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 Pt 2 Fe and an intimate intergrowth of Pt 3 Fe 2 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 UralianAlaskan-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 hightemperature 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 187 Os/ 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.

Residual platinum-group minerals from highly depleted harzburgites of the Lherz massif (France) and their role in HSE fractionation of the mantle

“…Pt-rich sulfides are quite rare (Melcher et al, 1997;Malitch et al, 2001). The PGM observed in the Pyrenean harzburgites are also similar to the most commonly reported PGMs in alaskan-type intrusions where Pt-alloys and Pt-sulfides are statistically more common (Johan et al, 1989;Malitch and Thalhammmer, 2002). More importantly, in both occurrences, the PGM are thought to have crystallized from S-undersaturated melts, a condition that was also fullfilled in our BMS-free samples.…”

Chromite composition and platinum-group mineral assemblage in the Uktus Uralian-Alaskan-type complex (Central Urals, Russia)

“…1;Betekhtin 1961;Razin 1976;Cabri and Genkin 1991 and references therein). In recent times, comparative study of PGE mineralogy in lode and placer deposits has provided support to this conclusion and showed that Pt-Fe alloys accompanied by Ir phases represent the most abundant constituents of the PGM assemblage (Cabri et al 1996;Makeyev et al 1997;Garuti et al 2002;Malitch and Thalhammer 2002). Such a mineralogical specialization is remarkably consistent with geochemical data showing positive anomalies of Pt or Pt plus Ir as distinctive features of the chondritic PGE distribution patterns of Uralian-Alaskan-type dunites and chromitites from all over the world (Fominykh and Kvostova 1970;St.…”

The activation energy values estimated by the Arrhenius equation as a controlling factor of platinum-group mineral formation

“…Hexagonal, coarse-grained Os-Ir-Ru alloys, ranging from 100 lm to 2 mm have been located in the Papua New Guinea placers derived from ophiolites. They are often characterized by an unusually high Pt content, an association with Pt-Fe alloys and have been probably derived from ophiolites of the Papuan Ultramafic Belt (Johan et al, 2000;Malitch and Thalhammer, 2002). Euhedral macrocrystals of Pt-Fe alloy from the Kondyor PGE placer, Khabarovskiy Kray, eastern Siberia, Russia, exhibit inclusions suggesting a petrogenetic association with the late apatite-magnetitephlogopite clinopyroxenite bodies rather than the dunitedominant main intrusion (Malitch and Thalhammer, 2002;Shcekas et al, 2004).…”

“…Some authors have suggested that coarsegrained PGM have formed at low-temperature, during the weathering of mafic rocks and sedimentation in placers (Augustithis, 1965;Ottemann and Augustithis, 1967; 0016-7037/$ -see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.12.009 Cousins and Kinloch, 1976;Bowles, 1986;Barker and Lamal, 1989;Bowles et al, 2000) whilst others suggested that such coarse-grained PGM have crystallized at relatively high temperature, in a magmatic environment (Cabri and Harris, 1975;Hattori and Hart, 1991;Johan et al, 1991;Slansky et al, 1991;Cabri et al, 1996;Johan et al, 2000;Malitch and Thalhammer, 2002;Hattori et al, 2004;Tolstykh et al, 2005).…”