THE NATURE OF ZINC CHROMITES FROM BANDED ILMENITE-HEMATITE QUARTZ SANDSTONE OF THE BEZYMYANNAYA FORMATION OF THE TUVA-MONGOLIAN TERRANE (SOUTHEASTERN PART OF THE EAST SAYAN, RUSSIA)

The paper presents the first data on detrital zinc chromites from banded ilmenite-hematite sandstone of the terrigenous Bezymyannaya formation of the Tuva-Mongolian terrane in the eastern part of the East Sayan. The zinc content of the accessory and ore chromites from the ophiolite complexes in the southeastern part of the East Sayan is not high and falls within the range of 0.2 to 1.5 wt. % ZnO. Scanning electronic microscopy has been used to study morphological and structural features and chemical composition of detrital zinc chromites. The study of chemical composition of mineral phases of titanium-ferruginous oxide interlayers, composed of ilmenohematite, Ti-hematite, pseudobrookite, pseudorutile and zinc chromite, has shown that Zn (12–27 wt. % ZnO) is present only in chromite and entirely absent in other ore minerals, i.e. that zinc enrichment in chromite is a feature of the ore body rather than the result of metamorphism or metensomatoses of terrigenous rocks in the source area. The ore bodies of the studied detrital chromites are associated with ultrabasites and basites, their metasomatized varieties, and podiform chromites from the ophiolite complexes in the southeastern part of the East Sayan. Metasomatic changes of mantle peridotites and chromitites and the effects of fluid/hydrothermal solution parameters resulted in substitution of Mg by Zn in the chromite. Zinc could originate from the subducting-slab rocks – volcanic-sedimentary rocks containing the fragments of submarine hydrothermal sulfide ore deposits, zinc-enriched and involved in metasomatic changes of peridotites and chromites.