Data are presented on chromitites from the northern and southern sheets of the Il’chir ophiolite complex (Ospa–Kitoi and Khara-Nur (Kharanur) massifs). The new and published data are used to consider similarities and differences between ore chrome-spinel from the chromitites of the northern and southern ophiolite sheets as well as the species diversity of PGE minerals and the evolution of PGE mineralization. Previously unknown PGE minerals have been found in the studied chromitites. Ore chrome-spinel in the chromitites from the northern sheet occurs in medium- and low-alumina forms, whereas the chromitites from the southern sheet contain only medium-alumina chrome-spinel. The PGE minerals in the chromitites from the southern sheet are Os–Ir–Ru solid solutions as well as sulfides and sulfoarsenides of these metals. The chromitites from the northern sheet contain the same PGE minerals and diverse Rh–Pt–Pd mineralization: Pt–Ir–Ru–Os and isoferroplatinum with Ir and Os–Ir–Ru lamellae. Areas of altered chromitites contain a wide variety of low-temperature secondary PGE minerals: Pt–Cu, Pt–Pd–Cu, PdHg, Rh2SnCu, RhNiAs, PtAs2, and PtSb2. The speciation of the PGE minerals is described along with multiphase intergrowths. The relations of Os–Ir–Ru solid solutions with laurite and irarsite are considered along with the microstructure of irarsite–osarsite–ruarsite solid solutions. Zoned Os–Ir–Ru crystals have been found. Zone Os82–99 in these crystals contains Ni3S2 inclusions, which mark off crystal growth zones. Different sources of PGE mineralization are presumed for the chromitites from the northern and southern sheets. The stages of PGE mineralization have been defined for the chromitites from the Il’chir ophiolite belt. The Pt–Ir–Ru–Os and (Os, Ru)S2 inclusions in Os–Ir–Ru solid solutions might be relics of primitive-mantle PGE minerals. During the partial melting of the upper mantle, Os–Ir–Ru and Pt–Fe solid solutions formed syngenetically with the chromitites. During the late-magmatic stage, Os–Ir–Ru solid solutions were replaced by sulfides and sulfarsenides of these metals. Mantle metasomatism under the effect of reduced mantle fluids was accompanied by PGE remobilization and redeposition with the formation of the following assemblage: garutiite (Ni,Fe,Ir), zaccariniite (RhNiAs), (Ir,Ni,Cu)S3, Pt–Cu, Pt–Cu–Fe–Ni, Cu–Pt–Pd, and Rh–Cu–Sn–Sb. The zoned Os–Ir–Ru crystals in the chromitites from the northern sheet suggest dissolution and redeposition of Os–Ir–Ru primary-mantle solid solutions by bisulfide complexes. Most likely, the PGE remobilization took place during early serpentinization at 450–600 ºC and 13–16 kbar. During the crustal metamorphic stage, tectonic movements (obduction) and a change from reducing to oxidizing conditions were accompanied by the successive transformation of chrome-spinel into ferrichromite–chrome-magnetite with the active participation of a metamorphic fluid enriched in crustal components. The orcelite–maucherite–ferrichromite–sperrylite assemblage formed in epidote-amphibolitic facies settings during this stage. The PGE mineral assemblage reflects different stages in the formation of the chromitites and dunite-harzburgite host rocks and their transformation from primitive mantle to crustal metamorphic processes.

The concentration levels and distribution features of the platinum group elements (PGE) in quartzsulfide and base-metal ores in deposits of the Sayan-Baikal Fold Region (SBFR) are discussed. Microfire assay neutron activation analysis (MF-NAA), which enables one to work on a nondestructive basis and allows one to avoid inaccuracies related to chemical sample preparation, was used as the main analytical technique. Three types of hydrothermal mineralization with elevated grades of PGE (especially Pt, Pd, and Ru) have been identified: (1) pyrite-pyrrhotite (massive sulfide) mineralization hosted in black shales of the Il'chir Sequence;(2) gold-sulfide ores of the Zun-Kholba, Tainsky, Kamenny, and some other gold deposits; and (3) silver-basemetal ores of the Dzhida-Vitim Zone. The PGE contents significantly vary, from global average values to tens of grams per ton. An absence of PGE minerals implies that these elements are finely dispersed in sulfide minerals and native gold. Taking into account difficulties in conversion of PGE into analytical forms, their nonuniform distribution in sulfide minerals, their high affinity to coordination compounds, and experimental results, cluster species of Pt and Pd in major minerals are suggested for the gold-sulfide and silver-base-metal ores in deposits, which are related to suprasubduction ophiolites and island-arc and intraplate settings in the SBFR.

No abstract