Contrasting Platinum-Group Mineral Assemblages From Two Different Podiform Chromitite Localities in the Pindos Ophiolite Complex, Greece

Abstract

Platinum-group minerals (PGM) have been identified in chromitite from two localities in the Pindos ophiolite complex in Greece, Milia and Korydallos. At Milia, the PGM are predominantly Os, Ir, and Ru, i.e., IPGE-bearing, as is typical of many chromitites in ophiolitic complexes; they consist mainly of IPGE alloys and sulfarsenides rather than laurite. At Korydallos, the assemblage is dominated by Pt-and Pd-bearing PGM characteristic of rarer Pt-and Pd-enriched ophiolites. These PGM include Pt-Pd alloys with the base metals Ni, Cu, and Fe, and traces of Au, PGM arsenides and sulfarsenides, sperrylite and members of hollingworthite -irarsite -platarsite solid-solution series. The base-metal-enriched minerals at Korydallos include a Cu-rich Pd-Pt alloy and a Ni-Fe alloy with 1-2% Pd. The association of Pt-and Pd-bearing PGM with base metals suggests that these PGE were collected by small quantities of immiscible sulfide liquid. The PGM are irregular in shape and are located almost exclusively in the altered silicate matrix interstitial to the chromite grains, on the edges of chromite grains, and in veins of silicate cross-cutting chromite. We suggest that the IPGE-bearing and Pt-and Pd-bearing PGM alloys and sulfarsenides are secondary and were formed by loss of sulfur and gain of arsenic during serpentinization. A further stage of alteration produced oxides and hydroxides of Pd around the PGE-bearing alloys and arsenides. SOMMaIReNous avons identifié les minéraux du groupe du platine (MGP) présents dans la chromitite à deux endroits du complexe ophiolitique de Pindos en Grèce, Milia et Korydallos. A Milia, les MGP contiennent surtout Os, Ir, et Ru, c'est-à-dire les éléments IPGE, tout comme c'est le cas dans les chromitites de plusieurs complexes ophiolitiques; on trouve surtout des alliages et des sulfarséniures des IPGE plutôt que la laurite. A Korydallos, l'assemblage contient surtout des MGP de Pt et Pd, caractéristiques des massifs ophiolitiques plus rares enrichis en Pt et Pd. Parmi les MGP à cet endroit, on trouve des alliages de Pt-Pd avec les métaux de base Ni, Cu, et Fe, et des traces d'Au, des arséniures et des sulfarséniures, la sperrylite et des membres de la solution solide hollingworthite -irarsite -platarsite. Parmi les minéraux riches en métaux de base à Korydallos, on trouve un alliage Pd-Pt riche en Cu et un alliage Ni-Fe avec 1-2% Pd. L'association de MGP de Pt et Pd avec des métaux de base fait penser que les éléments du groupe du platine ont été concentrés par une faible quantité de liquide sulfuré immiscible. Les MGP ont une morphologie irrégulière et sont situés presqu'exclusivement dans les zones altérées de la matrice silicatée interstitielle parmi les grains de chromite, le long de bordures des grains de chromite, et dans des veines de silicates recoupant la chromite. Nous croyons que les minéraux porteurs des IPGE d'une part, et les alliages et les sulfarséniures enrichis en Pt et Pd sont secondaires, §

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

“…The predominance of Os-, Ir-and Ru-bearing PGM in the chromitites having a chromian spinel with high Cr# [Cr/ (Cr + Al)] stands in contrast to the prevalence of Pt-and Pd-rich PGM in the chromitites hosting a chromian spinel with low Cr#. This further indicates that different types of magmas, having formed from varied degrees of partial melting of more than one mantle source (Economou-Eliopoulos & Vacondios 1995, Prichard et al 2008, were involved in the formation of the Pindos chromitites.…”

“…ophiolite complex has been investigated during the last two decades by Economou-Eliopoulos & Vacondios (1995), Tarkian et al (1996), Economou-Eliopoulos et al (1999), Grammatikopoulos et al (2007), and Prichard et al (2008). Their results indicate that two distinctive compositional groups of chromitites can be documented, solely on the basis of chromian spinel, high-Cr and high-Al types, each of which shows a relatively restricted compositional variation.…”

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

“…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 equilibrium constant for the transfer of an ion from one phase to another is directly related to Gibbs free energy and the partition coefficient, which are defined by the behavior of the elements. The apparent epigenetic desulfurization at relatively low (350-500˝C) temperature and partial oxidation of primary laurite crystals has been facilitated by strong brittle deformation and crystal deformation and dislocations, making it possible to remove the sulfur [3,13,48].…”

“…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].…”