Chromian Spinel Composition and Platinum-Group Element Mineralogy of Chromitites From the Milia Area, Pindos Ophiolite Complex, Greece

Abstract

The chromitites of the Milia area in the Pindos ophiolite complex, in Greece, were investigated for their platinum-group mineral (PGM) content. The chromitites are massive, more rarely disseminated in texture and occur as small pods. They are composed of magnesiochromite crystals with Cr# [Cr/(Cr + Al)] between 0.80 and 0.84, and Mg# [Mg/(Mg + Fe 2+ )] between 0.54 and 0.72. The total platinum-group-element (PGE) contents in chromitites are relatively low (≤170 ppb), although they may locally be higher (up to 1059 ppb). The IPGE (Os, Ir and Ru) predominate over the PPGE (Rh, Pt and Pd). The PGM assemblage, consistent with the geochemical data, is dominated by laurite and Os-Ir-Ru alloys that occur both as single and composite grains, generally less than 15 mm in size. Laurite has a wide range of Os-for-Ru substitution [Ru/(Ru + Os): 0.42-0.99], whereas primary alloys are enriched in Ru (up to 73.80 wt.%). Some laurite crystals exhibit an anomalous pattern of zoning. Such zoning requires an inversion of the normal T-f(S 2 ) trend in magmatic systems, and is herein considered to be due to postmagmatic processes. Some Ru-rich alloy grains contain relatively high Rh and Pt abundances, similar to those of residual sulfides in mantle peridotites. This feature suggests that these alloys may represent residual PGM phases from earlier episodes of melt extraction from the severely depleted mantle unit of the Pindos ophiolite complex. Combined compositional data indicate that the Milia chromitites formed from a hydrous boninitic melt in a suprasubduction-zone environment.Keywords: chromitites, platinum-group minerals (PGM), laurite, alloys, ophiolites, Pindos, Greece. Certains grains d'alliages riches en Ru contiennent des teneurs relativement élevées en Rh et Pt, tout comme les sulfures résiduels de péridotites du manteau. Ces grains d'alliage pourraient donc représenter des MGP résiduels après l'extraction antérieure de liquide silicaté, laissant un manteau très stérile dans le complexe ophiolitique de Pinde. Les données combinées sur la composition indiquent que les chromitites de Milia se sont formées à partir d'une magma hydraté boninitique dans un milieu au dessus de la zone de subduction.(Traduit par la Rédaction)

Gibbs Free Energy of Formation for Selected Platinum Group Minerals (PGM)

“…The presented (∆G f ) values for sperrylite (PtAs 2 ) (Table 1; Figure 5) seem to be consistent with its common occurrence in chromitites as an interstitial phase between chromite grains [5,[13][14][15].…”

“…Although the stability relationships between various phases can be studied using the experimental method, thermodynamics may give us an approach to the calculation of the Gibbs free energy and phase diagrams [28]. However, the requirements for the PGM stability and re-deposition remain still unclear, since the activity-composition models and relevant phase diagrams for PGM are very limited, and the PGM forming processes in natural systems are complicated [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]28].…”

“…The strong effect of As on PGE partitioning has been documented by the formation of stable high-temperature phases between As and Pt or Rh, including sperrylite (PtAs2), hollingworthite (Rh, Pt, Pd)AsS, and platarsite (Pt, Rh, Ru)AsS [5,[13][14][15]. The presented (∆G f ) values for sperrylite (PtAs 2 ) (Table 1; Figure 5) seem to be consistent with its common occurrence in chromitites as an interstitial phase between chromite grains [5,[13][14][15].…”

“…Platinum groupelements (PGE) can be classified into two subgroups: the Os-, Ir-, Ru-rich or IPGE (more refractory) and PPGE (low-melting and more soluble) (Ir group), interpreted to reflect compatible behavior (partition or distribution coefficient between solid and magma, D ě 1) during large-degree mantle melting and (Pt, Pd)-rich or PPGE assemblages, reflecting the incompatible behavior (D ď 1) of the PPGE, showing enrichment as a function of the differentiation degree [1]. The behavior of PGE during partial melting and crystal fractionation and which minerals are collectors of PGEs have been investigated and reviewed extensively [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].…”

Native osmium from the Guli Massif, Northern Siberia, Russia

“…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.…”