CHROMITITES OF THE URALS (PART 1): OVERVIEWOF CHROMITE MINERAL CHEMISTRY AND GEO-TECTONIC SETTING

Abstract

Published and unpublished compositions of chromite in 333 chromitite samples from 14 ultramafic complexes of the Urals are overviewed. The chromitites occur in the mantle unit and/or the supra-Moho cumulate sequence of ophiolite complexes, as well as in Alaskan-type intrusions. They vary in size from giant ore deposits associated with ophiolites (e.g., Kempirsai, Ray-Iz, Voykar-Syninsky) to sub-economic mineralization in the Alaskan-type complexes (e.g., Svetly Bor, Kachkanar). Mantle-hosted chromitites occur either as discordant, podiform, high-Cr ore bodies and sub-concordant elongated lenses of high-Al chromite. In the supra-Moho sequences of ophiolites, chromitite is mainly of the high-Al variety, and occurs as concordant layers alternated with peridotite and pyroxenite cumulates. In the Alaskan-type intrusions of the Urals, chromitite occurs as centimeter to meter-size pods and lenses having syngenetic or epigenetic relationship with the host dunite. Calculated melt compositions in equilibrium with chromite and comparison of chromite composition with those from various volcanic suites, and chromitites from different plutonic complexes, allow division of the Urals chromitites into four different compositional groups, corresponding to different geodynamic environments of formation: 1) The high-Al, low-Ti suite (Al2O3 > 20 wt%, Cr# < 0.70, av. TiO2 = 0.15 wt%, av. Fe3+# = 0.05, δlogf(O2) = -0.1 ÷ +2.3) includes most of the supra-Moho stratiform chromitites and some podiform chromitites hosted by the ophiolitic mantle rocks. These chromitites crystallized from MORB-type tholeiitic magmas (av. FeO/MgO = 1.0), produced by low degrees of partial melting of a slightly depleted source in subduction-unrelated geodynamic settings. 2) The high-Al, high-Ti suite (Al2O3 > 20 wt%, Cr# < 0.70, av. TiO2 = 0.80 wt%, av. Fe3+# = 0.20) is represented by the CHR-2 chromitite from the supra- Moho cumulus sequence of the Nurali ophiolite complex. The calculated melt in equilibrium with chromite differs from MORB in having higher FeO/MgO = 1.90, while the chromite displays characteristics of spinels in intra-plate basalts and chromitites in layered intrusions. The coexistence of chromitites derived from MORB and transitional tholeiites in the Nurali cumulate sequence is considered a feature typical of continental margin ophiolite complexes. 3) The high-Cr, low-Ti suite (Cr# > 0.70, Al2O3 < 20 wt%, TiO2 < 0.30 wt%, av. Fe3+# = 0.06, δlogf(O2) = -1.7 to +2.7) includes most podiform chromitites hosted by the ophiolitic mantle rocks and a few examples of supra-Moho stratiform chromitites. They have crystallized from high-Mg magmas with average compositions referable to picritic tholeiite and boninite (FeO/MgO < 1.0). These chromitites are typically found associated with subduction-related ophiolites of the Urals. 4) The high-Cr, high-Ti suite (Cr# > 0.70, Al2O3 < 20 wt%, TiO2 = 0.38-1.30 wt%, Fe3+# = 0.20-1.29, δ logf(O2) = +0.9 ÷ +5.9) is represented by chromitites from the Urals Alaskan-type intrusions and the East-Khabarny complex. They have crystallized from Fe-rich magma (av. FeO/MgO = 1.35) under oxygen- fugacity conditions well above the FMQ buffer. The melt is characterized by high-Ti, high-K, calc-alkaline composition, having many geochemical characteristics in common with ankaramites. It was generated by partial melting of a fluid-metasomatized mantle source, in a subduction-influenced arc setting. However, the close similarity with the zoned complexes emplaced in the Russian-Far-East craton suggests that formation of Alaskan-type melts may be not restricted to SSZ, island arc settings.