Metallogeny of the Chrome Ores of the Xerolivado-Skoumtsa Mine, Vourinos Ophiolite, Greece: Implications on the genesis of IPGE-bearing high-Cr chromitites within a heterogeneously depleted mantle section

Chrome ore deposits comprise less than 1% of the volume of a pervasively serpentinized dunite body (similar to 3.5 km(3)) that constitutes the Xerolivado-Skoumtsa Mine of the Vourinos Ophiolite in northwestern Greece. Ores have been examined to aid determination of their geological mode of occurrence, their mineralogy and mineral chemistry (Cr-spinel and olivine), and platinum-group element (PGE) geochemistry. The ore bodies are highly deformed and of the schlieren type, where Cr-spinel-rich and serpentine (after olivine)-Mich layers (1-20 cm thick) alternate. The Cr-spinel displays a limited range in Cr# [100 x Cr/(Cr + Al)] atomic ratio (77-85) and TiO2 content (0.05-0.22 wt%). The Mg# [100 x Mg/(Mg + Fe2+] of Crspinel shows a wider variation (52-74) indicating that subsolidus (Mg-sp-Fe(2+)ol) re-equilibration occurred. Remnant fresh olivine in the serpentine-rich layers is typical forsterite [Fo#: 100 x Mg/(Mg + Fe2+) = 9395]. The Cr-rich chromitite ores have total PGE and Au abundances that range between 743 and 205 ppb: they show a general enrichment in Ir and Ru over PPGE (Rh, Pt and Pd) resulting mostly in a negatively sloping Cl chondrite-normalized PGE profile with a quite strong Ru peak. Field observations indicate that the schlieren banding of the deformed ore bodies is a tectonometamorphic feature acquired within ductile deformation of the ophiolitic slab. Geochemical calculations demonstrate that the parental melts of the Xerolivado-Skoumtsa high-Cr chromitites were boninitic. Their high Ru content and variation in CrOsp values indicate a polygenetic origin from geochemically similar, but spatially distinct melt inputs. These melts originated within a hydrated mantle wedge beneath a forearc basin within an evolving lithospheric slab. We emphasize that the PGE budget of the Xerolivado-Skoumtsa chromitites was not inherited from interactions of the migrating melts with their parental harzburgites, but it is an intrinsic feature of the deep-seated and heterogeneously depleted mantle source of these magmas. (C) 2017 Elsevier B.V. All rights reserved.