Ru Os Ir Pt AND Pt Fe ALLOYS FROM THE EVANDER GOLDFIELD, WITWATERSRAND BASIN, SOUTH AFRICA: DETRITAL ORIGIN INFERRED FROM COMPOSITIONAL AND OSMIUM-ISOTOPE DATA

New constraints on the genesis and long-term stability of Os-rich alloys in the Earth’s mantle

“…Furthermore, our results are consistent with the empirical observation that the bulk of Os-bearing alloys found in nature are associated with placer deposits found in the vicinity of ophiolites (e.g. Nakagawa and Franco, 1997;Meibom and Frei, 2002;Malitch and Merkle, 2004;Pearson et al, 2007;Coggon et al, 2011), most of which appear to be related to back-arc basins. However, based on our results, the more depleted peridotites are also expected to include residual alloys phases, which are indeed found on occasion (e.g.…”

“…Therefore, even depleted mantle sources require significant melt extraction to allow for the complete removal of S from a source, and the exsolution of osmiridiums. It is not surprising then that most osmiridiums found in nature are directly or indirectly associated with highly depleted mantle sources that suffered considerable melt extraction, like the sources of picrites, boninites, and komatiites (Malitch and Merkle, 2004;Pearson et al, 2007;Barnes and Fiorentini, 2008). Back arc-settings, where mantle sources suffer large-scale melting episodes due to the fluxing of the mantle wedge with water released from a subducting slab, are ideal locations for the formation of osmiridiums (cf.…”

“…Walker et al, 2002). Osmium-rich alloys dated using the 187 Re-187 Os radiogenic decay system are known to record Re-depletion ages in the order of 3-4 Ga despite being subjected to equilibration temperatures in excess of 1000°C (Malitch and Merkle, 2004;Pearson et al, 2007; and references therein). Pearson et al (2007) have identified osmiridiums with model ages older than 1 Ga in ophiolite suites with much younger emplacement ages (e.g.…”

Native osmium from the Guli Massif, Northern Siberia, Russia

“…For instance, the Os-IrRu alloys from the Evander Goldfield, located at the eastern extremity of the Witwatersrand Basin (see Fig. 1 in Malitch and Merkle 2004), differs from those of the other six goldfields (i.e., East Rand, Central Rand, West Rand, Carletonville, Klerskdorp, and Welcom) in systematically higher Ru contents and different modal proportions of chemically distinct Os-rich alloys. Detailed evaluations of compositional trends of native osmium from a lithologically homogeneous source, that could facilitate the establishment of correlation between systematic substitutions and petrological evolution of the source rocks, are missing.…”

“…To avoid confusion with osmium as an element, we will refer to the mineral grains as native osmium. Native osmium has been described in situ from ophiolites (Dmitrenko et al 1985;Legendre and Augé 1986;Augé and Johan 1988;Ohnenstetter et al 1991;Ohnenstetter 1992;Melcher et al 1997;Augé et al 1998;Garuti et al 1999;Melcher 2000;Zaccarini et al 2005;Ahmed 2007;Shi et al 2007;El Ghorfi et al 2008;Kapsiotis et al 2009;Uysal et al 2009;Gonzalez-Jimenez et al 2010 and references cited therein) layered intrusions (Cabri and Laflamme 1988;Balabonin et al 1994;Merkle 1998), and predominantly from placer deposits all over the world (Feather 1976;Cabri et al 1996;Malitch 1996;Krstid and Tarkian 1997;Nakagawa and Franko 1997;Gornostayev et al 1999;Weiser and Bachmann 1999;Weiser 2002;Malitch and Merkle 2004;Barkov et al 2005;Tolstykh et al 2005Tolstykh et al , 2009 among many others). For many placers, the location and type of the source rocks (e.g.…”