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)

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The Pindos ophiolite complex, located in the north-western part of continental Greece, hosts various podiform chromite deposits generally characterized by low platinum-group element (PGE) grades. However, a few locally enriched in PPGE + Au (up to 29.3 ppm) chromitites of refractory type are also present, mainly in the area of Korydallos (south-eastern Pindos). The present data reveal that this enrichment is strongly dependant on chromian spinel chemistry and base metal sulfide and/or base metal alloy (BMS and BMA, respectively) content in chromitites. Consequently, we used super-panning to recover PGM from the Al-rich chromitites of the Korydallos area. The concentrate of the composite chromitite sample contained 159 PGM grains, including, in decreasing order of abundance, the following major PGM phases: Pd-Cu alloys (commonly nonstoichiometric, although a few Pd-Cu alloys respond to the chemical formula PdCu4), Pd-bearing tetraauricupride [(Au,Pd)Cu], nielsenite (PdCu3), sperrylite (PtAs2), skaergaardite (PdCu), Pd-bearing auricupride [(Au,Pd)Cu3], Pt and Pd oxides, Pt-Fe-Ni alloys, hollingworthite (RhAsS) and Pt-Cu alloys. Isomertieite (Pd11Sb2As2), zvyagintsevite (Pd3Pb), native Au, keithconnite (Pd20Te7), naldrettite (Pd2Sb) and Rh-bearing bismuthotelluride (RhBiTe, probably the Rh analogue of michenerite) constitute minor phases. The bulk of PGE-mineralization is dominated by PGM grains that range in size from 5 to 10 mm. The vast majority of the recovered PPGM are associated with secondary BMS and BMA, thus confirming that a sulphur-bearing melt played a very important role in scavenging the PGE + Au content of the silicate magma from which chromian spinel had already started to crystallize. The implemented technique has led to the recovery of more, as well as noble, PGM grains than the in situ mineralogical examination of single chromitite samples. Although, the majority of the PGM occur as free particles and in situ textural information is lost, single grain textural evidence is observed. In summary, this research provides information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing.

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The Yarlung Zangbo Suture Zone (YZSZ) of South Tibet is divided by the Zhongba-Zhada terrane into two subparallel ophiolitic belts in its western end. The peridotite massifs of the southern belt tectonically overlie the Tethyan Himalaya sequence. The Purang peridotite body in this belt is intruded by two groups of dolerite dikes, providing significant compositional, geochronological, and isotopic information about the melting history of the Neo-Tethyan mantle. U-Pb ages of zircons separated from dolerites show that peridotites of West Purang were intruded by an early generation of dikes at 138.5 ± 2.0 Ma (Valanginian). These dolerites show ocean island basalt (OIB)-type normalized multi-elemental profiles and Sr-Nd isotopic signatures [(La/Yb) N = 13-16], high initial 87 Sr/ 86 Sr ratios (0.70598-0.70765), and low ε Nd (t) values (-2.6 to -2.3). Zircons separated from this group of dolerites have slightly radiogenic ε Hf (t) values (+2.6 to +4.6). The next generation of dolerite dikes intruded the East Purang peridotites between 124.5 ± 2.5 Ma and 124.4 ± 3.2 Ma (Aptian). These East Purang dolerites show normal mid-ocean ridge basalt (N-MORB)type normalized multi-element patterns [(La/Yb) N = 0.6-0.9] with noticeable negative Nb and Th (±Ti) anomalies, and have high 87 Sr/ 86 Sr (i) (0.70295-0.70618) and high ε Nd (t) values (+7.7 to +9.2). Zircons separated from the East Purang dolerites show strongly radio genic ε Hf (t) values (+3.5 to +17.0). Semiquantitative geochemical modeling demonstrates that the parental magmas of West Purang dolerites were generated from 5%-10% polybaric partial melting of a deep-seated juvenile asthenospheric source enriched by plume-type components. In contrast, the parental melts of East Purang dolerites were derived from more than 20% melting of a juvenile spinel-bearing MORBtype mantle source that was modified by subduction-related melts/fluids to a minor extent. A possible tectono-magmatic model for the petrogenesis of the Purang ophiolitic massif could be linked to incipient continental rifting and subsequent oceanic seafloor spreading associated with decompression upwelling of an asthenospheric source contaminated by plume-type components. This plume-proximal seafloor spreading-system was succeeded by the initiation of Neo-Tethyan intra-oceanic subduction close to the active continental margin of Eurasia during the Early Cretaceous.

The Vourinos ophiolite complex, located in northern Greece, hosts various chromite deposits characterised by very low platinum-group element (PGE) grades. Total PGE (excluding Os) concentrations in the fourteen chromitite samples collected for use in this study varied from 200 to 300 ppb.Previous reports on the platinum-group minerals (PGM) from the Vourinos chromitites, obtained data by in-situ investigation on polished sections. Consequently, we used the technique of hydroseparation to study the PGM from the concentrates in the Vourinos chromitites. More specifically, we investigated two separate composite samples from Voidolakkos and Xerolivado chromitites. The Voidolakkos concentrate sample contains 74 PGM that include: laurite (Ru,Os)S 2 ; irarsite (Ir,Ru,Rh,Pt)AsS; erlichmanite (Os,Ru)S 2 ; ruthenium pentlandite; iridium (Ir(Os)); osmium (Os(Ir,Ru,Pt)); secondary phases composed of Ru, Os and Cu; alloys of Ir-Fe; Rh-and Ru-Ni-Fe alloys; and Os-Ir-Fe alloys. The investigation of Xerolivado concentrate sample yield 45 grains of PGM, including laurite, irarsite, erlichmanite, minor other PGE sulphides, Os-Ir-Ru alloys, iridium and secondary phases of Ru-Os alloys. PGM occur as both single and polyphase particles in both samples. The bulk of mineralisation in Voidolakkos is dominated by a finer variety (,10 mm) of PGM than the Xerolivado sample (mainly ,20 mm). The former occurrence hosts considerably more altered PGM grains, less laurite and a larger variety of PGM than the latter, whereas Os-Ir alloys are present in almost equal amounts in both samples. The hydroseparation process has recovered significantly more, as well as novel, PGM grains than were known from previous in-situ mineralogical examination of single chromitite samples. Although, most of the PGM occur as free particles and in-situ textural information is lost, single grain textural evidence is observed. The mineralogical and grain size differences between the two samples may reflect styles of mineralization and indicate significant remobilization of PGE. The latter possibility is suggested by the presence of secondary PGM, which may be related to the different alteration processes that affect the Voidolakkos and Xerolivado chromitites. In summary, this study provides significant new information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing of these minerals.