COMPOSITION AND PARAGENESIS OF Pt ALLOYS FROM CHROMITITES OF THE URALIAN ALASKAN-TYPE KYTLYM AND UKTUS COMPLEXES, NORTHERN AND CENTRAL URALS, RUSSIA

Abstract

Chromitites from the Kytlym and Uktus zoned Uralian–Alaskan type complexes in the northern and central Urals, Russia, contain abundant inclusions of Pt alloys compositionally attributable to one of three main groups: 1) Pt–Fe alloys with an isoferroplatinum-type composition, Pt3Fe; 2) Ni-rich Pt–Fe–Ni–(Cu) alloys with a stoichiometry of the type isoferroplatinum Pt2.5(Fe,Ni,Cu)1.5, tetraferroplatinum Pt(Fe,Ni,Cu), and ferronickelplatinum Pt2FeNi, and 3) Cu-rich Pt–Fe–Cu–(Ni) alloys with a stoichiometry of tulameenite type, Pt2Fe(Cu,Ni). The Pt–Fe and Ni-rich Pt–Fe–Ni–(Cu) alloys occur as primary inclusions in unaltered chromian spinel and are considered to have formed in a high-temperature pre-chromite stage. A sulfide-rich assemblage (erlichmanite, Rh–Ir–Pt thiospinels, cooperite, unknown Rh–Ir-bearing base-metal sulfides, pyrrhotite and pentlandite) accompanies the Pt3Fe alloy, indicating that it was a stable phase under relatively high sulfur fugacity. The Pt–Fe–Ni–(Cu) alloys formed at low sulfur fugacity, as suggested by the absence of sulfides and the exclusive presence of native osmium in the assemblage. The Cu-rich Pt–Fe–Cu–(Ni) alloys of tulameenite type are exclusively located along cracks in contact with secondary ferrian chromite, magnetite and chlorite, or constitute the metasomatic replacement of primary Pt alloys, and are considered to have a low-temperature hydrothermal origin. The secondary assemblage of PGM also includes potarite, prassoite, rhodian pentlandite, Cu–Pd alloys, native osmium, Ir–Fe alloys and oxides. These data strongly suggest that Ural–Alaskan-type chromitites are major contributors of Pt alloys that are encountered associated with nuggets in placer deposits of the Ural Platinum-bearing Belt.