SPINEL COMPOSITION, PGE GEOCHEMISTRY AND MINERALOGY OF THE CHROMITITES FROM THE VOURINOS OPHIOLITE COMPLEX, NORTHWESTERN GREECE

Abstract

The Vourinos ophiolite complex, in northwestern Greece, hosts abundant deposits of chromian spinel of historical economic importance. The chromitites have been investigated with regards to the composition of the chromian spinel and olivine, and the distribution of the platinum-group elements (PGE) minerals. Chromitites are composed of magnesiochromite, accompanied by magnesian olivine and locally abundant micro-inclusions of pargasitic amphibole. The composition of magnesiochromite varies significantly in southern Vourinos (Cr#: 0.58–0.81), and less in northern Vourinos (Cr#: 0.75–0.83). Olivine cores from the chromite deposits of southern Vourinos have lower Mg# and Ni than those of northern Vourinos. The PGE contents vary between 13.6 and 375.4 ppb, and are higher in IPGE with respect to PPGE, which is typical of mantle-hosted ophiolitic chromitites. The PGE abundances of southern Vourinos chromitites correlate positively with the Cr# of the ore-hosted magnesiochromite. Consistent with the geochemical data, the platinum-group mineral (PGM) assemblage is almost exclusively dominated by Ru–Os–Ir phases. Two distinct PGM assemblages have been recognized: 1) a primary one, formed at the early magmatic stage before or during magnesiochromite crystallization, and 2) a secondary one, formed by desulfurization–oxidation of primary PGM sulfide precursors (laurite). This laurite has a Ru# value ranging from 0.55 to 0.98. The secondary assemblage is dominated by oxygen-bearing compounds of Ru–Os–Ir–Fe–Ni, some of which exhibit intriguing internal rosette-like textures. Raman spectroscopy data on these PGE-bearing oxygen compounds did not reveal any OH − content, indicating that they might represent oxides. Compositional discrepancies regarding the NiO content and the Mg# of olivine, as well as the Os content of laurite, suggest that the chromian spinel deposits from the south and the north of the complex have been formed under different physicochemical [ f ( S 2 )–T] conditions. Combined geological, mineralogical and compositional data indicate that the Vourinos chromitites crystallized from a progressively differentiating melt of boninitic affinity.