Compositional fingerprints of chromian spinel from the refractory chrome ores of Metalleion, Othris (Greece): Implications for metallogeny and deformation of chromitites within a 'hot' oceanic fault zone

摘要

The chrome ores of the retired Metalleion mine of the Othris ophiolite are hosted in a small volume of a pervasively serpentinized, tabular harzburgite body. These ores have been studied to determine their geological mode of occurrence, mineralogy and chromian spinel (Cr-spinel) chemistry. The ores consist of massive chromitite (85-95% modal Cr-spinel) with mylonitic fabric in imbricate-shaped pods. Chromian spinel displays a limited range in Cr# [Cr/(Cr + Al) x 100 = 53-63] and Mg# [Mg/(Mg + Fe2+) x 100 = 59-73] and low TiO2 content (= 0.11 wt%). Minor- (Ti, Ni, V, Mn and Zn) and trace-element (Sc, Co and Ga) concentrations do not show any significant variations from Cr-spinel cores to boundaries and were not considerably modified by post-magmatic processes. However, Cr-spinel compositions show slight enrichments in Zn and V, and depletions in Ti and Sc when compared to the composition of chromite from the East Pacific Rise mid-ocean ridge (MOR) basalts. The composition of Cr-spinel from the Metalleion chromitites is quite anomalous on a global perspective as these ores are not clearly associated to a suprasubduction zone (SSZ) or MOR chemistry.