PT-FE NUGGETS DERIVED FROM CLINOPYROXENITE-DUNITE MASSIFS, RUSSIA: A STRUCTURAL, COMPOSITIONAL AND OSMIUM-ISOTOPE STUDY

Abstract

Combined structural, compositional and osmium-isotope data on selected Pt–Fe nuggets from economically important placer deposits closely linked to clinopyroxenite–dunite massifs of the Siberian Platform (Kondyor, Inagli, Guli) and the Middle Urals (Nizhny Tagil), Russia, are presented for the first time. Pt–Fe alloys investigated are ferroan platinum (space group Fm3m) with a composition close to Pt 3 Fe. This emphasizes the necessity of an X-ray study in identifying the particular Pt–Fe alloy species. Less common are compositions such as Pt2Fe and an intimate intergrowth of Pt3Fe2 and PtFe. Other platinum-group minerals (PGM) observed in ferroan platinum include a diversity of Os–Ir–Ru alloys, PGE sulfides [laurite, malanite, cuproiridsite, cooperite, and an unnamed base metal – (Ir,Pt) monosulfide], PGE sulfarsenides (hollingworthite, irarsite), Pt–Pd tellurides (moncheite, telluropalladinite) and stibiopalladinite. This suite of PGM is consistent with those from other zoned or Uralian- Alaskan-type massifs. However, unusually Ru-rich alloys included in ferroan platinum of Guli are characteristic of PGM derived from an ophiolite source and underline the transitional signature of the Guli massif between zoned-and ophiolite-type complexes. Pd-rich ferroan platinum nuggets indicate a derivation from clinopyroxenite source-rock, whereas Ir-rich Pt–Fe alloys suggest a chromitite source. The presence of numerous Os–Ir–Ru exsolution lamellae in ferroan platinum are indicative of a high-temperature origin of the PGM. The first Os-isotope data from Os-rich minerals from chromitites and placers closely associated to the Kondyor and Inagli massifs reveal low 187Os/188Os values with a very narrow range, indicative of a common mantle source of the PGE, implying that the PGM are of primary origin. Disintegration of parent ultramafic source-rocks and short-range mechanical transport of liberated PGM formed the placers. Os-isotope model ages in the range of 340 to 355 Ma constrain the formation age of the Kondyor and Inagli massifs of the Aldan Province at the southeastern part of the Siberian Craton, and closely match those from the Guli massif (370 Ma) of the Maimecha–Kotui Province at the northern part of the Siberian Craton.