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High PGE contents and extremely abundant PGE-minerals hosted in chromitites from the Veria ophiolite complex, northern Greece

2007; Elsevier BV; Volume: 33; Issue: 1 Linguagem: Inglês

10.1016/j.oregeorev.2006.10.008

1872-7360

George Tsoupas, Maria Economou-Eliopoulos,

earthquake and tectonic studies

More than 100 small, isolated chromite occurrences form elongated lenses that follow the NW–SE trend of the host serpentinized dunite within a 60 km2 area 15 km SE of Veria, northern Greece. They occur only within mylonite shear zones, characterized by a strong overprint of late brittle deformation. Detailed investigation of chromitites by scanning electron microscope with energy-dispersive X-ray analysis shows that chromite is dominantly of high-Cr composition, with the atomic Cr/(Cr + Al) ratio ranging from 0.55 to 0.84, and an average value of 0.7. Interstitial primary silicates in chromitites have been replaced by secondary phases consisting of mostly Cr-andradite, Cr-chlorite and serpentine flakes in lesser amounts. The most salient feature is the high total platinum-group element (PGE) content (up to 25 ppm), coupled with the presence of extremely abundant solitary platinum-group element minerals (PGM) within massive chromite samples located along a shear zone. These PGM fragments are angular, rarely corroded grains or fine-grained aggregates and occur in a matrix of Cr-garnet. They are mostly Os–Ru–Ir–Fe alloys containing small amounts of Cr, Co ± Mn, followed by irarsite. Laurite (RuS2) with variable Os and Ir contents occurs as remnants surrounding by Ru–Os–Ir oxides of varying composition. Available geological, mineralogical and geochemical data indicate a prolonged multistage evolution (at least four stages) for the PGE mineralization. The relatively high IPGE enrichment in chromitites seems to be related to post-magmatic processes encompassing a long period of deformation episodes, beginning with plastic deformation in the asthenospheric mantle and culminating with brittle deformation in the crust. Metasomatic fluids have, during a process of rodingitization and serpentinization at a low state of sulfidation, substantially modified the composition of primary laurite towards Fe-, Cr-, Co-, and Mn-bearing Fe–Os–Ir–Ru alloys and oxides. The most prospective locations for chromitite–PGM targets seem to exclusively be small chromite occurrences that are found along shear zones within ophiolite complexes. These shear zones post-date the initial magmatic deposition of the host chromitites.