Composition of minerals in mantle peridotites as proxy of ore-forming processes in the mantle: Evidence from ophiolites in the Voykar-Synya and Kempirsai Massifs

Abstract

It is shown that ortho- and clinopyroxenes in mantle peridotites, which host different-scale chromite deposits, are marked by significant differences in the structure of rocks, composition of olivine and chromian spinel, and, primarily, content and distribution of the major ore-forming oxides therein. Enstatites and diopsides in peridotites from the southern Kempirsai Massif are notably depleted in the major oxides as compared to minerals in harzburgites from the Voykar-Synya Massif. The central portion of some large enstatite grains in peridotites from the Kempirsai Massif is marked by high contents of chromium oxide and alumina. Correlation of the prominent strain-mediated zonation in large relict pyroxene grains with decrease in Cr2O3 and Al2O3 at their rims suggests an initially high content of these components in pyroxenes. Extremely narrow variation range of the composition of high-Mg olivine in harzburgites from the Kempirsai Massif and its relatively small grain size as compared to harzburgites from the Voykar Massif were also provoked by the high-temperature recrystallization of mantle rocks accompanied by the transition of Cr and Al from silicates (pyroxene) into ore mineral (chromian spinel). Redistribution of components and recrystallization of peridotites were likely caused by the emplacement of harzburgites by a large volume of fluid/melt, which reacted with the host rocks, and by the high rate of mantle deformation (contraction of olivine grains) during this process. Components (Cr, Al, Fe) needed for the formation of chromium ores were extracted from both rock-forming silicates and migrating melts.