Abstract
The paper describes disseminated tabular, podiform massive, and transitional chromitite deposits from a mantle section of the Kraka ophiolite massif, South Urals, Russia. The chromitite is hosted by dunite with no correlation between their size and quality and the size of the dunite bodies. Thick dunite bodies mostly host disseminated fine-grained banded chromitite; massive ores are composed of coarse-grained chromitite typically with a thin dunite envelope. The chromitite and host ultramafic rocks exhibit plastic deformation of silicates and chromite, which is expressed in microstructural features, preferred orientation of rock-forming olivine, and folding of the chromitite bodies. The ultramafic rocks are also characterized by deformation-induced textures leading to the formation of the small-size chromite grains on structural defects of plastically deformed rock-forming olivine and orthopyroxene. The formation of dunite bodies and associated chromitite is related to the localization of deformation of rising mantle flows under decompression conditions. Dunite was the most rheologically weak zone exhibiting a focused solid state flow and effective separation of mineral phases (olivine and chromite). The higher amount of the latter in dunite is a result of deformation-induced breakdown of enstatite and removal of trace elements from olivine. The structural features of massive chromitite aggregates indicate that they are a product of concentration and aggregation of grains under the influence of tectonic stresses at high temperatures and pressures, similar to pressure sintering.
Published Version
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