The redox state of chromium ores of the Polar Urals

Abstract

The redox state of chromium ores of the main industrially significant types, developed in ultramafic rocks of the Rayiz-Voikar complex of the Polar Urals, was studied. The chromitites occurring in various geological settings – rocks of the dunite-harzburgite complex and large dunite bodies – have been investigated. For the first time, on a representative sample of analyzes (more than 150 samples), an assessment was made of oxygen fugacity and temperature of olivine-spinel equilibrium in chromium ores of the Rai-Iz and Voikaro-Syninsky massifs. At each of the studied objects, the iron content of minerals increases linearly from chromitites to their host ultramafic rocks. The temperature of olivine-spinel equilibrium in chromitites varies within 550–800оС. The oxygen fugacity in aluminous chromitites averages FMQ +0.5–1.5 log. units, in medium chromium – FMQ +1.5–2.5 log. units, in high-chromium ones it exceeds +2.5 log. units relative to the FMQ buffer. The fugacity of oxygen in densely disseminated chromitites is 0.5–1 log. units higher than in poorly disseminated and rarely disseminated. The values of T-fO2 parameters correspond to the crustal conditions and are close to those established in the metaultramafites of the studied massifs. The data obtained may indicate that the concentration of the ore component and the formation of chromium ore deposits occur not only in mantle or lower crustal conditions, characterized by fO2 values close to the FMQ buffer, but also as a result of ultramafic crustal metamorphism occurring in an oxidizing environment.