FORMATION CONDITIONS OF PLATINUM-GROUP METALS IN CHROMITE ORES OF THE KEMPIRSAI ORE FIELD

Abstract

The distribution and occurrence forms of platinum group elements (RPGE) were studied in detail in chromite ores of the Kempirsai ore field. The compositions, phase relations, and fine structure of representative aggregates of PGE minerals and coexisting phases of various mineral classes were investigated by scanning and transmitting electron microscopy using microanalysis and microdiffraction of electrons as well as electron probe microanalysis. A high solubility of nonferrous metals was established in PGE minerals. Regularities in the fractionation of PGE and nonferrous metals between compounds of various classes and relative ages were revealed. The earlier RPGE minerals are represented by metallic solid solutions (MSS) of the Ir-Os systems. The later MSS correspond to the following systems: Cu-Os-Ru, Cu-Ir, Ni-Ir, Ni-Os-Ir-Ru, Ni-Ru-Os-Fe. The Ru-based disulfides were crystallized in equilibrium with MSS of the Ir-Os system, whereas later sulfides are represented by compounds with various stoichiometries and a wide miscibility of Ni, Cu, Ir, Rh, Os, and Fe. Such regularities are typical for the crystallization of the latest phases, represented by classes of arsenides, RPGE sulfoarsenides, and nonferrous metals. The following minerals were established in intergrowths with various RPGE minerals: forsterite, chromium garnet, perovskite, amphibole, quartz, anatase, halloysite, talc, muscovite, todorokite, nantokite, graphite, serpentine, brucite, zeolites, bassanite, kaersutite, and potassium feldspar. The following ultrafine multiphase intergrowths of relatively constant sets of components were revealed: Na, K, Mg, Si, Ni, Fe, Ca, V, Mn, Cl, S, and As, and often Os, Ir, and Ru. The study of the fine structure of phase relations confirms that the succession of deposition of RPGE minerals was defined by a general evolution of the mineral-forming system but not by a superposition of processes of different ages. We found that the leading mechanism of the RPGE mineral crystallization was their dendrite growth during gas-transport reactions in a highly concentrated supercritical fluid. In addition to RPGE, the fluid contained other metallic components not participating in the chromite formation, as well as the ions of Cl -, F -, C -, S -, and OH -. The formation of native metals and MSS in such conditions does not require the existence of highly reducing conditions but is reached by reactions of disproportionation. The representative technological sampling of 0.5 mln tons of a commercial ore allowed us to determine the average content of the RPGE sum in chromite ores of 0.71 g/t. Such a content is maximal among any known natural ores, including the Noril'sk sulfide ores. On the basis of these data and taking into account the total reserves of chromite ores, we evaluated the RPGE resources to be no less than 110 tons. It was concluded that the size parameters of RPGE minerals and their aggregates made it possible, at least partly, to extract RPGE into separate platinum concentrates.