Chemical zoning of chrome-spinel nodules and oxythermobarometry of nodular chromitites of the Engayskoe-3 occurrence in the Rai-Iz massif (Polar Urals)

Abstract

Relevance of the work. Chromitites of nodular texture are found in all chromite-bearing alpine type ultramafite massifs of the world. The question of the geological and thermodynamic conditions of their formation still remains debated. The geology, mineralogy and petrogenesis of the nodular chromitites of the ophiolitic massifs of the Polar Urals, the results of studies of which are presented in this work, are slightly highlighted in Russian and foreign literature. Purpose of the work. To study the chemical zoning of chrome-spinel nodules from the chromitite of the Engayskoe-3 occurrence of the Rai-Iz massif (Polar Urals), to establish the patterns of changes in the content of chemical elements in the core and rims of the nodules, to determine the T–fO2 parameters for the formation of the studied chromitites. Results. The nodules of chromitites of the Engayskoe-3 occurrence consist of the core composed of interspersed chromite, which is surrounded by a solid rim of chromite. The change in the ratio of trivalent cations in the spinels of a nodule rim (from its inner part towards the edge and core) occurs mainly by replacing Al3 ↔ Fe3+, as indicated by the inverse proportionality of the number of these cations in the unit cell of the mineral. Similarly, the composition of spinel grains changes forming an impregnation in the space between the nodules. For chrome spinels from the nodule core, the change in the composition of the mineral grain from the center to the edge is due to the isomorphic replacement of Al3+ ↔􀀁Cr3+. The central parts of the spinel grains from the nodule core have the oxygen fugacity values to 0.7 logarithmic units above the FMQ buffer, on average, about FMQ +2 logarithmic units. The edge parts of the grains are much more oxidized: oxygen fugacity (determined for them) is FMQ + 3.1...+ 3.3 logarithmic units. The inner part of the nodule rim is recomposed (fO2 (FMQ) = +1.2...+1.7 logarithmic units) compared to the marginal (fO2 (FMQ) = +3.3...+3.6 logarithmic units above FMQ buffer). The temperature of olivine-spinel equilibrium is at the level of 550–600 °С. Conclusion. It was assumed that the core of the studied nodules are fragments of earlier disseminated chromitites. The formation of nodules rims is associated with a stage of metamorphism, in which the paragenesis of chlorite, amphibole, and talc was formed in the silicate part of chromitites; and olivine retained its plasticity.