-hosting rocks of this structure become heavier from base to roof of its section. The isotopic composition trends of carbon and oxygen are similar in behavior to that of sulfur in ore sulfides. Carbonates of hydrothermal-sedimentary iron oxide ores associated with sulfide ores have significantly lighter isotopic compositions of carbon (-0.5...-5 ‰) and oxygen (-2...+5 ‰). It is shown that the isotopic compositions of carbon and oxygen depend on the content of Mn in Fe-Mg-carbonates.