Geological STRUCTURE, mineralogical and chemical FTATURES of ultramafic rocks and chromitites of Menzhinsky Deposit (South Kraka, the Southern Urals)
The largest deposit of high-grade chrome ore in Kraka ophiolite (Menzhinsky) is described. Chromitites, as few veins, 0.5-1 m thick, are localized in dunite body (about 1000 m lоng and 30-40 m thick) enclosed by a massive peridotite. The structure of the deposit is symmetrical: peridotite - dunite - chromitite - dunite - peridotite. This corresponds to a symmetrical mineral chemistry: 1) an increasing of Cr# and Fo from peridotite to dunite and chromitite, 2) a depleting of pyroxenes with Al, Cr in the same direction. Overall in ultramafic rocks, olivine and orthopyroxene grains show a high-T deformation pattern. Large enstatite crystals are separated by low-angle boundaries to several subgrains. They enclose numerous lamellae of diopside, Cr-Na-pargasite and many tiny neoblasts of enstatite, diopside, forsterite with new-formed micron-size precipitations of Cr-spinels, crystallized along their edges. The Cr-spinels are formed as rods and needles along subgrain boundaries in olivine and as lamellae in deformed enstatite. In all noted cases, the tiny Cr-spinel crystals formation is a result of plastic deformation of primary silicates. We infer that observed chemical and mineralogical zonation of studied deposit is a result of an ascending flow of mantle polycrystalline continuous substance, in which a hydrodynamic field have acted. Olivine and orthopyroxene have a strong rheological contrast of behavior, so all the enstatite grains were reduced in size (as a strong phase) and the olivine grains (a weak phase) have concentrated along the axis of flow (as dunite). As the flow was going on, the chromite and olivine grains were separating.