Genesis of chromitites from Korydallos, Pindos Ophiolite Complex, Greece, based on spinel chemistry and PGE-mineralogy

Abstract

The Pindos Ophiolite Complex, located in northwestern Greece, hosts various chromite deposits of both metallurgical (high-Cr) and refractory (high-Al) type. In Korydallos are encountered both types of chromitites. These are podiform chromitites that have small dimensions and occur sub-concordantly to the hosting peridotites. The Cr-rich chromitites contain magnesiochromite with high Cr# [Cr/(Cr + Al): 0.65–0.68] and low platinum-group element (PGE) contents (294.1 ppb), whereas the Al-rich ones host spinel with low Cr# (0.44–0.48) and elevated total PGE grades (28830 ppb). The former display a nearly flat C1-normalized PGE-pattern, whereas the latter show a positively sloped normalized PGE-pattern. The in situ mineralogical investigation of the Cr-rich chromitites revealed a platinum-group mineral (PGM) assemblage dominated by small (≤ 3 μm) sperrylite, laurite and erlichmanite grains (determined by recalculated qualitative analytical data). Textural relations suggest crystallization under conditions of high fS2 and fAs and/or low T. The in concentrates mineralogical study of the Al-rich chromitites showed that the PGM assemblage that they host is dominated by Pd–Cu and Pd–Au–Cu alloys. The vast majority of these alloys is associated with abundant secondary BMS (base-metal sulfides) and BMA (base-metal alloys), thus confirming that a sulfide melt scavenged the PGE + Au of the silicate magma from which chromian spinel had already started to crystallize. Both assemblages were affected by an invasion of an oxidizing aqueous fluid in the investigated chromitites. Combined data indicate that the chromitites from the Korydallos area crystallized from a progressively differentiating MORB/IAT melt, produced in a small backarc basin in a supra-subduction zone setting.