PGM in chromitites of Kraka massifs (the Southern Urals): diversity and origin

The paper provides results of study of platinum group minerals (PGMs) from 18 ore occurrences and deposits of the Kraka massifs, most of these located in ultramafic rocks of the upper mantle section (15), and several occurrences in a crust-mantle transition complex (3). It is shown that chromitites in the upper mantle section have refractory geochemical specialization (Os-Ir-Ru), while chromitites of the transition complex typically contain Pt and Pd minerals. The highest concentrations of the platinum group elements (PGE) are observed in chromitites of the transition complex (up to 2500 ppb of the total PGE). However, minor amounts of chromitites at these sites do not allow us to consider this mineralization type as promising in practical terms. chromitites in the upper mantle section are about an order lower in PGE (50–200 ppb of the total PGE). Analysis of the obtained data suggests the following explanation for various PGM types identified. PGMs occurred in chromitites of the upper mantle section at two stages: 1) disulfides of the laurite-erlichmanite series and, to a lesser extent, Os-Ir-Ru alloys were formed within chromite grains in result of subsolidus processes in the upper mantle restite during solid-phase segregation of PGEs initially incorporated in the crystal lattice of chromite; 2) sulfoarsenides and other PGE compounds with basic metals and antimony were formed by hydrothermal processing of chromitites in crustal conditions. Pt and Pd minerals were produced by differentiation of magmatic melts separated from restite; they were completely or partly transformed under the impact of hydrothermal processes.