MACROCRYSTALS OF Pt Fe ALLOY FROM THE KONDYOR PGE PLACER DEPOSIT, KHABAROVSKIY KRAY, RUSSIA: TRACE-ELEMENT CONTENT, MINERAL INCLUSIONS AND REACTION ASSEMBLAGES

Abstract

Euhedral macrocrystals of Pt–Fe alloy from the Kondyor PGE placer, Khabarovskiy Kray, eastern Siberia, Russia, have a relatively constant composition of Pt2.4–2.6Fe, tin and antimony contents up to 0.3 wt.%, and an unusually low content of all PGE except Pt. The millimetric crystals contain inclusions of fluorapatite, titanite, phlogopite, magnetite, ilmenite and iron–copper sulfides. The macrocrystals have a complex gold-rich rim, with four groups of gold alloy: tetra-auricupride (the most abundant gold-bearing phase), Au–Ag (98–54 wt.% Au), Au–Ag–Cu–Pd and Au–Pd–Cu alloys. The inner part of the reaction rim hosts a variety of PGE minerals, such as stannides, antimonides and tellurobismuthides of Pd and Pt. Stannides occur as copper-bearing (taimyrite–tatyanaite series) and copper-free compounds (atokite–rustenburgite series). The main antimony mineral is Sn-bearing mertieite-II. Tellurobismuthides are represented by Te-rich sobolevskite and an intermediate member of the moncheite–insizwaite solid-solution series. The reaction rim also hosts several unknown phases, such as Pd7Bi3, Pd3Bi, Bi2O3•3H2O, and a phosphocarbonate of thorium. The coarse crystals occur in eluvial and alluvial placers within the Kondyor massif, a zoned clinopyroxenite–dunite intrusion of Paleozoic or Mesozoic age. The inclusion assemblage suggests that the macrocrystals of Pt– Fe alloy are associated with late apatite – magnetite – phlogopite clinopyroxenite bodies, and possibly formed in a late–magmatic pegmatitic environment. The Pt–Fe alloy crystals have undergone hydrothermal overprint by NaCl-rich solutions bearing gold – silver – palladium – copper, which produced a wide variety of intermetallic compounds as well as Pd-bearing intermetallic phases. The aqueous solutions also had a minor but characteristic bismuth – tin – antimony – tellurium component.